Quadrajet Adjustment

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

 

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

 

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

 

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.

 

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

 

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.

 

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.

 

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

 

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.

 

Qudrajet Question

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.

 

I feel like an idiot

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 **** you on the prices but in the long run it is the ONLY way to go.
Have a good New Year, Steve

 

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

 

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.

 

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.

 

Gaasoline Issues

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