BB is $500 if you do the work yourself. With a 15t countershaft sprocket it has legs, great on the street.

 

[QUOTE=Klxster;516258]With 351BB and rejet and exhaust, you've spent $1000.00 or more. Plus you've replaced perfectly good components - cylinder, piston assembly etc../QUOTE]

i tend to agree with that. if i have my klx long enough to wear out the motor i definitely will try the big bore. i have 7,000 miles on it so i cant see swapping the cylinder just yet.

i have a 13t countershaft sprocket that i take on and off as my mood changes. my bike does have a higher top speed with the stock 14t. i know some folks have better top speed with the 13t.

i asked about the exhaust because i've had friends who upgraded exhausts on vehicles and got more hp but only at the upper rpm range and they lost hp and torque down low. thats fine if you need to run flat out but i don't. i didn't want to upgrade to an slip on or full fmf and loose my low end.

im not an engineer but back pressure somehow helps with low end. i had it happen when i straight piped my 92 dodge cummins. i lost low end grunt. welded a good muffler back in and i had my low rpm torque back and some good engine braking.

someone needs to make a muffler where you can tune the back pressure. supertrap?

 

I and some others are working on a recipe for getting the most HP and TRQ from the stock bore. The loss of low end your refering to is less a function of adding backpressure with a restrictive muffler than it is a correct sizing of the head pipe from port to muffler. I expected the Megabomb to mitigate the loss of low end ( the megabomb+PC4 have a larger I.D. head/midpipe than stock header+DG) but it did not. See it for yourself on my shoot-out thread.. Combined with the MCM, the stock header+DG slip-on and a 12-12.8:1 AFR kick major a$$ in the TRQ department...

 

No amount of "backpressure" is good. You don't need any. You need a properly designed exhaust capable of the appropriate exhaust flow velocity.

Supertrapps are garbage.

 

I have an FMF Powercore 4 on my bike, and it feels like it has a lot more top end, plus more bottom-end grunt, than with the stock muffler. I do almost all tight single-track woods riding, where I need the low end grunt/torque more than top-end HP, and really like the setup I have (FMF PC4 with a home-made db-killer inserted, Dynojet kit, snorkel removed, and 13/47 gearing.) With the stock muffler, it just felt neutered...

 

that statement about back pressure made me do a bit of research. i guess i have been misinformed thanks for helping me get my head out of the sand. i had the concept slightly screwed up. so I'm looking for a properly designed exhaust. not just a huge pipe.

 

again, thanks for getting me on the right track with regards to back pressure. i'll look i to the mods you suggested.

 

RS Motorcycles has a calculator to figure exhaust head pipe length I found on-line. When I ran the numbers for the 8200 rpm red line it came out to be around 1-1/2" ID 21" long head pipe to the muffler. 10,000 came out to 1-3/8" ID and around 19" head pipe to muffler. I didn't take the head pipe off to measure to the valve, guesstimating around 3". I forget where I got the cam values, maybe some from the Marcelino thread. It's been a few months since doing so.

Kind of demonstrated that a smaller head pipe complements the lower range power.

It checks out to an extent with the aftermarket head pipe I have in the garage and another from which I have measurements, it's about 1-3/8 and I think it was the right length. I have a reverse cone megaphone Pro Circuit slip on to go with it and a reverse cone megaphone is the trick for a single cylinder.

That said, I'm thinking the KLX300 pipe and muffler would be a good one to use IF the head pipe is actually different from the 250 for sure. I think the Marcelino cam re-timing showed some boost in the mid range on his dyno runs.

The Marcelino modification is easier than it sounds in the thread. When I did it I did a simple documentation of the steps minus the science and rationalization since it clearly helps the 250. It is in a Google Drive document linked in my signature. That document also mentions disabling the compression release, which is not needed with the electric start 250.

Here are the most common jetting mods that are published on-line by another rider:


I've done these modifications they work good.
You will smile.

16009-1912 Kawi Needle-Jet, N1TC
92037-1401 Kawi Clamp, Jet Needle
92143-1667 Kawi Collar
Needle clip slot 2 down from the top
92063-1069 Kawi Main Jet #125 or 92063-1074 Kawi Main Jet #128
92064-1108 pilot jet #40 Air screw at 2 turns out to start
drill the slide vent to 7/64ths

Airbox snorkel (14073-1577) high flow unit

open up the crank case vent hole in the airbox

10-4313-K250 big gun evo x exhaust system


air valve removal ( Air Injection System ): This won't do squat for performance but it will lower exhaust temp significantly and cleans up the under-tank area.
just plug: the hole in the airbox,
the vacuum port on the carb,
and the intake to the reed valve on the front left side of the head
( I took it off and made a cover to replace the reed and cover )

Here are the most common I have read about.

Muzzy full exhaust or you can drill a hole in the stock exhaust; there are several posts on how to do that.
#125 main jet (Size depends on several factors, see posting on jetting)
pre-'96 KLX300 needle (N1TC)
pilot air screw backed out to 2.5 turns
Slide vent drilled to 7/64th
Airbox crankcase vent restriction removed
KDX200 snorkel (hi-flow)
13t counter shaft sprocket (14t is stock)

The links below show the part#'s, how to remove the carb, how to install the carb parts, and Kawasaki web site for diagrams of carb, ect.

http://www.hmfengineering.com/forums...ead.php?t=1617
https://www.kawasakiforums.com/m_17800/tm.htm
https://www.kawasakiforums.com/m_34306/tm.htm
http://www.kawasaki.com/Default.asp?...D=2&intParts=1

Both my brother and I have done these mods plus a Dial-A-Jet to smooth out the transitions. Works great all around and not too costly. He's running a 300 pipe and I'm still running stock. The bike pulls fairly decently for a 250.

 

Monkey Wrench, If/when you do the MCM, I wouldn't even pull the exhaust cam out - just take the spring off the ACR ( which totally deactivates it ) and retime the cams... No one has reported ill effects on the starter from a deactivated ACR... THis will make the MCM much easier to perform...

 

thanks. i'll probably wait untill my next valve check. i think ill need to swap shims by then so i'll do the mcm at the same time.