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Low Compression advice

  • Thread starter Thread starter BlackStar
  • Start date Start date
B

BlackStar

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1978 Suzuki GS 1000 12700 original miles. 2nd owner, 1st was my uncle. Sat for 15 years. The engine has never been apart.

Iv'e got this thing up and running fairly good now, but I'm troubled by the compression numbers. The head has been re-torqued and the valves were found to be in tolerance. Some intakes were on the low side (none < .001), the exhausts were all about nominal. Re adjusted tighter ones closer to nominal. The compression has been checked both hot and cold with not much difference. The strange thing is the pattern of the numbers.

#4: 145
#3: 135
#2: 125
#1: 115

I have run through these things starting at #1 and then starting at #4 because I though my cranking speed may be contributing. Throttle wide open, once with the carbs completely off. Iv'e probably checked it 5 times separated by 150 miles or so.
Iv'e put about 800 miles since I got it running and re-checked again to see if perhaps the rings were stuck but have not seen any change at all. Iv'e also run some yamaha ring free fuel additive through it with no change. I put two shots from a trigger oil can in #1 to see what would happen to the compression. It shot up to 200 Psi which indicates rings. A thourough inspection of the head gasket perimeter revealed no visible indication of leaking. Just a little baffled by the pattern of the numbers.
I'm planning on a top end tear down this fall to see what is going on.
Anybody ever see a pattern like this before?
 
Seems strange. Not sure what to suggest other than ride the bike. The compression numbers are a little low but not dangerously so. Maybe there was some corrosion that damaged the cylinders? Not sure. Marvel Mystery oil is good stuff to free up stuck rings. A guy that used to work in the Saturn assembly plant in Tenn. told me that they recommended a good soaking to free up the rings in their early four cylinder engines since they tended to gum up the ring land on the oil control rings. Sounds like you have already tried a similar soak though so I'm not sure it would do any good at this point. Ride and tear down during the winter sounds like a good plan.
 
Since it sat for so long, it could have had some corrosion on number one piston if the valve was open from the exhuast. No way to correct the pitting apart from taking the thing apart. But now that said, take the plug out of one with the piston at the bottom of its stroke, fill the cylinder with transmission fluid and let the bike sit for a few days. the oil will leak down past the rings to the crankcase. If the ring are stuck this will clean out the crud and free them up. still with the plug out, turn the engine 180 degrees so the piston is at the top of its stroke, look in thereto see if there is still oil in there. If not, then step two is to put half a can of seafoam into your tank and drive that bike around the seafoam will clean the valves. Run through another tank and then recheck the compression with all the plugs out and the throttle wide open. If one is not better then eventually you will need to tear into the engine for a bit of a rebuild.
 
Those figures are unusual. They could be caused by the following senario.

If the engine was not run or turned over for 15 years there would have been some surface rust on the rings and bores of the cylinders that had valves open. Since the firing order of these bikes is 1,2,4,3 its possible that 1 & 2 cylinders both had valves partially open together. If you turned the engine over before pouring in some lube oil for the rings, you may have broken small pieces of the compression rings off these pistons. They would also have marked the bores to some extent. This situation will not improve dramatically by adding more kms to the engine. As Nessism suggests, ride it and pull the engine down when you feel you are ready.
You can probably do another 20,000kms on those figures.
 
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