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compression problems

  • Thread starter Thread starter vejesse
  • Start date Start date
V

vejesse

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Hello, maybe somebody here can help me out.
I have a 1982 gs850G. I checked the compression recently. From left to right: 50psi; 75; 75; 90. The one at 50psi went up to 80 with some oil in the cylinder. The one at 50 also had a fouled plug so evidently oil is getting past the rings. Added Oil also raised the reading for #3 cylinder a bit but not the others. The manual says the range is 125-175 with 100psi being service limit.
So here's my question: Will just running the motor with the unbalanced compression destroy my lower end? I haven't adjusted the valve clearance yet, but I suspect this won't help burnt valves and seats.
At this point I just want to ride it through the summer and worry about a rebuild later but I don't want to hurt the motor any worse than it is.

Experienced hands have any ideas?
 
Your readings are so low, I'm suprised it runs at all. Was the engine warm? Was the throttle wide open when you made your readings? These things make a difference.

Assuming the above were OK, poor compression comes from two sources, the rings or the valves (or both). The determiner is the procedure you followed with the oil - so it sounds like rings. Leaking or mal-adjusted valves can also cause poor compression.

As to running this way, I don't think anything can hurt the GS crankshaft. But maybe others will disagree...
 
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Run it, it may get better with more miles.....

Run it, it may get better with more miles.....

Or it may not. Definitely adjust the valves, and start looking for a motor to rebuild, so you are not down a few months when the time comes. Or find one to run while you rebuild yours. Motors are cheap, I just bought an 850 engine complete for $3 on ebay, just to get the pistons and cylinders for my 750. You're lucky, 850 engines stayed the same for a lot of years, it should be easy to find one.
 
Just want to emphasize doing your compression test with a wide open throttle. Makes all the difference.
 
Just want to emphasize doing your compression test with a wide open throttle. Makes all the difference.
Click to expand...

Ditto. Made that mistake before.

As said before, if a tablespoon of oil in the cylinder increased compression, you have bad rings. If it doesn't increase compression you have leaking valves. How many miles are on the motor?
 
:shock: I'm surprised it runs at all, must smoke like crazy when you decellerate!
Another thing is to make sure the battery is charged and remove all the plugs to get the engine spinning really well, that truely is not good for pressure.
 
more compression

more compression

Hello me again. I'll try the compression test again after adjusting the valve shims, really warmed up and with the throttle open.
By the way, the motor only smokes for about one minute when first started. I have no idea how many miles are on the motor (replaced speedo) but the guy I bought it from claimed around 20K and I've maybe put on 2k.
As long as I've got your attention here's something with the carbs.
There's a bad flat spot between 2500-3500 rpm. The carbs have been soaked and rebuilt with new parts. Synched last year. I'm not sure if the idle screws are set quite right but it starts right away and seems to idle fine. Somebody told me bikes of this era (1982) run too lean, but I can't image that performance was this poor when new.
I would have the valves adusted by now but last time I tried I couldn't get the cam cover to come off! It was like it was glued on.
 
You may have to hammer and pry (carefully) the valve cover off and plan on buying a new gasket.

I wouldn't concern myself with carb/performance tuning until you have the compression corrected. If those readings are even close accurate, it's amazing it runs at all - much less has a flat spot. Better to do one thing at a time.
 
Wow, i'm suprised your bike is running at all. Those numbers are beyond low. Like someone else said in this thread, just start searching ebay for engines, you can find them for dirt cheap. I just picked up a complete 650 for $4. Just takes some patience.
 
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