Bowhunter,
I think I know what Jonala is refering to. Over in the Yahoo XR400 group, there was a guy named Gordon Banks who recommended a bunch of simple mods centered around drilling out some holes in the end of the exhaust baffle (the Honda fiches refer to this part as the "diffuser"). Everyone at Yahoo refers to these as "The Gordon Mods". I have included an excerpt from his original post below and it will give you a good indication of where and how to drill the holes. I like his idea of making the mods "tunable" by using set screws. The Honda fiches show the XR250 exhaust system to be very similar to the XR4. My riding buddy did these changes to his 01 XR4 and really likes it - even removed a flat spot in the throttle. My understanding is in the later model XR250/XR400's (98+?), the baffle is welded in so to remove it would require spot-welding a rod to it and banging on it. I'm going to do the drill mod myself. If this text is too long and doesn't post, let me know and I can e-mail the mods to you.
Lance
*** Gordon Bank's Post ***
3. Spark Arrestor (Muffler Insert or Baffle)
The Service Manual does not show or mention any differences
between the different years, but we know that the 96-97
models had a two-piece unit where the spark arrestor and the
end-cap were separate pieces. The 98-00 models have these two
pieces combined into one. To use an aftermarket insert, we
generally have to use the separate end-cap piece from a 96-97
model.
NOTE: After examining the stock muffler and spark arrestor,
and running some flow numbers for the different areas involved
with the numerous plates and baffles (both in the muffler and
in the baffle insert), I came to the conclusion that the
primary restriction to exhaust flow is the small final outlet,
which has an i.d. (inner diameter) of only 20mm. Without removing
the baffle insert from the muffler, examine the exhaust tip.
Notice the actual outlet, which measures 20mm i.d. (0.787").
Around this is a larger 'bright finish' ring which may be to help
prevent the rider from coming into contact with the actual outlet,
which probably runs hotter. Down in between the 20mm outlet and
the bright-finish outer ring, there is room to drill 1/4" holes
into the baffle to provide additional outlets, and thusly additional
exhaust flow area. Holes drilled in this area will be 'inside' the
spark arrestor screen, so the spark arrestor function is maintained.
On my spark arrestor, the area to be drilled is large enough to
accept a 1/4" bit (0.25"), but there's a benefit to using a #2 bit
(0.21") which I'll explain later.
Since the stock 20mm (0.787") outlet provides a flow area of only
0.4862 sq.in., and a 0.21" hole has a flow area of 0.0346 sq. in.,
each 0.21' hole adds 7.1% more flow area. Just three such holes will
increase the exhaust flow area by over 21%!
First I ran the engine with the undrilled baffle, to get a feel for
the stock sound level at idle, and while revving the engine. After
drilling one hole, I could barely hear any difference. After drilling
a second hole, I could hear the difference, but it was slight. The
third hole made a bigger difference, but still not objectionably loud.
The fourth hole took it over my limit, however, adding a definite bark
to the exhaust note. This, I decided, was a bit too loud, so I used a
1/4" (0.25") set-screw to plug the 4th hole. Had I drilled the hole
with a 1/4" drill but, I'd not have be able to plug it with a 1/4"
diameter set screw. This effectively reverted back to having just
three holes, and it also indicates that I can plug anyr or all of the
holes with 1/4" set screws to return to the stock sound level if
necessary. I guess you could say that this modification is "tunable"
by inserting or removing set-screws from numerous holes.
A brief test ride with stock jetting and the stock air box snorkel
in place showed that the added three holes made the bike run a bit
crisper, but it was also running too lean, which made the engine
overheat. This more-or-less proved that the added 21.3% flow area
was beneficial, and it had cost me nothing but a little time, and
it's totally reversible by plugging the holes with set-screws.
It is NOT necessary to remove the spark arrestor insert when
drilling the holes. The metal chips will fall either outside the
muffler, or into the screened area of the spark arrestor. Once you
have drilled the desired number of holes, you can then remove the
insert and shake out the tiny pieces if you so desire. If you don't,
they will eventually fly out the exhaust outlet anyway, since the
spark arrestor screen prevents them from falling down inside the
main muffler chamber.
Later, with the drilled spark arrestor in place, the air box snorkel
removed, and the header inlets ground out to 0.944" instead of the
0.75" left by the welding flash, I rejetted the carb, eventually
coming to the same setup recommmended by CYCLE NEWS, a 160 main jet
and a #60 pilot jet. I later switched to a home-made #59 pilot jet
(a smaller pilot jet drilled out to be larger than a #58, but not
as large as a #60). Still later, I lowered my float level to 13.5mm,
and limited the total float travel to 19mm, to imrprove throttle
response in rough terrain.
Now I'd like to offer a few thoughts about aftermarket pipes ...
Gordon Banks
Huntsville, AL
2000 XR400R