Trail Bike Adventure Magazine

Evolution

Sunday, 30 November 2008 23:30

Issue #25 has finally gone so I ducked out to the shed to catch up on some maintenance.

The XR was the only bike there, so it was the lucky one. I slipped on the new pipe and fired 'er up, expecting to have to do a little tweaking with jetting. I was right.

Drive was strong through the mid-range and top-end, but it was hesitating straight off idle. After making sure it'd warmed up, and the problem was consistent, it was off with the tank and seat, out with the slide, and I dropped the needle one clip position.

With everything bolted back together, there was absolutely no difference.

So I pulled the seat and tank and carby slide off again, and dropped the needle another clip position, which meant it was as far as it could go. I bolted it all back together, and tried it again.

Still no different.

I didn't think I was that bad at tuning, but I've made a goose of myself plenty of times before, so off with the seat and and tank again, out with the slide, and this time, in case I had everything totally arse-about, I raised the needle the whole way.

Back together again, there was still no difference.

It was beginning to penetrate even my thick scone that something wasn't normal.

So I decided to put the needle clip back in the middle position and try the air screw.

Off with the seat and tank, out with the slide, slip the clip back in the middle position, and back together it all went. I fired it up, and it was idling away sweetly when I knelt down, screwdriver in hand to find there was no air screw.

There was gaping hole in the side of the carby where the screw should've been, and it was audibly sucking air like a Hoover on steroids.

Hmm...that would explain the problem.

So the pipe is on, but now I have to search around for a mixture screw for a flat-slide Mikuni.

Good on me.

The front wheel bearings also need replacing.

Not much to report this week, because I've been chained to the desk finishing the issue #25, but here we go:
• Longest Day is just around the corner. Yamaha has confirmed the TDM900 has been modified along the lines suggested in the Coast To Corner article and is on its way back here.
• The CRF is on its way back from the Sydney Moto Expo.
• I'm to pick up an RMZ250 tomorrow, and a YZF250 is on its way.
• A KTM690 Enduro is on its way.
• An Husaberg FE450 is on its way.

I don't know how interesting it is, but here's an indication of the evolution of competition exhausts.  They're all FMF pipes for the same bike - my XR - so it shows how times and attitudes have changed.

First up is the motocross pipe I used in the mid-1990s. I don't recall a sound level limit then, and when you look at the pipe I was using, that's a good thing. Here it is...  

As you can see, it's what's called a reverse-cone silencer, and it was designed to have a variable number of discs bolted on. The more discs, the easier for the gasses to escape, and the higher the volume.

I don't recall ever running any discs.

My bike's only a 250, but the open-classers - at that time, that meant mostly 600cc and above - ran pretty much the same set-up, and it was loud.

Imagine fronting up with a pipe like this now?

Next up, below, is the oval-shaped FMF I used for enduro when I stopped racing Thumper Nats.

You can see the discs in place on this one. Without changing the jetting it cost a little top-end horsepower, but offered a little more torque in the lower part of the rev range than the motocross pipe, and it was heaps quieter. I don't remember ever having been tested, but it was designed to comply with the sound levels accepted at the time. I'd guess it's about 106dB.

Finally, here's the 94dB, US forestry-approved, Q2.

It has a nice, fluffly note, and a mesh spark arrester. No discs, and no real way to vary anything. It looks as though the spark arrester can be removed, but that would allow the sound level to creep up.

These are all slip-ons, so the original headers are still in place.

Curiously, the pipes don't seem to vary much in weight.

Looks spiffy, but!

TF