The Kawasaki KT250 trials bike



My KT250 project

I think it was in 1999 that I was consumed by the sudden desire to have a running trials bike after seeing a KT250 advertised here in the SF Bay area. I rushed over, took a brief test ride, and bought the bike.

This was one of those purchases that looked a LOT better than it actually was. The chassis was in fair shape (straight rims, etc) but I could see a lot of rust in the tank.

I flushed the tank, and rust just kept coming out (and being visible inside), so I decided I should cut the tank open and do a good job of derusting. After I ground down the welds and separated the top and bottom I attacked it with a putty knife and was peeling big sheets of rust off. This didn't seem to be doing much good, so I put the tank in the blaster and had at it. Soon I started getting little pinholes in the tank, where basically all that was keeping the gas in was the rust and external paint.

A new tank was in order. I couldn't find an aftermarket plastic tank that was guaranteed to fit, so I built an aluminum tank for the bike, using the stock tank as a rough model for the new one.

I'd traded the stock front fender and mounts to my friend Julian Farnham, so I also made some new fender stays and mounted an Acerbis front fender. Below you'll see links to some photos of my bike and Julian's bike.

Modifications

As I mentioned above the squish clearances were pretty wide (about .020" too much IIRC). Since I didn't want to increase compression I welded up that area and made a fixture that let me hold the head in the lathe and recut the squish band. I had to grind out the chamber a bit to offset the extra metal in the squish band. After this I retarded the timing - generally running it as retarded as the stock slots allowed.

I had a 1974 Bultaco Sherpa T 350 that I'd bought new, and loved the grunty heavy-flywheel engine in it. My friend Jim Connely has a Montesa 348 that runs similarly, and Jerry Erickson loaned me his very nice 348 to ride in one trial. I wanted to try and make the Kawasaki run as much like the Spanish bikes as possible.

I borrowed a Montesa Cota 349 cylinder sleeve and piston from Jared Bates at SW Montesa and measured them and did drawing. The Kawasaki manual gives port timings, and I also got port timings for some Hodakas, a TY250 Yamaha and an early 250 Bultaco Sherpa T to compare. Louis Gattman also told me the timing he'd modified a customers KT to. The KT looks quite short on exhaust duration compared to most of the others, so I raised the exhaust (keeping the stock curve in the top of the port) about 4mm to bring it into the 168-170 degree duration range.

TRIALS PORTING

KT250 349 Cota 250 Bultaco Hodaka 100 Hodaka 125 TY250 Gattman KT250
Exhaust duration: 152 175 156 164.5 162 173 170
Transfer duration: 106 140 127.6 128ish 128 107.7 115
Intake duration: 136 135 122.2 131.5 139 reed 140


I had Scott Chapman, one of the local 2-line KT250 riders, try my bike at a trial with the squish head, retarded timing, raised exhaust port and Wiseco piston (a little shorter intake skirt so a bit more intake duration than stock). He told me that he thought my bike hung on a bit better at the very bottom of the rev range, and had better midrange punch. And this was with my bike running stock gearing, while his bike is geared lower than stock (but maybe he compensated for that by trying a higher gear).

After Scott rode the bike I built a new exhaust for it, with the hopes that a longer head pipe would enhance the low-end power and a new silencer would quiet it down. All the KT250s I've heard seem to have a lot of clanging and banging going on in the empty central portion of the exhaust. I replaced that with an accessory absorption silencer. I had a friend ride it who said that he thought the pipe did improve things, but the main thing I can notice is that it is a little quieter with the extra silencer. Here are the photos I took as I was building the exhaust - so if you've ever wondered how you might go about doing a project like this these photos should give you a good idea.

The stock exhaust is a 1.75" OD header pipe going into a sort of flattened expansion chamber, and thence to a secondary muffler/spark arrestor mounted behind the right hand rear damper.

With the way the Kawasaki ran the two downtubes I had to drop the pipe down and then to the side to get enough clearance with the front fender at full bump. I then brought it back up and over to the cylinder center line before heading to the back edge of the cylinder. I had serious thoughts about removing the bottom frame rails, squeezing the stock downtubes closer together and farther back towards the engine, but restrained myself. I decided that if I was going to do that I'd be better off just building a whole new frame from scratch, and doing it all the way I'd want it to be.

In Smith and Morrison's "Scientific Design of Exhaust and Intake Systems" (third edition) I found a design for a mechanical baffle silencer on page 129 that looked like it wouldn't be too hard to build. I wanted to avoid fiberglass packing as it either blows out or gets saturated with two-stroke spooge, and either way has to be repacked periodically.

The muffler was about 10" of the off-cuts of the 1.75" U-bends welded together. I then cut a two .250" wide slots 180 degrees apart down most of the length of the 10" tube. I then welded two half cones over the slots, so that the cone got bigger the closer it got to the front of the tube. The cone stopped about .25" from the front bulkhead. The far end of the tube was welded to a bulkhead that was solid in the middle where the tube was welded, but had some holes/slots at the top and bottom. After this was a short chamber, about 1-2" long, with another bulkhead with a 1" ID hole in the middle of it. That hole had a 1" OD tube leading back to the secondary muffler. The bulkheads were approx 3.5" tall by 2.5" wide - similar to many of the modern aftermarket silencers. I then wrapped some 20 or 22g sheet steel around the assy to close it off, and welded everything up.

This sees the exhaust coming into the tube, forced to reverse at the far end, exiting through the slots into the angled cone sections, reversing again at the front bulkhead and expanding as it goes to the second bulkhead, contracting a bit while going through the second bulkhead into the small chamber, and then out through the 1" tube.

M&S say that the slots with the cone cause the wave going in one direction to interfere with the wave going the other way, and the reversals and small chamber also help to break up the sound waves.

Sadly, after a day's work that muffler seemed no quieter than the stock pipe. In fact, the thin steel skin would "ring", making the noise even more objectionable. I could damp that ringing by grabbing the silencer with my hand and squeezing on the sides. I suppose I could have done something similar with some welded in cross tubes, but since the rest of the noise didn't seem much different from stock, I decided not to bother.

Bummer.

Plan B was to take a S. Miller Bultaco silencer that I'd bought ( it was a modern style, not the nice 70s fabricated aluminum unit with beading etc) and welded it on in place of the silencer I'd made.

The bike was definitely quieter as far as the clanging/banging in the exhaust, so that was counted as a plus for the project.

I rode in a trial on the Saturday after finishing it and frankly I couldn't tell a lot of difference. Since I'd ridden a trial just two weeks before I'd hoped that I'd have a good enough memory for the power delivery but it appears I just am not terribly sensitive (as a rider).

I did get my friend Craig (he's riding his TL250 on the 2 line now, while I'm still floundering on the 3 line) to take it out for about 20 minutes after the event. He'd ridden it briefly at the last trial, and came back and said he thought it was noticeably improved on the low end power, and felt it was a worthwhile modification.

I guess I was hoping that the exhaust would transform the KT's power int that of a 350 Bultaco or 348 Montesa, which may have been a bit overly optimistic of me.

This should give people a good idea of what goes into building a pipe from U-bends. It really isn't too hard - you just have to cut and file a bit, and have a good selection of various radii bends. The only part that was made on a lathe was the ring that goes into the exhaust port on the end of the head pipe. I hope you find this reasonably instructive and interesting.

I should mention that before I rode the bike with the new exhaust I extended the slots in the ignition stator, making them about twice as long, all in the retarded direction. I ran the bike at the trial with the ignition timing pretty much midway between TDC and the stock timing mark. I'm going to try it with more advance - roughly back to where it was when it was fully retarded in the stock slots.

I'd like to try some more flywheel on the bike, though my friend Craig thinks it has plenty. But I do like the heavy flywheel feel, and it can help compensate for ham-fisted throttle action in a section. Unfortunately Kawasaki didn't leave much room in the magneto cover. I can put a thin (about .200" thick) plate on the end face of the rotor, but any kind of ring on the OD of the rotor is going to require a new cover. Some bolt-on flywheel weights were sold "in the period". They were made of brass, which is heavier than steel, but a chunk of steel is a lot cheaper to come by so I'll probably just do that. If I like it and want more flywheel I can see about making a brass flywheel weight.

I've also been talking with John Laurent (see below for the link to his website) and former Team Green trials rider Curt Comer about big bore kits. The Tryals Shoppe sold a 300cc kit back in the 1970s (John has a copy of an article about this on his website that I sent him), and Curt said their factory bikes had a different big bore kit in them. Since there seem to be no shortage of riders who can outride me in a section on a gutless TL125 Honda the extra power is probably not going to make me an ace rider - but it might be a fun project. I've been looking at some different prospects for donor pistons, but haven't yet arrived at one that can be used without having ring locating pins moved, etc.

Here are some photos I took at an AHRMA/PITS trial at Frank Raines Park. My KT250 is the one with the aluminum tank. I know of 5 other KT250s besides mine in the greater Northern California area.

Here is a photo of the left side of the KT250 without the bodywork

Here is a photo of the right side of the KT250 without the bodywork


16 March 2008

In the past week, after a several year hiatus, I've resumed working on the KT250.

I pulled the wheels a few years ago to put on a set of new tires and found that not only were the stock steel spokes/nipples heavily corroded (stream sections at Frank Raines Park) but the front wheel was cracked. I was told by a friend who has a wheel shop that he thought that DID had tried hardening the rims a little more in the mid 1970s to keep them from denting, but got them too hard so they lost ductility and cracked instead.

A few days ago I finished measuring the rim offsets (which are only approximates since the rims weren't terribly straight and sent the bare hubs down to Buchanan's to have Excel rims and stainless spokes/aluminum nipples made. I'm also going to a WM3 (2.15") rear rim instead of the stock WM2 (1.85") as that is what the new trials bikes are running. There are a number of trials bikes using rim screws into the tire bead to retain the tire instead of rim locks, which saves unsprung weight and makes tire changing easier, so I'll probably go with that.

The next item was to start looking at the frame again. The other thing I'd noticed was that the under-engine frame rails were nearly touching the engine -- again. They are also heavily dented so I decided to remove them and substitute an aluminum skid plate. This was a common DIY modification in the late 1970s and many manufacturers also did this.

When I pulled the rear dampers I noticed that the swing arm would barely move unless I gave it a very strong push. When I removed it I found several problems. The center tube in the frame is .060"/1.5mm narrower than the space between the rubber silentbloc bushings. When the pivot spindle is tightened up the frame is drawn together, pushing the center bushing of the silentblocs in until they take up that clearance on the center tube. This removes the offset of the center bush on the outside of the swing arm and means the face of the swing arm rubs on the frame and wears as pictured below. I made a couple of small press tools to try and remove the silentblocs but they wouldn't budge with the vise, and I don't have a press. I'll probably just use a hole saw to cut the rubber out and then a small grinding point to cut through the outer case of the silentblocs. Bronze bushings will then be fitted.

Early August, 2008

The new pivot is completed. Externally it doesn't look much different from stock so there's no photo at this point.

14 August 2008

Now that the swing arm pivot is completed, the next task is to move the pegs down. There doesn't seem to be much reason to have them any higher than is needed to clear the bottom of the skidpan. I cut the pegs off with a hacksaw, cleaned them up, and made some new adapter plates out of 1/4" steel. I wanted to ensure I had plenty of weld area with the new plates. After fitting I set the frame on the welding table with a 1/2" rod in the swing arm pivot so I could level it with a Wixey digital angle gauge. This let me use squares to set up the adaptors and pegs. I welded both adapter plates fully on to give me as much access to the joints as I could get before sticking the footpegs in the way. Then I added the footpegs on their bits of original plate. The pegs are in the same position relative to the front axle (which is pretty much the same as on my Bultaco 325 Sherpa T) and are 1.5" lower than stock.


Links to KT250 sites

The Vintage KX/KT Website


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© 1996-2008 Michael Moore, last update for this page 14 August 2008

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