Very low compression problem solved - sort of...

[baz666]

I don't know how many of you remember my problem with almost no compression in all cylinders from about a month ago. It appeared the cam chain had jumped and put the exhaust cam off a tooth, affecting when the valves closed.
So I re-aligned the cams with the crankshaft marking as advised by various site members. As an added precaution I filled the combustion chambers with combustion chamber cleaner, let it sit for a few days, then blew the stuff out. It was black and chunky with old bits of carbon that had been stuck to the tops of the pistons. Shining a flash light down in the spark plug hole before using the comb. chamber cleaner and I saw just black sooty surfaces. Afterwards, I could see the metal surface of the piston top with a sheen of comb. chamber cleaner and some remaining carbon film. Hopefully that will get burned off if/when I finally get the bike running.
As a result of all that, here is the current compression picture.
From 30psi across the board originally, it is now at:

(From left to right while sitting astride the bike)

Cyl. #1 - Cyl. #2 - Cyl. #3 - Cyl. #4
140psi - 90psi - 90psi - 140psi

Any advice on why I'm getting the two different readings on cylinders 1 and 4 and then on 2 and 3? Is it due to my cam alignment set up?
Or should I check the valve gap clearances again?
thanks for all the help,
baz

[Nessism]

Compression needs to be judged with a hot engine, throttle held wide open, and some recent (hopefully hard) miles if the bike is coming out of long term storage or a top end rebuild. Needless to say the valve clearance needs to be correct. Regarding those two low cylinders, I doubt it's cam timing related or all the cylinders would be low.

[niclpnut]

Have you done a leak down te

Could help you narrow down where your loosing compression.

Checking the valve clearances cant hurt,

Hopefully no valves were damaged when the chain jumped.

Nic

[niclpnut]

Good #'s on 1&4

[Big T]

Glad to see you've sort of solved your problem

Check the valve clearances, ride it and see what your compression numbers are hot

[baz666]

I haven't tried to start it yet. I know I must change the oil after using the comb. chamber cleaner. I was just wondering if there's something I'm doing or could do that's made such a difference in compression between cylinders 1&4 and cylinders 2&3.
The other thing is; when I try to thread the compression adapter into spark plug hole #3 it is very loose. I can almost pull it out and feel the sides of it scrape against the threads on the way out. The weird part is that it threads in nice and tight in all other cylinders. So I don't know if I'm losing compression around the loose adapter or what. What a weird bike and situation...
Anyway, I'll change the oil and try to start it now that I've got most of the compression back then do a compression test after getting it nice and hot.
I just can't figure out why cyl. 2&3 would run at 90 psi. A bent valve wouldn't affect BOTH cylinders, would it?
Any advice is always much appreciated.
thanks,
baz

[wdavie]

Hi Baz if the cam chain jumping affected the timing would the valves for 2&3 cylinders be open together at the wrong time thus possibly bending them but not affecting 1& ? This could be why your getting lower compression on 2&3 but not 1&4.

[hampshirehog]

If it was bent valves your numbers would be worse than that. I'd ride the pants off it (red line and beyond) for a couple of tanks then check the figures again.

[mike_of_bbg]

I don't think you'd get 90 psi if you had a bent valve. My guess would be that the rings need to re-seat. Listen to Ed; cold compression readings don't mean much, though when one is faced with a bike that won't run there's little else to go on. Honestly 140 psi seems a remarkably high reading cold.

Will a plug tightly grab threads on #3? That might be something to look into before you finish stripping it.

Remind us what bike we're working with here, or just put it in your sig.

[baz666]

It's an 83 GS750ES with about 50,000 kms (30,000 miles), totally stock except for a 4-into-1 cobra exhaust.
It was running fine last fall when I put it away. First time trying to start this spring and a loud "Whack" noise from the top end. Afterwards, the bike would crank & crank, but no start. I checked compression and got about 30psi across the board.
I took off valve cover, took photos of cam positions. Some members said the exhaust cam was 1 tooth off. I removed tensioner, rebuilt it and while it was off the bike, undid the exhaust cam and moved it 1 tooth forward so arrows on sprockets lined up properly with cyl. head gasket surface.
A veteran wrench told me to fill comb. chambers with solvent cleaner and leave for a couple days then blow it out. Came out filthy and chunky.
Put tensioner back on and tried compression test again. got 140, 90, 90, 140 from left to right. That's dead cold.
Tried comp. test again today and got some variation on the original readings, 120, 90, 90, 110.
#3 spark plug hole is pretty loose. I used some plumbers thread tape to try and tighten it up. That helped some. The strange part is that the spark plug screws into #3 plug hole just fine, no looseness. So I don't know if it's the threads on the comp. tester adapter or what.
This bike has thrown some really odd curveballs at me...
I'll change the oil tomorrow and try to start it finally.
Any advice much appreciated.
thanks,
baz

[niclpnut]

Does a spark plug fit snugly in the thread or is it just the adapter for the tester?

Run some Seafoam in the gas when you get to fire it up. Help to clean up the chamber and free the rings.

Nic

[baz666]

yep, the plug fits nice and tight but not the compression tester adapter. I think maybe the adapter threads are further apart or something. The weird thing is the adapter fits nice and snug in the other three plug holes.

Anyway, I'll change the oil today and then try to start it and go from there.
thanks for everybody's help and patience,
baz

[Nessism]

Did you verify the cam position using the information in the service manual? Set to 1-4 T, 1- mark pointing at the cam surface, count the pins between 2 and 3? It's good to listen to people on the forum and all but you should always try to verify things are correct by yourself.

Regarding that carbon remover stuff, I'm not a fan but too late now. The engine will develop carbon as a matter of course and it doesn't hurt anything. You run more risk by loosening all that crud up than leaving it alone.

At any rate, please stop fussing around with checking compression until you run the bike for some good hard miles. If the compression is still low after you put a couple hundred miles on the bike, and check the compression hot, then you have an issue.

[SVSooke]

Give this advice to Pete when he gets into these moods.Dude you're over thinking it.Start it up and ride it for a bit.Then you can tell what going on.Yes it's the same advice you have already got numerous time.Go and do it PLEASE.

[baz666]

Your 'free the rings' comment made the lightbulb go off. That may have been the problem all along after the bike sat for several months. And now after running comb. chamber cleaner and realigning cams, perhaps the rings are freeing up slowly. That might explain the disparity in readings from one comp. test to the next, just a day apart. I'm wondering if I should run more comb. chamber cleaner through it.