Rough running Merc

[Kingzrt]

I have an MCM140 On an 83 Sunbird the engine runs smooth at fast speed but is jumping/ vibrating when at slower speeds, and at idle. I am thinking the timing might be an issue. I have tried everything else changed fuel filter checked plugs cleaned carb, any suggetions? Thank you for your help

 

[flabum]

Re: Rough running Merc

Have you checked the compression?

 

[Kingzrt]

Re: Rough running Merc

Have you checked the compression?

No i have not

 

[flabum]

Re: Rough running Merc

You might want to do a compression test first...... timing doesn't just go out of adjustment. Post the results.

 

[Don S]

Re: Rough running Merc

By all means do a compression test, but do it correctly so it means something. Like THIS
When was the last time you did a full tuneup, points, condenser, cap, rotor, and sparkplugs.
You also need to set the point dwell, not just gap them with a feeler gauge.
Then you reset the timing, it changes along with dwell.
Then, you need to adjust the idle mixture and rpm.
But it all has to be done at one time if you want your engine to run correctly.

 

[Kingzrt]

Re: Rough running Merc

By all means do a compression test, but do it correctly so it means something. Like THIS
When was the last time you did a full tuneup, points, condenser, cap, rotor, and sparkplugs.
You also need to set the point dwell, not just gap them with a feeler gauge.
Then you reset the timing, it changes along with dwell.
Then, you need to adjust the idle mixture and rpm.
But it all has to be done at one time if you want your engine to run correctly.

Thanks for the info i will do a compression test following the instructions you provided, Thanks. The last full tune up was probably 5 years ago. But the boat sat for 2 years before i used it again, then My mechanic just changed plugs and wires and that was about a year ago, Should i start with a full tune up then go with the compression test or do compression test first?
Thanks for all your help truely appreciated.

 

[Don S]

Re: Rough running Merc

Follow the instructions and do the compression test. If it's ok, then do a tuneup. It's WAY overdue.

 

[Kingzrt]

Re: Rough running Merc

Follow the instructions and do the compression test. If it's ok, then do a tuneup. It's WAY overdue.

One more thing i failed to mention, I have been having a hell of a time getting this engine started. If it sits 2 days or 2 months i can wear batt's down to zero before it finally starts.

 

[Don S]

Re: Rough running Merc

Not a surprise.

 

[Kingzrt]

Re: Rough running Merc

Not a surprise.

Just a quick question do i have to use all marine parts for the tune up or can i use Standard engine part's from an auto parts store?

 

[ziggy]

Re: Rough running Merc

do yer compression test 1st. no point in gettin tune up parts if ya don't pass the test...... get yer parts at the boat store, or if ya must shop at a autoparts store, specifiy marine parts only. there is a difference. yes. get marine parts........ ya might get away with auto plug wires..... other than that....... marine duty only...... as far as not starting goes. why don't ya do a compression test, if it passes, get yer tune up parts and set it up to the specs in yer oem service manual for yer engine. bet it'll start much eaiser then......

 

[flabum]

Re: Rough running Merc

Always do your basics first.... check Compression, check for water in fuel and check the spark.


Check the compression and post the results here.....we will get it fixed for you but you have to give us the info we need

 

[Kingzrt]

Re: Rough running Merc

Always do your basics first.... check Compression, check for water in fuel and check the spark.


Check the compression and post the results here.....we will get it fixed for you but you have to give us the info we need

Ok Guys i did the compression test on all 4 cylinders i have 150psi, i did it as instructed per Don S. So i guess do a complete tune up will be next on the list? I will get all Marine parts. Thanks for the help guys.

 

[Don S]

Re: Rough running Merc

I've done a lot of compression tests on 3.0L engines, and never had one with all cylinders exactly the same.
If the compression test is ok, yes, do the tuneup.
You mentioned early in this thread that you "Cleaned the carb" what exactly does that mean. Did you take it apart, soak in carb cleaner, blow out all the passages with compressed air, reset everything to specs with a new gasket kit? That, is how you clean a carb.

 

[Kingzrt]

Re: Rough running Merc

I've done a lot of compression tests on 3.0L engines, and never had one with all cylinders exactly the same.
If the compression test is ok, yes, do the tuneup.
You mentioned early in this thread that you "Cleaned the carb" what exactly does that mean. Did you take it apart, soak in carb cleaner, blow out all the passages with compressed air, reset everything to specs with a new gasket kit? That, is how you clean a carb.

Initially i just soaked it with Carb cleaner, I will pull the carb off and completely clean it as you specified as part of the tune up. The compression was not exactly 150 on all #1 150 #2 148 #3 147 #4 150, the service bulletin you sent said as long as it was above 100psi it was good. I have a service manual i just picked up and will do a complete tune-up including carb. Thanks for all your help in guiding me in the right direction, Don.

 

[JustJason]

Re: Rough running Merc

when you change your filters. Make sure you get the one inside the fuel pump too, and check the carb. Sometimes there is a filter behind the fuel inlet.