Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Lean pop when secondaries start to open - marine Q-jet

<< < (2/2)

Tom-in-KY:
Back to the boat issue - I tried a completely different Q-jet that came off of a stock 454 boat engine - MerCruiser version of the Q-jet.  I was getting the same issues as with my other carb.  Oddly enough, the lean pop seemed to ease greatly, and almost go away, when I pulled the flame arrester off and was running the boat with the hatch up slightly for observation.  I also ran it with the flame arrester off and the hatch down, and it seemed to act up again.  At this point, I am starting to think that I have some type of ignition problem.  I am suspecting that something is breaking down with the heat, and the cooler air helped to alleviate the issue.  I suppose it is possible that it is an air delivery issue with the carb too, but it ran fine for years with the hatch down and the arrester installed.

Nevertheless, I got your book on Friday, and have already burned through it twice over.  I have come to the realization that I was way off base when I rejetted my stock Q-jet originally.  I was probably on the rich side - I had a "B" hanger on the secondary with "CC" rods.  It looks like I have the smaller CFM (750) carb with the smaller primaries.  I am getting a case of booster drip at idle with either carb (neither has an idle air bypass circuit).

Your book mentions the benefit of having as much CFM as possible on the primary side for marine engines.  Do you think it would be worthwhile to find a carb with the larger primaries for this application?  Of course, I would have to close off the venting for the float bowl.  Alternately, I was thinking about removing the outer ring from the primary boosters on one of my marine carbs as described in the book.  Would you advise doing this?  I have an extra carb body, so it wouldn't be a major sacrifice if it didn't work.

I am going to order one of your HP kits after I figure out if I have an ignition issue.  Also, if I am going to remove the booster rings or go with a completely different carb, I probably will need different rods and jets.

Thanks for the help, Cliff!

Cliff Ruggles:
Havne't tried removing the outer booster rings for a Marine application to date.

We have built later model large cfm castings for Marine applications, with great success.  I always use the linkage found on later carburetors that allows more throttle angle from the primaries before engaging the secondaries.  These were common on mid to late 70's models, Oldsmobiles in particular if memory serves me correctly.

I've set up plenty of factory Marine carburetors for big block applications as well.  The need to be calibrated exactly for the application just like anything else.  You can add some bypass air by drilling the casting and baseplate as shown in our book.  Drilling the throttle plates will work, but not as well as actually having bypass air installed......Cliff

Tom-in-KY:
Thanks for the answer, Cliff.  Do you think I would be better served to try removing the primary outer booster ring from one of my existing carbs, or would I be better off trying to find one of the large bore Olds Q-jets like you described?

Cliff Ruggles:
Tough question.  When you loose the booster area(s) the carburetor becomes less effective/efficient at fine metering.  This results in increased fuel consumption.  You are basically making it work like a small bored Holley carburetor. 

For Marine applications with "warmed over" engines, and all of our race only carburetors,  I like to set them up to run straight off the jets and not use primary metering rods in them......Cliff

Navigation

[0] Message Index

[*] Previous page