Lumpy idle - Mercruiser 5.7L V8 petrol

[Franknstein]

Hi all

I recently recommissioned my Maxum 2300SC from winterisation, and all went well, except the starter motor which died a death during the cold season, so replaced that - all was well, engine started first time.

Upon running the engine though, after it had warmed up, I noticed it was idling very lumpy, to a point it was wanting to cut-out, revs dropping very low, up and down perhaps once every second - continually.
Other than this it runs perfect throughout the rev range. I took her out on her maiden voyage for the season this sunday, and it performed faultlessly throughout the rev-range... When I came back to the mariner to tie-up - it kept cutting out on me, which was infuriating and stressful, as getting her into her berth is difficult enough with power.

Anyhow, I suspect a dirty carb - the idle circuit perhaps? I never did any preventative maintenance on the fuel system at the end of winter, and probably am suffering the consequence now. I have however replaced the water/fuel sep filter.

I was hoping to spray some carb cleaner somewhere into the carb to clean it out, and hopefully not have to remove the carb from the inlet manifold.... is this possible to get carb cleaner into the idle circuit in-situ? Any tips on doing this? Engine running or not? Would I have to remove the idle adjustment screw perhaps completely and spray in there or something? Its an Edelbrock carb, not sure on the model number - but yeah, the normal big dumb edelbrock carb from those early 90s 5.7L GM/Merc motors.

Any help appreciated.

Many thanks

 

[dpb]

I would check that the auto choke is working properly first. as it might not be opening.
What type do you have?

 

[Franknstein]

Thanks, it has an electric choke.
Carb is Mercarb 43mm two-barrel, 3310-861245.
Are you thinking it permanently staying chocked, starving the engine of air throughout the rev range, and hence running rich?

 

[dpb]

Something like that, mine had a mechanical choke that stuck closed, only seemed to effect mix at idle though, ran fine at high revs, can't remember why!

 

[Vitesse]

Other possibles that come to mind-

Poorly spark plug lead. Had similar symptoms on a car, lumpy/stalling at idle, fine at higher revs.
Ignition breaking down (run the engine in the dark and look for sparks. Don't touch them though, like my mate did.)
Old fuel

 

[lynall]

Rotor arm and cap and leads wont break the bank, repower marine decent prices

 

[rosssavage]

And don't forget HT leads. Russ of this forum had a similar problem, after trying choke, carb, and just about every other fuel related component he traced it to duff HT leads.

 

[ARE]

Before you go to far, how old is the petrol in it?

I had the same problem with a 5lt V8 last year, drained petrol tank and refilled with fresh petrol and 20min of fast cruising sorted the problem.

Anthony

 

[Peanuts]

You beat me to it about the petrol, I remember my next door neighbour at home had a speedboat with a 150 on the back, start of the season would not even get on the plan, spent money and ages trying to sort it out, changed the fuel and problem 'sorted' The fuel was last seasons, so less that 1 year old.

 

[James L]

Might be obvious but have you cleaned the flame arrestor, I had these same problems when that got gunked up, actually didn't look too bad but I cleaned it and it made a world of difference.

Also I sprayed in carb cleaner, the one you spray in while the engine is running, shut down, wait and then run at highish revs and watch smoke go everywhere, seemed to make an improvement, but I can't remember the name of the stuff I used.

I also used Oil cleaner when changing the oil, but by the looks of it my boat was older than yours so might not make a difference to you.

James.

 

[gordmac]

If you have problems with leads, cap etc it is more likely the engine would run badly under load rather than be a problem at tickover. Check the choke works. Stale fuel could be an issue. We used to clean dirt out of carb jets by putting the palm of a hand over the carb intake.