REBUILT MERCARB...FUEL MIXTURE SCREW....SCREWY

[johnnypac]

1989 searay, 3.0mercruiser, 2bbl mercarb
I rebuilt the carb, new gaskets, accel pump, everything....
It starts and runs great..... BUT as I do the idle adjustments, I disconnect the throttle cable, turn the idle speed screw til it idles at 600rpm, I turn the fuel mix screw in.... ALL THE WAY IN (gently) and that's where it runs the best.... Turn it out half a turn or more and it starts coughing.... Keep going out and it dies.

I've checked the throttle plates, they appear to be nearly completely closed....

Is it the float too high? Too rich? Can't be, that doesn't affect the idle circuit...or Does it? But it runs GREAT with the needle all the way in, no hiccups at all...half throttle, wot (very briefly) great acceleration....I'm tempted to just leave it like that, but I know it ain't right.
Idle screw is perfectly clean, I've checked for debris, nicks etc, sprayed out the hole (where the screw goes) with carb cleaner...

I AM on muffs right now.... That's the only thing I can come up with, maybe I just need a load on it?
Any help would be appreciated.
Thanks

 

[alldodge]

1989 searay, 3.0mercruiser, 2bbl mercarb
I rebuilt the carb, new gaskets, accel pump, everything....
It starts and runs great..... BUT as I do the idle adjustments, I disconnect the throttle cable, turn the idle speed screw til it idles at 600rpm, I turn the fuel mix screw in.... ALL THE WAY IN (gently) and that's where it runs the best.... Turn it out half a turn or more and it starts coughing.... Keep going out and it dies.

I've checked the throttle plates, they appear to be nearly completely closed....

Is it the float too high? Too rich? Can't be, that doesn't affect the idle circuit...or Does it? But it runs GREAT with the needle all the way in, no hiccups at all...half throttle, wot (very briefly) great acceleration....I'm tempted to just leave it like that, but I know it ain't right.
Idle screw is perfectly clean, I've checked for debris, nicks etc, sprayed out the hole (where the screw goes) with carb cleaner...

I AM on muffs right now.... That's the only thing I can come up with, maybe I just need a load on it?
Any help would be appreciated.
Thanks

Air passing the venture's sucks fuel out of the idle ports and into the engine. The idle screws adjust the amount of fuel going in at idle to around 1200 rpm. Your closing the idle screw down and should shut the fuel off and kill the engine. So either your float is to high and your leaking fuel in over the top of the bowl, or your needle seat and screw are damaged and the fuel is passing the seat

 

[fishrdan]

Sounds like it may be procedural...

Lightly seat the idle mixture screw, then back it out 2 turns. After this rough setting is made, start the engine and adjust the idle speed screw to 700+- RPM. Then adjust the isle mixture, then idle speed, then idle mixture, etc., etc., until the engine is running smooth. (Adjust the idle speed about 100 RPM over the engine's spec'ed idle RPM as it's going to drop when the boat is in the water and in gear.)

If you can still screw the idle mixture screw in all the way without killing the engine,,, with the engine running, try squirting some carb cleaner around the carb base to see if the RPM's drop/raise due to an air leak. Try this around the manifold to head gasket too..

Your final carb adjustments need to be made with the boat in the water, engine warmed up, idling in gear without the boat being restrained (tied up). Monkey with the idle mixture and idle speed screws until it purrs like a kitten while in the water.... You want to be able to drop boat into gear without the engine dying, struggling or stumbling.

 

[johnnypac]

I followed that exact procedure probably 4 times now.
I checked for vacuum leak and didn't find any, then I thought, if it had a vacuum leak wouldn't it be running lean and require the mixture screw to be further out than what would be "normal"? ....ie, it would be getting more air than intended and require more fuel to run smooth.
I'm getting extra fuel from somewhere but just can't find it yet. Gonna ck power valve to make sure the gasket is there, and ck float.

 

[alldodge]

I followed that exact procedure probably 4 times now.
I checked for vacuum leak and didn't find any, then I thought, if it had a vacuum leak wouldn't it be running lean and require the mixture screw to be further out than what would be "normal"? ....ie, it would be getting more air than intended and require more fuel to run smooth.
I'm getting extra fuel from somewhere but just can't find it yet. Gonna ck power valve to make sure the gasket is there, and ck float.

My guess is the float level, needle seat or the float is sinking and needs replaced

 

[johnnypac]

K, thanks alldodge, I'm gonna run through it again and look at all those. This will make the 7th time I've opened it back up.
Thanks again.

 

[Kevtheman]

I followed that exact procedure probably 4 times now.
I checked for vacuum leak and didn't find any, then I thought, if it had a vacuum leak wouldn't it be running lean and require the mixture screw to be further out than what would be "normal"? ....ie, it would be getting more air than intended and require more fuel to run smooth.
I'm getting extra fuel from somewhere but just can't find it yet. Gonna ck power valve to make sure the gasket is there, and ck float.

Johnnypac:
Its, been a long, long time since you had this problem on your Mercarb, but I have the exact same problem. I've have checked everything, measured everything, changed gaskets, float heights... removed the power valve piston, changed gaskets. Can you remember, what did you finally do to solve the issue? It could be one more clue for me. Thanks, Kevtheman