No Compression

[GSJim69]

I got this '82 GS850G a couple of weeks ago. I am doing the basic maintenance items for one of these new bikes. I have found that anything the previous owner did was f'd up. Stripped bolts, stuff not connected or missing everywhere.
I pulled the carbs to give them a proper cleaning. I noticed that the intake and the cylinder side of the carb on cylinder 1 was black. That worried me. So, I did a cold compression check. The rest of the cylinders were between 115 and 125. I figure that is OK for a cold engine. Cylinder 1 had no compression or at least too low for my guage to read and it goes pretty low. I added some oil the the piston to check if the leak is from the rings. The compression increased a little, but not much. So, it's probably a valve.

My question for you experts, should I go ahead and adjust the valves as I had planned and run the bike a bit to see if it improves or would that be a waste of time and I should go ahead and pull the head and do a valve job on it?

Oh, yeah it has about 39,000 miles on her.

James

 

[Suzuki_Don]

Check your valve clearances first before you do anything else.

If you have a tight valve on that cylinder and one of the valves is being held open all the time and not allowed to close then you will have no compressionon that cylinder.

And putting oil in to seal the rings will not do anything. First, set all your valves, tappets to the correct setting and then do another compression test.

Get back with the results.

 

[Nessism]

I agree with Don, do those valves now. Squirt some Marvel Mystery Oil down the cylinder as well to free up the rings in case they are sticking.

It really amazes me how little mechanical sympathy some people have; many a GS has died because some meat head beat the crap out of the bike and never adjusted the valves. Hope you caught this one in time.

 

[GSJim69]

OK, I will continue on with the valve adjustment and let you guys know what I find.

Thanks

 

[GSJim69]

It's shimmed

It's shimmed

I shimmed the valves properly. The intake valve had a 2.60mm shim in it to get it to a clearance of .003" it took a 2.40mm shim.
I redid the cold compression check on cylinder #1. It increased to 68 psi. A lot better, but still not good.
I also added a little Marvel Mystery oil into each cylinder and let is soak overnight before I did the compression check.

What do you guys think, should I put it all back together and run it for a while and recheck the compression or do I need to pull the head and have a valve job done?

I hope the answer is run it and recheck. But, I do not want to destroy the engine.

Thanks

 

[rustybronco]

Jim, how long did this engine sit before you received it?

if it sat for a very long time, then I would say run it and see what happens.
but my gut tells me it has a burnt valve.

can you turn the engine over with a wrench and look through the #1 plug hole at the valve faces, or do you know anyone with a bore scope?

 

[Nessism]

Nothing to loose by running the bike at this point. Put a couple hundred HARD miles on the bike and then test the compression again. Remember to check compression with the engine warm and while you hold the throttle wide open.

After doing a baseline compression check, add a teaspoon of oil to each cylinder and check compression again. If the compression comes up noticeably, that indicates the rings are worn.

 

[duaneage]

Do an oil change first then do another in 500 miles. Lots of boogies in there to clean out.

Did you hold the throttle wide open for the compression test? Readings are low with a closed throttle, like 40 lbs too low.

If it gets to 100 you can probably just run it for the summer. Teardowns are a winter project, not riding season work.

 

[blowerbike]

if you feel like it...
remove the carbs and take a compression reading after you have ran it.
i bets its over a hundred anyways.
its hard to draw air through vaccum carbs even with the throttle open(slides are still down).
in the end...buy or borrow a leak down tester and follow the instructions.
then you will know exactly where your leaking.

 

[tkent02]

Run it for a while and see what happens, can't hurt anything that isn't hurt already. Chances are good it will come around. These things are not made to sit.

 

[GSJim69]

I am not sure how long it sat before I got it. I do know that it has ran at least 100 miles since It was bought trip here and to register it.
The cold compression test was done with the carbs removed.
I guess I will go ahead and run it a while and then do another compression and valve check and see how it looks. Maybe I will get lucky and it will be good enough to last until the riding season is over.

Thanks

 

[mike_of_bbg]

I don't think the cranks usually twist there, but you could turn your crank about 90 degrees right between TDC and BDC. Get a slender rod and compare the distances between the tops of #1 and #4 pistons and their spark plug holes. My crank was twisted between the chain piece and #2, and my compression #'s for 1 and 2 were around 70 psi. Just something to eliminate...

 

[GSJim69]

but my gut tells me it has a burnt valve.

Are the intake valves as likely to be burnt since they receive a blast of cool air during every intake stroke?
What have you guys found that do valve work?

 

[asetech]

Intake valves burn when they are not seating properly because the valve is not dissipating the heat through the head. If the valve was improperly adjusted there is a good chance it burnt. If it were me I would put it back together run it and see if it comes back to life.

 

[GSJim69]

I've put about 800 miles on the bike since I adjusted the valves and checked the compression.
I just checked it again (warm engine and wide open throttle) and cylinder #1 is still low. The results were:
#1 70
#2 120
#3 125
#4 130

I added a little oil to cylinder #1 and the compression test increased to above 90. So, I think it is definitely one of the valves.

Any suggestions from the experts here?

Thanks

 

[tkent02]

Did you look in to the spark plug holes as you turn the engine by hand yet?
If it is burnt you should be able to see it. Also a leak down test will tell you more.
It is sounding more like the head will need to come off.

 

[duaneage]

A compression increase attained using oil or transmission fluid points to the rings, not the valves. The Oil gets into the rings and improves sealing temporarily. If the rings are OK and a valve is bad the oil won't change the compression much (and in this case going from 70 to 90 is not a big jump) since oil won't seal up a bad valve seat well. I think you have a questionable valve.

You could also try and remove the carbs and exhaust to gain access to the valves from behind. With the valve closed (you can tell by piston position and overall timing) you could introduce a small amount of kerosene into the port behind the valve. If it leaks out rather quickly then you know. The exhaust valve is most likely to have damage or carbon build up. If the intake valve is leaking it could be a foreign object is hung up on the seat.

It's not unheard of for debris to get pulled in and captured by a valve, especially when cranking. An engine with a bad spark plug could load the exhaust seat with carbon that causes leaks and poor sealing. Overheating could cause a warped valve or stem, but that is rare. Carbon could build up on the stem and prevent the valve from closing, but after 800 miles that should not be a problem.

The seats won't easily clean themselves. Dis-assembly is usually required.

Solutions range from pulling the head and replacing a valve to attempting to clean the face off with cleaning solutions applied from behind. With winter approaching I would just ride it for now and do a head yank this winter, provided it is just a valve. And you now have a simple way to do a leak down test from behind each valve.

 

[GSJim69]

I expected that I would need to pull the head.
I am going to ride it until the winter. Then I'll pull the head.
Once it is off should I take it to the machine shop for inspection or is there anything else that I could do to try and improve the situation without machining?

 

[almarconi]

Once it is off should I take it to the machine shop for inspection or is there anything else that I could do to try and improve the situation without machining?

Once you have the head off, measure..measure..measure. The service manual has an exhaustive explanation of all the head components and their measurements. Once you know then you can make a more educated guess as to what needs to be fixed.

 

[Guest]

This can cause a no compression scenario