Compression Readings

[Salty Tricks]

1991 Rude 200HP

I was told by someone(?) that loop charged engines have lower compression. My numbers are all around 70-75 psi. These sound really low but I did this last year and while the boat is slow to plane it does top out at 30-32 knots which is okay for me. The planing issue may be a prop issue if I can ever straighten out my tach problem. Is this motor tired?

 

[rebuilt]

Re: Compression Readings

I can't answer your question, but have one of my own. Same topic. I hope you don't mind someone jumping in on your thread. '89 Johnson-60 hp 3 cyl. What are respectable compression #'s? I know even is good. My 35 has great #'s, but it uses dome top pistons too. This motor uses flat tops. How do I learn about "loop charged"? What is it? What does it describe? Is this '89 60 hp "loop charged"? I've heard that phrase used alot in posts. Thanks, KR

 

[JB]

Re: Compression Readings

Guages and techniques vary too much to put a number on compression.

What matters is that all cylinders are within a 10-15% range.

 

[tashasdaddy]

Re: Compression Readings

salty tricks, i would do a decarb on that engine. planing depends on the hull it is on and the proper engine height, trim, and prop.

 

[Salty Tricks]

Re: Compression Readings

Thanks TD
This is on my 23 Trophy. It started life with 3100 lbs dry and a 150 Force. I swapped it out for a really clean 91 200 Evinrude. I must admit I have loaded this boat down with alot of stuff over the years adding weight. To add....I have yet to solve a tach/rectifier problem( that's tomorrow's project) and suspect this prop is wrong for the boat. Installed height really shouldn't be a concern...I have this mounted to a hydraulic jack plate and basically can start it at various engine heights. It has hydraulic trim planes as well. I am not unhappy with the motor..just concerned that the numbers are low and she takes a little longer to plane than expected.

Tashasdaddy can you look at my waterpump question I posted earlier...I had some problems with the install.

 

[Silvertip]

Re: Compression Readings

75 PSI is on the low side and will affect power output although the engine may run fine. Changing the prop will help with hole shot but the real problem is lack of power due to low compression. 90 PSI or better would be "normal". Check compression with another gauge and on a warm engine. Decarb as was suggested and then check again.

 

[rebuilt]

Re: Compression Readings

On this train of thought, shouldn't a straight re-ring get compression up? Without an inside snap guage, and a set of quality micrometers, it's hard to tell how much wear the bore itself is showing. Maybe a nasty ring ridge would be a tell-tale. Thoughts?? KR

 

[Silvertip]

Re: Compression Readings

Low compression can be caused by a number of things -- among them are: 1) worn rings, 2) an engine that had been overheated one or more times and the rings have lost their tension, 3) excessive carbon build-up causing rings to stick tight in the ring lands on the piston (decarb may loosen them and raise compression), 4) a leaky head gasket due to overheating or head warpage 5) the engine is simply showing you it's tired and worn.

Without tearing the engine down you really don't know why compression is low. However, doing the decarb would take care of the carbon build-up if that was the problem. Pulling the head is not a major operation and enables checking the cylinder bores for evidence of wear. You can rent the inside micrometers but a ridge at the top of the bore and absence of a hone pattern would tend to point to simple wear.

 

[rebuilt]

Re: Compression Readings

Thanks, I'm looking at re-ringing an '89 60 hp 3 cyl. I'm pretty new to big 2 strokes. Been foolin with 4 stroke car and truck engines for years. I cannot get over how much help this board has been. Someday, I hope to be able to contribute with actual answers that apply. Not just 10,000 questions. Sometimes the tech manual just doesn't cut it. I've got a Seloc manual for my other motor, then lucked up and found a JE shop repair manual for my motor on eBay. But there's nothin like bouncin ideas off of real people. Thanks, KR