I have a 1996 V6 vortec carburated boat motor that was rebuilt once about 50 hrs ago. Lately it burned up a piston on a lean condition and we just got the pison replaced and back together. The problem is it has low vaccum and behaves like there is a leak somewhere in the induction system. I have tried 3 different carburators and all behave the same way. Idle screw is turned in all the way for it to idle. I have also plyed with the idle mixture screws and to no avail.
If I partly close the choke it revs higher like as if I was plugging the leak. I have sprayed either around the intake manifold and carburaotr gasket but it does not inticate a leak anywhere. No vaccum ports or lines on this motor.

When the motor was rebuilt, I had the block decked about
.020 and the intake runners lining up were off just a little when assebling the intake to the heads but not to a point where one would think it would cause a problem.


A compression check shows all 6 cylinders are 190 -195lbs each so a sticky valve is out. All the plugs look white with no oil burning indication.
Does anybody know of another test that I could try to find where the leak is?
I am really stumped here. Does anybody have any ideas?
Thanks - Gary

intake gasket....make sure you are using the paper type and not the rubber o-ring style..

what is your vaccum reading? A valve problem would be indicated with a unsteady vaccum reading,flucuation.Just one Burned piston I'd have to go with an intake gasket also....

Retarded ignition timing.

Runs better with choke, idle screw in, white plugs, low vacuum, burnt piston.

But either doesn't show it. Hmmm, fuel pressure ?

Maybe PCV valve or it's connection to the carb. (if it has a PVC)

Gaskets sound more likely.

If you have had this problem after rebuilding the engine I would check your cam istallation. We installed an engine that a customer built, had 190 compression but was a dog to run with 6in vacuum. We pulled the cam gear/chain cover and found he had installed the gears one tooth out. Installed the gears correctly in it runs great now.

I was just about to suggest valve timing.

If you think you have a leak, then spray carb cleaner around the intake - anywhere you think it could possibly leak. When the RPMs pick up, you have found the location of the leak.

Thanks for all the replies. Heres more....
The vaccum guage indicated a fluctuating 10-12 inches of vaccum at idle. When I rev it to 2000 rpm it comes up to 19 inches and appears to be almost steady with a slight variation of about a 1/4 inch.
Also, every once in a while the motor will back fire through the carburator!

When I ran it, the boat accelerated well and made its target speed (50mph) once getting past the bawg from idle and again when the 4 barrels opened. I ran it for about 10 minutes and it did not seem that valve timing was the issue. Plenty of umph!
Ignition timing was also checked.
No PVC valve

If the intake gasket was leaking, wouldn't it show up if carb cleaner or either was sprayed around it?
Or does anybody think it could be leaking underneath?

Gary

not all the time, if leaking from below in the lifter galley you would not know....what type of gaskets did you use ?