squish, compression ratio, and a few questions...

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No, what I am saying is that the port if ever fully uncovered, is only for a few degrees rotation,

The other, say 350 degrees, it is slightly, largely or completely obstructed.

Ok that makes sense. The piston does dwell at bdc for a short moment as the rod changes directions, so it does spend more time there relative to the rest of the stroke (except for tdc dwell). Improvements there would be capitalized on as much as is possible due to the dwell. But like you said, it probably isn't worth worrying about.

Good thread though! At least for folks like us who find the inner workings of a 2 stroke interesting. Thanks again for all the responses.

 

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Okay just a quick follow up, I cut the head yesterday and measured the compression (the same method F5 uses, leveling cylinder and filling though spark plug hole with piston at tdc, except I use rubbing alcohol instead of fork oil, much less messy). I added 6cc's to the chamber, this gave me a static of 13.7:1, and 220psi on the compression tester gauge when kicking it over. I can cut another 2cc's out to get a 12.7:1, or cutting one more cc out should give 13.2:1. I can get non-ethanol 95 octane gas here, any opinions on whether I even need to cut anymore out? I have read one thread were someone was running around 14:1 on pump gas (93?) and said the bike was running good. Thanks again for any response.

 

[(F5)]

You did put a smear of grease around the bore before bringing piston up to TDC yeah? Just concerned using thinner viscosity fluid have to be sure its , erm. . .water tight so it doesn't leak away.

 

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You did put a smear of grease around the bore before bringing piston up to TDC yeah? Just concerned using thinner viscosity fluid have to be sure its , erm. . .water tight so it doesn't leak away.

Yes, a good coat of o-ring and seal grease. I use a 30cc syringe to draw the alcohol back out after I measure, I pull almost as much out as I put in, no sign of anything leaking down the bore.

What do you think about the 13.7:1 compression. Would that be acceptable with 95 octane fuel? Or is it still too high? Thanks again.

 

[(F5)]

Well I think it is pretty borderline & I'm not experienced in what your gas behaves like. There is the RON vs MON rating & I don't keep up with who's got what.

It is a square engine & watercooled which is a bonus, but the cooling is quite variable in the bush so its not a roadrace engine that can be kept in a narrow margin.

Also you don't want to make it too hard to kick over when you've fallen over & stalled on the slide of a slippery hill, picked the bike up & have to start it again. A single prod wins.

 

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Yeah I have decided to go ahead and knock it down to below 13:1. Better safe than sorry. Thanks.