getting the new motor right

[snacob14]

I just had my motor rebuilt. I broke it in and took it to my last race. The previous motor used a wiseco dome top piston with a stock head gasket. I changed to a cr250 piston which is flat top and bumped the compression with just the metal part of the new gasket. The problem I'm having, and had at my race, is that when I get into the throttle, about 3/4 on, it breaks up. It feels like its running rich, but before I go changing jets, thought I'd put it on here and see what you guys think. I was coming through the pits at my xc race, tapped out, and it was like I had a fouled plug or something.

[1promodfan]

You know, my motor is doing the same thing. I'm gonna go down one more jet just to see. Also, I thought of borrowing my buddies coil, and just see if thats it. Maybe you could try the same thing. I checked my plug yesterday, and it looked a little dark. Try your jetting, or check your plug to see where you're at.

[Honda 250r 001]

if you are running a flat piston, i believe you to run a flat top cylinder.

[snacob14]

hopefully thats not true, the guy who built the motor has been building them for the last 20 years.

[machwon]

Originally posted by snacob14
hopefully thats not true, the guy who built the motor has been building them for the last 20 years.

Changing to a flat top piston without doing significant head modifications will drop the compression ratio alot. Lower compression ratios are way less forgiving on jetting so it will have to be right on, or it will foul plugs. I'd check with your engine building about what if anything was done to the head. Without considering the head design on a piston change like that would lose everywhere on power.

[snacob14]

even running a smaller headgasket wouldn't make up for the loss of compression?

[Honda 250r 001]

Im going to have to say no, but you need to get a compression tester and see, you are going to have to get the head milled im betting.

[snacob14]

The piston is from a 97-01 cr250

[Honda 250r 001]

contact wiseco and see what they say. they have the trx flat top conversion kits for sale.

[machwon]

Lets say for discussion sake you lost about 10cc by using a flat top piston vs a stock piston. This means you have added 10 cc's to the combustion chamber volume. But, using a different gasket removed around 3.5 cc's from the chamber volume. If a 250 motor normally had 12:1 compression ratio and you added 6.5 cc to the combustion chamber, the new motor would only have a compression ratio a little over 9:1 without modifying the head.
Hope this helps explain a possible situation. As the Wiseco catalog notes for the flat top conversion "head modifications req'd, for experienced builders only".