Just been looking into a bit of power gain with a new exhaust and jet for my 09 250s. Bit b of information around but confusing. Is it alright to just get muffler and tune up or do you need new headers to match. Fmf seem to be a popular choice. With what kind of header. Any info would be appreciated.
Thanks clint

 

Do the Bill Blue 351cc thing and Jet the carb. Most bang for the buck[s].

 

Turns out to be pretty simple. But first - A "muffler" or slip-on is actually a midpipe and a silencer. If you choose a FMF slip-on, you can install it with the stock header or a FMF header (Megabomb or Powerbomb). Not all slip-ons can do that as the FMF headers, as well as all aftermarket headers that I have found or ever seen, are a larger diameter than the stock header. For instance, my DG-R slip-on will not fit into my Megabomb header correctly, while it does fit into the stock header.

A slip-on + stock header is easier to setup the carb for - and once it's set up correctly power levels will be noticeably increased from off idle to a little over 9K rpm. My latest member report is that you will loose about 6 mpg running my "max power" carb setup for stock header + slip-on. (75 to 69 mpg) EDIT: A newer report shows practically no decrease in MPG.

A full FMF or other larger-than-stock complete system is more difficult to "jet"(setup) for if you want the most power the system can make. Once correctly fueled, power levels will be noticeably increased from off idle to redline(10,000+ RPM). Power above 6.5K (plus or minus) will be much stronger than with a slip-on + stock header. Top speed will also increase as the bike will pull higher RPM in the top gears.

BTW, I believe that the only way to achieve all the power that a full system can make
is to run without the airbox lid. So if lidless is not an option for you....

Any performance exhaust item will also help make power with a big bore kit - so the money isn't wasted.

 

Agreed to an extent. It is expensive, but if I had the money I'd do it over anything after jetting the bike in and opening up the intake into the air box.

Over the years for the money, exhausts have usually produced the smallest difference of the three things to do - jetting, exhaust, big bore. In fact usually on in-line fours the exahaust would add a few peak horsepower, but usually lose some in the middle. Of course that made the bike feel good when it hit the power burst, masking the true effects. Plus they were easy to do... especially if you ignored jetting as so many in-line four riders did. Lots of blued head pipes.

The problem is no magazines do any sort of real side-by-side testing anymore. If that was done, a rider would learn a lot more about what happens with exhausts. Most of what I've learned has been from a combination of decades old motorcycle magazines (no work done like that anymore), automotive magazines (they do) and what individuals have experienced (including me).

In this case all I can say is from calculations I made with an on-line exhaust calculator, the 250 should have about a 21" long 1.375" ID head pipe to the can for the power peak it is built to run. Power bomb style set ups are a science probably beyond my understanding, although I keep wondering if they're based on some of the two stroke expansion chamber design. But unless there's a sudden change of path in the media, that will never come to light.

Suffice it to say after this wordy essay:

• The intake is stuffed up, the OEM snorkel likely has less than 1/4 the opening area of the hole it is plugged into.. On the SF either pulling the snorkel or, if you can deal with intake roar, removing the lid opens up breathing significantly.
• The jetting is stupid lean even with the stock snorkel, so do the KLX300 jetting from the known mods using Kiehin parts or a Dyno Jet kit.
• The muffler might be high performance for a moped, but with that pea shooter end it is as bad as the OEM snorkel.
• The KLX250 head pipe is way small, but keep in mind and try to learn what a pipe will do to your power. Gaining 2 horsepower while narrowing the power delivery a significant amount isn't worth it.

If you haven't done the first two things, do them first. They cost less than $100 if you can do the work yourself. At 900 feet elevation the KLX300 jetting is virtually a plug and play. It is linked in my signature. It was knowledge generated in 2006-2007 by other KLX250 riders and can be found on virtually any site dealing with the KLX250. I just documented and linked to it.

If they are done, do the exhaust or even just a can. Unfortunately so many are so loud. Do some research, somewhere on this forum there was a rider who got a good sound level from an aftermarket slip fit.

 

Thanks heaps for replies and all that info guys snorkel is already off and just with the jetting I'm In Australia is the jetting for the klx300 meant for Au model or are Au models set up different

 

I had the local shop do my 351 install. My 09 KLX had the FMF Q4 system on it[and jetted] when I got the bike. Seems like the 351 kit is around $500.00 or so about the same as the BB carb or the full FMF. Guy I ride with has a stock 09 with the BB 351 and the carb jetted it seems to run fine. I vote 351[with jetted carb, duh] first. Then a full FMF PB Q4.

 

Look in the KLX250 Cheap Mods link in my signature, it details the jetting with Kawasaki part numbers. Numbers will be the same regardless of country. It may or may not be same as the Australian model, my bet is they are.