High RPM low Power?

I have an 81 XJ650 that suddenly decided to go from running great to something else! It has lost alot of its power from low speeds to high speeds. But, it seems to be running at the usual rpm's. Can someone give me their opinion as to how to approach this problem?

 

what kind of oil are you using?

 

Castrol 20W50 and yes it did start after I changed the oil, but not immediately.

 

Are there other symptoms like poor idling? It sounds fuel/ ignition related and I guess Chuckles suspects the clutch is slipping although you say the rpms are normal?

 

if the revs go up & the speed stays the same, it's the clutch slipping.

 

There's not any other symptoms. The number 4 cyclinder runs at about about 90psi. The other three are at about 120psi. Was starting right-up. I thought the problem might be the carbs so I rebuilt them again last fall after I put it away for the winter. Now I haven't been able to get it started since the rebuild. But, I'm ready to get it going again. If the clutch is slipping is it an adjustment, replacement, or an oil problem. Where should I start. Just before this started I also changed the tires. I wondered if it could be a rear brake problem also.

 

I'm pretty sure those are awful compression numbers. 120psi is about the lowest that is acceptable, and the 30 psi spread means you'll never get that cylinder to run right. I'd say first figure out why you're compression is so bad. Do the oil test to determine if it's the rings or the valves. Have you checked the valves? Also remember to do the compression test with the throttle fully open. That can make your numbers seem low also.

As for the clutch, I would think adjustment come before replacement. RickCoMatic has a good write up somewhere about adjusting the clutch, and even repositioning the arm if you can't take up all the slack with the cable adjustment.

 

Definitely check your valve clearances, as this could be the source of your compression woes.

 

You need to provide us with two Compression Check values.
1. Standard
2. Wet Test (Tablespoon of Oil added to Spark Plug Hole.

Conduct the Test under IDEAL Conditions
Fully Charged Battery
Throttles held Wide Open
Spark Plugs Out
TCI Unplugged.

Get --> Dry Numbers.
Write-down results.

Re-Test.
Add Oil to Cyl during the Test.
New set of Numbers.

Generally speaking ...
A Dramatic Improvement in the PSI -- "Wet" -- indicates a Piston Ring ~ Cylinder Wall situation.

Modest or No Improvement in PSI -- "Wet" -- indicates a problem related to Valves. Burned, Tight; Stuck.

That's Step One.