Mercruiser 140 loosing power after warming up.

[drkaras]

I have a 140 Mercruiser that runs perfectly when cool. After 20-30 minutes it starts to loose power and I go from 4500 RPM to 3000. At that point the RPMs can vary widely and at times go back up to full power.

It had a blown head gasket and cracked head when I bought it, so I have already installed a new head and a rebuilt carb. I have tried running from a clean gas can with a new filter without improvement. Fuel pressure tester quickly gets to 9 per recommendations in "Adults Only." Compression is good in all 4 cylinders. Timing and dwell are dead on.

What am I missing?

 

[bigbrownbuku]

Re: Mercruiser 140 loosing power after warming up.

try a new ignition coil. make a little alligator clip test lead and run b+ to your igniton coil also. you mention dwell, check your distributor shaft side play and play with the points gap a little too. dwell after idle is vastly different if your dizzy shaft loads to 1 side under higher rpm.

 

[marlboro180]

Re: Mercruiser 140 loosing power after warming up.

deleted by author

 

[jtybt]

Re: Mercruiser 140 loosing power after warming up.

They weren't one of Merc's success stories...trouble is, they don' really have many real good stories to brag about. One thing, Merc is a step up from the old OMC.


Marlboro,
The thing is, you're really not giving us much to go on to diagnosis.

 

[marlboro180]

Re: Mercruiser 140 loosing power after warming up.

Sorry, but boatfan had a loss and seems really distraught about it. I hope he doesn't head for my motor....or anyone else's....

So, when you're running now with a rebuilt carb (did you do the rebuild , if so how? or from a shop? or a re-man?) and a new head . I presume valve lash , etc. done?
Is the motor running at a decent temp. now after the new head?
If it is running in a varied fashion I would tend to lean toward fuel delivery, i.e. vapor lock High heat in the bilge), float height/ drop improperly adjusted. BTW - what carb?

You mentioned running from a gas can. Good idea. Are you on the water while doing this?

I suppose a really sloppy dist. shaft could come into play, but since that wasn't messed with....and it would run crappy from the get go.



regards

 

[jtybt]

Re: Mercruiser 140 loosing power after warming up.

Marlboro,
Sorry, I got you mixed up with the poster. My apologies.

 

[marlboro180]

Re: Mercruiser 140 loosing power after warming up.

jtybt- no worries, think I did the same

drkaras- more info on what you have for motor, carb, etc. will yield better results for help.

 

[drkaras]

Re: Mercruiser 140 loosing power after warming up.

It is a 1982 Mercruiser 140/3.0. It has a rebuilt double barrel Mercarb from a carb shop. The ignition coil is brand new as well. I changed the impeller when I bought it and it runs nice and cool. I installed the rebuilt head myself, so I cannot promise that the valve lash is perfect.

Again, it runs perfectly for about 30 minutes. I cannot imagine an engine running more smoothly. Then it will slowly start to loose RPMs. Over about 10 minutes at WOT it will go from 4000 to 3000 and loose plane. At times it will surge and sometimes even go back up to full power for a short time. The whole time, the engine still sounds like it is running fairly smooth.

I have eliminate the gas tank as a problem. It ran just as poorly running from a gas can on the water. The fuel pump has a new filter and seems to be creating the proper pressure. It had the same problem with the old carb, so I do not think carbueration is the issue. If we close the choke halfway we do get some improvement in performance. All 4 spark plugs look the same and do not seem to be significantly fouled. We sprayed WD40 on the manifold gasket with no change. Timing and dwell are on. I am running out of things to check. I can't imagine what is changing so significantly when it warms up.

 

[drkaras]

Re: Mercruiser 140 loosing power after warming up.

Also - what does "make a little alligator clip test lead and run b+ to your igniton coil also" mean, bigbrownbuku?

 

[marlboro180]

Re: Mercruiser 140 loosing power after warming up.

It is a 1982 Mercruiser 140/3.0. It has a rebuilt double barrel Mercarb from a carb shop. The ignition coil is brand new as well. I changed the impeller when I bought it and it runs nice and cool. I installed the rebuilt head myself, so I cannot promise that the valve lash is perfect.

Again, it runs perfectly for about 30 minutes. I cannot imagine an engine running more smoothly. Then it will slowly start to loose RPMs. Over about 10 minutes at WOT it will go from 4000 to 3000 and loose plane. At times it will surge and sometimes even go back up to full power for a short time. The whole time, the engine still sounds like it is running fairly smooth.

I have eliminate the gas tank as a problem. It ran just as poorly running from a gas can on the water. The fuel pump has a new filter and seems to be creating the proper pressure. It had the same problem with the old carb, so I do not think carbueration is the issue. If we close the choke halfway we do get some improvement in performance. All 4 spark plugs look the same and do not seem to be significantly fouled. We sprayed WD40 on the manifold gasket with no change. Timing and dwell are on. I am running out of things to check. I can't imagine what is changing so significantly when it warms up.

Hmmm, seems like you are doing the right things. Just curious about the choke- so it runs better when you half choke it? In what way?

Regarding valve lash- warm motor , back off 'till play (up and down) is felt, snug 'till lash gone, then 1/2 to 3/4 turn more and move on to next?( lots 'o debate between 1/4 turn to 1 full turn. Merc says 1 turn, some say less --search threads)

Have you run it w/ the cover off? I presume so w/ the gas can run on the water..

Maybe someone else will chime in?

 

[bigskiohio]

Re: Mercruiser 140 loosing power after warming up.

its 3/4 turn but only do 1/4 turn at a time then wait for engine to settle then another 1/4 repeat till 3/4 turn. going all at once could cause engine damage.

 

[drkaras]

Re: Mercruiser 140 loosing power after warming up.

Thanks Marlboro180. I know I over tightened the valves the first time and had to back them out. That was last year. It is easy enough to check. That can be done correctly in the driveway, right?

When I close the choke 1/2 to 2/3 the RPMs seem to go up a bit. Maybe just from richer fuel. Or does it mean that there is an air leak somewhere?

Also, my father mentioned that he could feel some blow back at the carb as we were loosing power. I imagine that a stuck valve could do that.

 

[marlboro180]

Re: Mercruiser 140 loosing power after warming up.

Thanks Marlboro180. I know I over tightened the valves the first time and had to back them out. That was last year. It is easy enough to check. That can be done correctly in the driveway, right?

When I close the choke 1/2 to 2/3 the RPMs seem to go up a bit. Maybe just from richer fuel. Or does it mean that there is an air leak somewhere?

Also, my father mentioned that he could feel some blow back at the carb as we were loosing power. I imagine that a stuck valve could do that.

Sure, that can be done on the driveway.

BLOWBACK from where?

 

[drkaras]

Re: Mercruiser 140 loosing power after warming up.

When he put his hand over the opening for the choke, he could feel some back pressure. I don't know if that is normal, or if it means we are loosing some compression from a valve that is not seating.

I readjusted the valves last night with the engine running. Lets just say that my face was well lubricated afterwards.

I have also checked that my ground is good. I don't think I have too much play in the distributor shaft. New points and condenser are on the way.

 

[marlboro180]

Re: Mercruiser 140 loosing power after warming up.

http://forums.iboats.com/showthread.php?t=167561

check it out.

 

[Ratboy14]

Re: Mercruiser 140 loosing power after warming up.

My engine has the exact same problem with the surging. Year is '84. Coil, Points, Condenser and rotor are brand new. Temp is fine so I don't see where the problem is.

 

[drkaras]

Re: Mercruiser 140 loosing power after warming up.

I may be on to something. I checked compression, cold and warm, and it was perfect. By then it was idling poorly, so I threw the tachometer on it to see where it was. I had dropped about 75 RPM at idle.

Then I gave a little push on the distributor and the RPMs shot up and the engine started running smoothly! I sure hope this is it. A new distributor is better than a new block.

 

[futz]

Re: Mercruiser 140 loosing power after warming up.

So pull the cap check the rotor is making contact on the contacts under the cap as it rotates, make sure cap is on tight. The dizzy tight. No play.

 

[drkaras]

Re: Mercruiser 140 loosing power after warming up.

Great news! I replaced the distributor and it ran perfectly all week! I am still not sure why it caused problems only when it warmed up, but to all of you who are losing power after 30 minutes, push on the distributor a bit and see if your RPM's improve.