uneven compression 4hp Evinrude. (update)

[steviecops]

Hi<br />I've got a 1980 Evinrude 4hp that I'm trying to get running. The compression readings are 100 psi in number one, and 80 psi in number two. I've had the head off to clean out the water passages and the pistons and cylinders looked in very good shape. No dings or scoring, and no excessive carbon. I used a new head gasket when I put the head back on.<br /><br />What else could be causing the uneven compression?<br /><br />Thanks<br />Steve

 

[DJ]

Re: uneven compression 4hp Evinrude. (update)

Another possibilty is that the rings are stuck, or worn, on number two.<br /><br />Get some Sea Foam or other deacrb product and let it sit on top of the piston(s) with the engine at full tilt up position.<br /><br />You'll have to make sure the ports are closed on the piston you are soaking.

 

[steviecops]

Re: uneven compression 4hp Evinrude. (update)

Thanks DJ, I'll give it a try.<br /><br />Merry Christmas<br />Steve

 

[Paul Moir]

Re: uneven compression 4hp Evinrude. (update)

If that doesn't free it up, try taking off the exhaust cover plate and poking at the rings with a soft implement (ie, plastic) to see if they're stuck. I had a 4hp have the same sort of problem with the bottom cyl when a drip of water somehow ended up in the cyl. The ring was stuck from corrosion crusties and so the decarb solution didn't free it. Poking at it with a stick did free one but not the other and I ended up taking it all apart to get it free.<br /><br />Good luck!

 

[steviecops]

Re: uneven compression 4hp Evinrude. (update)

Thanks Paul.<br />I'd taken the exhaust cover off to clean the passages and now it's back on with a new gasket! I'll just have to take it off again........ carefully!<br /><br />Thanks<br />Steve

 

[Joe Reeves]

Re: uneven compression 4hp Evinrude. (update)

On some of the small horsepower models which incorporates a fuel pump, the pump is positioned in such a manner that if the diaphram fails, that failure will have a grave effect on the compression of one cylinder.<br /><br />To see if your model might be one of those, simply insert some sort of blocking material between the fuel pump and the block.

 

[steviecops]

Re: uneven compression 4hp Evinrude. (update)

Hi Joe.<br />This engine doesn't have a fuel pump fitted, it's gravity fed from the integral tank. Although there is a hole which would take the fuel pump, it is blanked off with a small nut. I will check to see if it is leaking around the hole. Is there any recommended method of blanking the hole , ie, should I use gasket sealer on the thread?<br /><br />Thanks<br />Steve

 

[Meaux]

Re: uneven compression 4hp Evinrude. (update)

My father was a locksmith and used to drill a hole in old combination locks to visually align the discs and establish the combination. He knew the proper place to drill. To seal the drilled hole, he would simply press a small lead shot into the hole and trim off the excess. Good as new. I think this method of sealing holes would work on any hole or port on a carb. Give it a try.<br /><br />Meaux

 

[steviecops]

Re: uneven compression 4hp Evinrude. (update)

Here's an update.<br /><br />After letting the OMC engine tuner sit in the cylinders for a few days, then running the engine,(sort of!!), the compression readings are now just over a hundred psi in both. However, I'm still having a few problems.<br /><br />Thanks for the help<br />Steve