It's been brought up before that the stock exhaust and stock 300 exhaust headpipes have a gloppy interior weld where they meet the head. I didn't think much of it until yesterday when out of curiosity I pulled the header off and peaked inside. The weld is a continuous bead around the inside and was in excess of 1/8th tall all around, thats over 1/4" in diameter restriction!! I spent some time with an air grinder and dremel and knocked it down to less than 1/16" and tapered the leading edge of the bead so it wasn't just a sharp lip. Blew the filings out of it and remounted it.

Results:
Low end unaffected

Mid and high end more responsive. Able to pop the front tire over water holes in 3rd and 4th gear, unable to do so before grinding.

Smoother motor above 7500 rpm.

Noise level seems unaffected as well.

In the dirt its more noticeable coming out of corners, 3rd and 4th gear will break the tire loose coming off the berms. Down the straights coming up through the gears it will keep the TM2 rear spinning through the top of 4th.

On the pavement the only real noticeable difference is the lower vibes at high rpm and it revs more freely up to redline.


Overall I'd say it probably wouldn't show that much difference on a dyno but it is a difference you can feel and use. I was talking to a buddy of mine who is into the 80's turbo Buick's and he said that this mod is very common for those cars, they call it "porting the header".

 

right on bro. a free mod is always good.[sm=smiley20.gif]

 

Ive read about this mod, and was under the impression that an external weld was neccessary (vibration, and durability?)??? Id do this IMMEDIATELY if its otherwise??

 

You can't weld the pipe externally or the flange wouldn't be able to pull the header tight to the head, or it wouldn't pull evenly. Notice I didn't grind the weld completely off, I only thinned it out, besides if it is a proper weld the metal beneath the globs should be fused together anyway, that extra crap on top is not where the main joint strength is. Its not like soldering or brazing where the joint strength is based on the filler material.

 

Good point, does it look like its got good penetration from the factory???

 

I also am under the distinct impression that the only differences between the KLX 300 header and the 250 header are length (250 is longer (mo better IMO), and that immediately before the muffler the 300 header steps up, correct?

 

The overal headpipe (ie from head to just before the expansion on the can) I think is the same so no its not like a difference between long and shorty headers in a car/truck. The mounting location between the can end and headpipe is more toward the rear of the bike on the 300 so yes that part is physically longer. The mounting flange on the 300 header is a larger diameter than the 250, so much so that I couldn't use the 250 header clamp without totally bending it out of shape. By step up thats what you mean right, an increase in diameter?

 

I got the call from the dealer tonight, HMF slip on is in (YEAH!!), BUT.........all the carb parts arent (still removed the entire exhaust tonight!!). Gonna do all the mods at once over the next couple of days (Ive got kids, I wont get one block to do it all at once!!), carb rejet (n1tc needle, 128 main, and 40 pilot, drill the slide), uni filter, crankcase breather mod, ditch the snorkel and air injection (smog), install the slip on, and tommorrow Im working on the stock pipe, there are twohuge welds in it, one at the flange, easy as it seems to take care of (and yes, as stated above, if its properly welded, itll hold, cause Im going to smooth that baby out!!), and another before the stock gasket insert, and its HUGE, but its like 2 plus inches inside the pipe, so Ill be on a hunt fora long die grinder bit to smooth that baby out!! I see the issues with the stock pipe, but I dont think they will be that hard to take care of! All of this with about 100 miles on it!!!!

 

A die grinder is easier on the arms but I'm of the opinion that a good round or half round file will actually take the material off faster. My grinders both were plugging with steel. A sanding drum on my dremel actually took the material off better. So yeah, I'd look for a medium coarse half round file, about 1" across and you should be pretty good, slightly narrower maybe.