Yeah, weird that our very similar bikes with a lot of the same mods can be so different.

 

My comment was meant to be very general with the only variable being the displacement.

Better breathing creates a need for more fuel.

[quoteOne other issue with the KLX that I've been curious about that you may have an opinion on. The KLX250S...at least for sure the '06/'07 models, has the valve reliefs running down the crown of the piston to almost touch the first ring land. Now, it's not the first time I've ever seen that, but it's usually not the ideal design for combustion chamber/fuel burn efficiency from what I've been told and read. When you install an OEM 300 piston or bigger, the reliefs are in what is normally the more common and probably more efficient combustion chamber to valves to piston configuration. And yes, I realize there are variables to this concept too. Still, I'm wondering how much this might affect the jetting discussion we're having here.[/QUOTE]

Pockets in the combustion area seems to be going against the norm for combustion efficiency. I haven't seen the piston that you are talking about. Does there need to be such a deep relief to clear the valves in normal operation? Or could it be to prevent valve to piston contact in the event of a valve remaining open (broken timing chain) when the piston goes to TDC?

Ride on
Brewster

 

Pockets in the combustion area seems to be going against the norm for combustion efficiency. I haven't seen the piston that you are talking about. Does there need to be such a deep relief to clear the valves in normal operation? Or could it be to prevent valve to piston contact in the event of a valve remaining open (broken timing chain) when the piston goes to TDC?

Ride on
Brewster[/QUOTE]
I'm pretty sure the stock KLX piston will hit the valves in an untimed camshaft scenario, but it's not so much the depth of the valve reliefs as the location that I was wondering about What I'm just curious about and have no basis in fact is that it looks like the 300 piston and head were made to go with each other. It looks like the 250 was an afterthought due to the extreme edge location of the valve reliefs...way down into the far edge of the piston, almost to the first ring land.

Now I realize that the KLX started life as a 250 before becoming a 300. I was curious if the valve location in the head and the combustion chamber on the original 250 is the same as the 300 and/or the the '06 and later model 250's. It's the valve relief location on the '06 and later 250 piston that causes me to be curious about this. For example I know the early 250 didn't have a case opening large enough to accomdate even the 300 piston without maching...as least from all reports I've seen...never tried it myself. I think most of that input came from Australians who've had this older model since the beginning.

Well, instead of erasing this post I'll just leave it and answer my own question...perhaps. I notice Bike Bandit's site shows the piston in the '94 KLX250 (2nd year of production) is the same part number as my '06. So I'd assume that the valve placement is the same on all KLX's from day one...even the 300. So that seems to put to rest both my curiosity and the idea that the head design works better with a 300 or bigger piston. Still, the extreme valve relief on the 250 piston strikes me as weird for a design that started with a blank sheet of paper. We now return you to your regular programming...LOL!

 

This I can partially answer. The combustion chamber on the early 250 is smaller than on the 300. They both have an 11 to 1 compression ratio. Don't know about the valve location.

Ride on
Brewster

 

132 and 40

 

138 and 38.

 

Now that's interesting. I seem to remember something about this but can't recall now. The intake valves on all years of 250's and 300's seems to be the same by part number. The exhaust, however, shows the older 250's and the 300's having the same exhaust valve part number but a different number for the '06 and later 250's. Of course looking up the bare head part number between the 300 and the '06 and later 250's doesn't help, as the 300 doesn't use the air injection if I'm remembering correctly.

Any of you Aussies who've been heavy into these engines know about some of these possible nuances between the older 250, the 300, and the '06 and later 250's? Particularly about valve location/position, valve size, and where the valve reliefs are in the older 250 piston.

 

94 KLX250R 'D' series and 'E' series bikes made 32 HP at 8500 rpm, although among other things the advance 40º from 3000 rpm may have something to do with the power!

 

It appears that both the 250cc and 300cc bikes were available from 1993 in Japan so from an engineering point of view, using the same head for both and adjusting the piston to suit (you'll need a different part anyway due to the difference in cc) is the logical step.

Of note. It's only this year for the first time that KTM have tuned their airbox to suit each individual model of bike and engine size (different box for each bike.) Just goes to show that corners are cut even with competition bikes, and there is still room for the little guy to get a little more from the bike they buy.

 

Thanks for the additional 250/300 info. On that 32hp figure, however, is that really possible as a stock hp for this engine? It just sounds too high for this engine in stock trim. It probably has to at least be at the flywheel.