John I saved your string that was on two stroke world for the street taco, in fact have it printed and have used it for reference,’love the husky head on it, but the Rotax head is very intriguing. Is that the 160mph bike? Would love to hear more...
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They're actually one and the same bike. When I finally "get this land speed thing out my system" - my wifes words, not mine - it'll go back into street trim.
I'd kinda underestimated the potential of the air cooled engine and thought it lacked the horsepower to set records. I think I was wrong about that but was getting frustrated with the engines limitations. That's when the other engine project started in earnest. What I wanted was something with a more modern port layout than the old Pursang engine, and with a better bore:stroke ratio too. I chose the Rotax cylinder not because it has more potential than anything else - it doesn't - but because the cylinder mounted reed housing made it easier to adapt to the Bultaco crankcases. I could've just used a complete modern engine but I wanted something that was still fundamentally a Bultaco that looked like a Bultaco.
The Rotax uses a 72mm stroke, while the longest stroke Bultaco used was 64mm, and even these are a tight fit. I had to basically cut the walls of the crank house out and weld in some machined rings to make room for the crank, and of course these were positioned right where the original case studs were, so these were repositioned too. The crank itself was machined from a KX250 unit and uses a long rod from a LT500 and a Husky piston. Mallory slugs were pressed in for balancing. The cases were grouted for strength.
Aluminum deck plates were welded to the top of the cases to allow the Rotax cylinder to be bolted down. The port layout was determined with EngMod2T and while the porting is relatively mild it's about as far as I can go without welding. It was designed from the start to run methanol so needs no radiator, just a couple of small water tanks with an electric pump and thermostats to get it warmed up quickly and prevent the head from getting too hot.
The pipe was based on EngMod2T as well, the original max speed being 9800, this being raised later to 10500 and then 11000 when the engine proved to run smoother than expected. Many happy hours were spent on the dyno, not just getting the numbers out of it but also making sure it was able to withstand prolonged running at full load and RPMs. To hear it run like this on the dyno for a minute or so continuously sounds cruel and unmerciful but that's what has to be done. Depending on the weather and the tune power is between 70 and 80hp. With a bike as light as the little Bul it accelerates quickly, even with the LSR gearing.
The fairing chosen was an old Peel Mountain Mile design and seems to work OK. The first couple of years were a bit of a flop really, though records were set. It just didn't make the power on the lake that it did on the dyno and I had no idea why. Eventually it dawned on me that the fairing - which was almost sealed at the front due to not having a radiator and in an attempt to minimise drag - was creating a low pressure area at the carb inlet. It'd pull very strongly off the line but as it picked up speed it got progressively weaker and weaker. When I finally woke up to what was happening I cut an air intake slot in the front of the fairing and ran ducting from there back to the carb. This made a massive difference, and it was a huge relief to see the bike finally show it's potential at its last run. The fading power with speed was gone, but wheelspin became an issue even at 120+.
I'm pretty pleased with how it's turned out so far. It took a lot of work to get the clutch up to the task - they're quite anaemic - and so far the gearbox has been fine. I'm still battling some torsional vibration issues but I'm confident these can be overcome. Sounds pretty simple I guess but there were teething problems of course, and wrong trees were barked up. But it's been a fun project.
