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Compression numbers for my 1982 Evinrude 90hp, over propped damage?

M

Mawnee

New member
Tested the compression today and got 121, 121 on one head and 115, 110 on the other.


A quick history:

The engine is mounted on a recently bought 1985 16' Phantom bowrider(similar to glastron, bayliner ect.). The first time we took it out it ran fine for a few sprited runs while I cruised around and got used to it. Nose would go right up when I punched it and it would plane up within a couple of seconds. I did notice that the boat seemed to be overpropped as it topped out at 4100rpm doing about 37mph(engine plate calls for 4500-5500). And it was a bit hard to start, though I initially thought it was my fault not the boats(improper use of high idle setting ect). On the way back in(towards the end of the longest nonstop run so far) it started to lose power. At full throttle the nose would go up but it wouldnt gain speed or get on plane, it just didnt want to rev up. It also started idling rough. When I got it home I found a fitting on the underside of the carb leaking fuel when I squeezed the primer bulb. I replaced that fitting on both carbs and put the carbs back on with new gaskets. Tested it on the muffs and the boat fired right and idled perfectly.

So our second time out the boat pretty much did the same thing. It ran great on the way out(10 minutes idle cruising and 10 minutes cruising on plane). Then on another 15 minute high speed run it started to lose power, idled rough, stalling, hard to start, ect. I stopped and let it cool a few times on the way back in and pulled the motor cover during the breaks to look for anything obvious. In the course of this I broke one of the coil wires getting the cover on/off which didnt help (I found this once I got it home). Broke off right at the coil pack :mad:.

Hopefully that covers everything. The boat does have a strong coolant stream. I have double checked WOT throttle settings. With the throttle all the way forward the carb butterflies are horizontal(roll pins vertical).

My new prop arrived today, I got the pro-pulse so I can adjust the pitch and get it just right. I've ordered new coils and the gaskets to pull the head. While my compression numbers arent way off, I figured since its only $15 worth of gaskets to check it out why not. Especially since it may just be a bad gasket.

So what do you guys think is the root of my problems? Am I overheating due to being overpropped? Could it really be that simple? Any other suggestions for things to check out?

Thanks in advance for any advice :)

Phantom.jpg

Motor.jpg
 
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when was the last time carbs were cleaned? if you don't know go in and take a look and make sure there are clean and free of dirt and junk should be nothing in there but gas and oil. also next time it goes down on power, when you have the cowling off, place your hand (with the motor off) on the heads if you can keep your hands on them you are not over heating. you also need to check your over heat alarm. make sure it is working.
 
Sorry forgot to mention I went through the carbs when I fixed the fittings on the bowl. New gaskets, o-rings, floats, ect. But they didnt need it, they were completely pristine inside.
 
I just had it running out on the driveway again and noticed something odd. There is a vacuum line going to the Bypass cover on block behind the fuel primer valve(at the low compression cylinder). It is spitting fuel out around the fitting like that port is blocked or recieving back pressure. Could this be a symptom of a gasket failure allowing compression into the block?
 
I got burned.....

I went ahead and pulled the bypass cover. I could see a small nick on the piston surface....crap... So I pulled the head and found this. The engine had brand new spark plugs when I bought it, they are all still perfect. Looks like he lost the electrode to a plug and let it chew on it for a while. The kicker is that you can tell someone tried to "fix" the head by porting away some of the damage....

Pistondamage.jpg


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Headdamage.jpg
 
what is the compression number on this piston? do a leak down test on the cyl and let us know the number.
 
This motor has or had broken piston rings -----------TIME TO PULL THE BYPASS COVER AND HAVE A LOOK AT THE RINGS !!
 
That piston was the low one at 110, but that was cold. With all those dings in the head and piston I dont doubt its detonating once its warm. This explains my power loss after running for a while. I'm not going to put the head back on it to test it. That piston is gonna have to come out and I'll replace the head. Luckily the cylinder wall looks fine(which is why I supect spark plug electrode and not ring).
 
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