Does porting affect compression?

MURDER YAMAHA

MURDER YAMAHA

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I have a ported redhead, new Yamaha pistons, rings, gaskets, everything.
when I put it back together, I am getting uneven compression numbers.
wondering if the porting has anything to do with it. They read as follows.
107/120/105.
Tried it with thin head gasket, stock head gasket, and also tried 2 different heads with the same results,
 
Yup! Raised exhaust timing will lower compression but in your case, having different compression numbers is very odd. If the heads/head were not machined (matched to porting), ALL 3 should show lower compression. I hope Mr. Viper chimes in on this one.
 
Using stock head. Like I said, have tried 2 different heads and gaskets, but the amount of change between the cylinders stays the same.
I ordered a new set of rings just for peace of mind, but want to make sure Im not missing anything before I re assemble again.
Only thing that makes sense is maybe that the porting may be slightly different between cylinders ?
But, I cant really wrap my head around that being the cause.
All cylinders are within spec, all pistons and rings are new Yamaha.
Maybe I should measure from top of the cylinder down to top of the exhasut port and see if there are any differences.
I would assume that would be the only thing that may affect it ?
 
Ok, I think I have figured it out. The one cylinder that has more compression was one I had to have redone, because of my own stupidity. It’s exhaust port is considerably lower from the top of the cylinder than the original 2 cylinders, so I think that’s why that cylinder has more compression than the original two.
 
you dont make static compression till the piston goes up to the top edge of the exhaust port, so a higher exh port would have less compression then a lower one would on the same cylinder head. if the cylinder has been redone it might have needed welded on topedge of port and they dont get them back to original alot of times. also in many cases if the cylinder was blown up they will deck the top of the cylinder like .003-.005" to clean up the edge from detonation removing material for plating it. this will usually not be a problem as the head gasket will take up the slight difference, but it does change the deck clearence in the cylinder, would make slightly more compression as it would move the whole piston up in the cylinder the milled amount.
 
Thanks Don.

As you said in your email, that is a possibility.
But, the first set of cylinders we used had not ever been blown up to my knowledge.
And then when you redid the center cylinder for me after I wrecked the original, I tried both the original cylinder you did(I had replated) , and the one you redid, with the same compression results, which were about 13PSI more than the outer 2 cylinders.
Both ported cylinders yielded the same compression on the center position.
But, the motor is now back together using the original ported set you did for me.

Numbers are now as follows using a Peak performance head with 20cc dome, 2 layer head gasket.
PTO 127 w/ .053 squish
Center 135 w/ .051
MAG 126 w/ .050

I think Im close enough now to calm the nerves.
I will just put some miles on it and see how it goes, but should be good now.
Rings may still not be seated properly since this motor has only went about a 1/4 mile total so far.

I richened up the main jet to a 155, and will now run better fuel.
Will use 2.5 gallons of VP 110, and 8 gallons of 91 octane.
Should give me about 95.5 octane.
 
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