1987 evinrude 100hp - Compression good then a major drop in all cylinders?

[tuggeyt]

Ok been slowly pretty much rebuilding this motor since I got it like 7 months ago or so....
Here's whats up with it now... When I purchased the motor and checked the compression, it was 111,115,113,111.
Now I checked the compression and it and cylinders read right around 78 +/- 1... What the heck would cause this?
Replaced stuff on it like starter, plugs, rebuild carbs, nothing with the actual motor.
I ran it on the hose a few times, no more than 10 minutes, most times under 3 minutes.
Ive taken it out 3 times on test runs, ran first time about 15 minutes, second time about 10, third time about 45 minutes.
Made a post about a month ago on running problems.. Here's the link
http://forums.iboats.com/forum/engi...g-way-it-starts-runs-and-possibly-overheating

Any ideas? I can't play with it till next Friday but first thing I'm doing is trying another compression tester.
Pretty sure it's good though cause I used it on another motor a day prior...

 

[tuggeyt]

Any idea?

 

[tuggeyt]

Bump

 

[GA_Boater]

Did you try the other comp. tester? You didn't say.

Is the battery fully charged? How long are you cranking? Until the gauge stops increasing?

 

[emdsapmgr]

Slow cranking (less than 250 rpm's) will result in lower compression readings. (weak battery) You have not run it much, but if the powerhead was severely overheated while runninbg, it can cause the heads to warp, causing low compression. I'd probably pull both heads and check the seal rings on the head gaskets. If they are intact, that's a good sign. Then put both heads on a perfectly flat straight edge or plate glass to see if they are perfectly flat. While the heads are off, I'd also inspect the top ring on all 4 pistons to be sure they are all in once pc. Hard to see, but if you move the piston around by hand in the cylinder and shine a light on the piston, you should be able to see if the ring is continuous or not.

 

[racerone]

Is the oil injection in service ?--Have you checked that.----Or are you mixing at 50:1 on it.---All older cylinder heads warp with time , overheat or not !!!

 

[boobie]

Before you do anything, check the compression again with another gauge and get back to us.

 

[tuggeyt]

Ok sorry for the delay but I tried a new compression gauge and got same exact readings... I also poured a little oil into the cylinders to see if it would go up at all and it did by like 1-2 points, nothing major... The vro was disconnected and it's 50:1 premix... If the heads warped enough for this, wouldn't it be over time, not such a short amount like this? I also left the compression gauge hooked up to it after cranking... After 10 minutes it drops like a half/1 point... That's it

E: after 30 minutes of leaving compression tester on there it hasn't dropped more than that initial half/1 point

 

[emdsapmgr]

Adding oil the cyls during compression testing will raise the comp #'s slightly. These high numbers are not valid. If you keep cranking for another 30 seconds, the extra oil will dissipate and your compression numbers will drop right back to the original number, which is the correct reading.

 

[racerone]

Leaving a compression tester in there for 30 minutes proves nothing !!!