Some advice needed.

So long story. I bought my xj 650 2 summers ago, it ran fine all that summer untill the end when it started to run a little funny. Winterized it, got it out early spring, ran fine, thought nothing of it untill a month in it started running rough. Clean and tried to sync the carbs, it was still running bad. I tried to plug chop it and but it and retune and retune, finally striped the carbs and cleaned them again, just to make sure, tried to resync, still no luck. It was late in the session so I put it away. So I was planning to just take it to a shop and have them clean and sync the carbs but before I took it in my father suggested I check the compression. So I check the compression today and I don't think its good news. I didn't have the gas tank hooked up, I took off the TCI box, I took all the plugs out, charged the battery (not fully charged but chargedish) left the charger on and did all the test. I did all the test dry first and got 90-105-90-120. Then I put some oil in and got 210-175-240-240. Now I've never done a compression test before so I very well could have done it wrong. I cranked it for around 8-10 cranks and held the throttle all the way open. The question is, if i did it right what kind of job is replacing the piston rings and how much would parts cost me? Thanks for reading this long winded intro, and thanks for any suggestions.

 

You used too much oil on the wet test. The extra oil takes up space, raising the pressure. There's just supposed to be enough to get the rings wet.

When testing compression you need to make sure the gauge is sealed well, the battery well charged, spark plugs out, throttle WIDE open, and you need to crank it until the gauge quits rising.

Ideally the engine should also be warmed up, but since none of us likes being burned we seldom do that.

If you pay a shop to do the carbs you'll likely be wasting your money. Check out Rick's write up in XJ FAQ SUGGESTIONS, do them right and save a ton of money.

 

I did follow ricks write up and I may have done things close to right but if my rings are bad I have a feeling that it is the big reason things werent working out. Im pretty sure now that I put to much oil in and that explains why the numbers were so high but am I right to assume that 90's are WAY to low. I was under the impression that around 150 was good and 120 may work but high is really the correct compression. So Im I right about that and if so I need new rings?

 

did you ever check the valves

 

Good point. I glossed over that because the numbers came up wet, but didn't really think about the amount of oil skewing those results.

The 90s are way low. IIRC the spec calls for a minimum of around 130 dry.

I'd fire that baby up to get the excess oil out and re-do the measurements. Only use a small squirt of light oil for the wet test.

I'm wondering if compression is your problem since it sounds like it comes and goes. Compression doesn't come back.

It's not running well cold and then falling apart when it warms up is it? That's a symptom that raises suspicion the valve lash is too tight.

 

^^^+1. I'd suggest a little less than 1 cc of oil Disco, this means a few drops.

 

All right, as of last fall i couldnt get the bike running at all, Ill try to readjust the carbs, get it running, and then redo the test again.