I'm doing little reseach on optimum exhaust system for these bikes but I'm missing one key piece of information. Size of the exhaust valves.

Does anyone know the diameter of the 250 or 300 exhaust valve?

--
Mikko

 

Outside diameter 24.4 - 24.6mm.

Ride on
Brewster

 

The 300 exhaust valves are larger. Don't know the size.

Ron

 

Hey Brewster, great information!
And just to double check, that is 250S exhaust valve size? Or 300R?

--
Mikko

 

That is the spec. from the KLX250R, KLX250 service manual (1997). The KLX300 service supplement manual doesn't have the spec. for the KLX300.

Ride on
Brewster

 

From what I can figure out, the older KLX250, KLX250R and KLX300R share the same cylinder head, including valves. These newer 250S and 250SF bikes, supposedly have smaller valves.

It's very good to know the 300R valve diameter, from that I can calculate the optimun header size for that motor. Assuming the 250S valves are 2mm- 1mm smaller, I can also estimate optimum header diamterd for this motor.

Wil be interesting to see how those numbers jive with the actual 300R and 250S header diameters.
--
Mikko

 

I'm curious about this. Anyone notice how far to the edge of the piston crown the valve reliefs are located on the 250S piston? They go right over the edge of the crown. When you look at the 300 OEM piston, the reliefs are in a more "normal" location. The reason I mention this is that it almost looks like the head's valve locations were ideally designed for the 300, and that the 250's piston was just an afterthought. Now, I'm not claiming this is the case, but I just thought the 250's piston in relationship to the valve location in the head appears unusual. My meager understanding of combustion chamber design would suggest that having the valves correspond to the piston at the extreme edges of the cylinder and even lapping down the edge of the piston would not be ideal for flow or flamefront upon ignition. Again, I'm a novice on the real dynamics involved here.

I know a little about the history of the KLX250, then to the KLX300, and now to the KLX250S, but I'll bet there are some interesting nuances between these engines that most of us are not aware of. For example, most of us know the original KLX250's cases will not accept the 300 cylinder...at least without modification perhaps. There may be other interesting tidbits out there. I'd like to know if the combustion chamber size and valve positioning has been identical on all 3 variants of this engine. I'm just curious...not argumentative.

 

Hmmm! I'm interested too. As we have a "frankenbike" KLX250R down here in AUS. I wonder which head we get? Could be the answer to the extra performance questions, compared to the USA models (along with a few other KLX300R parts our AUS bikes come stock with.) Won't be whipping the head off just yet but if anyone down here has had a look and measured the exhaust valves then post it up.

 

Hello All,
The KLX250 & KLX300 have the same size valves in them I=39 MM E=25 MM. Kiblewhite makes a O/S intake valve for the KLX 250 it is 30 MM it is only 1 MM larger than stock. There isn't very much room for O/S intake valves. I recently installed O/S valves in the head on my KLX 351 also had the head ported it made some difference the engine revs a bit quicker now also installed a TM36 pumper carb that made a big difference but the gas milage suffers now. Now have 34 HP at the rear wheel using a experamental exhaust.
Bill

 

Thanks Bill for clearing that up. Just points out that old saying about not believing everything you hear on the enternot. Bill, do you have any thoughts or direct knowledge about that issue of the valve reliefs on the 250S piston? I've been into quite a few engines and had never seen the valve reliefs that far out to the edge and side of the piston crown, but I'll repeat what I said before...I'm no expert and have hardly seen all the piston-to-valve designs out there. My first thought was that the combustion chamber size and valve placement was designed for the 300, but the application of the 250S piston and bore came later. I realize there was a KLX250 before the 300, but that really different valve relief deal on the 250S just got me to thinking. Just speculation on my part and trying to learn something about piston valve reliefs.