Compression numbers

[weekendrider]

Cold motor, nothing removed or propped open. Nothing added (like squirting oil in the top) but a test plug to keep from frying things.
Electric start to turn it over.
Left side,

Right side,

That's almost a thirty % difference?
And these readings are quite a bit different than the readings 6 months ago.

 

[5twins]

Did you at least roll the throttle wide open? That's all I ever do and it gives me good results. I don't remove the carbs or prop the slides open. But I usually do test warm motors.

 

[weekendrider]

No didn't touch anything but the starter button. I do need to test warm for a comparison.

 

[5twins]

Hold the throttle wide open. You want the butterfly plates open so air can more easily be sucked into the engine. You will get readings with the plates closed, as you see, but they take much longer to build up. Testing again on the cold motor but with the throttle held open may give you better numbers.

 

[weekendrider]

OK I'll do that. And test a warm motor.

 

[TwoManyXS1Bs]

We used to read the character of the compression tests, like watching the needle variations during carb syncs, to get a better clue to whut's going on in there.

On a good engine, the compression would come up rapidly, reaching final reading after only 3 strokes. On an engine needing work, sometimes it comes up slowly, taking numerous strokes to reach a final reading, or it comes up rapidly to a low number. One of those indicates valves, the other rings.

And, dang it, I can't remember which is which now.

Edit: There's also the wet test.

 

[gggGary]

Yes fully shut throttle plates will drop the reading, doesn't have to open all that much. Slides don't need to be moved they have a plenty big gap. I actually did back to back compression testing on various open closed stuff on an XS650 so it's not just hot air. I can get several pounds more compression by foot than with the e-start.
Both sides are low old man, but saw your oil and know the motors coming out anyways.... I like the TDC and put air in through the plug hole/ compression tester, test. Turn the compressor dial down to 0 and slowly bring it up while listening at intake, exhaust and open oil dip stick hole, The problem area is usually pretty obvious.

 

[weekendrider]

I can get several pounds more compression by foot

But the right knee is already swollen and Ima pussy.

 

[peanut]

suggest you reset your valves a little looser...like 0.006.012" "and retest with such a big difference it has to be a burnt valve or one thats not seating I should have thought

 

[grizld1]

I'm having trouble believing those numbers, WR; unless something ugly happened on the road home (besides the chain breaking on you), your scoot was running too well in the rally for those numbers to be right. With 65 psi on the right your motor would have been running on one cylinder. Even if both readings are low and both jugs were developing enough pressure to fire, your motor would have been shaking like a dog passing peach pits with such a big difference between cylinders, and that wasn't happening either. Peanut may have called it; could a valve(s) have tightened up?

 

[weekendrider]

Grizld1 when I first got the bike running a cold compression check with no carbs on showed
120 left and big fat 0 on the right. I ran some ATF through the right cylinder and rode it to the Ozarks rally with those cold numbers.
At that rally (John will attest) I had no trouble steppin off if I wanted.
Per the suggestions I next tried a cold motor with an open throttle and got.
Left cylinder.

And the right.

That is looking better with only <20% difference in the cylinders. So the amount of air you let the cylinders have does make a BIG difference.

Now lets look at gggGary's test it warm suggestion. It didn't take maybe 10 minutes to bring the temp from.

To (and actually it gained a bit even after I shut down because of the delay between what the motor is to heating the oil which is what the gauge is reading)

At this point you will want some gloves to remove the plugs unless you one tuff sumbeech or you have no nerves in your fingers, so have a pair handy.
Also as a caution/warning even though I ran the carbs dry and only one cylinder had a plug it still tried to fire off and run for a moment.
Warm motor WOT.
This is the left.

Warm and WOT. Drum roll for the right.

And just for sheets and giggles.
Warm and closed throttle.

So if you have rescued a bike and think you have bad compression on a cylinder.
Test it correctly and ride it for awhile, the numbers may improve. That instant right now, get into the top end to fix it may not be necessary.

Side bar as the cam chain is into aluminum it will be time to freshen up the top end this winter and I fully expect with a set of new rings, cam chain and front guide, and clean up the head it will be good as better than new.

 

[gggGary]

Hey wait, how many miles on this beast?
One oft neglected item is a good seal on all the gauge threads couplings and shraeder valve. If the gauge doesn't hold a reading steady bit of silicone lube will fix it right up. C'mon, do the DIY leakdown test!

 

[grizld1]

125 psi on both sides; that's within spec and tested like Mama Yama says, WOT, warm motor, spun at 200 rpm, which is about what the starter motor will turn if it's healthy.

 

[gggGary]

125 psi on both sides; that's within spec and tested like Mama Yama says, WOT, warm motor, spun at 200 rpm, which is about what the starter motor will turn if it's healthy.

Especially considering how far off the valve timing is with the guide missing!

 

[peanut]

I was referring specifically to the Boyer Bransden ignition system