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Compression test on GS1000

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What would be considered good compression????? I got a reading of 120 psi on the first two cylinders, and 90 psi on the 3rd and 4th cylinders.
 
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Are these numbers from a warm engine with the throttle wide open?
If yes the motor will run.
Put a little oil in the cylinders and see if the number rises. If it does the low numbers are ring related. If not the valves are the issue.
 
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First thing I would do iis check the valve clearances. Get it running and beat the sh$t out of it for an hour or two. Then recheck the compression
 
No, this was with a cold motor, just cranking the engine, not actually running the motor.
 
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120 is fine, particularly cold. 90 psi cold might be OK, it should go up when the engine is warm. Compression tests are supposed to be done warm; but when a bike's not running we do what we can. Always with WOT, but you shouldn't have gotten near 120 if you weren't wide open. The 30 psi difference is not good. Is the bike running? Was it warm? Might just be rings that aren't quite seated.

I'll throw in the "nightmare scenario" not to scare you - but it did happen to me - it's something you can check for. My comp #'s on 1 and 2 were quite low (<80psi) with about 120 psi on #3 and #4 (82 GS550L). The roller bearing crank engines use a pressed crankshaft instead of a single machined piece. My crankshaft got "twisted" so that one of the shafts turned inside one of the weights, and my #1 and #2 cylinders were between 20 and 30 degrees ahead of where they should have been. You can use a wooden dowel rod to measure the distance between the spark plug hole and piston top and compare #s 1 vs 4, and 2 vs 3. If they're significantly off you may have a problem. BTW, as you would expect thinking about it geometrically this is much more evident directly between TDC and BDC than at either end. I had my cylinder head off with #2 and #3 at TDC and couldn't tell - as a friend was turning the crank I visibly noticed the height differential between #2 and #3 in mid-travel.
 
No, the engine runs, just not real good. It feels like it is missing some. I know I have a weak stator, and there is only about 8VDC at the coils, so I need to do the coil mod. I just wanted to look at the compression before buying a stator. I didn't do the check with the motor warm, or with WOT. I just now rechecked 3 and 4 after putting a little oil in, and with WOT, and the compression went to 230 psi on both! :confused: I doen't see the significance of WOT, but apparently, it does make a difference, so now I am not sure where I am at. Maybe if I had did the initial check at WOT, 3 and 4 might have read ok? :confused:
 
I thought that if compression reads on the low side and by adding oil it (compression) goes up then it is rings not valves. Conversely, if adding oil does not bring compression up then it is a valve issue...maybe just an adjustment
 
I have to do something this morning, but when I come back this afternoon, I am going to take the bike for a blast, and then recheck the compression with a warm motor and with the throttle wide open. Hopefully, all will be well. Then I will get a stator. My charging voltage never gets up to 13VDC, at least by revving it up to 6k rpm's, so this is probably the main reason, it is not running to good. Plus there is a big voltage drop going out to the coil, so the spark is even weaker.
 
Before you order that stator, check the ground wires on your Regulator and Rectifier (I think they may be seperate on your bike). Poor grounds on these circuits are very common and will cause low charging voltage. Standard recommendation here at GSR is to extend the grounds to run directly to the battery negative cable.
 
Thanks, I will do that. I checked the stator output, and it was 60VAC at around 5k rpm's.
 
You want your bike to be able to take in as much air as it can during a compression test, hence the wide-open throttle. Are we talking 230 psi? That's way too high, unless you're running a diesel bike! 230 kPa would be way too low. How much oil did you put in? 1/2 to 1 teaspoon should do it - crank it over a couple of times to distribute and re-run the test. If you put in too much oil that can reduce your head volume temporarily and cause too much compression.
 
gs850cmc said:
I thought that if compression reads on the low side and by adding oil it (compression) goes up then it is rings not valves. Conversely, if adding oil does not bring compression up then it is a valve issue...maybe just an adjustment
Click to expand...
You're right! I fixed it.
Thanks
 
60vac is ok for an aftermarket stator. Mine reads about that. I would go through the stator papers from here point by point & you'll soon find the problem.

Dan :)
 
mike_of_bbg said:
You want your bike to be able to take in as much air as it can during a compression test, hence the wide-open throttle. Are we talking 230 psi? That's way too high, unless you're running a diesel bike! 230 kPa would be way too low. How much oil did you put in? 1/2 to 1 teaspoon should do it - crank it over a couple of times to distribute and re-run the test. If you put in too much oil that can reduce your head volume temporarily and cause too much compression.
Click to expand...


Ok, I re-checked the compression after taking the bike for a little ride this afternoon. This is after I burned out all of the oil I had put in through the plug holes. It smoked like crazy! :eek: When I checked each cylinder when I came back from the ride, I got a reading with the slides wide open of 150 psi on all but #3, here I got a reading of 145 psi. So I am relieved that my motor seems like it is in pretty good shape. :dancing: I am going to do the coil relay mod tonight and check the bike out tomorrow, cause now I almost sure that I don't have a strong enough spark on the plugs, hense the sluggish performance. What else could it really be???

Oh, and thanks for the explanation of why you have to check the compression with a wide-open throttle, and with the motor warmed up. Why didn't I think of that!!!!
 
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salty_monk said:
60vac is ok for an aftermarket stator. Mine reads about that. I would go through the stator papers from here point by point & you'll soon find the problem.

Dan :)
Click to expand...

Thanks Dan, when all else fails, read the instructions. :D
 
Yep, those are great #'s. Maybe I should make a compression test guide for BikeCliff's site. It's really one of the first tests which should be done in evaluating a motor.
 
mike_of_bbg said:
Yep, those are great #'s. Maybe I should make a compression test guide for BikeCliff's site. It's really one of the first tests which should be done in evaluating a motor.
Click to expand...

Good idea. But the first test is really a plug read. If things look grim, a compression test should follow. There are some who will say not to do the compression test until correct valve clearances have been confirmed.

If you've just bought a bike, valve clearance checks in the first few weeks do make sense, unless it's running like a champ, of course!
 
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