Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Marine Quadrajet fresh rebuild fine yesterday, overflowing today...

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Walleye4Days:
Not sure exactly what it was, but I put a new float, needle, and seat in and it works as it should now. The only issue left is that the idle air screws bog out the engine when all the way in, but they don't seem to bog it out when backed all the way out. So, I'm having trouble finding the sweet spot because with the idle at 650, the primaries closed off, and engine barely chooching along, the screws work great when I screw them in/it bogs out the engine. But, when backing them back out trying to find the second bog point (to split the difference) it doesn't bog out...

Kenth:
In my book the 17059298 is an 4MC marine Qjet.
These units have .070" upper and lower idle air bleeds and .052" idle channels with .037" idle tube restrictions.
No control on rich side of the idle screws indicates a too lean idle circuit and the idle tubes needs to be opened some. I suggest at least .038" idle tubes, maybe a couple of thousands more after test.

Cliff Ruggles:
The upper idle airbleeds in most Marine Q-jets and really tiny and in the airhorn, not the main casting.  Common idle tube sizes are .026" to .028" and rare to see them much bigger than that.

Most will go "pig" rich if you open them up much past .032" especially on a stock engine with the correct Marine cam in it making a lot of vacuum at idle.

The exception to that are some of the Volvo Penta Marine units as being discussed here which Kenth mentions, which were built from Oldsmobile type cores and have larger upper idle bleeds in the main casting.  They are easily to ID because they will be either hot-air or electric choke, not divorced.

Just adding info here for those rebuilding or working with Marine Q-jets, but where folks really screw the pooch with Marine engines is installing cams with tight LSA and shorter seat timing.  That never works out nearly as well as a gentle cam on a wide LSA or original Marine replacement cam right to start with. 

Marine carbs don't use idle bypass air either, not even drilled for it and no reason to unless for some reason engine mods were made to lower vacuum at idle speed.

Many Marine engines have very low idle speed settings, and some even "kill" the ignition for a moment when the Morse controls are moved to engage the Marine gear or outdrive.  This is done to keep them from "slamming" into gear and damaging components.

So keep all this in mind when you make plans to "modify" your Marine engine and stay as far as you can away from most of the cams companies like Comp and others offer for them unless you have the tuning skills to get those engines to idle well at really low RPM's.....FWIW......Cliff

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