need help blowing fuel out carb.

[dguess]

1985 3.8L OMC.

I have it at the Mechanics he is telling me he cant fix it. These are the problems:

runs great up to 2500 RPM, then it starts blowing fuel out the carb. Idles great starts evertime no problem.

They have rebuilt the carb., replaced the carb, replaced the intake manifold, check the compression and say it is good (160), they think it is exhaust getting into the fuel but not sure where.


any ideals, I am open to just about anything.

thanks for any help

 

[Bondo]

Re: need help

Re: need help

they think it is exhaust getting into the fuel but not sure where.

Ayuh,... I'd sooner think the exhaust is just plain ole Plugged....

 

[dguess]

Re: need help

Re: need help

the exhaust was working when I took it up there because it was blowing black smoke out the rear. they had to take the manifolds off in order to get the tapped covers off to check the valves. I am almost to the point of trying to find another motor and have it installed if that would fix the problems.

if the exhaust is exhausting out the back then how would it blow back up the carb? just trying to understand.

 

[Howard Sterndrive]

Re: need help

Re: need help

here's the key question:

How did this problem begin?

did it happen with a bang- not doing it one trip and doing it the next?
did it come on gradually? getting worse each time...
did it begin after work was done?
other?

engine history?

 

[Howard Sterndrive]

Re: need help

Re: need help

they had to take the manifolds off

doesn't mean the manifolds aren't plugged up inside

ask them if they might have thought to stick a screwdriver down the bellhousing exhaust ports to see if the exhaust shutters are in there- you pull the screwdriver out fast and listen for the dull thud of the rubber coated plate flapping shut- it's spring loaded

lots of techs have no idea these old 800 series even had shutters

 

[dguess]

Re: need help

Re: need help

frrom the beginning:

bought the boat last year took it out twice run fine then the upper gear box froze up tried to rebuild myself so I winterized the motor and then had the stern drive rebuilt back in late feb., took it out for the first time in late march, would not run was blowing gas up thru the carb. took it to the mechanic they have it where it idles smooth sounds great, and you just have to bump the key and it fires right up, except that it will not go over 2500 RPM, it starts blowing fuel again they are about ready to give up on it. they have replaced the intake, done a tune up, checked the valves, rebuilt the carb.,

it does allow the exhaust to go out the back of the boat, when it starts blowing gas out the carb it also starts blowing black smoke out the exhaust.

I am calling them Monday and talking to them again I would like to be able to suggest or see if they have tried things.
So any help would be great
Thanks

 

[Howard Sterndrive]

Re: need help blowing fuel out carb.

did the upper gearcase get changed to one with a different gear ratio?

how is the boat for power up to the 2500 rpm? like it's dragging the anchor?

 

[dguess]

Re: need help blowing fuel out carb.

I am not sure on the gear ratio I will ask about that tomorrow.
It seems fine up to 2500 RPM no lag it jumps right up but then feels like we are dragging the trailer behind the boat. the engine seems to make a ticking noise and the boat seems to shudder.

 

[Howard Sterndrive]

Re: need help blowing fuel out carb.

well, this list from Don S. is a great summary of what can hold engine rpm's back:
http://forums.iboats.com/showthread.php?t=295070

especially the fuel related ones..

it's the blowing fuel out the carb that has me stumped...

thus more questions than answers...

I wonder if the timing chain might have skipped a tooth.

 

[Alpheus]

Re: need help blowing fuel out carb.

I would be checking each and every one of your exhaust manifold passages. Pull out the spark plugs one by one and look for ones with discoloration. Black smoke = carbon burning. Only one thing can make your carb start "throwing up" and that is positive pressure in your intake...

 

[Howard Sterndrive]

Re: need help blowing fuel out carb.

yeah, and that leads back to the exhaust shutters.

 

[dguess]

Re: need help blowing fuel out carb.

thanks for all the help I have not been able to get ahold of the mech. yet but will ask him about the shutters when I do.

Thanks again this is a great site with lots of helpful people and suggestions