OMC 140 Compression PLEASE HELP!!!

[sccheech9]

I just changed my head gasket on my 1975 OMC 140. My compression numbers before the change were:
#1 - 130
#2 - 90
#3 - 105
#4 - 120

Now after I changed the head gasket all my numbers are between 90-100. Whats wrong? Why I try to start it, it backfires a lot and I see a puff of smoke shoot out the carb. Why it will fires finally the whole engine shakes violently.

PLEASE HELP!!!!

Thank You

 

[mkast]

Re: OMC 140 Compression PLEASE HELP!!!

Check the firing order again.

 

[sccheech9]

Re: OMC 140 Compression PLEASE HELP!!!

Yes, That was the problem. stupid me. thank you.

Why has my compression dropped so far?

 

[sobitthen]

Re: OMC 140 Compression PLEASE HELP!!!

If your OMC 140 is the Ford 2.3 OHC motor, or any OHC motor, then you may have the cam timing off, 90-100# compression is low for that motor. Did you have head resurfaced or checked for 'true'? [If resurfaced, then you will have slightly modified your cam timing in relation to your crank according to the Ford forums, not enough to make much difference in our applications luckily.]
You followed the torque sequence and torqued in 2 stages? 60 then 90 was for my 89 motor.
Turn the engine to TDC then pull the sight plug for cam and make sure it is aligned, you may have to turn over motor more than once and check since cam and crank spin at a 1:2 ratio...

Warm up engine, pull all plugs, remove coil wire, check you compression again if alignment was good and post your results.

 

[sccheech9]

Re: OMC 140 Compression PLEASE HELP!!!

No, mine is the gm 140 not the ford. I got the head checked out and they said it was fine.

It is the right head gasket. I torqued the head down very slowly. I did 10 lbs to start, then went to 20 then did 20 pound intervals after that.

I have to have the throttle wide open the do the compression test right? I didn't do that before changing the gasket either. How much of a difference will this make?

I did remove all the spark plugs. Maybe my starter isn't turning the engine fast enough?

 

[sccheech9]

Re: OMC 140 Compression PLEASE HELP!!!

How exactly do I check the timing like sobitthen was talking about?

 

[NoKlu]

Re: OMC 140 Compression PLEASE HELP!!!

By tightening pattern he means the order that you tightened the headbolts, not the torque on the bolts. The tightening sequence is very important. What pattern did you use. I would get the torque sequence from the manual and re torque the heads exactly as described in the manual.

 

[mkast]

Re: OMC 140 Compression PLEASE HELP!!!

Why has my compression dropped so far?

Thirty-four year old engine. Other than change the head gasket, did you do anything else to the engine?

 

[sccheech9]

Re: OMC 140 Compression PLEASE HELP!!!

Yes, I went by the sequence in the book. 1 through 10.

I changed all the spark plugs a week ago. When I took the head off, the rings looked ok, so I didn't worry too much about those. Changed the oil and filter. What else do I need to do?

 

[sccheech9]

Re: OMC 140 Compression PLEASE HELP!!!

bump. Anyone?

 

[mkast]

Re: OMC 140 Compression PLEASE HELP!!!

What else do I need to do?

Rebuild the engine.

 

[sobitthen]

Re: OMC 140 Compression PLEASE HELP!!!

How exactly do I check the timing like sobitthen was talking about?

On that motor you would have to pull the timing cover off since the cam is in the block, but you shouldn't have changed compression like the Ford engine since you did not have to align the cam during assembly.

Did you decarbonize the head and pistons? Pour 0.5 OZ of some engine oil into each cylinder through spark plug hole before retesting to see if numbers come up, maybe you have bad rings from using some tool to remove the carbon , if you decarbonized when off. If numbers come up near normal, then you have bad rings.
If numbers do not come up then it is in the valves. [If you left debri in the cylinder perhaps the debri is stuck on the valve seats.] Does that motor have adjustable valve train, if so, perhaps you overtightened the lifters, there is procedure where you align motor up to certain timing mark and adjust lifters cold, if you can't find the procdure, you could loosen all up until push rod spins with your fingers, tighten until you cannot spin it and give additional 1/2 turn, rotate motor checking for loose ones and tighten as needed, this is a rough way to get a cold adjustment, then perform compression test.

Try this and let us know...

 

[Maclin]

Re: OMC 140 Compression PLEASE HELP!!!

Why did you change the head gasket? Did a leakdown test show it had blown out somewhere?

I would do a leakdown test now. If you did not do one before this then the head gasket was just throwing a dart at the wall. The leakdown test will show you what is leaking compression.

 

[martyh]

Re: OMC 140 Compression PLEASE HELP!!!

On that motor you would have to pull the timing cover off since the cam is in the block, but you shouldn't have changed compression like the Ford engine since you did not have to align the cam during assembly.

Did you decarbonize the head and pistons? Pour 0.5 OZ of some engine oil into each cylinder through spark plug hole before retesting to see if numbers come up, maybe you have bad rings from using some tool to remove the carbon , if you decarbonized when off. If numbers come up near normal, then you have bad rings.
If numbers do not come up then it is in the valves. [If you left debri in the cylinder perhaps the debri is stuck on the valve seats.] Does that motor have adjustable valve train, if so, perhaps you overtightened the lifters, there is procedure where you align motor up to certain timing mark and adjust lifters cold, if you can't find the procdure, you could loosen all up until push rod spins with your fingers, tighten until you cannot spin it and give additional 1/2 turn, rotate motor checking for loose ones and tighten as needed, this is a rough way to get a cold adjustment, then perform compression test.

Try this and let us know...

Ditto..
Check the valves for being too tight. If the only thing you did was change the headgasket and it was torqued down correctly thats were I would start. Try a squirt of oil in each hole and retest.