RM250 head mods

[trailrider121]

Does anyone know if you can interchange cylinder heads from 2003-2006 RM250's? I have a 2003 rm250 and need to replace my cylinder head with a new one.I was thinking if a 2005 rm250 head will work,maybe I can run pump gas with it.

Thanks,

[Dave2122]

I would not interchange the heads. The head volumes may be different and you could loose power. If you have money to burn then by all means get the 06 head and measure the cc's and make your calculations. For the time it is not worth it to me. I can run pump gas in my RM 250 with no problems.

[ALLEYCAT50]

Hey Trailrider121. I have a suggestion before you spend money you do not have to.

I have a 2003 rm250, pump gas in my area is only 91 octane. I added an extra base gasket which lowered my compression ratio, now I can run 91 octane, without any trouble. While you have your cylinder off check the edges of the ports for burrs, mine was really bad, I cleaned these up and knifed edged the ports.

Something else you can try is mixing race fuel and pump gas 50/50, you should notice a difference. You can run race fuel if you don't mind the price, but it gets expensive.

BTW- I assume that you are having trouble, a bit of a stumble between 1/4 to 3/4 throttle that is why you want to change heads. The 2003 rm250 has this problem.

Let me know how you make out.

[trailrider121]

Thanks alleycat50,

I have ran race gas and it runs better,but it does get expensive.I was thinking today about putting another base gasket to see it helps.I dont mind if it changes the port timing,I run the bike more on the top end anyways.I have retarted the timing alot and the bike runs way better.It still has a bad knock to it while under a load.

As you mention about the stumble,I never had this problem.The engine has great throttle response.Thanks for the tips and I'm going try out the extra base gasket....

[ALLEYCAT50]

Hey man listen to this, I never would have thought it, but you may have too much squish.
Friend of mine purchased a '05 yz250, it spark knocked all last year. When he took off the cylinder this year he found some detonation marks in the head, so he sent it to get machined, the shop told him that he had too much squishband thickness, not the width but the depth. The shop told him that in a 250 the thickness should be .030" to .035", you need .020" clearence between the piston and the combustion chamber. My bike is apart so I checked mine, .057"

I thought the shop just wanted another $50, but they sent him home with the head. They said to place the head on to a piece of med-fine sand paper, on a flat surface and remove .020"-.030".

I did some searching and these numbers are right. Most said that you can get away with removing upto .040" without changing the compression ratio much.

I am going to try it. We won't know for awhile how we made out, too cold and snow here.

It is something you may want to look futher into. If you retarded the timing and the sparkknock improved, then this theory would make sense to me, but that is just me.

PEACE!!!

[ALLEYCAT50]

Here are a couple of links that I found with some info on squishbands.

This one is for watercraft but still 2 stroke theory.
http://www.hpt-sport.com/keeppuln.htm

The second is a 2-stroke hop up guide, I liked this one.
http://www.lukesracing.com/luksr/tips.html

[oldandbroken]

I use a software package to check heads. It calculates Maximum Squish Velocity and Compression ratios to estimate octane requirements.

Check out his site.
http://ericgorr.com/twostktech/egportweb.html

[ALLEYCAT50]

That is pretty good info. Is the software supposed to be there too? I would like to get that. I have read a bit about it. The TSR software? I think. Searched for it, Couldn't find a download.

[oldandbroken]

I bought the TSR software direct from the producer. It came on CDs and is DOS based. He has some examples on his web page, but not downloads.

http://www.tsrsoftware.com/

I've talked to him several times and emailed with questions, he was always good to respond.