compression?

[bigfish34]

I have a 1984 Johnson 235. I just tested the compression and it seems a little low but it is uniform. the cylinders measured 80 psi on 5 of 6 with one reading 85. Are those measurements low enough to make it not rev up under a load?

 

[emdsapmgr]

Re: compression?

Is it a standard 235, or the 235S model? What heads are on it? A stock 2.6 should run around 115 lbs. A base 235 perhaps 10 lbs lower. At 80-85 lbs, it should run, but will be down on hp. I'd run a can of engine tuner through it, then check the compression again.

 

[jbjennings]

Re: compression?

I'm curious why you decided to check the compression. Is the motor new to you or did it suddenly start running bad? If it's new to you, I'd check your compression gauge on a motor of known compression. In other words, I'd suspect your gauge is wrong. If it still reads 85 I'd agree with the engine tuner advice (decarb) and hope for the best. Most of the time, you're not going to see all the cylinders on an engine with that many cylinders being low. It'll be just one or maybe two. The cylinders being all close seems to suggest your gauge is wrong and your motor is fine.
JMO,
JBJ

 

[bigfish34]

Re: compression?

This is a new motor on my boat. I changed from a inboard to an outboard and accidentally forgot to add oil to the gas. The engine ran for about 3 minutes before it quit at which point i realized what I had done. That was what precipitated my checking the compression. Also the motor has sat for maybe 7-8 years on its side.

 

[jbjennings]

Re: compression?

Oh no. That's bad news. that would definitely ruin all cylinders at once. I'd still check the compression gauge before getting too upset. they're bad about having bad valves and such which cause screwy readings. Before I turned it over again after rechecking the compression, I'd spray some light oil in the spark plug holes on all the cylinders. come to think of it, you could check the compression without spraying oil in the cylinders , then check it after. If there's a huge difference, you probably have scored cylinders and pistons. About all you can do I would think is lube the cylinders and get some good mixed fuel in the system and try it and see how it works. then again, you might wait until a real expert checks in on your thread---I'm NOT!
Good luck, sorry for the bad news,
JBJ

 

[emdsapmgr]

Re: compression?

Make sure you do your compression check with all the plugs out and that the battery is well charged. Sraying oil in the cyls will give to an erroneous reading. Just make sure that the normal oil/gas fuel is lubbing the pistons via the carbs during the comp test. If the compression does not come up any from the engine tuner, pull the heads and some of the side covers. Look for scuffed the pistons (aluminum transfer to the cylinder walls.) I'd also carefully check the bridges in the intake and exhaust ports to make sure none of them cracked during the lean overheat lockup.