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GS 550 '79 - low compression on one cylinder after sitting 2 years

  • Thread starter Thread starter Stigy
  • Start date Start date
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Stigy

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Hi.
I started to work on my GS 550. It sat for over a year. Cylinders 1,3 and 4 give me 135psi during compression test. Cylinder 2 however gives only 80psi. I've put some (1ml) oil in it, through the sprak plug hole and it gave me 180psi. So, I know it's the rings problem. I'm wondering is it possible that the rings are stuck on piston because of this long sitting period. It's quite odd that other cylinders are more or less ok.
So i heat cycled the engine two times (5 minutes of running without a load). Nothing happened - still 80psi on cylinder 2. Now i got 3 options:
1. engine flush - kerosene or diesel (diesel could be a problem for clutch),
2. put new engine oil in it and ride it - hoping that the rings could unstuck if somehow this is the problem,
3. tear down top end and see whats happening with cylinder 2 - possible rebuilt.

What do You think about this case, GS gurus ?

Cheers !
 
Last edited:
Ride it. Put some Marvel Mystery Oil in the gas tank since that will help reduce any possible ring sticking. About 1.5 oz or so with a full tank, for the next few tanks, and see what happens. Needless to say, you should be checking the valve clearances right away since you could have carbon build up on the valves if you haven't checked them regularly.
 
I bough the bike one year ago. I haven't done anything with it yet. Valve clearances were next after compression test, on my to do list. The clearances was waaaay out of spec. The limits are 0.03-0.08mm. Most of my clearances were 0.25-0.3mm. The question is - could it be because of the carbon build up around valve faces ? If yes than i should disassemble the engine right away :(
 
The only time i've seen clearances that large are when there were bent valves. You wouldn't have any compression at all if that was the case though.
 
So whats the best action to take here ? What would You do Nessism ?
 
With those numbers I would have to ask if you are doing the check the correct way. It doesn't sound like you are. Have you been to Bikecliffs site to get your download-able factory service manual? There is a right way to check valve and also a wrong way to do it.
 
You mean this ?

valve.jpg

Yep, I'm 100% sure i did it correctly. One of the valves (4 cylinder exhaust) was 0.05mm the res were out of spec:
1 intake 0.35 exhaust 0.15
2 intake 0.25 exhaust 0.35
3 intake 0.3 exhaust 0.25
4 intake 0.15 exhaust 0.05

Compression pressure test:
1 135psi
2 81psi
3 135psi
4 141psi
 
The fact that you got 180 PSI when you shot some oil through that cylinder indicates the valves are sealing OK and the issue is with the rings.

Heat cycle / ride it. If they are just stuck they should come good with use. There is no reason to tear the motor down at this stage if it is running OK and not blowing smoke.
 
Ok but what about valve clearances ? Just set them with shims ? Isin't it odd that they are out of spec so much ? Didin't know the previous owner, it came from Netherlands so i don't know how the bike was serviced but that much clearance ? How could this become so huge ?
 
Probably a good idea to check what shims are in there now - the previous owner might have just got it wrong.

Not sure what the shims go down to, but most shim kits will have from 2.30 to 3.00 so if the shims are on the lower end the PO may have just shimmed them incorrectly.
 
Also - are the cam caps set properly? Correctly numbered / in the right spots? If the cams are floating at all the valve specs will be out.
 
Are you sure you wrote those numbers down right? Are you using metric only feelers or are they inch with the metric approximations?
 
hillsy said:
Heat cycle / ride it. If they are just stuck they should come good with use. There is no reason to tear the motor down at this stage if it is running OK and not blowing smoke.
Click to expand...

Since I have a similar issue (cylinder with pistons in it are now being soaked with Marvel mystery oil.

What is the proper way to run such a (Italian tune up) cycle?
Hard acceleration till redline? Or few miles 1000 rpm below redline? I am reading many different things on the web.

I'll let you guys know what the oil soak did to my rings/compression in a few days
 
What you do is rev it up and let deceleration slow you down. Repeat and repeat some more.
So, get out on the road and run it in , say, 3rd gear at varying speeds
 
hillsy
I'll check the cam caps. Didn't look chec it yet.

cowboyup
I'm sure i wrote the numbers correctly. I'm using metric feeler gauges.
 
A motor sitting this long can get rust on the valve faces and seats. More so on valves/seats that are open when sitting. A completely closed valve may have little or none. After riding 50 or so miles, recheck clearances and see if they come down. The clearances you have will not harm the motor at all. Rust on a valve and seat will cause the clearances to be high......Billy
 
Great thanks Billy. That's a very useful tip. I'm going to check if this kind of problem occurred. Worth remembering for the future.
 
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