I would recommend an aftermarket header....larger diameter than stock. I had a 4Stroke Works muffler on for awhile. Too damn loud!. Went back to stock muffler with end cap removed. Very small reduction in power and meets the sound level requirement for use on public land. That's on a KLX300.

Instead of trying to wring out the last 1/4 HP, put your $$ and effort into the handling, unless all you want to do is go in a straight line on pavement.

Ride on
Brewster

 

The complete 300 system, uncorked, on the 250 will allow you to build up to 22HP/16lbft TRQ - or same as a stock header+slipon .. Add the FSW KLX300R header to the stock 300 muffler for a potential of 23+ HP..

This is a HUGE gain from stock (16HP/11.5lbft TRQ)..

All numbers above are SAE corrected @ rear wheel..

 

Found a calculator for head pipe length on-line. For the KLX250 at 8600 rpm redline the ideal was 1-3/8"ID and 21" long. 10,000 rpm was a shade bigger, I think maybe 1-7/16"ID and 19" long. That first set rang true with the aftermarket head pipe I have and is close to some specs I got on a stepped pipe from another rider. The stock pipe is probably more like 1-1/4 ID or less.

Do not for a minute think that bigger is better. If it was then tuning would just be to slap a big tube in there without need for any calculation. For proof just look at car headers. Big ID primary head pipes on big displacement lower reving engines, shorter length too, small ID longer tubes on small high rev engines. It's all about proper flow, not plain old open flow.

So if a rider sourced some good mandrel bent (not that muffler shop machine bent crap) tubing a rider could make a better flowing head pipe. It is usually available in various radius 180 degree bends with about 8-10" of straight tubing at each end, 90s and 45s can be had too. We built a nice high pipe for my SR500 some years back. Looked good enough to elicit a comment on a card from a Kerker/Supertrapp rep.

All a rider would need to do is follow the general pattern of the stock pipe... and either be able to weld or know someone who can.

 

Header design/optimization is pure science now days. It's calculations involve the fields of fluid dynamics and acoustical dynamics.. Fluid dynamics control the timing of the high and low pressure waves of the gas pulses and acoustics allow timing control of the reflected acoustic waves.. Back in the slide-rule days, we knew only diameters, lengths, and collectors (collector configuration can allow sharing of low pressure waves on multi-cylinders and proper reflection of acoustics.) We are not so limited now days in our abilities. Don't worry if this seems too complex - I have the advantage of a larger brain case..

We are better now, you can't judge a modern header/system using the "slide-rules" of long ago -

 

I already have a 2 bros M7 with stock header, but it's still too loud IMHO. Plus it backfires (cracks) on deceleration like a ****! Trying to quiet it down and kill the cracks as much as possible...wondering if a 300 header/pipe would help....

 

Of course doing all the tricks is science, as is doing the math to develop the straight diameter header to the proper ID and length. Not all science is high level. In addition even the straight head pipe deals with the acoustics and fluid flow - always has. The more black art of expansion chambers gets even deeper into the acoustical wave patterns, as one finds out when they go to build an expansion chamber for their two stroke. Gordon Jennings presented the whole thing well in his Two Stroke Tuner's Handbook. Of course those were simple chambers a rider could cut, roll, and weld. The factories were able to play with flipper cones and other unique features to squeeze a bit more from them. Thing is those simple three cone chambers were very effective and gave the biggest bang for the buck over stock pipes, the others made smaller steps in gain. Fact is the two stroke drag and flat track pipes, which aren't restricted by having to "fit in" tight confines, were extremely close to that simple design described by Jennings.

Now that we "flexed our science muscles"... The basic straight diameter tube head pipe is easily calculated and built by anyone with the specs, the tubing, the hacksaw, and the welder. The results will be significant compared to the undersized KLX pipe. Which is why I posted what I did. Now they have the correct numbers. Anyone can make the pipe in their garage.

If a rider wants all the tricks with chambers leave it to the companies who can afford the research with everything in sight having time and money to do so to eek out the nth bit of power. But if you simply want to make a big, but lower cost step better than the stock KLX250 or 300 pipe, the specs are there and easily done.

Summit Racing stainless tubing bends, 1-3/8" ID 2-1/2" bend radius 18 ga. is $9.97, other sizes range. Check it out.

Of course sometimes those trick pipes don't seem to add as much as one would expect. Remember, sometimes the simplest first step is the one that gains the highest percentage gain, the rest just adds a bit more from there.

 

As usual, you are a wealth of information...but unfortunately I am not able (yet) to weld and bend, so am just wondering if the stock 300 header/pipe off a 97 klx300 would help out my backfires, and quiet it down a bit... I'm not terribly interested in squeaking out the last 1/32nd bit of power out of it, right now it is an absolute weapon, point and shoot.. Loosing 1hp I doubt I'd even notice the difference...

 

No Dash8, your backfire problem has nothing to do with your header or slipon. I assume your air injection system is removed and properly capped, and your exhaust system is tight and leak free from the head seal to the midpipe/muffler joint. If so then your fueling during deceleration is incorrect/flawed. Fixing this with the CVK, is as simple as making sure the idle is set to 1300 rpm and then richening the idle mixture with the fuel screw.

 

Ya, I realize the pipe has nothing to do with the backfires, but also looking to make it quieter.

I've already had the exhaust off and resealed everything (I hope lol). I'll go thru the air system and double check again. Maybe the plate on the cylinder is not seated probably.

Other than that, I have a Tm36, so time to start looking at jets again it runs well, just the backfires are annoying

 

My brother picked up a bit more power installing the full KLX300 system. A bit louder, probably would block out a bit of the popping through the baffled, not straight thru, muffler, but not eliminate it. He had a turn down tip for the end of the 300 muffler, welded up using the same diameter tubing as the larger outlet. to go in place of the small diameter tip. He lucked out finding the full system for a good price on ebay.

The popping through the exhaust happens on my 650 and is simply the bit of leanness in carburetion at idle on the carb. All I have to do to stop it is to simply lightly crack the throttle and it stops.

I don't know for sure that it can even be jetted out since it isn't dealing with the actual necessary jetting for idling, but rather a higher rpm of the engine drawing a mix that isn't really meant to exist at that rpm - saying the mix drawn in at the higher rpml is too lean to fire properly, but if jetted to stop it the idle may be too rich. You end up with popping through the exhaust on the overrun. Lets face it, an engine spinning 3000 rpm isn't going to get proper fuel mix with the throttle closed. It may get mix too lean to ignite properly, when it builds up a bit you get a random pop.

I don't get too worried about it. As long as it isn't the extreme loud bangs, but rather just a light popping or crackle sound you don't really have any problem. You hear it from virtually every bike and car with carbs and even some with EFI, running more open exhausts. The stock ones baffle the pop to a light ping sound.

As for my exhaust construction tip, go make friends with TIG welding skills. I really think the KLX250 exhaust would be easy using only one or two of those bend. The flange can be cut from some flat stock and we made the step up in size by cutting a small ring of the tubing, splitting it and welding it on. I made one mistake when we did the pipe for the 500, we were test fitting without the gasket in it - I had to grind 1/4" off at the head to get the pipe to fit. Don't make that mistake! I was fortunate to have that situation when we welded up the SR500 pipe and the chamber for the MB5. Maybe after I retire I will buy a TIG welder to play...