Piston compression question

[birdgod]

Just finish rebuilding a 1994 omc 90hp v4 engine. When I test the compression is read 102, 102, 102, 125. The 4th piston have a high compression read. Is this namal?

 

[emdsapmgr]

The 102 readings would be normal for your engine with the factory low compression heads on it. The 125 is pretty high, if it's a realistic number. Based on that 125, you should be running premium fuels only. 125 was normal compression for an 85 hp engine of the late 70's, back when they had high compression heads and ran leaded 89 octane fuels. I'd pull the head off for a look. Was that piston ever replaced? Do the markings on that piston crown match any markings on the other 3? Unlikely, but you have a mis-matched rod on that piston, so check it's range of movement vs. the other 3. One other thing-the carbs on your original engine are jetted to keep the pistons cool during firing. My concern would be that the high compression piston may be running on original (stock) jetting-that could lead to a piston failure (due to overheating) if run that way.

 

[racerone]

In this case " rebuilt " means all new O/S pistons or are you using the old parts over again ?

 

[birdgod]

I am using the old part. Also the test was done with cold engine. Should I try worm it up first and test again , or just put in new rings for the piston?

 

[SparkieBoat]

you should measure your cylinders and check for out of round. then slide your rings into the cylinder and check end gaps. something is surely wrong. 23 psi difference is too much. needs to have all cylinders with in 10 psi. with in 5 is expected and better. within 2 or 3 for a fresh rebuild is what I would expect. you should also check your heads and make sure they are not warped.

 

[birdgod]

Just ran the test for a few more time, it turn out the measure I got was wrong, the real reading is 100, 100, 100, 110.

 

[SparkieBoat]

that will run .. but I would expect better numbers out of a fresh rebuild. did you have your heads smoothed? and new head gaskets?

 

[birdgod]

Turn out I put too much oil in to run the test. I am going to hook it up and run it warm to to the test again.
Well this engine was working before. My friend decided to take it apart to see how it works, and it has been apart for years all clean. I got all new gasket for it. But all other parts is the original.

 

[SparkieBoat]

you should have at least honed the cylinders and new rings and smooth the heads. since you had it apart.

 

[emdsapmgr]

If you oil the cyls before the compression test, you will get an initial higher (inaccurate) compression reading. That number will quickly go down as the excess oil dissipates in the cyl. After another 10 rotations, the cylinder should read normal compression. That's the correct compression reading.