1985 Mercruiser 140 Runs Rough after warmup

[greg.brady]

Hi,

I'm having a problem with my 4 banger Merc 140:

When the motor is cool, I can run it for between 15-30 minutes on plane.

The motor will start missing, and will continue to get worse until it is hardly running.

If I let it cool down and it will start up and run great again until it gets warm and starts missing.

Water Temperature and Voltage are reading good.

Things I have tried:

- new gas / checked gas for water
- checked pump for volume - looks good
- checked anti siphon valve
- checked fuel like for cracks
- checked tank pickup for cracks
- yellow fuel pump failure line is empty
- added new condenser/cap/rotor , spark plugs and wires are pretty new
- no play in distributor shaft

I did have an issue with a burnt gasket between 2 and 3 a few years ago but it cleaned up good and new gasket has worked properly.

But... the heat of running is causing an expansion of metal somewhere that is causing my problem (unless of course it is fuel or spark).

I've never seen anything like this, your suggestions are greatly appreciated, Greg

 

[Don S]

Re: 1985 Mercruiser 140 Runs Rough after warmup

HOW TO: Testing your boats fuel system.?/ 283269

 

[greg.brady]

Re: 1985 Mercruiser 140 Runs Rough after warmup

HOW TO: Testing your boats fuel system.*/ 283269

Excellent informaion, I will report back with my findings.

Thanks, Greg

 

[greg.brady]

Re: 1985 Mercruiser 140 Runs Rough after warmup

Hi,

#1 - I did a compression test:

1- 145
2- 141
3- 140
4- 138

Pretty good numbers I think?

#2 - I put a piece of clear line in the fuel system, No air bubbles, fuel looks good

#3 - I checked the plugs , they look typical not really clean but serviceable.

I'll regap them in the morning and swap out the points and see if that makes a difference?

At this point I'm thinking either electrical or possibly valve adjustment.

- When it runs on muffs you can hear the miss happening every two or three seconds at idle

Any help appreciated!

Thanks, Greg

 

[thumpar]

Re: 1985 Mercruiser 140 Runs Rough after warmup

Try taking the gas cap off when it is having the problem. It could be a vent issue. You could also run it off an external tank to rule out that side of the system.

 

[greg.brady]

Re: 1985 Mercruiser 140 Runs Rough after warmup

I checked the vent and filler and both are clear of obstructions. Fuel looks good, im going with an electrical issue for now till I exhaust that option.

Thanks, Greg

 

[jimbo_jwc]

Re: 1985 Mercruiser 140 Runs Rough after warmup

added new condenser/cap/rotor , spark plugs and wires are pretty new .

I had a condenser that I replaced when I tuned up my truck that acted like this ,put old one back in and it solved my problem. The new one was faulty .