by theory if it was fuel pressure or float it would do it from both so most likely the idle air bleed is plugged or a problem with a gasket or yes the butterfly could be wacked out and one is open more than the other causing too much air flow past the booster which will draw fuel in. the only adjustable part that you can play with is the idle mixture screw. they are simple to work on if you have the book so I would just pull it apart and make check that all the gaskets are correct.

It sounds like something was missed when he rebuilt the carb. I would almost bet the float adjustment is not correct. Either that or a piece of dirt got stuck in the needle and seat. That is about the only thing that will make a Q-jet run rich like that.

If you want to eliminate an external issue then swap the carbs between the two engines and see if the problem follows.

Idle jets are rarely adjustable.

Again, I have never been inside a Quadrajet, but I would look for a small fixed brass idle jet and a small fixed brass idle air jet. They might be behind covers.

However, the quantity of fuel might be bigger than what could possible be delivered through the idle circuit. As Bill says, check the needle valve and float one more time.

Also, swapping carbs will help you understand for sure that the carb is the culprit.

Ok, a small update. Haven't found the problem yet, but made some detective work. I turned the mixture screw rather lean, and it ran nicely for a day, no fuel in water from the exaust etc. mild stench and still nozzle drip. Just for an excercise i blew canned air with modest pressure in from one if the idle air intakes just to see if there is a piece of dirt, it probably raised the main metering rods, and the result was that it is very bad again. Fuel in water, actual pebbles of soot dropping from the exhaust etc.
And then i turned the idle mixture screw to rich (not lean), to give it more fuel, and the nozzle drip stopped. Still fuel in water and a nasty smell everywhere.

I will recheck the needle and seat, and fuel pressure also.
I will update when I have done that.

Another update. I messed around with the carb, the fuel pressure is steady 5.5PSI, the needle seat etc seem fine, jets seem fine, rods seem fine, air passages seemed open. Could not find the cause. I suspected perhaps previous owner had tuned the carb and I cannot detect it visually. So I bought a new carburetor.

It runs better, no stink but there is still some fuel in water behind the boat, and the idle fuel consumption is about double when compared to the other engine (15-20 liters/hour vs 6-12 liters/hour).

Im mystified now. Can it have something to do with the Thunderbolt IV timing jump I have present?.

Any ideas?

Did you every try swapping the carbs between the engines ? That would help verify that the carb is the problem and not something else.

Hi,

I actually did not. The other one ran so nicely that I was afraid to touch it in a way . But since I tried three carbs on that engine, one of them brand new, so I presumed it cannot be that...
But you may be right. If nothing else, I will then try to interchange them... so I will have tried four carbs.

Ville,

I'm far from an expert, but....

It is entirely possible you are chasing two demons at once. Since you replaced the carb, and, while the problem got better, it didn't entirely disappear, I'd be very inclined to start hunting bad ignition. Get yourself a spark checker (they are cheap) and make sure you're getting spark in all cylinders. My guess is that you're down at least one, and you're using carb tune to compensate. This is causing the engine to run rich and spit unburnt fuel out the exhaust.

Thanks. Brad.

I actually had a similar idea, there has to be something else too going on. Because of the timing jump (with light at idle) I suspected the Thunderbolt IV module. I´ve searched around in forums and people with issues with jumping timing do not seem to have a solution where that comes from, except they all "suspect" the module. No-one has closed any threads with a solution at least that I found.
But yeah, I guess I have to start hunting somehow. Perhaps interchange the modules, and spark check.

Thanks!

I discovered another symptom that I can repeat. This happens with the new and old carb.

After returning to idle after revving past secondaries opening, ~2000-2500+rpm or running on WOT, the engine bogs down. The bog down is a sputter where you can keep the engine running by giving it more throttle. When not being careful enough it still dies. When trying to restart it takes a lot of cranking and giving it throttle and then sputters to life, and after a while runs fine again. This was happening to me when returning to harbor after planing, and was very annoying because I lost control of the boat in tight spaces and had a couple of near misses with other boats. I always thought that this was the old carb issue, but now I could repeat the issue on dock with the new carb. I just have to rev it up enough warm, and then the symptom repeats itself.
Also, there was a couple of backfires around 1000-1500rpm... but after I while I could not repeat that.

Could this be an issue where the timing advance does not come back, or comes back slowly after being past some point? That can only be the module?
There was the timing jump issue too on idle, which seemed to go away at about 1500rpm (when checked with the timing strobe).

Talk about double, or triple demons...