XR400 looking for head mods

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#1
Acctually I have a friend that has a 400EX fourwheeler but as I understand the motors are nearly identical? I was going to do some head mods mainly a poilsh job but maybe port match the intake and exhaust to their respective manifolds and maybe re radius the intake runner. Just wondering if all this (mainly the polishing of the combustion chamber and runners) is worth the time and effort for some gain in flow. I wish that I had a flow bench to verify this but i do not. any other head mods recomended?
 

SFO

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#2
If you want to put a real cam in it you will need shortened guides and a spring kit, so while it is apart I would suggest the KMPI stuff.

They make SS valves for this motor too.

Blend the seats, have your valve job guy open up the throats and unshroud the seats on the chamber side, I think the short turn is pretty good on this intake if I remember right.

If you end up putting a big cam in it make sure the valve job guy has it, (the cam) so he can check the valve to valve clearance, things get close in these rfvc heads pretty quick.
 
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#3
Thanks! but I really do not want to go as far as replaceing the cam or vavles for different ones, (my friend likes to ride low in the powerband) so i'd rather stick to the polishing and to a lesser extent the porting.
 
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#4
Originally posted by TheJunkMan
so i'd rather stick to the polishing and to a lesser extent the porting.
Even with the stock cam and valves unless you can properly rework the throat and seat entrance and exit there is no point in touching the head. You could krazy glue M&Ms upstream in those ports and not hurt the performance, but botching the throat or seat area can kill flow in a big hurry.

Polishing should be reserved for the wheels on your tow vehicle. :confused:
 

XRpredator

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#5
*note to self: purchase krazy glue and M&Ms for next XR400 tune up . . .

:confused:
 
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#7
Originally posted by SFO
plain or with peanuts?
Plain, they are more aerodynamic. :confused:
 

Patman

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#9
JunkMan The intent of my XR284 build was a grunt bike with the possibility of some light race duty. The mods on my head are pretty much what SFO & Rich suggested (gee I wonder why? :think:) and all I can say is WOW!!!

BTW I am pretty sure they left the M&M's in the candy dish.
 

TexKDX

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#11
Hey Super-Pee -

My 278 will be a 12:1 284 this week. My 278 bore was not very straight and the apparent cause of my oil consumption woes.

Won't be a total rip-snorter like yours but should do a little better than the 10.5:1 278 that was in thar... specially at altitude.

The XR6 BWP is down now too, getting a stator re-wind for mas lights, heated grips, and heated vest. Will be like 200 watts out when we get done with it.

The CRE260 needs the forks taken down and the springs checked. It is a bit soft in the front. If I do that too this week, I may not have anything left to ride but the Monty!! Spent 3 wonderful days in NM over the holiday on the CRE. What a great bike. Great motor, handles like a Honda. Way different than a bud's '03 300EXC. Glad I got the CRE.