Low compression, runs fine....any thoughts?

dgsoles · Post by **dgsoles** » Wed Nov 05, 2008 6:48 am

I'm a little worried about my low compression numbers. I just installed new piston rings and honed the bore. I also have a new head-gasket installed. Yet, whenever I take a compression reading (at wide-open throttle, warmed-up), I consistently get 50-60 PSI. The engine runs fine.....I'm using a tester with a hose on it....could that be a problem?

Wheelman-111 · Post by **Wheelman-111** » Wed Nov 05, 2008 9:48 am

Greetings:

I'll assume you were careful with your assembly. Did you insert the rings into the bore and measure gap? Also that you slathered everything with plenty of KY Jelly - no, check that - Genuine Soichiro Honda-Approved 2-Cycle oil when you screwed it together. No obvious external compression leaks is also assumed. You mentioned a new head gasket but crancase air volume can also be lost at the cylinder base.

Advice:
1. Repeat with a different measuring device. Your tester may be flat wrong. If a second, known good anemometer also reads 55,
2. Squirt a few drops of Genuine Soichiro Honda-Approved 2-Cycle oil into the plug hole and repeat your test compression test. If it's still low,
3. Check the reed cage. If it's allowing air back out the intake, it will decrease your apparent compression.

If all appears well, and the bike runs OK, run it. When you repeat the test after a few miles, and you know you will, you may find it's improved as the rings bed in. Please let me know how it goes, as I am about to emark on a similar journey.

dgsoles · Post by **dgsoles** » Wed Nov 05, 2008 9:59 am

I was quite careful with my assembly, however I did not take just the rings and insert them into the bore....I did slather plenty of 2-stroke oil over everything before assembly. I also used a brand new crankcase-cylinder gasket. When I put a little oil into the combustion chamber, the PSI goes up by maybe.....3 or so, not much. As for the reeds, I was wondering about that. I noticed that one side doesn't like to seal all of the way (an extremely small amount). I very carefully took them apart and cleaned everything off, and it seemed to help. Now, when I look at them, about half of the time they're closed all of the way, the other half that little (it's so small, you can't even see it from the intake side, you need to flip it over and see it from the crank-case side). How much would a slightly faulty reed affect compression? It feels like there might be some air coming back out of the intake (it's a little hard to tell the difference between suction and compression with my hand over it). But, as I said, the bike does seem to run fine (other than my dirty carb issues, which, hopefully this last cleaning took care of, I still need to reinstall the carb).

Bear45-70 · Post by **Bear45-70** » Wed Nov 05, 2008 11:59 am

You need to check your compression tester to a known good one (some auto parts stores have loaners or rental tools) or use it on an engine with known compression numbers to verify it is reading correctly. I've seen many engine brought in for repair because the owner's compression tester was bad.

Wheelman-111 · Post by **Wheelman-111** » Wed Nov 05, 2008 12:40 pm

Greetings:

Even a small air leak at the reeds will have a significant effect on the compression measured at cranking speed.. The closure of the reed is in part dependent on the pressure applied from within the crankcase as the piston descends. If some air blows back out the reed cage, there's less starting volume/pressure to transfer through the ports to the combustion chamber.

That said, I'm not so sure that a very small leak has has ANY effect on performance as the engine's RPM, Temperature and pressures increase - most likely slamming that read firmly shut. This would explain (?) why it runs just fine. IF it gets low on power, you might consider replacing said reeds, but until then, just ride it.

dgsoles · Post by **dgsoles** » Wed Nov 05, 2008 12:44 pm

I'm willing to bet that is my problem, then, because it does run very well, even at idle, which would be impossible if it did, actually, have only 50 PSI compression. I suppose I'm making a mountain out of a mole-hill here. Thank you guys for the advice.

Wheelman-111 · Post by **Wheelman-111** » Wed Nov 05, 2008 1:48 pm

Glad to help whenever I can. I have received more good advice from this Forum than I have given. Just be sure to let us know how it goes.

Bear45-70 · Post by **Bear45-70** » Wed Nov 05, 2008 2:07 pm

If you had a reed standing open enough to effect cylinder compression that much then engine would be a nightmare to start. Start with the simple thing, check the compression tester first.

dgsoles · Post by **dgsoles** » Wed Nov 05, 2008 4:17 pm

Well, I think I can do that. Just have to find one around here...I'll keep you guys posted. I might get to it this weekend.

DENMANSHAFFER · Post by **DENMANSHAFFER** » Wed Nov 05, 2008 4:53 pm

My two 86's both run around 90 on my tester, and both run great.

Kenny_McCormic · Post by **Kenny_McCormic** » Wed Nov 05, 2008 5:41 pm

Dead volume in cheap testers makes a big difference when your dealing with 50ccs.

dgsoles · Post by **dgsoles** » Wed Nov 05, 2008 8:55 pm

I read that somewhere, due to them just not having the volume to move the air in the hose sufficiently....I think....

Honda Spree and Elite 50 Forums

Low compression, runs fine....any thoughts?

Low compression, runs fine....any thoughts?

Compression

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