CB360 compression question

R

robmurray12

New Member
Hi guys,


Got a new CB360. Did a compression test to see what kind of shape the engine is in. Results: 110psi left cyl. 100psi right cyl. Note: these readings were with a cold engine(garage temp @ 30F) and NO oil squirted into the cylinders.


I've read that a warm engine should be around 170psi? Also as long as the cylinders are within 10% of each other its ok to have a differing pressure reading? Thoughts?



From my info, would you guys suggest an engine rebuild or leave it be? Thanks.
 
Have you checked and adjusted the valve clearances yet? If not, do that and then check again. Also, you did the test with the throttle wide open and the choke off right? Try the oil trick after these readings to see if they jump up.


If the numbers don't increase after all this you're looking at a top end rebuild. 100 to 110 psi is way below rebuild specs. It may run but you won't have the power it should.
 
If possible, do your compression test with the engine warmed up and with the throttle all the way open (bike not running, of course). It helps to remove the spark plug from the opposite cylinder as well.

If the numbers are still low, then you can try it with a squirt of oil in each cylinder. If the pressure goes up by a fair margin, then you problem is probably the cylinders/rings. If no change, then it's probably the valves.
 
Engine does not run at this point so having a warm engine is out of the question. I'll check valve adjustments, then try again. If there is no improvement I"ll ad some oil to see if its the rings. I was doing the compression test w/out the opposing cylinder's spark plug in.
 
Google it and there should be a few available.
If you're still having trouble finding it, send me a few bucks on Paypal to cover shipping and I'll send it over on CD.
 
Thanks. Ok, I went ahead and squirted some oil in the cylinders, Left cyl. read 150psi, so an improvement over the initial 110psi. Right cyl. read 60psi, so it acually went down which confuses me. Either way it looks like an engine rebuild is in order. Previous owner said the right cylinder had lost power so this should be no surprise. Also at 26,000mi on the odo I believe this is fairly high mileage compared to most. Could I possibly get away with just re-ringing it and doing some top-end work, or do I need to plan on boring over and getting new pistons?
 
To begin with, check the valve clearances like Deacon mentioned. This will give you a bit more info with which to work.

After that, questions will only be able to be answered after disassembly. There's no way to say how much work needs to be done until you open her up. :)
 
Once you do get it opened up, here's a few things to try:

1.) Put some acetone into each of the intake and exhaust ports. If ANY of it leaks by the valves and into the compression chamber, that's a problem.

2.) If the cylinder walls look a bit scarred, run your fingernail over the scars. If you can feel it, you'll probably need to get the cylinders bored. A machinist will tell you how much you'll need to go over, but pistons and rings are readily available at +.25mm, +.50mm, +.75mm, and +1.00mm over.
 
Thanks for the tips sonreir, its so damn cold here in CO right now that I can barely even work in the garage. I may have to wait a few days for it to warm up before I really get serious about tearing into it. I also need to organize the garage before I start tearing her down so that I can catalog parts effectively.
 
Hi guys,

I'm currently in the rebuild stage and i'm working on the motor. I am checking the compression and the engine is out of the frame, and it's significantly low. I plan to add some oil to the pistons to see if that helps anything. My question is, do the carbs and exhaust need to be attached in order to get accurate reading? Or, can I leave them off without affecting anything?
 
you should have checked it after it was warmed up ie a running motor,you dont need pipes onnit and having the carbs off is actually a very good thing with a cv carb
now that you are rebuilding it dont even worry about it
you are going to put hands on and check every thing that matters,valves,valve guides,valve seats,valve springs,cam journals, cylinders bores and piston clearance etc etc etc
all the checks and specs are in the factory service manual
 
Those numbers are completely normal when you don't open the throttle plus have choke open
It's probably impossible to spin motor fast enough when it's out of frame
 
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