Anyone know anything about 85cc engines?

plynn41 · Post by **plynn41** » January 17th, 2011, 5:50 pm

Sorry everyone for posting this question on here, but I've posted it on two different forums and got no response. I figured my luck would be better here.

I'm doing a total rebuild of the kid's yz85 and thought I might tweak the compression. I use race fuel, and don't mind sharing the jug with the kid. When I measured his squish band, I found it to be 0.50mm, which seems real tight. We bought the bike used and I know it's been gone through before. We've put two pistons through it, but this is the first time I've done the whole business on this engine.

Does anyone know what a good squish clearance is for an 85cc motor? Is it likely that this head has already been cut?

Thanks!

maddog1927 · Post by **maddog1927** » January 17th, 2011, 10:15 pm

yz250 is .070ish stock

.045 is considered tight for the yz250.

at .050 my guess is the head has been cut.

you may be able to take a little more off. probaby cheaper to get a thinner base gasket

2strokeforever · Post by **2strokeforever** » January 18th, 2011, 4:09 am

its 80cc, its gonna have a tighter squish than a 250
your best bet is to ask someone who does lots of 80s

http://www.youtube.com/user/500bigbore · Post by **Roostius_Maximus** » January 18th, 2011, 6:00 am

flat top piston or the dome chamber?

plynn41 · Post by **plynn41** » January 18th, 2011, 6:40 am

Dome top piston.

BTW, it's 0.50MM, 0.020 inch. I mean REALLY tight. But the bike runs great on pump gas. I'm thinking it wouldn't be wise to go any tighter just for the mechanical clearance issues.

I posted the question on TT yamaha 2-stroke forum and got nothing. Same on TT honda 2-stroke forum under generic "85cc squish band" question--nothing again. Everyone I personally know who races them has their machine work done by someone else, and wouldn't know the specifics.

Post by **MojoScojo** » January 18th, 2011, 7:29 am

Are you wanting to change the power curve? What kind of power are you trying to get out of it?

plynn41 · Post by **plynn41** » January 18th, 2011, 7:45 am

We don't want to change the timing or the way the motor hits. Not interested in cutting the bottom of the cylinder, and certainly don't need the timing to be any more radical. But if there were some cheap bottom end that could be gained by increasing the compression a tad, then we'd be do that. I suspect that the head has already been cut though, and wondered if anyone could confirm it.

Post by **AlisoBob** » January 18th, 2011, 8:04 am

Time and money spent on boingers beats any head mod.

Post by **AlisoBob** » January 18th, 2011, 8:07 am

http://kustom-kraft.com/yz80bigborekit.html

Done

plynn41 · Post by **plynn41** » January 18th, 2011, 8:09 am

AlisoBob wrote:Time and money spent on boingers beats any head mod.

So I've heard (and I believe it). But I don't spend money on head mods. I've got my own CNC router table.

And I just spent about $600 on the little bike for a new crankshaft, piston and new Barnett clutch. That's all the boy's gonna get this year!

NightBiker07 · Post by **NightBiker07** » January 18th, 2011, 11:54 am

Get on Everything2stroke.com. or call Denny at Flotek, he might know.

http://flotekperformance.com/

he knows his stuff, he ported my 300R cylinder. he is definitely a hoon, and would recommend him for services, especially for banshee work.

BannedCR500Riders.com

Anyone know anything about 85cc engines?

Anyone know anything about 85cc engines?