good read-not sure if its been posted up on the forum before
http://www.e-dirt.co.za/forum/forum_posts.asp?TID=27063We'd heard about those Baja bikes hitting 115 mph or more, while some experts have flatly stated that no production motocross dirt bike will run over a hundred per. Even with the tallest possible gearing to be had.
Well sir, the arrival of a 495 KTM and a genuine maniac Six Days rider appeared to us to be the perfect combination. Rod Bush is the Service Manager of KTM, U.S.A., and has a pretty good track record to back him up. Three golds, a silver and a bronze. He's also the eleventh-ranked enduro rider in the country.
A 16-tooth countershaft sprocket was delayed from Austria, so we had to go with a 15-fang version. Internal changes were needed to make up for this. A 20-tooth fifth gear from a 390 enduro bike was slipped in. Primary drive was 31/79.The gearing at the rear was limited to 48 teeth. Not enough for what we had in mind. A YZ250G rear wheel was found to be a virtual slip-fit, just by making some simple spacers. This let us drop to a 44-tooth rear sprocket, which is just about worth one more tooth on the countershaft.
To play it safe, we also installed a gigantic 150.80 x 18 Metzeler, two-ply, nylon tire. This massive tire is almost 29 inches tall and noticeably dwarfs a 4.50 x 18 Metz. A larger tire is much like gearing a bike up. Rod felt that he could get some extra rpm out of the engine by changing the timing.
The deck height was changed to 1.4mm, from a normal height of .8 to .9mm. Intake and exhaust manifolds were carefully matched, but no porting was done. Carburetion was left standard. Ron ran premium gas with a 50:1 Duralube mix. Klotz Octane Booster was added to get rid of any possible detonation at the high engine speeds we'd be running at.
After the bike was thoroughly checked out and broken in, we headed out for famed El Mirage dry lake, home of many high-speed crazies. The lake was in reasonable condition, with a bit of mud and water at the edges. We had enough room for our needs. Two markers were set up and Rod took some initial passes. We checked all three stop watches and double-checked the distances.
Our first dozen passes had us in the 108 to 110-mph range. Rod leaned out the main jet in the 40mm Bing carb and slipped into a set of Danaise road racing leathers and a full coverage helmet with a shield. He also felt he could get a greater speed with more distance available before the traps. Because of the limitations of the wet part of the lake, Rod had to try to get the bike up to a high speed on a bumpy fire road. We estimated his speed at near 90 before he got on the smooth lake bed. It worked! On his next pass Rod went over 120 mph!
More runs and a semi lay-down position got the speed even higher. Our best run of the day was 123.75 miles per hour! .............................
