YZ80 powerband doesn't flow in

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#1
I have a 87 YZ80. The bottom end recently went. I put a 88 bottom end in. Somehow the piston came 1/4" out of the top of the cylinder. I don't know if it's in the piston, cylinder, or rod. I made a spacer for the bottom of the jug and got the piston right at the top of the cylinder at TDC. I had to mess around with the timing to get it close, but the mark on the flywheel is wrong so it isn't exact. The power band is there but when i start off i have to slip the clutch to make it come in. Once i am in the power band and moving, i can down shift to stay in it. It has 90psi compression and stock jets. When i try moving the timing it doesn't get any better. thank you in advance
 
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#2
Well I am pretty impressed that you were able to make this work. perhaps engineering has a bright future waiting for you. So think math classes!!!!!

The biggest issue will be the ability of the fuel charge to be able to get the proper draw volume and speed from the "draw" and "squish" of the bottom of the piston against the crank area and flow properly up through the transfer ports to the top of the piston. How to overcome this will be an incredible challenge the easiest will be the proper crank.

D
 

Patman

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#7
Stock bore and you made it an 88? So this is a stroker kit piston? Typically I see the extra in the rod but I guess it's possible. 1/4" above the cylinder would seem to be quite a bit more than an extra 8cc or so though. You sure about all this?
 
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#8
the cylinder, piston, and head are all 1987. the lower end including crank and rod are 88. the piston is a stock wiesco replacement. it was all right when the 87 parts were on the 87 lower end. sorry i don't really see what you are asking, patman.
 
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#9
glowpowered said:
the cylinder, piston, and head are all 1987. the lower end including crank and rod are 88. the piston is a stock wiesco replacement. it was all right when the 87 parts were on the 87 lower end. sorry i don't really see what you are asking, patman.

Have you done any research on the rods? The 88 model rod sounds to be longer from reading your post. That spacer you installed under the cylinder has lowered your compression and has shifted the power curve to the upper end or has just interfered with proper cylinder flow of incoming gas/air or the purging of exhaust gases. I could be way off, never messed with that year bike or model.
 

Patman

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#10
I read it as you made it an 88cc engine not an 88 model year, my bad. Still if the piston is sticking ABOVE the cylinder there would seem to be a problem that making an adapter might not be the real solution for. I would guess you have changed the timing on the ports by placing the space under the cylinder hence the screwed up power delivery.
 
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#11
A quick reference check from wiseco says the bore and stroke are the same. Oem parts fiche backs this up with the same part numbers. So exactly what have you done???
 

Patman

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#12
Glad I'm not the only one scratching my head.
 
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#13
Maybe got the wrong piston? Boxed wrong?

Where did you get the 88 bottom end? Maybe it has a stroker crank?

Rotor
 
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#15
quick reference check from wiseco says the bore and stroke
Wrong 87 part number 520MO4800 and 88 part number 569MO4800 so there have it they are diff part number.

I belive the 88 bottom end with 87 Piston will not work reason 88 Piston Pin hole are diff place and think there small difference in case deck height.

To bad you didn't own a Honda CR80/85 can you get way with diff model engine years 1986 thur 2007 there very Compatible unlike other
The thing only you have watch for
Cylinder Head there are 2 type has to do water hose intake placement
Cylinder there are 2 type has to do Exhaust Flange