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Compression Check

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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Checking compression tonight in my '78 GS750E. What's low enough to rebuild motor? I suspect rings, but after wet/dry compression test, will no more. Just wanted an idea before I started.

Thoughts/Ideas/Opinions?

Thanks
 
I don't have a manual for the 750, just for 1000 but could be close.

Normal compression = 128-184 psi
Minimum compression = 100 psi

Hope this helps.
 
On most motors 120psi or less is in the worn stage. Below 100psi is rebuild time, if not before getting that low.
 
Tested it last night. I started her up to warm her up a bit so I could get accurate readings. It ran, not very well, but at least she started. I had two cylinders at 100 psi and two at 55 psi. I ran the test several times to insure my numbers were correct. Wet & dry tests. I could not believe it was still running with that kinda compression. The last time I rode, I got about 15 miles out of it before plugs fouled. It ran like crap and was terrible on gas mileage. The the engine oil smelled like fuel. I was looking at timing and carbs. Guess I should have compression tested it sooner.

So, looks like time for a rebuild! I am going to wait and see the condition of the cylinder walls before deciding if I need to hone or bore. Will know more after tonite. For some of you that have bored a 750E over size, do you have any advice? How much over did you go? Where'd you get parts? Bearings? Cam chain tensioner? Stuff like that.
 
It can be your valves carboned up and not sealing. Put it on TDC and take the valve cover off. Lightly tap the valves...not to hard as to make a keeper come out. Then check them again...I recently had a 1150 that I was very concerned about...thats all it was...that cylinder was down 40 pounds. Your valve clearance can also affect your readings if your valves are tight. A leakdown test would be much more accurate as to what is wrong.
 
I did the leak down test too. When I pressurized the cylinder I could hear gurguling in the oil pan. I also notice that the small layer of oil that has built up on the outside of the motor around the headgasket was bubbling too!!! I have been nursing the bike for two years and got about 25 miles out of it last year. I should be shot for not doing a rebuild last year!!!

Got the cams out last nite to find that both bolts holding the timing gear on the exhaust cam had worked loose and just started to "rub" on the inside. I was close to MAJOR PROBLEMS!

how much oversized on the pistons do I need to go to get to 850cc?
 
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