compression issues

[suzi]

Hello fellow GSers. Have a question that I hope can be answered.
I have a 79 gs1000 with 47,000 miles on it. The number 4 cylinder is about 25 psi compression lower than 1-3. It runs rough at idle and low
range rpms. It runs excellent at mid and high range. I have a reputable mechanic working on it with good exp. on these old jap bikes, and he is perplexed as to what is causing the lower comp.
We pulled off the #4 plug cap while engine running at idle, and it barely made a diff in the way it idled (which as I said was rough).
However, when pulling off any other plug cap, the engine nearly died.
So there is significant difference and #4 is barely firing at all.
The valves have all been checked and #4 is .001, which is within normal range.The rings are all prestine in appearance, so piston wear would be unlikely i would think.
The leak down test was PERFECT and no leak was detected.

I have searched and read previous threads and some mentioned sitting for long periods can cause this problem.
I am a fair weather rider and probably over the last few years have put less than 2,000 or so miles on it.
Could this cause this type of problem? not riding it enough? If so, what would you expect to see as a tell tale sign? When I first bought it 12 years ago it had about 36,000 miles on it and it ran fine. There is no smoke from exhaust.
Any thoughts or suggestions would be greatly appreciated or any more
info needed let me know.

Thanks, Doug

[barnbiketom]

rings 'pristine in appearance'???
does this mean you had it torn down?
what was the cylinder taper?
what was the ring end gap?
what was the piston taper?
I would surely have had the cylinders checked by a machinist and at the very least installed NEW RINGS and gently honed the cylinders!!!!
also .001 is at the low range and I' would rather see loose than tight.

but as to the rough running?

I'll bet compression has NOTHING to do with it.

Clean your carbs . you can verify this problem by turning the idle mix screw.. i bet it does nuttin!!!

Also if that #4 carb is dumping fuel.. it may have the rings washed dry of oil, thus resulting in lower compression.

just my thoughts!!

[hampshirehog]

0.001 is right at the bottom end of the 'normal range' for your valve clearances. I would whack a thinner shim in there for sure.

Don't worry too much about 1 cylinder being down compared with the others - 25lbs is inside spec. Chances are if you take the bike out and give it some beans things will even up - the leak down test and no smoke points to that.

Your rough idle hints at a carb problem in #4 = probably a blocked pilot jet but it could also be a bad synch, broken fuel screw etc.

[suzi]

I took it in to the shop to have the cylinder base and head gaskets replaced because they were leaking. So yes it was torn down. I had already rebuilt an thoroughly cleaned the carbs. The mechanic also went through the carbs and all was well. Carb synch was also DEAD ON. As for the piston and cylinder tapers, I have no clue.
Again, as I said previously, It ran MUCH MORE roughly when the other three caps were pulled, and made almost no difference when pulled off #4. So, I guess not sure what that means exactly, but I know it's not a carb/fuel problem.
Thanks for your input.

[hampshirehog]

That compression difference won't make it run roughly. Try swapping your 1 and 4 plug leads over and see if that makes a difference.

This may seem a bit harsh but if your mechanic thinks the compression difference is making the bike run roughly then I would go for the carbs again. You have to be meticulous with these and a lot of guys take short cuts and it doesn't work.

[gearhead13]

So, what are your compression numbers? 25 psi lower than what? 100? 140?
Were the cylinders lightly honed before it was reinstalled? Or just slapped back together?

[suzi]

the numbers were mid 90s 1,2&3 and about 65 on #4.
The cylinders were not honed, but it should be noted that
it runs NO DIFFERENTLY than it did before I brought it in
(before it was torn down and reassembled).
Thanks

[hampshirehog]

Is your gauge ok and are you doing the check correctly? Standard is 128 to 184psi, service limit is 100psi so your good cylinders are low. Max difference between cylinders is 28.5 psi.

your figures are so low that if you're doing the test correctly something is well adrift.

[suzi]

I, m pretty confident that the mechanic is doing the checks correctly, but of course, can't be sure. If these are indeed true numbers would it not run poorly at all speeds and rpms? I wonder.
Could I possibly need a rebuild, even if the rings do not look worn and it doesn't smoke?
I'm really a bit confused and I guess I'm at the mercy of my mechanic, and trusting his ability.
It was suggested earlier that I should put a little thicker shim in #4 and think that's probably a good idea.
Also wonder how much a role, if any, the lack of riding it has to do with anything?
Thanks

[gearhead13]

I, m pretty confident that the mechanic is doing the checks correctly, but of course, can't be sure. If these are indeed true numbers would it not run poorly at all speeds and rpms? I wonder.
Could I possibly need a rebuild, even if the rings do not look worn and it doesn't smoke?
I'm really a bit confused and I guess I'm at the mercy of my mechanic, and trusting his ability.
It was suggested earlier that I should put a little thicker shim in #4 and think that's probably a good idea.
Also wonder how much a role, if any, the lack of riding it has to do with anything?
Thanks

Do you have anything to compare it to?
.001" clearance is real tight, fix that first. Thinner shim!
Personally, if I had the top end off a 50k mile engine I would have installed new rings or at least honed it.
But it could just need some good hard run time to help the rings seal.

[suzi]

could someone please explain what a leak down test measures. I thought it was another indicator of compression. Also, with compression numbers so low even in the good cylinders, how would the bike be expected to perform.
I'm getting a strong impression that it may indeed be time for a top end rebuild/bore.
Also, does it make any sense to anyone else, that with comp of#4 cylinder being only 65, wouldn't it make sense that it would barely run/fire on that cylinder (at idle, as I explained in the first post)?

[Mekanix]

Not just compression, well actually not compression at all. A leak down test will tell how well the combustion chamber is sealing.

A bad test would even let you hear where the air is hissing out of and point you in the right direction.

It helps by telling you how good your valves are sealing and how well the piston rings are sealing.

You end up with a percentage of loss. You simply put air into the cylinder and see how easily it leaks out, there is a gauge that will show if you have an acceptable loss or large loss, and if its too bad you will actually hear the air hissing.


With such low compression its funny that it is even running right at all. 3 cylinders are just enough but the 4th is almost dead weight. However once you spin it fast enough that cylinder will end up helping. So that's why its ok on top end but not at idle,

It might be rings,(doubtfull) or intake valves or exhaust valves.
Go for the shim'(s) first.

What about valve timing? Is it set correctly or do you have aftermarket cam's that need to be degreed? Being off by 1 tooth will cause some low compression numbers like that,

[Homie]

the numbers were mid 90s 1,2&3 and about 65 on #4. The cylinders were not honed, but it should be noted that
it runs NO DIFFERENTLY than it did before I brought it in
(before it was torn down and reassembled).

If those numbers are in fact accurate I'm surprised the engine runs at all - those numbers are LOW. With #4 being so much lower than the others SOMETHING should have been indicated with the Leakdown Test. As "Mekanix" stated, with the Leakdown Test you are basically pumping a measured amount of air into the cylinder and then measuring the amount (differential pressure loss) and location (intake, exhaust, breather or head gasket) of the air that leaks out of the cylinder. With those kinds of numbers it has to be going somewhere other than staying in the cylinder.

Putting the original rings back into the cylinders without at least giving the cylinders a light hone is perplexing. The rings break-in and set to the cylinders. When you separate them you break that seating and they need to be reset. A light hone can usually accomplish this and enable the rings to set with the cylinders.

Another thing to consider is the ring gaps. The service manual is very specific as to where each ring gap should be in relation to the piston. If #4's ring gaps are not properly aligned it could cause excessive blow-by. But then again, the Leakdown Test should have detected excessive blow-by through the crankcase breather if this was the case and the test was conducted properly.

[chef1366]

No offense intended but get another mechanic or better yet do it yourself.

[Nessism]

If leak down is good, compression would be good too. Something wrong with the info you are receiving. The bike won't even run with the compression numbers you mention.

As mentioned, .001" is too tight. Most likely the mechanic doesn't even have a .001 feeler gauge as they are almost non existent.

I agree with chef, get another mechanic or wrench yourself.