Compression test results

[arvo92]

The problem is - I do not know it myself. The bike odometer is 32000miles but the engine no and frame no do not match. So it might be that a swap with the engine has been made (a bad sign).

Could it be that by "trashing" the engine the compression rate figures could go down as well?

And I would not put my bets that the valves etc. have ever been changed as the bike has been properly abused as shown by other parts of the bike.

I do have new rings, gaskets etc on hand but I can not take the left hand side muffler connection to the cylinder off. It seems the only way is to cut it off which I am a bit weary to do it properly.

[Barry]

Not withstanding Tony's comments about a squirt of oil in the cylinders not being quite so applicable on a horizontal cylinder, did you try it anyway to see if the compression figures improved. Also assume you did the test with engine hot and carbs off or throttles open and pistons lifted.

[arvo92]

I have not tried oil. But other than that I did the compression test as described.

The spark plugs were burning hot when I removed them and grounded them on the cylinder.
I opened the throttle fully and pumped the starter for 10 second for both cylinders.

I could squirt a spit of oil in there to see if I get any change at all and report it back here tonight.

[arvo92]

Results of squirting oil into cylinders.

Left - 115 to 130
Right - 120 to 128

So oil did affect the results somewhat. Not a huge leap but still noticeable. Left cylinder compression, which had lower results increased much more than right cylinder.

If I am correct then oil indicates only piston ring wear and tear, not the valves?

[Barry]

If I am correct then oil indicates only piston ring wear and tear, not the valves?

That's the usual interpretation.

[Luca]

I opened the throttle fully and pumped the starter for 10 second for both cylinders.

But did you lift the slides?  Spinning the engine with the starter won't make enough vacuum to lift them up and the engine will be starved for air, lowering your compression test figures.

[arvo92]

Lift the slides? Do you mean pulling the throttle cables as far as they come out of their brackets to get the butterfly slide opened up?

these compression rate figures are already low. Do you want to tell me that I have a bucket instead of a piston in there  

[Luca]

Twisting the throttle opens the round, flat butterfly valves that rotate with the throttle shaft.  The slides (with the diaphragm and jet needle) are indirectly controlled by vacuum from the intake port and slide vertically in the carburetor body.  There will not be enough vacuum to lift them when spinning the engine with the starter motor.

To properly test you need to hold the throttle wide open and hold the slide up while cranking, or just remove the carburetor altogether.

Your compression figures will go up, not down!

[Bob_Roller]

If you remove the intake tube from the carb, you will see a the slide, it moves with engine vacuum not the butterfly plate that the throttle cable moves, without the engine running, it almost blocks the carb throat and restricts airflow .

I also operate the starter for about 45 seconds on each cylinder .

Make sure your battery is fully charged before you do a compression check .

[Justin B.]

Hmmm, I dunno 'bout cranking 45 seconds - seems like starter abuse!  I crank it over about  10 revs and important do do the same number of cranks on each side.  You can watch the gauge and it will "squirt" a little higher with each turn but IIRC after about 5 revs you don't get much more to speak of...

[Tony Smith]

Hmmm, I dunno 'bout cranking 45 seconds - seems like starter abuse!  

Inclined to agree. I reckon that 5 compression cycles is more than sufficient. I also think that given the vagaries of gauges, technique etc that the best advice is to look to see that there is no significant difference in the measured compression between the cylinders.

if the difference is small and the bike is now chewing oil, or having trouble outrunning grannies on electric mobility chairs, I'd be inclined to leave it alone, unless i was goign to pull the heads off to do the valves, in which case a set of rings is a no-brainer.

[nhmaf]

I always completely remove the carbs to eliminate any intake restrictions from messing up the results - I think it is simpler to do rather than holding the butterfly open and making sure the carb slide are fully up  -- otherwise your test results will be lowered/inaccurate.

[arvo92]

properly test you need to hold the throttle wide open and hold the slide up while cranking, or just remove the carburetor altogether.

Your compression figures will go up, not down!

You were correct on this. The results were different. I removed the carbs totally and pumped the engine on twisty roads so that the engine was completely hot before commencing the test. All the abovementioned requirements for the test were fulfilled as well (throttle wide open, battery full, crank the starter for 10 seconds - i could still see some movement in PSI after 5 starter cycles) Results are:

Left - 125 psi
Right - 130 psi.

I am leaving myself out of the debate about oil in horizontal cylinders. I can only report what my findings are.

Left - from 125 to 150!
Right - from 130 to 147

So percentage wise, the left cylinder with lower compression rate, increased much more than the right cylinder. Nevertheless - cylinder compression rate increased 21 psi on weighted average.

Question - could these compression rates mean that my bikes 32k milage (50k kilometers) could be correct under certain circumstances?