Will a weak starter effect compression test?

I just ran a compression test on my recently acquired '79 KZ750 and I'm not thrilled with the results.

It came out at 110 for both cylinders. I ran the test once while the bike was cold and again just after a short ride with the same result both times.

110 seems pretty low, though the bike seems to run well. I'm wondering about a couple of things that may be contributing to the low test result.

I know my starter is a little weak and have been planning to rebuild it. How much will that effect the results of the comp test? The starter is strong enough to start the bike and most of the time does an adequate job. But sometimes, especially if its a little chilly out, I have to hold the starter button for about 2 seconds.

Another known possible culprit, I've got a very slight leak at the head gasket on one cylinder (but not the other).

Overall I suppose I'm wondering how worried I should be about this compression test result. 110 seems really low. Is there any way to know if its a piston/rings issue or a valve issue? Should I not worry until I've rebuilt the starter and then retest?

Hi Topper, Your battery should be fully charged to be able to crank the engine pretty fast. Did you have the plugs out? Did you have the throttle wide open? Put some oil in the test cylinder and redo it. If the compression is the same, it probably needs a valve adjustment (which should be done before any major dis-assembly).Give it a try and let us know what happens. Bob

I did have the plugs out, plug wires grounded and throttle wide open.

I'll try adding a bit of oil and testing again.

Valve clearances have been set recently. At the time the exhaust valves had no clearance. That's been fixed now.

I recall the service limit for compression on the 750 twins was about 115 (hot). If it ran with valves having no clearance for long, the valve seats could be burned. Also, a leaking head gasket certainly could drop compression.

Generally, relatively equal compression in all cylinders is a good sign.

Could confirm compression readings by re-testing with a different gauge.

A leak-down test would give a better idea about engine condition, and help identify the source of any compression loss in each cylinder, such as a specific valve, whether escaping past the piston and rings into the crankcase, and sometimes a leaking head gasket.

Exhaust smoke when first cranked, that dissipates after a mile or so, usually indicates worn valve stem seals.

Continuous exhaust smoke may indicate an oil ring issue, or possible fuel contamination of the crankcase oil.

Good Fortune!

What year, oh yeah 79. Use the kicker if you got one, and keep kickin it till the guage stops completely - should be 3-5 kicks or so. Starters suck!

If you gotta use a starter - hookup your car batt, it works hell-a-awsome and dont hurt a thing.
Crank the starter for like 4-5 seconds, till that comp guage stops completely.

If you had a head gasket leak - it would (usually) show a significant drop in compression - not like 110.

PS. If it hauls ass - dont be to worried about it - less you're aming for perfection this season.

I would just ride it and enjoy it. You can always tear into it in the colder months :laugh:

Peace bro.

TeK9iNe wrote: ...If you gotta use a starter - hookup your car batt....

Just be sure the car engine isn't running while batteries are jump connected.

Good Fortune!