Rough Idle

[ignisuti]

1989 Maxum I/O with 3.0L Mercruiser engine.

My boat has had a rough idle for years now. It'll only stay running at idle if I rev up the engine real good. Even that doesn't always work.

I can cheat by adjusting the idle speed high, but I'm looking for a real fix.
Any suggestions? Use Seafoam?

[mcoffey]

I would start with a tune up if not recently done. Also check and verify timing is set correctly. Next verify idle mixture is set correctly based on your altitude. Also check that choke is open all the way when warm. I assume this happens at all temps and not just when motor is cold? If all mentioned is correct, it should idle fine for you. Seafoam would not hurt though as it could be a bit gummed up since it is pretty old.

[ignisuti]

Originally Posted by mcoffey

I assume this happens at all temps and not just when motor is cold?

This issue is significantly worse with a cold engine.

When cold it simply won't idle. Once warm, it'll idle, but it's a rough idle and occasionally dies if left idling for more than a minute.

[Phillbo]

Maybe time for a carb rebuild .

[mcoffey]

Most of these things are cold natured beasts, I know mine is. Mine simply won't idle when cold until it nears 150 degrees. As far as warm idle goes, could be ignition, could be choke partially closed, could be idle mixture, could be it needs a card rebuild. I don't like to do a rebuild untill all else fails though. So I would try some Seafoam (make sure to use a double dose as for cleaning, that is what you have to use). I would also verify everything is adjusted correctly (idle, idle mixture, timing) and that it does not need a tune up. I had issues with mine last year where it would act like it was starving for fuel after it was hot and went through a no wake area and then tried to accelerate again. All it needed was a good tune-up. Seems the last mechanic who worked on it and replaced the engine failed to take spark plug class. All plugs were gapped wrong and 1 was even cracked. They also used the cheapest of the cheap cap and rotor and the wires were original

So a new set of AC MR43T plugs, gapped correctly and anti-seized, new quality cap and rotor and a shiny new set of wires and that problem just disappeared

[shrew]

It's an '89. More than likely it's simply time for a carb rebuild. This is nothing more than getting a rebuild kit and stripping down the parts. Soaking them for 24 - 49 hours in submersible carb cleaner then blowing them out with compressed air. You might need to go back with compressed carb cleaner for some fine tune cleaning. Replace the jets, replace all the gaskets, them reasseble and install. Carb si much more susceptible to old gas. GAs goes old in recreational vehicles since they don't get run daily like cars, or even weekly like a lawn mower. If gas has been untreated, particularly E-10 gas, then varnish builds up and gunks up the works.

Is this the only cure for a rough idle? Absolutely not, a good tune up, timing, plugs, can all go a long way. However you specifically stated that you need to increase rpm's either through the throttle or the idle screw. This sounds like a fuel delivery problem to me. It may be one of many, but this is a easy fix. Carb's need to be rebuilt periodically anyway, so it certainly wouldn't hurt.