No compression on one cylinder

[jdion81]

Hi, Im sort of new to the forum, I have been reading non stop for some time now as I have been trying to find what I want to do with my 81 GS850. I recently took it to the local bike shop because it started intermittently missing on one cylinder and progressively got worse. It eventually would not fire on one cylinder and I assumed a carb issue. While sitting in the garage over winter I noticed a puddle of what looked like dark oil that would keep re-appear after each time I cleaned it up. I thought that I had a leaky gasket on the crankcase. I did an oil changed and pulled about 3 gallons of fluid from the crank. It appears the gas tank emptied into the crankcase. After the oil change she started up and ran on 3 cylinders and was then taken to the shop. The shop came back with 100% compression loss on one cylinder. They said $300 to come back with a full diagnosis and well over $1000 for a fix. That was mainly labor. So I am thinking that I can probably fix this with some support and guidance from the forum here.

Any ideas on what I can check on this? I know that the valve clearance has not been checked in a long time. I am hoping that I am not looking at a bad piston or head.

[Nessism]

Lack of maintenance could have burned a valve. I'd check the valve clearances and clean the carbs. You might get lucky and the problem will go away.

[jdion81]

Once checking the clearances and carbs would it be a good idea to check the compression again? or try starting?

[Nessism]

A proper compression test requires the engine to be up to operating temperature. Also, the compression will increase with mileage if the bike has sat for a long time. I'd get it running, flog it good and hard for a couple tank fulls, then check the compression if the bike still isn't running the way you want.

[tkent02]

Check it again, if the valve was too tight to close, there could be no compression. With luck you may not have done any more damage than that. But if it does need valve work, it's fairly inexpensive if you do it yourself.

[rustybronco]

So I am thinking that I can probably fix this with some support and guidance from the forum here.

You think correctly.

Beg, borrow or buy a set of feeler gauges and get that cam cover off. Personally I don't think the compression is going to improve much with a 100% loss of compression in one cylinder.

I've a spare head in the garage...

[Big T]

[QUOTE=jdion81;1602819]Hi, Im sort of new to the forum, I have been reading non stop for some time now as I have been trying to find what I want to do with my 81 GS850. /QUOTE]

So, did you read BassCliffs 10 common things?

http://members.dslextreme.com/users/bikecliff/images/GSR_Greeting.html

http://members.dslextreme.com/users/bikecliff/

Ed's Top Ten Newbie mistakes?

http://www.thegsresources.com/_forum/showthread.php?t=171846&highlight=top+newbie+mistakes

If so, the valves would be adjusted and the carbs cleaned already

I'm just guessing here, but I'll bet it's cylinder #2 that's not firing. You know , the one that filled up with gas before it leaked into the crankcase.

New spark plugs, an oil change, a new petcock and another shot at compression are in order after the basics are done

But, tell me if it's #2 that's bad (or not)

[jdion81]

So, if they checked the compression by just cranking the engine while cold their results could be questionable? I think I may have a set of metric feeler gauges. I plan to get that cover off tonight and check the clearances.

[jdion81]

[QUOTE=Big T;1602895]

Hi, Im sort of new to the forum, I have been reading non stop for some time now as I have been trying to find what I want to do with my 81 GS850. /QUOTE]

So, did you read BassCliffs 10 common things?

http://members.dslextreme.com/users/bikecliff/images/GSR_Greeting.html

http://members.dslextreme.com/users/bikecliff/

Ed's Top Ten Newbie mistakes?

http://www.thegsresources.com/_forum/showthread.php?t=171846&highlight=top+newbie+mistakes

If so, the valves would be adjusted and the carbs cleaned already

I'm just guessing here, but I'll bet it's cylinder #2 that's not firing. You know , the one that filled up with gas before it leaked into the crankcase.

New spark plugs, an oil change, a new petcock and another shot at compression are in order after the basics are done

But, tell me if it's #2 that's bad (or not)

I have called the shop to see which cylinder has the compression loss. I have also asked how the mechanic checked the compression. I should know soon. Unfortunately I don't know which carb filled with gas then leaked into the crankcase. I have read the top ten list and wanted to be sure that I might not be looking at piston, head or valve damage. If this might be possible I would have to rethink my working on this bike.

[jdion81]

It was #2 and was tested without running the engine.

[Nessism]

It was #2 and was tested without running the engine.

If the petcock fails, gas will flow backwards down the vacuum line and into the #2 cylinder. I'd dump some Marvel Mystery Oil down the #2 spark plug hole and get busy adjusting the valves. The MMO will help unstick the rings in the event that they are stuck due to fuel varnish.

[mike_of_bbg]

Well, there are only so many ways the pressure can escape the combustion chamber:

Piston damage
Leaks around the head (cracks, spark plug hole)
Leaks around the valves (hanging open, burnt/cracked/chipped valves or seats)
Failed head gasket
Cylinder wall damage

Do you have your own compression tester? If not, you can probably get one at a store near you for ~$20. I'll bet that, if you're willing to do the work yourself and have the tools, you could get it running on all 4 again for less than they want to charge you for diagnosis.

The easiest places to start are looking for piston damage down the spark plug hole, and checking those valves! You can also do a wet/dry compression test (dry first, then pour 1 tsp oil down the spark plug hole and re-test): if the compression goes way up it's likely rings or less likely cylinder wall damage. Cold compression numbers aren't great, but when a cylinder won't run to get warm there isn't much else to go on. Make sure when you run compression tests that you have a fully-charged battery and all spark plugs removed; just the tester in whichever cylinder you're working on. Battery fully charged so your motor will turn over as fast as possible.

Oh, yeah - always hold the throttle wide open for your compression tests!

[Big T]

It was #2 and was tested without running the engine.

As above, put some MMO or good penetrating oil down #2 and turn the engine over a few times. Most likely, your rings are stuck from all that gas flowing thru there

Then, adjust your valves

And, order up a new petcock

Once you've got that covered, go out and ride it up and down the highway, lots of varying throttle, and see if some of that compression sneaks back into the cylinder

[tkent02]

Again, there can be no compression if the valves are to tight to close.
Damage comes very soon after this!
Check them first!

[jdion81]

I think my brother may have a compression tester. I am going to try find one before the weekend. I have the battery on the tender, hopefully it will be fully charged for the weekend. When I check the valves and I find that I need new shims, what is the best way to go about getting the ones needed? While I am at it would it be worthwhile to replace the lower head gaskets? I know I have a leak at the valve cover, but I am not 100% sure the others aren't leaking.