Low Compression

[Guest]

Got a 1977 GS750 which has not run since 1993 and in a bit of a state! Engine is in the frame, but the exhaust and carbs are not fitted. The engine turns over with the kick start. The wiring was in a bad state, replaced the loom and turned the engine over doing a compression test: Results were 1: 75psi 2: 55 psi 3: 65 psi 4: 53 psi which I believe is a bit low as it states 100psi in the manual?
Would the figures be affected because there was no exhaust or carbs fitted? Or am I better to wait and complete the build and try again?

Thanks for any information regards this problem

Little Wagon

[phydeauxmutt]

No exhaust or carbs will not affect compression in a negative manner. In fact, no carbs is the best way, as you have to hold the throttle wide open anyway.

Not run since 1993? You reckon the rings might be a little "stuck"? Did you do a 'wet' compression test or just a 'dry' one?

I think I know the answer to the next question, but I'll ask it anyway: did you check valve clearances?

With those numbers, an engine could fire up and run, if the carbs were clean and the valves were adjusted. Run the bike for a couple hundred miles (or more), exercising the limits of the tach. Run in an "energetic" manner for a while, the rings should free up and seal better.

Ideally, compression tests should be done on a warm engine that is properly adjusted, and the throttle held wide open. On a cold engine that has not been run for over 20 years, you are lucky that you got ANY numbers.

[tkent02]

What he said, if it sat that long it's lucky to have the compression it does, especially if it was parked because the valve adjustments were neglected and it ran like crap or not at all.
Very very common for the valves to be neglected, there can be little compression if the valves don't close, even less if they have been burnt from not closing for years.

Probably half of the old bikes I pick up have not had the valves adjusted for decades.

[bwringer]

Also, you're going to get garbage for compression numbers if you're just using the kick starter. You can't spin it fast enough or long enough with the lever to learn much of anything.

Count yourself lucky to get any reading at all.


If you bolt up everything else that's needed (make sure you do check the valve clearances), odds are excellent that it'll be just fine. You got some sort of readings and they're reasonably close, so there's certainly nothing horribly wrong in the bores.

[greg78gs750]

yep, plenty to worry about before compression

[little wagon]

Thank you for the reply's
Did the compression test with the starter motor as have sorted most of the wiring! (Well so far) Didn't think I could kick it over that fast lol
No did not check the valve clearances just thought I would see if it had any compression before going any further on rebuilding the bike
What is the difference between wet and dry compression testing?
With what you have all said I will finish cleaning the carbs and exhaust and refit, check the valve clearance, and try to start the bike

Cheers
Little Wagon

[phydeauxmutt]

The difference between wet and dry:

When encountering low(-ish) numbers, you don't know if it's because of rings or valves. Adding a couple spoonfuls of oil to the cylinder and trying again will seal the rings. If the numbers go up, your low numbers were because of the rings. If they don't go up, it's likely the valves, but could also be a leaky head gasket. Since all your numbers were relatively close, I would not suspect a gasket problem.

As far as waiting until you "complete the build", ... personally, I would verify a good engine before doing much of anything else.

[tkent02]

Meh, set the valves and run the Hell out of it.

[little wagon]

Will give it a go with the oil and see what the results are then? What size spoon are we looking at or is it easier with a amount in a syringe?

Thanks again

Little Wagon

[tkent02]

Again, all the compression testing in the world is useless if you don't know how the valve clearances are set.

I am outa here.

[phydeauxmutt]

Use a "spoon", use a "syring", drip some in, splash some in, do whatever you want. A "spoonful" kinda hints at kitchen-sized spoons, either a teaspoon or a tablespoon, but will gnerally amount to a couple of ccs or a fraction of a fluid ounce. If using a syringe, try to put the oil toward the intake side of the cylinder, so it will run around the rings toward the exhaust side. A few strokes of the piston should help distribute it, but at least let it start on the upper edge.

[Nessism]

Again, all the compression testing in the world is useless if you don't know how the valve clearances are set.

Agree. Adjust the valves or you are wasting your time.

[greg78gs750]

Agree. Adjust the valves or you are wasting your time.

yep yep. and yep is 10.

[bwringer]

Agree. Adjust the valves or you are wasting your time.

The thing is, when you find a GS that hasn't been run in many years, it was very likely parked in the first place because tight neglected valve clearances caused it to be hard to start. One day, the battery ran down before the frustrated owner could get it going, and it got pushed to the back of the shed to deal with "later"...

This is why we always harp on checking the valves before doing anything else. It's the #1 reason a bike gets parked for 20 years.

On the pre '80 bikes with points, neglected points were another very common reason a bike was parked. That little rubbing block ran dry of oil in a few years, and pretty soon you had no spark.

[tkent02]

There are other reasons too, didn't maintain the electrical system so it wouldn't charge is a big one, lost the key is another, scared themselves or scared the wife, got henpecked into submission, parked the bike so long the carbs got plugged up, etc. The neglected valves is by far the most common, and usually on the bikes that are still in use the valves are too tight anyway.