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  The Autonerdz Community Forums › › › Running compression too high?
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Running compression too high? (Read 269 times)
James H
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Running compression too high?
Feb 20th, 2020 at 7:48pm  
If you look at these In cyl compression tests (cranking and running on a 2 cylinder V Twin) can anyone explain and possibles reasons why the Rt Cyl Running compression is so high?
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ICCT_Lt_Crank.psdata (2691 KB | )
ICCT_Lt_Running.psdata (2491 KB | )
ICCT_Rt_Crank.psdata (1159 KB | )
ICCT_Rt_Running.psdata (1370 KB | )
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rayray
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Re: Running compression too high?
Reply #1 - Feb 21st, 2020 at 9:10am  
The right Cyl Running compression is too high because the the intake valve is closing too early.

When an intake valve closes too early, the piston moving up on the compression stroke will have more time to compress more air in the cylinder.
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Motorcycle_good_running_compression.gif (160 KB | )
Motorcycle_good_running_compression.gif
Motorcycle_right_cylinder_high_running_compression.gif (159 KB | )
Motorcycle_right_cylinder_high_running_compression.gif
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James H
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Re: Running compression too high?
Reply #2 - Feb 21st, 2020 at 11:19am  
Thanks for having a look. That is what I determined as well. cam timing and valve clearance is correct. It is a single cam push rod motor and I was thinking I would find a worn cam lobe, rocker etc but when I got it apart there is some wear but does not appear to be bad enough to cause this problem hence my post.
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rayray
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Re: Running compression too high?
Reply #3 - Feb 21st, 2020 at 12:26pm  
To measure where the True compression TDC is and any valve timing accurately, we should do the measuring using the ckp sensor along with the transducer in the cylinder.
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James H
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Re: Running compression too high?
Reply #4 - Feb 21st, 2020 at 12:31pm  
I wish, 1 pickup/pulse per crankshaft revolution, welcome to carbureted motorcycles.
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TrevorSchlientz
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Re: Running compression too high?
Reply #5 - Mar 3rd, 2020 at 3:00am  
Just spit balling here. If my memory serves don't v twins have a seriously Un even firing pattern? I would think loosing half of that pattern would drastically effect the cranks velocity on whichever cylinder is dead.
I've never seen a relative compression from a v twin. What does a known good RC pattern look like?
When I get to my PC this one will be fun to play with.
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HarleyMike
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Re: Running compression too high?
Reply #6 - Mar 3rd, 2020 at 8:03am  
Low vac. and higher compression. The tower seems to lean a bit.  (speaking about RT running capture)

I'm thinking like a "supercharge effect" from some extra air being scavenged threw a leaky valve.

Kinda like when only one cylinder in a car engine get all the EGR from stopped up ports.

Just food for thought.
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StitchB
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Re: Running compression too high?
Reply #7 - May 13th, 2020 at 10:49pm  
Food for thought.
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Screenshot_2020-05-13_23_22_49.png (253 KB | )
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