mercruiser 140 runs rough at high rpms

[jwh17]

Hey there! new to the forum.. I have a mercruiser 3.0L 140 that runs great at idle and low-to-mid range rpms, but once i open it up, she starts to misfire and run rough above about 2500 rpms and gets up to only about 3900. I think its running too rich, but i have rebuilt the carb, and set everything to specs as far as i know. (forgot to mention, its a mercarb.) I also set the timing and dwell. So does anyone know how to fix this issue? Any help would be greatly appreciated!!
-Jim

 

[bhammer]

Re: mercruiser 140 runs rough at high rpms

Since you just rebuilt the carb, I'd start there and double check that everything is set correct. How did you adjust the mixture? Experts will probably have more.

 

[JustJason]

Re: mercruiser 140 runs rough at high rpms

I can double check your numbers....
1. what is your timing set at, both idle and full advance?
2. what is your dwell set at?
3. when was the last time you changed your fuel filters?
4. have you checked for water in your fuel yet,
5. how exactly did you "rebuild" the carb

Personally that is where i would start... However your problem definitely lays in here:
Engine Won't Reach Operating RPM. Check
• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation (Full tuneup done recently)
• Remote control cables and linkage for proper travel to open throttle plates fully.

 

[JustJason]

Re: mercruiser 140 runs rough at high rpms

How did you adjust the mixture?

Idle mixture has nothing to do with WOT performance.

 

[bhammer]

Re: mercruiser 140 runs rough at high rpms

Idle mixture has nothing to do with WOT performance.

Don't some of those carbs come with an idle mixture as well as traditional mixture adjustment? I ask cause I don't know... Long time ago, at least the way I remember it, I had a truck that ran like crap at high rpm and it was one of the mixture screws. Once I set the mixture by vacum, it resolved the problem.

Your list is much better than what I came up with.

 

[JustJason]

Re: mercruiser 140 runs rough at high rpms

Don't some of those carbs come with an idle mixture as well as traditional mixture adjustment?

I can't speak for every carb ever made but I have not ever seen that on a car or boat motor. I have seen a "high speed adjustable jet" On PWC (jetskis)carbs and some lawn and garden equipment.

Don't thank me for the list... 1 through 5 is me but everyting else is courtesy of Don S.

Try answering my 1-5 questions first... then go from there. Before everyone chimes in and say "check this, that and the other thing"
Give me some numbers

 

[whywhyzed]

Re: mercruiser 140 runs rough at high rpms

......

 

[jwh17]

Re: mercruiser 140 runs rough at high rpms

sorry about the delay, well, the thing that is getting to me about it is that i got it running like it should, like an ansolute champ for about 3 weeks. This was after a trip to the machine shop with the head. All new valves and remachining. It ran fantastically, but now it started doing the same thing again. I dont really have the tools to get the numbers, but with a light i know the timing is correct, and the dwell is where the manual says it should be. I believe the issue to now be electrical, but i don't know where to start.. any advice?

 

[jwh17]

Re: mercruiser 140 runs rough at high rpms

also, it will sometimes start running good again, but it is intermittent. thanks, -jim

 

[bhammer]

Re: mercruiser 140 runs rough at high rpms

How's your fuel and your fuel / water seperator filter? Have you checked the filter for water? Also, are you sure timming is correct for the advance timming?

Does it still run good at idle and lower RPM ranges or is it rough all the time now?

 

[chiefalen]

Re: mercruiser 140 runs rough at high rpms

hows it look under the dist.cap.it advancing ok maybe sticking a little.its not backfiring?