Cam mod results with 351
#32
07-08-2011, 10:35 PM
Nobrakes:
Yes, I changed the jetting. It was for sure too rich, fuel mileage went down, engine was choking with air box lid on.
Bike always ran better with lid off, but I hate the noise.
Try this: Riding the bike, open the throttle to 3/4, now back it off at a medium speed. Feel the bike. IF it accelerates, more fuel can be added because that is what you are doing.
Air changes velocity faster than fuel, so by backing off on the throttle you are richening the mixture. This tells me to pick up the needle one notch.
Now same thing, except engine stumbles (you will feel it) when backing off on the throttle. Its too rich. The air is slowing down and fuel is not. Engine bogs because its getting too much fuel. Drop the needle one notch.
It works the same for the main jet except you can only mess with it from 3/4 throttle to full and full back to 3/4.
If the bike feels like its only accelerating a little or does not bog at all, then the jetting is really close. You just have to learn to turn the throttle back the right speed and right amount for what you are testing.
I have marks on my throttle for 1/4 and 3/4 at the hand grip so I can see them. 90% of my riding is done in the first 1/4 of the throttle since the 351.
David
Yes, I changed the jetting. It was for sure too rich, fuel mileage went down, engine was choking with air box lid on.
Bike always ran better with lid off, but I hate the noise.
Try this: Riding the bike, open the throttle to 3/4, now back it off at a medium speed. Feel the bike. IF it accelerates, more fuel can be added because that is what you are doing.
Air changes velocity faster than fuel, so by backing off on the throttle you are richening the mixture. This tells me to pick up the needle one notch.
Now same thing, except engine stumbles (you will feel it) when backing off on the throttle. Its too rich. The air is slowing down and fuel is not. Engine bogs because its getting too much fuel. Drop the needle one notch.
It works the same for the main jet except you can only mess with it from 3/4 throttle to full and full back to 3/4.
If the bike feels like its only accelerating a little or does not bog at all, then the jetting is really close. You just have to learn to turn the throttle back the right speed and right amount for what you are testing.
I have marks on my throttle for 1/4 and 3/4 at the hand grip so I can see them. 90% of my riding is done in the first 1/4 of the throttle since the 351.
David
I always wondered, though, on the KLX250 - why such a large difference in performance between it and other "performance" dirt bikes, i.e., KX250F, CR250F, etc. Displacement is the same. You can fit a non-CVK carb to the KLX250S and that doesn't do it. Is it the cam timing? Compression? Intake flow, etc? Probably all of the above, but I'm just thinking that M's mod might close the gap a little bit.
BTW, as far as I know, no one is deleting posts. If you're seeing posts disappear please report it. I know I have not removed anything. There may be a problem, though, just let us know if you make a post and it disappears or something. None of the mods here are in the business of deleting honest discussion.
#33
07-08-2011, 11:14 PM
Nobrakes, good question on the power output of the KLX. While the engine is a watercooled, DOHC, 4-valve engine, it is the same 80's technology in the KLR600/650 vein and not in the vein of a KX250F and others. The KLX design was of a design that was overbuilt and understressed. It was never designed as a race platform. The 2-strokes in that era were the race bikes, and while many raced XR250's and KLX250/300's, they were not intended to be the serious contenders...and they weren't except in the hands of some exceptional riders.
Engines like the KX250F were somewhat the result of emissions as racing 2-strokes have come more and more into conflict with past, current, and future regulations. In light of having to make very competitive 4-stroke racing engines, they are designed and built way more on the edge of what is possible and what is reasonable for the public to buy and use for racing. 2-stroke power was cheap and easy. Real 4-stroke power is not. More fragile valve trains, serious weight saving measures in the engine, high rpm requirements, and other issues make them poor candidates for real dual sporting. So, the older but solid designs like the KLX live on. Even the highly vaunted WR250R is of a similar design and relatively overbuilt and understressed with not much more OEM power than the KLX. After all, the WR is basically a slice off of an '07 Yamaha R1 street bike. And as old as the KLX design is, even among the technological possibilities of the Japanese big 4, it and the Yamaha are really the only somewhat modern 250 DS bikes IMO. The only reason the KLR650 still exists is that it was way ahead of anything else in its category in 1984 and is surprisingly still ahead of the other main Japanese DS bikes in the engine design. If no one is really pushing you from behind with much more technology to take your market, why should a manufacturer push the envelope?
Durability, relatively long service intervals, good fuel mileage, and similar elements just aren't part of the KX250F-style motorcycle. What we got is what we got, and it is what it is.
Engines like the KX250F were somewhat the result of emissions as racing 2-strokes have come more and more into conflict with past, current, and future regulations. In light of having to make very competitive 4-stroke racing engines, they are designed and built way more on the edge of what is possible and what is reasonable for the public to buy and use for racing. 2-stroke power was cheap and easy. Real 4-stroke power is not. More fragile valve trains, serious weight saving measures in the engine, high rpm requirements, and other issues make them poor candidates for real dual sporting. So, the older but solid designs like the KLX live on. Even the highly vaunted WR250R is of a similar design and relatively overbuilt and understressed with not much more OEM power than the KLX. After all, the WR is basically a slice off of an '07 Yamaha R1 street bike. And as old as the KLX design is, even among the technological possibilities of the Japanese big 4, it and the Yamaha are really the only somewhat modern 250 DS bikes IMO. The only reason the KLR650 still exists is that it was way ahead of anything else in its category in 1984 and is surprisingly still ahead of the other main Japanese DS bikes in the engine design. If no one is really pushing you from behind with much more technology to take your market, why should a manufacturer push the envelope?
Durability, relatively long service intervals, good fuel mileage, and similar elements just aren't part of the KX250F-style motorcycle. What we got is what we got, and it is what it is.
#36
07-09-2011, 01:14 AM
Join Date: Sep 2010
Location: Western NY
Posts: 695
Rev limiters and wheelies are wonderful things.
David
Here are my racing numbers. Font is comic sans.
Its my age!
Last edited by David R; 07-09-2011 at 01:21 AM.
#40
07-09-2011, 02:35 AM
When comparing the KLX's to the KX250F, keep in mind the operating range that each operates in. The KX250F goes well above 12,000 rpm and the KLX's top out around 9,000 rpm. Horsepower is proportional to rpm if the torque remains constant. That alone would account for over 30% increase out of the KX250F.
The Dynojet dyno run puts the max output of the KX250F near 32hp.
http://www.dynojet.com/pdf/2113.pdf
Ride on
Brewster
The Dynojet dyno run puts the max output of the KX250F near 32hp.
http://www.dynojet.com/pdf/2113.pdf
Ride on
Brewster