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Compression: 78' GS750

  • Thread starter Thread starter DannyMotor
  • Start date Start date
D

DannyMotor

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Just did a test.

Cyl 1: 80psi
Cyl 2: 70psi
Cyl 3: 75psi
Cyl 4: 85psi

The motor is completely dry (no oil). There are no carbs on the bike. Basically hooked up a fully charged battery and the solenoid to the bike to just crank it 5-10 times per cylinder. I did the test twice on all cylinders.

Will filling the motor with oil make a difference? I didn't want to risk having to fill the bike with oil to do a full tear-down after the fact.

They all seem even enough... but about 2/3rds of where they should be... Normally 100psi means a rebuild, and 120-150 is "normal".

Any tips?
 
Why would anyone crank any engine without oil in it?? Good way to scar the bejesus out of cylinder walls.
 
DannyMotor said:
Just did a test.

Cyl 1: 80psi
Cyl 2: 70psi
Cyl 3: 75psi
Cyl 4: 85psi

The motor is completely dry (no oil). There are no carbs on the bike. Basically hooked up a fully charged battery and the solenoid to the bike to just crank it 5-10 times per cylinder. I did the test twice on all cylinders.

Will filling the motor with oil make a difference? I didn't want to risk having to fill the bike with oil to do a full tear-down after the fact.

They all seem even enough... but about 2/3rds of where they should be... Normally 100psi means a rebuild, and 120-150 is "normal".

Any tips?
Click to expand...

Put oil in it, the cam bearings are not roller bearings, they will get messed up.. Yes the oil will make a difference. Did you open the throttle to do the test? If not those numbers are about normal, actually they are high. Are the valve clearances adjusted correctly? Tight valves effect compression. Those numbers are high enough to run fine, so run it. Couple thousand miles. With oil in it. Then check the compression again, with a warm engine.. If the bike has sat a while, the compression will come back up if you ride it a while.
 
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What Tom said. Warm engine and wide open throttle. Shut it off and pull all the wires to crank and not have her fire over. I think its a Yamaha product called Ring Free thats well talked about for loosening the rings and getting them seated as well as possible. Other in the know old timers say a few good squirts of a 50/50 ATF and Acetone treatment setting over night will also do magic .
 
Well I got the bike as a non-runner... so I don't want you guys to think I'm abusing a baby. Haha.

God knows what the PO did... this thing is just slapped together from about 3 bikes worth of parts.

The motor wasn't cranking when I got it, and I was doing a ton of electrical tests (rats-nest of a wiring harness) so I just wanted to test the solenoid and the starter...

Again... motor was cold, empty and had NO carbs on it (open intakes).

If those numbers seem OK, then I will move forward with a full re-do of the motor. Worst case scenario, it's completely clean, re-painted, full of oil, and runs... and THEN needs rings.

As of right now, I'm going to dump a little seafoam in the cylinders and start the re-painting/polishing, and electrical tune-ups. Then, come back with things in working order (hopefully).
 
How long has it sat? How many approx. miles? Chances are it will smoke a bit until things get worked and freed up...dont panic and throw in the towel. Seafoam will for sure work them a bit looser. Take off the oil pan and be sure the pickup screen is clean and no holes and see what kinda goodies are in the bottom of the pan..metal chunks..shavings..just basic sludge. . Get things such as valves, carbs, etc etc in order and fire her up. If the compression is ggod on the warm engine and she still smokes a bit then I would look at the valve stem seals..Just get her running first then take things asn they come.
 
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