Quadrajet Adjustment

[ChillrMn1]

At what point should the secondary throttle valves start to open relative to the position of the primary throttle valves? I have been told they should start to open when the primaries are about 40 - 50 degrees or should they start to open at a particular RPM considering the particular boat for a better power curve?

I ask this because I recently purchased a remanufactured carb, and as received, the secondaries don't open until the primaries are wide open. I know this isn't correct. Possibly a mismatched linkage assembly, however bending of the tang will correct this.

Thanks for any help.

[woodboat]

Well I don't have one handy but have used them for twenty years now. Don't know the exact percentage but they need to start opening around that 50% mark because from that point on the secondaries need to catch up so that they both reach 100% together. In other words with that last 50 percent you need to open the secondaries from 0 to 100 percent.

[ChillrMn1]

Thanks Woodboat. That confirms the 40 - 50 degree suggestion. I'll set it up by bending the tang.

[Richard Petersen]

If you were given the correct MARINE carb. as a replacement the linkage will be perfect. If NAPA auto- could be wrong. Marine cams suck fuel at 1000 rpm- early gradual opening is correct. Play with the tang a few times and it will break. Take the carb back. Rich.

[ChillrMn1]

It's definitely a marine carb. I compared the opening positions of the throttle valves of the old carb, and essentially adjusted the new one to match these positions.

EDIT - Did some checking ti I.D. the numbers stamped on the body of this carb. Apparently, this carb was originally manufactured to meet California emission standards. Looks like the rebuilder used various parts and assembled so this would meet marine requirements. It'll be interesting to see how the rods and hanger assembly match up with readjustment of the throttle plates. Purchased this carb from California, I'm in Florida so it's not that easy to return

After test run, I'll post back.

Thanks.

[ChillrMn1]

Utera,

Don't have the slightest idea what that link is saying. I'll check back with you after I complete my foreign language course.

Have a good day,

Bob

[Richard Petersen]

I am no walking marine carb expert, but, I doubt if he could or would change all the air and fuel metering -jets -air bleeds- metering rods- to keep it richer than a Calif. for use in a boat. Ask him if it will pass a U.S.C.G. inspection or will he and you both have to go to court and pay the fine. He should want to swap with you. Best of luck. Rich.

[ChillrMn1]

Carb performs much better since syncronizing the secondary throttle plates to start opening when primary throttle plates reach about 50% open. Slight adjustment now required to secondary air valve to address minor bog issue. This should be no problem.

Thanks for everyone's input.

[gonzo]

There is much more to adjusting a Q-Jet than angle of throttle plates. It also has to have the right jets, power valve and springs. The reason so many people think they don't perform, is that they try to use any carb setup. Check the manual for your engine application and make verify the jet size and the springs(they are color coded)

[ChillrMn1]

Your absolutely right Gonzo,

I'm not an expert on Quadrajets, but do know you can properly tune a quadrajet to outperform most others - when done right with a lot of patience. For me, next time to replace, it will not be with a remanufactured unit, but with a new quadrajet set up specifically for a particular application or a new spreadbore Holley with vacuum secondaries. Life will be simpler for me this way. In the meantime, I'll get it close enough to be satisfied. As a note, when we were kids with our "HOTRODS", we would immediately get rid of the quadrajets and replace with Holleys, only because we understood them better. I KNOW, I KNOW - I will catch hell for that statement about replacing the quadrajets.

Have a great day!

[Richard Petersen]

It is a known fact that rebuilding or recalibrating a QUAD is a lot worse than reprograming a fuel injected system. In all fairness -carbs are more forgiving- computers for fuel injectors give it up completly if they do not have battery reversal and short circuit protection.

[gonzo]

Q-Jets have mechanical or vacuum secondaries also. Haynes publishes a book for calibrating these carbs. I haven't checked if they include marine applications, but they have a chapter of theory of operation.

[ChillrMn1]

Yes, heard of the book and was tempted to buy. But found this link which was very informative, although application was automotive, assumed to generally apply for my application also: http://www.442.com/oldsfaq/ofcrb.htm. Excellent info on quadrajets, everything from rods, hangers, other tricks, you name it. My boat is a '78, 26' Pacemaker Flybrdge w/350 CID remanufactured rated at 260 HP, single inboard. This is not a hypo speed demon, but a sloppy 16 knot cruising speed platform used for fishing. This engine performs as good as could be expected, except possible slight improvement in fuel efficiency, but at this point, probably negligible. After consideration to time spent dissassembly this carb, dollars to replace and try different combinations, I feel I can accept as is. But then again, being the type of person I am, I could be tearng this carb apart tomorrow just to see what if. On the other hand, for $200.00 more than this rebuilt cost me, I could have purchased the correct carb, no questions asked. We'll see what the next few days bring. Everthing is fixed on the boat, so it's about time for me to break something on purpose so I'll have something fix. It's more peacefull down at the boat than hearing the dog bark at the cat and the daughter argue with the wife. Thanks for all the input and Happy New Years To All!

Regards,

Bob

[woodboat]

I had an improperly tuned Quad in my 26 ft Trojan with a 307 Chevy. It used 25 gallons in about three hours. Turned 3800 WOT. When tuned would do 4200 WOT and fuel mileage easily tripled. The power valve was opening way too early and the secondaries were too rich. Simply replaced the power valve spring and got leaner secondaries. also lowered the front metering rods a bit. Gathered up a few pairs and simply ran WOT tests.

[ChillrMn1]

All right Woodboat - you did it to me, got my curiosity, now I have to check this to satisfy myself. My engine is a remanufactured long block, reverse rotation, 350 CID, supposedly rated at 260 - 270 HP., the GM block casting number indicates the block itself manufactured in 1984. This engine was in boat when I purchased it and was replaced in 2001, presently has 580 true operating hours. I have no engine paperwork and don't have a clue who it was marinized for, but the previous owner said Mercruiser. This I question as he had given me lip service regarding other specifics. I would think the same carb requirements would apply as for a Quadrajet carbureted Crusader with the same displacement and HP. I know my present engine was not dyno tested so HP probably closer to 220 or 230.

My problem now is trying to find what the correct specified jets, rods, power spring, hanger, etc., would be for this application. Any ideas where I could gather this info? I've performed web searches but to no avail. I have a good marine parts man I'll be checking with today. Buying the parts will be no problem, I know where to source these. Just need to know what I need and don't want to go thru the trial and error routine. If it was a car, then trial and error not so bad, but in a boat so many variables to deal with, ie., different water conditions, currents, traveling to get to speed zones, etc.

Everybody on this forum has been very helpful and this is appreciated.

[woodboat]

In my opinion coping a working config would only be a starting point. How YOUR engine is proped, as well as cam and cam timing will have a huge effect. My particular engine had a bigger cam that was also advanced 4 degrees. This caused a lower vacuum at cruising speeds. So I had to change my power valve spring so that it wasn't opening on a light cruise. It was three boats ago so I am not 100% sure but think I went to a 3-5 inch Vacuum power valve spring. I got my number by using a vacuum gauge while running on a light cruise. As for WOT I borrowed a bunch of secondary rods from a friend that had used them to tune his Corvette. Knowing I was way too rich meant I only needed rods that were leaner. I then simply switched rods and tested. No carb disassembly is required. Simply switch rods and retest WOT. When I purchased them rods were about $5 a pair. I got three or four pair skipping a few to get a nice test field. Ended up very lean. It was after all only a 307 with small heads. SO find out what is in there now and buy one richer and two leaner and give it a try. You may find that what you have is best :) but at least you would know.
http://www.hioutput.com/tech/qjetrod.html
http://buickperformance.com/qjet.html
http://members.dandy.net/~k0xp/Oldsmobile/QJet/QjetTuningHelper.htm

PP Spring Notes
7037851 72 W-30 very soft (lean)
7029922 69 W-31 soft/medium
7011967 69 W-30 medium
7036019 70 W-30 auto stiff (rich)

Install the leanest first, run the car, and then try the next, and so on. Stay as lean as possible here as these springs affect idle more than power.

Next, we'll work with the primary metering rods. These are the skinny littly guys which are stuck in the primary metering jets. [Graphic of primary & secondary metering rods]



from http://archive.classicoldsmobile.com/html/108.html

[Richard Petersen]

Interesting fact. A stationary power plant 305 or 350 GM engine has a torque curve plotted by the factory at 1" of vacum @ manifold. HP and torque were about 160's each. I am glad I have 220 hp in my barge. Rich

[ChillrMn1]

Little follow up to results on my Quadrajet carb changes: Before changes - Idle speed in neutral was 700 RPM, no changes to air mixture adjustment or idle speed. Also, since installation of this unit, adjustment to the air mixture screws had no response. Replaced secondary metering rods identified as "CH" to "CL". These parts were removed from the previous carb that was so fouled that the secondary passages could not be cleared from blockage, thus reason for purchasing present remanufactured unit. The secondary rod hanger was the same "H" so this wasnt changed. Also shecked the PP spring tension but did not replace. The spring considered for replacement was a higher tension spring and did not want to create a richer situation. Reassembled Carb. Upon start up with replaced metering rods, idle RPM out of gear was now 1100 RPM. Readjusted idle speed adjustment back to 700 RPM. Adjusted idle mixture screws and these are now responding.

If I understand the metering rod chart correctly from the previous link provided, the "CL" rods are slightly larger in diameter and longer resulting in a leaner flow. This may explain having to adjust the idle speed adjustment to allow more air due to the richer flow the "CH" rods were allowing and resultant lack of response from the air mixture screws. It seems I've made some headway, but not sure if my assumptions are correct. One more thing, was able to do a test ride on short stretch of water, and the legendary bog was no longer there when the secondary throttle plates opened, another good sign. Was unable to notice an increase in WOT due to the short stretch of water and not being able to put trim tabs into play. Will be able to get better idea of improvement when making that trip back to the swordfishing grounds. This is usually a 42 - 43 gallon trip for me.

I could have probably purchased a manual for this but more enjoy getting into things cold and figuring out as I go. This way of doing things seem to stick with me longer as well learned lessons. Trick is, just to be smart enough to know when you're in over your head and stop.

I appreciate any and all suggestions, comments, negative or positive. Again, thanks for the time you fellows have spent helping me with advice. Anxiously looking forward to any other thoughts. The boat runs great now and would pass as such. I might just have have thing, maybe OCD, but really get a sense of satisfaction when knowing mechanical machines are operating at their optimum peak performance levels.

Thanks Again And Looking Forward To Hearing From You!

Regards,

Bob

[Richard Petersen]

About 35 years ago I screwed up carbs more by fiddeling than improving. Got smart and bought a exhaust gas analyzer. Bingo- saw that the carbs were set right out of the box. Now I use my improved system. Replacement must run better or equal to, than old unit. Carefully drilling in a riser would be needed in a water cooled system.

[ChillrMn1]

Your right Rich,

Same concept as I've used years ago to analyze combustion gas flues/vents for boiler efficiencies - ratio of fuel to combustion air, incomplete burn, carbon monoxide PPM for lack of oxygen, etc. I know excellent shops down here for remanufacturing USCG approved alternators, starters, etc., meeting the appropriate SAE requirements. BUT...................Made a mistake on saving money buying a remanufactured carb, especially a quadrajet that can be modified for so many slight but variable engine modifications which may result in less than acceptable performance and or fuel efficiency. The way I know some of these carb shops operate, carbs in pieces laid out on big benches, a part from here, a part from over there, bolt it together, and out the door. Learned my lesson. In the future, It'll be a new carb for the specific engine application. And if I had twin gas engines, they would be EFI, no question. My problem with a single engine EFI, would be if I broke down 20 miles or so out, I'm afraid of not being able to get it back up and limp in. I feel more comfortable with a carbureted engine due to the simplicity. Guess I'm one of the old dinosaurs when it comes to the newer technologies.

Bob

[Richard Petersen]

67 young and new 2002 Chaparral has 305/ carb 2 bbl 220 hp -- thunderbolt V distributor. VHF and cell phone charged up. Best I can do. Long live the dinosours.

[woodboat]

On the PP spring, best to stay as lean as possible. Regaining control of the mixture screws is a very good sign. As far as the secondary rods, regardless of tip length they both should seal the hole at idle and not effect mixture. I suspect something was assembled incorrectly and you fixed it, knowingly or not. Yes, according to the chart the longer tipped CL will run leaner once the secondaries begin to open.

[ChillrMn1]

Woodboat,

You mention I may have unknowingly fixed something - The Power Piston assembly did appear to be stuck when I was trying to remove to examine the spring type/tension. In other words, this assembly would not slide up & down with the force of the spring. I even had a little difficulty removing this assembly from the "cylinder it rides in", thought I might even be damaging it while removing with or pulling out with pliers. Upon reassembly of the PP & spring assembly and fine polishing of the piston, this assembly would now slide up & down freely with only the force of the spring.

With the PP in a fixed position (primary metering rods out of primary jets), seems like this would provide a certain fuel flow at all conditions in lieu of only providing a flow when the spring tension & manifold vacuum was in control of the piston. Is this correct? I think I may have just answered my own question, but any thoughts as always are appreciated. I also had a heck of a time aligning the primary metering rods into the primary jets properly. Maybe the rebuilder never had both within the jets. Seems like this might explain why I had no control with the air mixture adjustment screws.

As a hotrodding teenager back years ago, I don't recall Holleys being so critical nor be able to be so finely tuned. Yeah, you could change jets, etc., but it seems with Q-Jets, one can really dial these in. After this (my first real foray into a quadrajet) I can really start to understand how these Quadrajets are able to be so finely tuned to meet the specific demands of a particular engine especially once one understands the different combinations needed to produce the desired results. This I still have a ways to go on. This has been a good experience for me. I've also gained new respect for Q-Jets, these are really well designed carbs. Like most things, one just has to take em apart to get an understanding of the inner workings. They're really not as complicated as I've heard others comment.

Thanks For Everyone's Help & Advice,

Bob

[woodboat]

With tht PP, vacuum pulls the rods down at an idle. As the throttle opens vacuum drops off. The spring then pushes open the metering rod alowing more fuel in. If the rod was stuck open then fuel would have been entering the engine and turn down the mixture screws would have done nothing. Back to the PP spring: the softer the spring the less vacuum needed to hold it down thus a leaner mixture. There is also a depth setting for the rods. On car carbs you can pop out the plug and adjust the depth without taking it apart. I don't think the are "complicated" but there are many, many combinations and other than secondary rods they are difficult to change without disassembly. With that said I would much rather have a Qjet than a holley.

[oldgeardad]

Hello,
The "mechanical " linkage on the secondary throttle plate should start to open at aproximately 40 degrees. This ,however has little to do with actual proper secondary system operation as there is another "butterfly plate" that is located directly above the secondary throttle plate that needs to work in conjunction with the secondary plate.
This other plate is technicaly refered to as the "secondary air valve". This "valve" is designed to open on demand as required by the engine load and RPM. If the " valve " is set to open to early, it will cause either a hesitation or possible backfire on acceleration.Also, if it is opening early, once the boat gets on plane(assuming you have a planing hull) the engine may sputter or have intermitent small backfires as the engine runs higher in the RPM range.
If you want usefull information in resolveing the current problem with the carburetor I WILL be able to help you out. I will , however,need more information from you.
1) Was the carburetor rebuilt to the same specifications as the original i,e. same jets,metering rods, metering rod valve,float setting etc.,etc. or was it a "generic one size fits all" replacement?
2) What engine size,horsepower and drive sysem do you have?
3) What is your wide open throttle RPM now and what was it prior to the carburetor change.
I am not trying to complicate or confuse the issue .Hopefully, all you will need is a simple "secondary air valve" opening adjustment but it would be unwise on my part to explain that procedure if other issues may be causing your problem.
If you are willing to provide the information I will help resolve your problem.
If you want to learn more about Quadrajet Carburetrs than most people would be able to stand you might consider obtaining a copy of "Rochester Carburetors" by Doug Roe,HP Books,The Berkley Publishing Group, New York ,New York.
I will look forward to your answers ,
Steve B.

[Steve B.]

Hello Bob,
I am new to this website. That is my only excuse for writing a reply to a problem that has allready been solved. I obviously did not follow the thread all the way thru.
Anyway, I thought I might add one more bit of information. Allways set a marine carburetor to run on the rich side of correct, i.e. 13:1 even 12.7:1 in some applications.
Unlike automotive applications marine engines are allways runing under a heavy load . This along with todays poor quality ,alcohol reduced low octane fuels tends to get a marine engine into a state of constant pre-ignition or in worst cases, ongoing detonation. You probally know that running under these conditoins can cause severe engine damage such as burn't pistons, broken rings, smashed bearings, etc.
Setting engine timming a couple degrees retarded and running a prop that allows the engine to operate towards the upper end of the manufactures Max. RPM can help a motor that is running a little lean , but obviouslly its best to have the carburetor set up right.
To complicate matters marine engines are next to imposible to get accurate air/fuel ratio numbers on because of thecooling water being introduced into the exhaust gases.Sparkplug readings used to be possible but with unleaded fuels with additives they CANNOT be trusted.
You hit the nail on the head when you said that any future carburetor purchases would be O.E.M. material. They kinda rape you on the prices but in the long run it is the ONLY way to go.
Have a good New Year, Steve

[ChillrMn1]

Hi Steve B.,
The spreadbore Q-Jet I'm dealing with was purchased via internet remanufactured supposedly for a 350 CID/5.7 Liter G.M. Carb I.D. #s 17085586. This code includes a California Emission Standard. This carb appears to have been assembled with other parts in order to meet the marine requirements. This carb had CH secondary rods and, well the sychronizing of the throttle plates didn't appear correct either. When I bought this boat, the carb on it was a Q-Jet #17082403 which would be for a Crusader 350 CID. Tried to have local reputable rebuilder repair the carb I bought with boat, but water and other contaminates in carb seems to have blocked secondary fuel passages, would not respond when secondaries opened (throttle plates). Rebuilder unable to clean these passages. Took original carb apart and used the CL metering rods and also fixed problem with PP, seems this was stuck or primary rods not properly installed in primary jets. This carb runs better now. The secondary air valve also appears to be set very close to proper. I have made very slight adjustments to wrap spring, and don't experince any bog now when secondary throttle plates open. Seemed much improved after changing to CL rods.

Also please note, I plan to take the 17082403 carb, soak and do what I can to throughly clean and rebuild, and hopefully fine tune to optimum for my engine. It's now a challenge for me and fun.

The following are answers to your questions:

1. Appears generic, one size fits all, that's pretty obvious to me now. Carb was not offered for any specific engine or HP or model #.

2. The engine is a 350 CID, remanufactured, reverse rotation. I don't have paperwork on this engine, but was supposedly marinized to be a Mercury, 270 or 260 H.P. This is what previous owner told me. No Mercury I.D. plate on engine. My guess is this was purchased as long block from a remanufacturer and the parts from the old engine put on. This engine has definitely been marinized, not an automotive application. Engine has 580 hours true operation hours.The drive is straight inboard with a Borg Warner Velvet Drive, 1.52:1 ratio. NOTE - The GM casting number on the block translates block to 1984. ADDED NOTE - Manifold vacuum @ 15.5" at idle speed with .5" deflection. Initial timing set at 10 degrees BTDC at 600 RPM. Distributor is mechanical advance, prestolite breakerless ignition.

3. WOT was 4400, haven't checked since I made changes to carb. NOTE - Boat was originally overpropped with a 15 x 13 3- blade. With this prop, WOT only 3900. When I had prop shop select new prop, my priorities was to hit good efficiency @ 3200 RPM, 10 -- 11 GPH @ 15 KNOTS. I try not to run engine over 3800, it's used primarily for fishing, not a go faster boat. Prop selected by shop was a 15 x 11 Med cup 3-blade. This selection came close to my goal. Feel it's still a little overpropped, should probably drop to a 10" pitch from the 11. Before prop change, plugs always black, higher than acceptable fuel consumption and just bad performance. Now plugs running closer to the tan color, but as mentioned, fuel additives can fool you on this, at least me anyway. I'm a mechanic for A/C chillers, not automotive/marine.

I know that the right people know the combinations for the specific engine requirements to get these Q-Jets to perform like champs. I just don't have access to this info. Any tips are greatly appreciated. I'd like to set up the 17082403 carb this way, and plan to buy Doug Roe's book. Every web seach on Q-Jet info always brings a mention to his books.

Actually, the power plant is running real good, no problems, most would accept as great - feel engine could be a little smoother at idle out of gear though. With the price of fuel, I would like the satisfaction of knowing it's tuned as well as possible. I've taken care of all major issues this boat had, I'm just at the fine tuning/finishing stage now.

Appreciate any and all info.

Happy New Year!

Bob

[woodboat]

My feeling is 4400 WOT is real good. If you drop to a 10 pitch you will loose too much efficiency and burn more fuel. I have always been told to prop between 4000 and 4500 WOT with small block chevy. Current boat, 50 ft burns craft, hits 4150 WOT. Would NOT want less prop. @2900 it still wallows in the water. It takes 3300 to "feel right". If I reduced pitch it I would have to cruise @ 3500 or 3600.

[gonzo]

Steve B: modern fuel is better than the old stuff. As for low octane, you get what you pay for. There is 98 octane at the pump. Any engine with 10:1 or less compression will run fine with available gas. Using gas with too high octane is counterproductive because it burns slower and reduces power. It may also cause overheat. Most engine manufacturers specify the correct timing for the octane.

[oldgeardad]

Hello Gonzo,
I agree with most of your statements but would like to go a little deeper into the subject. You stated that 93 octane is good for any engine under 10:1 compression ratio. This is borderline true. All 10:1 motors are not identical. Depending on the camshaft and engine intake air efficiency (manifold,carb,heads),engine operating temperature,outside air temperature and other variables the engines actuall cylinder chamber pressures will vary greatly. It is cylinder pressure in P.S.I., not mechanical compression ratio, that determines fuel octane requirement.
I agree that running unnecessarily high octane fuel is a waste of money,can cause poor perfomance and poor fuel consumption but I am confused as to how higher octne fuel can lead to overheating. All other things being equal ,a higher octane fuel wiil burn cooler and slower than a lower octane fuel.
I dissagree about todays fuels being better or equal to earlier leaded fuels. First, depending on where you are in the country and the time of year you purchase fuel you will NOT get identical fuel even if you purchased from the same supplier in different States.Fuels are blended with different addiitives depending on the time of year and your specific location Winter blends have additives that cause the gasoline to vaporize at lower temperatures and pressures for easier engine starting.There are additives such as methanol and ethanol that are added to fuel stocks to artificialy raise octane numbers and are less expensive than petroleum based stocks. A fuel blender does not even need to inform the public at the gas pump of these additives unless they exceed 10% by volume. Alcohol provides less efficiancy and you have to burn aproximately 1 1/2 times the alcohol by volume than gasoline to get the same output of energy. This causes problems with carbureted engines vs not being an issue in an automobile engine with variable fuel injection, Further, there is no tetra ethyl lead in todays gas. Good for our health and enviroment. Bad for stable and consistent fuel octane numbers and cylinder head exhaust valves without hardened valve seats.
I realize these points are fine but they are pertinent and I mention them to be informative,not arquemenative. Boaters with older engines as well as the newer engines should try to keep a good tune up on their motors as well as buy fuel from a name brand supplier who sells enogh fuel to make sur you get a fresh load. Thank You for your reppy,
oldgeardad

[ChillrMn1]

Hi Woodboat,
Thanks for the input on the 10" pitch. After reading you post, the 11" is probably most appropriate. This is a Pacemaker with semi-hull and seems to propel pretty decently at this RPM, considering the boat itself. Don't want to be less efficient and run higher RPM for same speed considering how my boat is used. Did purchase Doug Roe's book, lot's of Q-Jet info, light on marine specific applications. Will rebuild original 17082403 carb myself and see if I can't do a better job than rebuilder in cleaning out all passageways. Would really like to know which secondary rods was specified in this unit for a Crusader 350 CID. This engine is probably very close to mine. Parts books list same carb I.D. number for both 350 & 454 engines. Don't know which Crusader (350 or 454) this carb was purchased for since this came with boat. One wants to assume the other bought it for the right engine, but you know how assumptions go........... And it's entirely possible the 2 carbs are identical in every way. Would Rochester have built 2 carbs with same I.D. number but with different internals (metering rods, PP Spring, etc.)? Unable to find info this specific.

Have A Good Day!

Bob

[Richard Petersen]

If you decide to rebuild the carb. Vacum clean the work benchfor a distance of 5' all around. put down a clean piece of WHITE paper from a supermarket meat room, 6' OK. Wash the tools in a solvent, dry, coat with a thin coat of oil. Have a roll of tape to remove any metal fillings sticking to a tool tip. Read the 1 st take apart set of steps. try to take apart 1 or 2 items and immediatly replace them with new. If you need to stop, mark the last complete step done. My age will now show. If you can not easyly remove a jet, Don't butcher it. Take it to a rebuilder with the instructions. Special tools are sometimes used in the assy. and disassy. He can only refuse. Most will smile, warn you what your into, help you, and give you their bussines card. As far as setting up a car or boat carb for max power. the logic of the book will cover all your questions. Foot to the floor is the same in a car or a boat. Roe's procedure is right. clean-clean-clean!!! Cloth or paper lint strands- barely visable- can get into the metered tunnels and ruin you best efforts. Don't be surprised if it does not run better. This is your first attempt. Pro's turn out duds.-- Compessed air - need a cotton or fuel filter, metal type, to stop grit from your tank, line. Also a pressure regulator set to about 10# max. Good luck, Rich

[gonzo]

Yes, it is true that the camshaft will affect the octane you need. If you don't advance the timing with higher octane, because it burns slower it overheats. It will act like any engine with a retarded ingnition, the fuel will still be burning when the piston is at the bottom. This causes a lot of the energy to become heat instead of force pushing the piston.

[ChillrMn1]

Rebuilt the 174082403 carb. Found main problem was fuel well under secondary jets completely restricted. This took considerable time and patience to clean out by repeated soakings, running a fine wire thru, and flushing with an eye dropper. (This is where the "Professional Carb Rebuilder" failed when I first bought the boat. As soon as the secondary throttle plates and air valve would open, complete bog. Paid $150.00 for this. Gave up on this carb at the time and purchased a generic remanufactured carb off internet, another mistake. Lesson learned).

The engine has never ran so good, since I purchased boat, from idle thru all ranges to WOT. Can't honestly tell if fuel consumption has improved due to the short test run. Flowscan appears to be lower, but could be wishful thinking. Will be able to tell next time out swordfishing.

A tip for first time Q-Jet rebuilders: Upon reassembly of the air horn assembly to the throttle body, lightly snug down with 2 screws. Check the power piston "PP" for free operation. Insert appropriate tool (I used a very small screw driver) thru forward vent stack and lightly press PP down. PP should return back under spring pressure. Mine would not. Found the "retaining spring" for the primary rods to the hanger assembly was interfering with a guide for the PP in the air horn assembly. Take air horn back off and carefully reposition this spring on the hanger. This should now clear the guide of the air horn when you reassemble. Check again. This is very important, Doug Roe in his book states "If the piston sticks down, primary system won't function. If piston sticks up, gas mileage will drop as much as 50%."

I would like to thank everybody for their input, advice, and technical information provided.

Bob

[woodboat]

Anyone got a line on secondary rods and PP springs. I found out why my one engine is such a dog, wrong rods. I need a pair of AV rods with an L hanger.

[ChillrMn1]

Hi Woodboat,
Couldn't find exactly what you're looking for, but try these: http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=3820&prmenbr=361 and this: http://store.summitracing.com/default.asp?target=esection.asp&N=120+4294925239+4294925135 (under edelbrock Q-Jet) . Maybe you can choose a combination that will satisfy your needs. Summit also offers a kit for four different PP springs which cut-in/cut-out at different manifold vacuums.

Bob

[woodboat]

Thanks. I had already called Summit and they were no help. However JEGS helped me out. I needed an AV rod and an L hanger. They didn't have them but had an Ay rod and a K hanger. That has got to be closer than what I had. I also ordered the PP spring assortment.

[crazed chicken]

I have twin 1988 Crusader 270s with 900hrs and Qjets. The engines appear to run great however at the dock idling I notice a gas sheen in the cooling water from both engines. No water in engine oil. The current consensus is the engines are idling rich and the idle mixture screw needs adjustment. Any ideas?

[woodboat]

try adjusting them per specs. Any good manual will give the procedure.

[gonzo]

Are the points, condenser and plugs in good shape?

[ChillrMn1]

The procedure I use knowing all else is OK as mentioned previously "points, plugs, no missing, proper ignition timing, etc.,": Connect vacuum gauge to a port reading true manifold vacuum (not a timed vacuum port), adjust idle speed screw to idle speed RPM per engine specifications, turn idle mixture screws (both & usually clockwise) to know you're running lean, slowly back each screw out until you reach lowest vacuum reading attainable and stop. Gauge needle should not fluctuate more than 1/2 to 1 inch, you may have to adjust each a little at a time, then turn each mixture screw 1/8 - 1/4 turn lean. Keep adjusting your idle speed screw as necessary to maintain specified idle speed RPM. If it's a relatively stock engine, a good steady vacuum at idle speed would be 15 - 19 inches, give or take. Once you start to play with it, you'll get the feel for it.

If you can't get a response by adjusting idle mixture screws at specified idle speed RPM, then you may have a stuck power piston, manifold leak, throttle valve shaft bushings may be worn, leaking fuel well plugs if they haven't already been epoxied, etc.

[crazed chicken]

The plugs, wires, cap and rotor were replaced last summer. It has factory electronic spark. The engines are 100% stock. Revving one of the engines produces slight black smoke and gas odor from exhaust.

[woodboat]

Of course black smoke is bad. That means it is way too rich. If you rebuild the carbs make sure you know everything there is to know about qjets as they are quite complicated.

[crazed chicken]

The black smoke was slight and not continuous. At what point do I decide to rebuild?

[woodboat]

One of my carbs was very rich. It would leave the plugs black, did not make full RPM and burned more fuel. I just redid the power valve spring and metering rods to make it leaner. I did NOT rebuild it as it was functioning correctly but was not set up correctly for efficiency and power. With that said it NEVER was so rich as to puff smoke so I would have been inside the carb way before now.

[specop1]

I didn't know I wasn't supposed to touch the power piston stop screw. I rebuilt my carb, and removed this screw. I didn't record where it was at. Now I have slight black smoke at idle, it runs good, but when I come to a complete stop it acts like it wants to die, but doesn't . But I do have rough idle, black smoke , and strong burn your eyes odor. Hiow do I get this screw adjusted back where it needs to be. I have it all the way down , and I'm pretty sure it was all the way down or very close when I took it out, but I guess not, because now I have this problem. Any step by step ideas to get me back to normal operation? Where are most of them set ?

[ChillrMn1]

it's possible your power piston might be stuck in the up position thereby keeping your primary rods up and out of the jets resulting in a rich condition. during normal idle, the manifold vacuum should keep the piston down with rods in the primary jets. as you increase rpm, less vacuum occurs and allows the power piston to lift starting the power mode. with engine off, try fitting something into your vent to assure you can depress the piston down and feel the spring pressure lift it back or return. refer to my previous post on this thread where i mentioned mine hung up after reassembly. of course, this is all assuming it's not just your idle mixture screws too rich. as for adjustment of the power piston screw, i don't know. this adjustment is not in my copy of doug roe's book. good luck. let us know how you make out

[woodboat]

I got my boat running today. I installed edelbrock secondary rods and hanger that closely match the good running port engine as well as the leanest power spring available. Idle is very good and smooth @ 580 RPM. Power is good. I picked up 250 RPM on the top end. I suspect fuel economy is up as well as everything is leaner.

[ChillrMn1]

Glad you're running better. I should have tried a leaner spring myself when I rebuilt mine. Next time if I have to rebuild her again. I'm sure the fuel economy is improved, let's us know what you gain.

Thanx

[woodboat]

Problem is I don't really have a baseline for fuel economy. It was flat in the middle when revved in neutral. It had a miss around 3300 RPM. It was missing a few hundred on the top end and all the plugs were black after only a few hours usage. Right now they are both running about the same which quite frankly seems very good. For small blocks pushing a 50 Footer I am actually impresed.

[SCSeaside]

I am rebuilding a QJet 17082403 that I removed from my port Crusader 454 350 HP Engine. I suspect that the float was sticking and allowing gas to flow past the cylinders and into the oil pan. Oil was very thin, watery in fact, and had a heavy gas smell. I took out 21 qts and the pan normally holds 7 with a filter change. Any pointers on rebuilding are greatly appreciated. I noted the power piston advice above. Very helpful.

[SCSeaside]

PS...I'm assuming that WOT in this string refers to Wide Open Throttle.

[ChillrMn1]

SC -
That's alot of extra fuel/oil you removed. May I also suggest the following if it's not carb problem: If you have a mechanical fuel pump, there is the possibility the diaphram in the pump is leaking. At the bottom of the pump is a fitting which should have a fuel line attached returning to the carb air horn. This is provided in case the diaphram fails so the fuel will not leak into your bilge but be "dumped" in the carb. If it is the fuel pump, then that will need to be replaced. And yes..WOT is Wide Open Throttle.

Let us know how you make out.
Bob

[maresca]

I am the first owner of a 1988 350 King Cobra 260 hp with 500 hours on it. The engine was stored for several years before first use. Since its first start I never was able to get the engine to the operating range 4200-4600 rpm. I have installed a 15X16 pitch propeller getting 4000 RPM and a 15X17 pitch getting 3800 RPM and maybe 2 miles more speed. The engine is working fine but the iddle is very unstable specially when cold and I have almost closed the idle needles to reduce raw unburned fuel but it seems these adjustement wont correct the real situation. Because of the iddling condition I took the carburator to a shop and they installed a new kit (probably automotive), but there was no change or improvement. After reading the diferent comments on this forum Im starting to think that my carbuator needs fine adjustement probably I have the wrong power valve spring, metering rods and jets or a combination. Will this items be part of a carburator repair kit or have do they have to be purchased separatelly.
Thanks for the advice!