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another low compression question

gustovh

gustovh

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Past Site Supporter
The engine is a 82 1100E in a 80 L. Last run was 620 miles in 11 hours. Arrived at home with a fried battery, found that out the next day when I went to fire her up and move it to the shop. Enter new battery and fired right up, rode it from the back yard to the shop and parked it to deal with the charging issue later.

A week later, the battery was still good, so I tried starting the old girl and couldn't get anything but a snort once and a while. Darn, darn ,darn, darn, so I checked the compression and was floored to find...30, 60, 30, 35 for numbers, they were all pumping 125 or better before I left on the 2000 mile jaunt. Oil level was always good and the bike ran flawless.

Any ideas, just curious.

V
 
I checked the compression with 2 different gauges and have not looked at the valves yet. I find it amazing that it could loose 90 lbs compression overnight, or over 2000 miles for that matter.

V
 
gustovh said:
I checked the compression with 2 different gauges and have not looked at the valves yet. I find it amazing that it could loose 90 lbs compression overnight, or over 2000 miles for that matter.

V
Click to expand...

There can't be much compression if the valves don't close. Check the clearances before you burn some valves.
 
Yes I held the throttle open during the tests. I haven,t ridden it because it won't start, Really curious why in the space of 2000 miles the valves would loose that much.

V
 
No, they tighten that much, how many miles since you last checked the clearances?

You've been on this forum long enough to know how this works.
 
I checked them just before the ride, so a total of 2200 miles and they were spot on. I'll give it a look today if I get a chance and report back. I have ridden allot of miles on a bunch of bikes over the years and never lost compression numbers like this before.

V
 
Compression dropping like that is perplexing. Almost as if the cam timing has jumped. 125 psi is getting low mind you. Maybe the engine just decided it's time for a rebuild?
 
520eek said:
How often should valves be checked?
Click to expand...

When you buy a bike, before you sell it, after any engine work involving the head, again a thousand or so miles later. A middle aged engine they don't need to be checked very often. Until everything seats in, more often, it probably depends on how you use the engine too. I think the book probably says every 5,000 miles or so, but that's probably overkill in most cases. Each engine is different, if you have checked yours a drew times and it hasn't changed much it's probably safe to go a little longer.

Almost every GS I have ever bought had at least some of the valves too tight, with the few exceptions being a few bikes I picked up from forum members who actually enjoy working on this stuff.
I also check on a new bike that the cams are timed on the right tooth, it's amazing how many are off by one or even two.
 
In my experience of this and other forums and my own compression testing on my bikes:

The compression testing of a motorbike is the least reliable and useful test of any of the tests we carry out on our beasts. The results almost always lead to the wrong diagnosis.

Mostly, after a while, we discover that its either that we've used the equipment (and usually its cheap Chinese rubbish) incorrectly or that we have not prepared the machine. There are simply so many variables. First thing to do is not to panic. Do all your maintenance tasks. Check that there is not gas in the oil or any visible signs of an oil leak.

It's suspicious to my mind that the bike ran flawlessly and now, just because it's been sitting a while, it has bad compression. Perhaps it will come to it's senses when you fix the charging system. These machines have feelings to you know!
 
Well if the engine jumped timing, it did it when I shut it off The last time. When I get around to it I will squirt some oil in each cylinder and see if that changes anything. Certainly has me stumped without pulling the thing apart.

V
 
It wouldn't jump timing unless you turned the engine with the tensioner out. I'm curious, very odd if the valves are adjusted correctly.

Do you have access to the equipment for a leak down test?
 
Vince, If the petcock is leaky the cylinders could be washed down with fuel. Pull the plugs, spin it around some, squirt some oil in the cylinders and test again.
 
Yup, I am aware of that one Bob, no gas in the oil or carb throats at the time. The tank has been off since. Going to oil up the cylinders, maybe tomorrow and see what I get.

Thought this problem was a curious one as well.
Consider that I have completed all the obvious tests, this one has me stumped because there was no warning or symptoms prior.

V
 
The only way I know of to lose compression quickly is to burn valves, but that won't happen without the valves being too tight, and it wouldn't be all 4 cylinders so evenly.

Holing a piston is another possibility, but that would be zero compression. Curious to find out what you learn when you do the leak down test.
 
I don't have a tester for a leak down test so I won't be able to give you any feedback on that one.

Am in the middle of a construction project at this time, so wrench time is at a premium at this time.

V
 
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