compression test numbers?

Checked compression on the XJ650 last night. 120 across the board except for #4 it was 130. Is this ok?

 

Not really, Standard 156 psi, Min 128 psi, max difference between pots 14 psi. Was that wet or dry? & W.O.T. ?

 

Could also just be your gauge. The percentage difference is more important than the actual reading. If you're concerned, put some oil in the cylinders and see if there's any noticeable change. Then get a "second opinion"---borrow a different gauge.

 

dry, closed throttle. does that make a difference?and will valve adjustment affect?

 

If the valves are tight it will, but try it again with W.O.T.

 

Slo, YES a closed throttle will make a difference!
You need to crack it open as far as you can to allow the engine to gulp down as much air as it can. I'll bet a retest of your compression will show an improvement.
I'm with Fitz, uniformity across the cylinders is most important, save the pressures are down in the dumps. I've seen 120 psi that ran just fine with clean plugs so I wouldn't let that slow me down. 100 psi would give me pause.
All cylinders should be within 10% of each other with no more than 14 psi between any two cylinders. The specs in the Haynes show max 171 psi and min as 128 psi with 156 psi being standard pressure.
Hope that helps and don't forget to ground the spark plugs or disconnect the TCI, we don't want to hear that you burned out the TCI!

 

Ya learn something new everyday on this site. I oopsed on the plug wires, I dont think I hurt it tho. Still runs thank goodness.I havent ran the valves on it either, so I'm guessing now that they are going to make a difference as well. I dont have any idea if the P.O.'s ever touched them. By the looks of it I'm gonna say no. So my journey begins..... A hopefuly better baseline on the compression, a good carb cleaning, and some fork seals, so hopefully by warm weather, the wife can get in the wind as well.

 

Hi Slo, the bike I am refurbing at the moment came without HT leads & I spun the motor for comp tests, then tested the coils when I pulled them, both amped out fine, I say always err on the side of caution & earth the plugs, but I doubt you will have harmed them.

 

Yeah I've spun mine over a few times with the plugs disconnected before I considered the TCI unit; nothing's damaged but I know better now.

How many miles on the bike? Valve adjustment is supposed to be done every 5000 miles and was quite often ignored by POs. Not uncommon to find a 25K XJ motor that is still running (not well) with untouched valves.

I would definitely check/adjust valve clearances before worrying about a compression check.

 

You do have all 4 plugs removed, don't you? Just checking, as the first time I did this, I didn't, and got readings similar to yours.