The consequences of power
I haven’t posted much about Black Butte in a while, but it’s been going well. Devin of Motion Left Mopeds recently provided me with 4 prototype pipes to compare with the St-Ripper, which has proven a little too rev-crazy for my build. I haven’t tested it again with the HPI and lightened weights, so maybe I should soon. I’ll avoid posting a photo of them in order to not reveal too many of Devin’s new designs, but they are all pretty different and give me a lot of options.
A reminder of current setup (the important things at least):
- Athena with about 172 degrees of exhaust duration and opened up transfers (not widened)
- Stock reed block with boyesen dual-stage mod reeds (more on these later)
- Stock intake ground out to about 20mm (this will soon be replaced with a new intake with material added, ground out to 21mm+ and a smoother entry into the reed block
- Puch magnum exhaust header to radiator hose intake hose deal
- 21mm PHBG with velocity stack
- Malossi MHR Delta clutch springs, the blue middle strength set (still ridiculously strong)
- Derbi 6 roller variator with 15g of weight
- 1977 house brand BX5 belt
- HPI ignition, set to 2mm BTDC
- Terry Dean Cain edition lightened and re-angled clutch bell, matches the angle of the derbi driven face
- Starter clutch pulled, rope-start thingy attached to HPI rotor
I have been running the prototype pipe with the largest chamber, and it’s a real winner. Lots of torque all over the place, and it revs out nicely to the low 60s. I can get to 50 in about 10 seconds and just cruising around and then throttling on around 30mph is a blast. It’s a real winner and I think it could be finalized and sold as is as a competitor to the MMM Destroyer. Note that I haven’t run the Destroyer in a long time so I am not directly comparing them at the moment. I put about 60 miles on the bike during the Toledo rally and the bike performed perfectly, besides being somewhat tricky to start with the rope starter. The crank spins up incredibly quickly due to the HPI, which is great, as it makes the clutch grab much more firmly and with less slip, which puts power to the wheels more reliably and quickly. It also allows me to cruise around 30 with no big effort, as the clutch can stick at that speed. The torque is great, because I can throttle on to pass slower bikes or just have fun blasting down an open road, or slow down to take a corner and then throttle on through it.
Ryan cooked this rope start up for me. The pull-start setup simply won’t work with the HPI rotor, and I know Julian had been using a similar disc on his bike with good results, so I gave it a try. It does add 5oz of weight to the rotor, but it’s still laughably light compared to the nearly 2lb stock flywheel. I’m still figuring out how to reliably start it; it’s usually a combination of a hard pull and a quick blip of the throttle.
So since coming back from Toledo, I’ve wanted to test out the other 3 prototype pipes, and went down to the shop to do so. One of them seemed similar but not as fast as the pipe I was running, but that isn’t conclusive, as at the end of the night I noticed the freewheel on the rear wheel was rubbing against the pipe and ground a nice big groove into the inside. Slowing me down? Maybe. I pulled the pipe and installed another one, which looked to be designed for higher RPMs. Initial tests seemed interesting, but when riding back to the shop I noticed a strange sound, and when I stopped the bike stalled out and wouldn’t start back up. After checking for blown seals and any obvious leaks, I pulled the frame off the subframe, pulled the carb, and found this:
My bike ate my reed. :( This bike has maybe… 300 miles on it? I guess I blasted too hard… but I can’t be replacing these dual-stagers every few hundred miles, so I’ve ordered the .4mm carbon reeds and will see how those perform. I ran carbons once before on my yellow bike (which is a drastically different setup) and they just didn’t seem right AT ALL, but I’m hoping that they will be a good match for this setup.
Since I had pulled the clutch to check the seal, anyway, I decided to give it a quick sanding to get some fresh pad material against the clutch bell. Check out what strong springs do to a stock clutch:
What you are seeing here is that the arms of the clutch are bending and only the dark parts of the clutch pads are engaging the bell. This isn’t terrible, but I’m interested to see how long this clutch lasts. I should probably get a backup clutch on deck, and remove the black MHR springs from that other clutch I have to avoid premature stress. The real key is to sand down the pads and clean off the inside of the bell every 50-100 miles to avoid slipping. It’s a pain, but it’s the price you pay for running a setup like this. You can honestly do it in under 5 minutes if you are fast, though.
So probably no more pipe tests until this weekend. I’m sending Christian two of the MLM protos for testing on his new build (which I hope he posts about soon), but keeping the torquey pipe and a higher RPM pipe for my own use for now.
Oh, I also opened up my variator to check the weights. After 250-300 miles (estimated) they were beginning to flat spot a bit, but nothing terrible. I ordered 5g Dr. Pulley sliders and tossed 3 of them in there. I also compared the TJT ramp plate to the stock Derbi 6 roller plate. The difference was fairly obvious: the TJT ramp plate features an aggressive flat angled ramp surface that begins in the middle of the plate and continues to the edge. The Derbi plate is a smooth curved surface to the edge. The edges are exactly the same at the end, but I’m not sure if the flat surface would cause the variator to more fully open or not, resulting in a higher final “gear”. It appears to me that the Dr. Pulley sliders would match very well with the shape of the ramps on the TJT ramp plate, while only a small portion of the sliders would be pressing against the Derbi ramp plate. I put the Derbi plate back on, but am going to be testing the TJT plate as soon as the bike is back together and I am able to do some good testing, instead of changing like 4 things at once.
More to come later, of course.
-ShaneB
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