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.

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

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!

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.