Compression readings

I have a line on a 1984 XJ750RL. THe seller agreed on a compression test. We did it together. Engine was warm when we performed the test, here are the readings:

#1= 170
#2= 157
#3= 162
#4= 165

I have to say that this wasn't easy to screw the tester hose in #2 and #3 hot head without burning my hands. Lower readings on these cylinders may be a result of the tester not being as tight as for the outer cylinders.

The seller said he ajusted valves last year (swapped some shims, changed others).

What are your thoughts about these readings?

I rode the bike, it is smooth, no stumble at any rpm below 7K. Engine is a 900cc, by the way.

 

I would say those numbers are pretty good
definitely a viable motor!
I wish my numbers were have that good
I got 120's

what is the mileage by chance?

 

The 900cc engine has around 52 000 kms.

 

What is supposed to be the ideal difference between cylinders?

Less than 5%?

 

You need to check your service manual, my manual for my 550 SECA states the difference should be no more than 14 psi (same for my other bike a 900 but different make). For your 900, it may be different and you should confirm using a service manual specific to your bike. But your numbers don't look too bad even with the 170 high and 157 low. Like you said, maybe if you do it with the tank off and have tester tighter on those two cylinders, you may be already within the acceptable range.

Also, if the bike has sat a long time, the numbers on that low cylinder may come up. Was the bike ridden at all by the P.O.? Or did it just sit for a long time? These can all be factors with the compression. If it has been sitting for a few years, just ride it as is for a couple of hundred miles. Then give it another compression test. See if things even out.

One more note: I know you said the P.O. adjusted the valves. But it wouldn't hurt to confirm his adjustments. The low cylinder could also be due to that.

 

This bike has been his daily driver until last month, if I understood well.

 

No problem, was just covering different possibilities without knowing any of the history of the bike. I would say do the test again on that cylinder but make sure the compression hose is on tight. Might be a none issue.

 

The bike is not close to my place (more than 1h30 of travel) so I don't think I will do the test again!