Rough Idle..Where to start?

[jedimaster]

3.0 mercruiser, 1988 140hp

The engine is running great while under throttle Getting 3600rpm at wot with a 19 pitch 3 blade alluminum and good power out of the hole.

But it idles really rough.
I have set the timing, has had a can of seafoam run through, rebuilt the carb(idled rough before as well), Replaced the plugs and wires.

What next? I have the RPM's at idle set at 800. If I raise it to 1,000 rpm idle it seems to idle fine? Is the idle mixture out maybe? or should I just turn up the idle rpm's?
It will eventually stall as it almost sounds like its running on 2 cylinders, its really rough.

 

[Zero Balance]

Re: Rough Idle..Where to start?

Put a vacum gauge on it and see whats what. Could be one of several issues, vac leak, air mixture, bad fuel.....

 

[Bondo]

Re: Rough Idle..Where to start?

Rough Idle..Where to start?

With a Full,+ Complete Tune-Up.............

I have the RPM's at idle set at 800. If I raise it to 1,000 rpm idle it seems to idle fine?

Idle spec is 650/ 700rpms, at idle, in gear, in the Water..........
At 800/ 1000rpms,...... All you're going to do is Trash the Outdrive.........

 

[Don S]

Re: Rough Idle..Where to start?

You may have more wrong than you think, your WOT rpm is about 1000 rpm LOW. you should be seeing between 4400 and 4800 rpm at WOT.
Run a proper Compression Test before you do a tuneup.

 

[jedimaster]

Re: Rough Idle..Where to start?

Sorry, meant to say 4600rpm at wot not 3600
Where do you attach the vacuum guage?

 

[jedimaster]

Re: Rough Idle..Where to start?

Compresion is good on all 4 did the head gasket this spring.