Very little compression in new OS 105 HZ-R

[Rctintin]

Guys, I have just removed the head from my new 105 HZ-R and added the additional .2mm shim because I will be using 30% Optifuel.
 
I have just installed a new OS no8 plug, and it has very little compression. Is this normal for these motors?
(Don't want to carry lower % fuel to the field as well the 30% that gets used in the N5c) 

I have just taken it apart again, and checked the piston ring is in the correct position, centered to the anti rotation pin in the piston. Re-built with the original shim only, and again little compression.

At TDC the piston crown is flush with the top of the liner. I have added a few drips of thin oil, and you can hear a hissing as you turn it towards TDC.

Any ideas guys please?

[Jean-Luc Bolduc]

As the breaking-in occurs the ring will seat and compression will increase, no worries.

[Rctintin]

Cool, was a bit concerned as it has less than my well used YS56!

N7 is coming along nicely, don't want motor troubles holding me up!!

[Jean-Luc Bolduc]

I hear you, you want that N7 purring !

[1helimech]

where is the hissing coming from  ?   from the head area or the exhaust port ?  If from the head better have a look at the head to block seal ie head shim/gasket.. just a thought that poped in to my head when I read your question....

[Matt Botos]

91HZ-R and 105 both have ring pressure holes in the top of the piston. When rotating slowly the compression will be minimal as the air escapes past the ring/liner and pressure holes. When used in normal operation, the fuel/air mixture combined with the compression of the rising piston increases the pressure on the back side of the ring which increases the ring to liner seal. (I believe this is an old NASCAR trick to gain more compression.)
The ring is also a split ring which keys on a pin inside of the ring groove. This split in the ring will allow a bit of oil/fuel to pass by.

Thanks,

Matt

[Rctintin]

Thank you sir, I sure was worried because its my first OS105 having used YS91's for the past 5 years!!

[John]

Play around with the shims a bit once you get it broke in, mine is running great on 30% with just the .2 shim in the head.