Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
Help with Marine Quadrajet
rozicrc:
Hi,
I purchased a kit/book and rebuilt a 1988 17083522 quadrajet as well as replaced a fuel pump. I set the float for a 1/4" within 1/64", and made sure the metering rods were straight/check valve seated/passages were clean/needle and seat passed vacuum. I checked fuel pressure and the new pump is putting out 5.5 psi on 4 clicks on the pump.
Being on a boat, there is one issue I had. I initially used the 1/4" thick paper gasket that was supplied on the kit as I wanted to keep heat out of the carb. On testing, the carb performed very well without issue. However, when I was setting the idle mixture screws and stalled the motor out (too lean), I had ~1 ounce of fuel leak on the primary side just under the throttle body. Looking into the primary side with choke open, I could see fuel pooled on top of the primary butterflies. I let the system dry up, and tried it again, with the same result. Turning the engine on and off with a key or moving through RPM ranges and turning off would not repeat the fuel leakage.
I figured there were 2 problem, one is getting the fuel there initially, the second is getting fuel outside the intake. I swapped the gasket to a conventional style and got rid of the external leak.
However, with this being a boat I want to understand where this is coming from before taking it out on the water. Is this a float hanging up or a common occurrance with the engine stalls on lean at idle that i never noticed before with the good carb to intake gasket?
Thanks in advance for the help
rozicrc:
I neglected to say base timing is set at 6 degrees and the motor is a Mercruiser 3.7 (190 HP). The idle mixture screws are adjusted to 4.5 and 5 turns respectively
Cliff Ruggles:
The gasket used under the carb would not have any effect on the carb leaking internally or externally. There are only a few places the carb can leak below the fuel level on the bowl side, which would be the bottom plugs. I haven't seen too many Marine carbs with leaky bottom plugs, especially the later production units, but they still need to be pressure tested during the rebuild.
If you go lean with the idle mixture screws when fine tuning, the fuel may start flowing from the main boosters. I've seen this happen a few times when setting them on our test engine. The fuel flow should not continue after shutdown, unless the fuel level was too high in the bowl, or the upper idle airbleed is plugged for some reason and the fuel is syphoning out of the bowl.
I get in a LOT of carbs here that were recently rebuilt/having issues and in a lot of cases find them to be leaking under the fuel inlet seat gasket, or at the needle tip when a poor quality needle/seat assembly was used during the rebuild.
The gasket surface must be clean and free of any defects, pitting, etc or there can be a leak from the pressure side. Most small leaks will only cause troubles at idle and after shut down, a big leak causes flooding and poor carburetor performance......Cliff
rozicrc:
It appears to only happen when adjusting idle screws but did continue after shutdown for a bit. I will try again with normal shutdowns over the weekend. Is 4 to 5 turns too aggressive? Is the carb is whistle clean and 2.5 starts to stall and it starts to slow down at 7
Cliff Ruggles:
Marine carburetors can be pretty sensitive to mixture screw settings because they are NOT emission calibrated nor to they use any idle bypass air.
As soon as the engine slows any at all when turning in the mixture screws, stop and back them up about 1/4 to 1/2 turns. Repeat this for the other screw and you are pretty much done with idle mixture adjustments.
Float level can also be super critical with a Marine application, as the engines are often "tilted" more than automotive. Follow the instructions very closely for float level, some need a much lower float setting than others to work correctly.....Cliff
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