Marginal compression. Effect on performance?

[Redman]

Let me ask this again:
My compression readings are 120-130-120-140. Any comments?

I know the expected is 114 to 175. Service limit is 100 or difference of 28 is listed in manaul as requiring overhaul.

I asked before and comments were moslty like "thats not real bad" or "see how it runs". This is a GK that I got in November (meaning havent riden it much at all). Was running real bad. Have checked compression, check all four sparks okay, have taken apart carbs for cleaning. Just recently got back together and now is starting and running much much better.

But I am still wondering how much performance I am loosing with this marginal compression. Any comments?

Knowing that 150psi is equivilant to 10 to 1 compression, then 130psi is about 8.8 to 1 compression, which to me seems like a big differernce. Or with the compression pressure measured at such slow rpm (more time for leaking loosing compression), is this low reading then not that much of an effect at running rpm?

Note: Compression taken with throttle open, but engine not warm. With oil in cyclinders reading more like 165-170, so know problem is related to ring/cyclinder as opposed to valves/cams.

Was near 40 the other day, so had to, had to go for ride. Did pull hard, but not really what I was expecting for an 1100 (with 40% more CCs than 850).

So... thats why I am thinking about the compression readings again (that and not having any snow to shovel).

[tkent02]

First of all ride it....

Compression can't be normal if it has sat a long time. Rings can be stuck, running with clean oil can unstick them. Adjust the valves (valves too tight will lose compression even if the engine is in great shape) and tune it as correctly as you can, then run it several hundred miles. Then see how it does. Numbers mean nothing, there are too many variables like altitude, cranking RPM, cam, valve clearances etc. What matters is the difference in between cylinders. Yours are different but not totally way out. With that compression difference I would expect perhaps a rough idle, nothing worse.

[focus frenzy]

Compression can't be normal if it has sat a long time. Rings can be stuck, running with clean oil can unstick them. Adjust the valves (valves too tight will lose compression even if the engine is in great shape) and tune it as correctly as you can, then run it several hundred miles. Then see how it does. Numbers mean nothing, there are too many variables like altitude, cranking RPM, cam, valve clearances etc. What matters is the difference in between cylinders. Yours are different but not totally way out. With that compression difference I would expect perhaps a rough idle, nothing worse.

DITTO on the recomendation, put it together come down for the nice warm spring Hill country ride we are having this weekend, and decide after that.

[DaveDanger]

Comment: It's a bike... Ride it!
Seriously, I'm not sure that you'll find an engine with anywhere near maximum compression once it's departed the showroom floor. These GS bikes we're riding and working on... are generally pushing 25 years old, unless you've overhauled it recently. I don't know that it's realistic to expect high compressions on any engine that isn't ridden on an absolute daily basis, and even then, you have to be aware that just like we humans, who're dying from the day we're born.... a mechanical construct as an engine begins to wear out from the minute it is started the first time.
I do believe that the temperature of the engine has a large effect on the compression check, as far as an absolute pressure reading. I don't necessarily think that a hot engine will make a low cylinder match up to a higher one, but it will bring all readings up across the board, all other things being equal.
BTW, did you crank for a specific amount of time, number of rotations or until the pressures stopped rising ?

[Redman]

Thanks Kent, Yep I am going to ride it - after getting cleaned up and back together. All the plastic is still in the basement. Was near 50 today, about 10-12 degress above average for this time of year.

Hey, FF Leon, thanks for the invite. I really did concider making the trip again for the Hill Country ride, especailly when temperatures were in teens and single digits a couple weeks ago. I enjoyed the ride and meeting you guys so much last year. But cant really swing it. Last year we also visited friends at Ft Hood, so thats how I was able to justify the trip to the Household Activitys Committee. But thier is another possibility for next year: Our daughter may possibley be at Baylor/SanAntonio next year, and garsch, as a good parents we should visit her some time, say maybe in 2nd week of March, eh!

DaveD, compression press was after cranking with starter for about 5 seconds. Although would have reached its maximum after only 3 or 4 pulses, but I would keep cranking just to be sure. And keep battery charger on to be sure battery didnt get run down. But engine was cold (not in running condition at that time). And did hold throttle wide open.


Oh, yeah, I intend to get it on the road here shortly. Is a few things I want to cleanup before I get all the plastic back on it. Will see how it goes after a couple hundred miles.