1983 xj750mk compression reading

when i first joined the site you guys advised me to do a compression test. i got 120 on all 4 which was surprising. For shits and giggles i did it again today when it was less windy and got 125 on all 4. I'm very surprised that all 4 are so close to each other. but i don't know what the specs are. ive hear between 120 and 130. any ideas? thanks

 

All four close is good, but have you tried a different tester to be sure its accurate? You can go to the major auto parts stores and borrow one. On the 550's those numbers would be expected, 125-135psi, but I think on your bike they should be higher like around 145-150psi. Of course even so 125psi is plenty to achieve combustion and power.

You are doing the test with a FULLY charged battery, throttle wide open, and the engine either only warm or cold right?

 

You're just above Low-specs.
119 and lower is bad news.

If it ain't fouling plugs, eating oil or chuffing smoke, ... you can get-by with that.

Wind is not a factor.

 

The bike was left setting. This tends to drop the compression readings.
You are now in the process of reseating the rings. Your compression readings will go up as the process progresses. Run the bike for 300-500 miles. Then change the oil. Keeping fresh oil in the engine will allow the rings to seat evenly again, and eliminate any sludge buildup in the engine.

I suspect that the engine will top out around 130-135 psi on all cylinders.

Also.
Check those valve clearances, and get them in spec before you risk burning a valve.
If you haven't done so already. Do a vacuum sync on the carbs so that each cylinder is doing the work evenly.

Ghost

 

Ive got 120 across all 4 cylinders as well. I have receipts from the PO's dated 1994 and 1995 that show it had the exact same compression nearly 20 years ago.
So you can ride the bike for at least 20 years with the CR you are getting.

 

i didnt have a full charged battery the first time. the engine was cold on both tests. it sat for 2 years. ive put 30 miles on it. gunna change the oil tomorrow. the plugs are a little past tan. it runs and starts well. going to do valves check soon. waiting to get paid and get a good set of feelers and new gaskets and bolt boots. gotta price things out and go slow to make sure it all good when done. thanks for the input.