Topic: CB650 Compression test-- Anyone know the psi rating per cylinder?

[Namba1032]

Hi there, I have a 1980 CB650C and Im going to do a compression test on it. Does anyone know what kind of psi Should I be getting out of each clyinder?

Brian

[crazypj]

about 160/170, still ok down to 120 but dont perform as well.
They should be within 20psi of each other but I prefer 10psi max difference

[eurban]

Getting accurate readings means . . .valves adjusted, all plugs removed, choke off & throttle wide open or carbs removed, battery and starter in good shape / battery charged, and a good tester with a good seal to the head.  .  . . .

[Namba1032]

Ahh ha, I may have been doing it wrong then. I had all the plugs in except the one I was testing, and the carbs weren't wide open. I'll have to try it again.

Regardless to the fact that I had done the test the wrong way, I was getting about a steady 90 psi out of each cylinder but then I'd stop cranking it over and bump the start button again and it would jump to 120. This happend on every clyinder except the number 2 cylinder which topped out around 110 psi. This cylinder I believe is getting washed down with oil from looking at the plug and the fact that the bike is smoking quite a bit when running (blue smoke). Could this be due to the fact that the valve are out of adjustment? The compression tester is a cheap one that I picked up at walmart... so it may be off, but I figured as long as they were all close it would be good.

Any ideas about the #2 clylinder? I was thinking possibly the valve guide seal is bad. I was planning on picking up a seal kit to replace it and the head gasket. It leaks a little oil out of the left side of the head which from what I understand is pretty common.

Any thoughts?

Thanks for your replies, I will have to do another test (the right way) and I'll let you guys know how I turn out.

Brian
 

[bryanj]

If its burning blue its either valve stem seal, along with possibly a worn guide, or rings, If you are pulling the head its not much more work to pull the cylinder block and check the rings and you will have disturbed the base gasket anyway just pulling the head! Dont forget the new "O" rings for the head and base gasket!!

[Namba1032]

Alright, I know its been a while but I finally did another test with much better results doing it the correct way.

Clyinder #1- 130 Psi
Clyinder #2- 125 Psi
Clyinder #3- 125 Psi
Clyinder #4- 135 Psi

Are these results decent??

Im going to be picking up an engine seal kit for this thing anyway and will probably have the jugs off it to replace the base gasket while its all apart. Should I consider replacing the rings while Im at it? I dont even know where I would get some nor how much they would cost, but just out of curiousity what do you think? I've done a lot of work to this bike and plan to do a lot more over the winter as long as funds prevail.

Check it out so far.

http://good-times.webshots.com/album/556157080lmoaOd

[Ibsen]

Is that with a cold or a warm engine?

And like bryanj said, when the head is off it isn't too much job to take off the cylinders either. And the o-ring for the oil ways that are located between the cylinders and the crankcase are prone to leak with age, so it's a good idea to replace them.

One thing you should have in mind when you reassemble the engine is the cam chain tensioner unit. Make sure you fit it's lower end in the correct groove in the upper crankcase half. If you miss that, the lower end of the tensioner will rub against the starter clutch.

[scunny]

as crazypj sais compression should be around the 160 mark, done on a warm engine. then pour a tablespoon of engine oil in the bore, crank her over again, if the compression rises your rings are worn.if not it could be leakage past the valves. but if it's running alright I wouldn't panic

[Namba1032]

When I did this the engine was cold.

I guess I'll have to do another test to check it out when its warm.

Thanks for the info guys, I really apreciate it, especailly the heads up about the cam chain Ibsen. Scunny, I have a question for you. If I put a table spoon of oil down the cylinder, wouldnt I want the compression to rise? And if it did why would this tell me I needed new rings? Is it possible that you made a typo or am I missing somthing here?

Brian

[scunny]

re the oil down plug hole. this is how you tell wether it's rings or valves if you have low compression. put the oil in, wind the bike over so the oil is spread in the bore then do your comp test. if the rings are shot the oil gives them a temporary seal and the compression will rise, no change means it's your valves

[Ibsen]

130 psi with a cold engine isn’t bad. Testing it with some oil in the cylinders will give you an indication about the state of the piston rings and their ability to seal, but if you measure the ring gaps, you might find that it is well within the service limit, and slightly deglazing the cylinder bores might be all that’s needed to re-establish a good seal. But since you are dismantling the top end, new rings along with deglazing would make a better option though.

The best way to test for compression leaks is a leak down test by the way.