Compression difference cylinder 1 to cylinder 2

[Fast_Frankenstein]

Hi All, I'm awaiting parts for a build I'm working on  - story to follow. I'm reading the service manual for shits 'n giggles in the meantime (baby its cold outside) and note that the compression pressure on cylinder one and two are different during testing according to the factory manual. I've never run across a motorcycle with a compression stagger, and am hoping for enlightenment. Is this a possible side effect of the 270 degree crank, and electric start? Combustion chambers appear the same, and pistons share part numbers... so (??)  

[robbo10]

Well, I read somewhere (I think it was the Rebel 1100) where each pots does different things as to power and torque....!

[Triple Jim]

It may have to do with the uneven cranking speed.  One cylinder has 1-1/4 turns of the crankshaft getting up to speed before its top dead center, and the other one has only 3/4 of a turn before its piston gets to TDC.  It seems plausible that the crankshaft isn't spinning as fast for that one, so the pressure reading is a little lower.

[mossrider]

There is a compression release system active on #2.

[Triple Jim]

1 hour ago, mossrider said:

There is a compression release system active on #2.

Thanks, I need to look at that in the manual.  But it would have seemed like if a compression release were active during cranking, the pressure would be lower than that.

Edit:  I just looked at the "decompressor" system in the manual.  It looks like it should work on both cylinders equally, but again... if the engine's speed changes as it rotates, maybe one cylinder has more time to bleed down than the other.  Or maybe I'm still missing something.  

Edited February 11, 2021 by Triple Jim

[AP996]

Decompressor system works on one valve on both cylinders so can’t see how that would be the difference.

 

[mossrider]

On 2/11/2021 at 6:36 PM, AP996 said:

Decompressor system works on one valve on both cylinders so can’t see how that would be the difference.

 

No it doesn't.

Oops, my bad. This motor does have decompression on one exhaust valve per side.