I just got an FMF Q4 for my bike and of course am excited to put it on. I'm planning on getting a header too, but not sure when (some other things are more important at the moment). Am I able to go ahead and install the Q4 now and the header later? Or will that cause issues? Also installing a jet kit when I get the header, or sooner, haven't decided yet. Any info you guys can give me is appreciated.

 

I'de suggest jetting immediately, and picking up an aftermarket filter if possible.

Someone else may want to chime in, but I'm pretty sure the increased air flow from the muffler would make your jetting a bit lean?

I have the K&N filter, and a DynoJet kit. Order a couple of cheap jets online or ask the local dealer if they have any. The header is not nearly as important as these two things IMO

 

I think the jetting should be done even on a stock bike. They just run better all around with the proper jetting.

 

According to FMF, you don't "need" to rejet with just the slip on. It's important to follow the instruction and use high temp sealant at the header/exhaust joint and let it set for 24-hours before firing-up the bike. After 24-hours snug it up a little bit more and you should be good to go. You want to avoid an air leak, which is why you need to seal it properly.

 

Elevation is the biggest factor. Seal Level, Re-jet from the start.

There is no benefit in an intake/exhaust mod if you don't "Need" to re-jet.

 

With needing to use the silicone, I'm thinking it'd be best to wait and install the whole exhaust at once, or not get the header.
For those of you using the stock header and aftermarket slip-on, do you like that setup? Does it work well? I know and FMF header would be better than stock, but if it doesn't make that big of a difference, maybe I'll just leave the stock one on it.

 

There are varying opinions on the performance advantages of an after-market header. Unless I stumble across a killer deal on a header I'm sticking with the OEM one.

 

Here's an older article but it still applies after all this time... It goes over the lazy throttle response and the small diameter header. I'm sure since it's the internet we are all way faster than Larry roeseler that no one will think anything of this lol

Stroker's KLX 300 Stage 1 Mods

 

I got the powercore 4 and the instructions said no need to rejet so I put it on first and found that to be the case. Bike was much improved from stock by unplugging it alone. Next came airbox mods and jets as intake mods really do mess with jetting. Again an improvement was noted. I later added a powerbomb header and it was a small but noticeable improvement with no real effect on jetting that I could tell. I did the mods one at a time so I could see just what did what and this was my experience. All the changes were made around the time the bike would be considered broke in between 500 and 1000 miles. has around 8000 or so on it now and runs the same or better now as when the changes were done.

 

I think you should rejet as soon as you put the slip-on on the bike. I would recommend adding a high flow filter (uni is my choice) at the same time and do the jetting once. You should feel a noticeable difference with just those mods, regarding throttle response and power. You may be able to get away with adding a header later without jetting again.