Dyno tuning

[WTF304]

You sure your I.D and O.D isn't mixed up ?

[scottly]

The needle has an OD of .071", at the point in the taper where the needle meets the ID of the needle jet, with the slide fully open.

[scottly]

Wide open throttle tests on the dyno with the drilled needle jet showed an improvement in the air/fuel mixture from 3200 RPM up, but now the bike is waay too rich at lower speeds and small throttle openings. Next up is a Mikuni 36mm carb that was fitted to the same XR500 that the WBX-1 cam came from.

[scottly]

I put the bike with the 36mm Mikuni on the dyno today, and found that the air/fuel was actually rich at small throttle openings, when I thought it was lean. The carb has a #35 pilot, while others have reported using a #25. The #220 main jet appears to be very close to perfect.
Just because you can snap the throttle wide open at 2700 RPM, it doesn't mean you should, as the motor can't take that much carb until about 4000 where the Mikuni starts overtaking the stock carb. It's interesting that the HP basically flat-lines from about 5600 RPM to about 6850, while the stock carb started dropping off.
Just for grins, I'm posting two versions from the same two dyno runs; the first is the stock carb in blue compared with the Mik in red, plotted against RPM, and the second is plotted against MPH. As usual, tests are run in 4th gear, WFO.

[WTF304]

Nice ... that mik has an accelerator pump?  .... this is good stuff to know.

[scottly]

After swapping the #25 pilot in, the hesitation was even worse! At 1/8 throttle, the bike ran poorly, even in neutral, and the spark plug was sooty. I happen to have a VM 32 on hand, and comparing the two carbs I found there was a brass washer missing on the VM 36 between the main jet and the emulsion tube/needle-jet/main jet holder, which may have allowed for fuel to leak past the tube. Sorry, no pics of the washer, as it's now on the carb on the bike, but I've pointed out the shoulder where the OD of the washer would seat, drawing the tube firmly down.

[scottly]

With the missing washer replaced, the bike ran much better, enough to ride it to a bike show last Saturday.
Yesterday I ran it on the dyno with the #25 pilot, and found it to be lean compared to the #35, but the tests with the #35 may have been skewed by the missing washer.
I made 3 runs back-to-back; the differences at the right side of the graphs are due to the rate of opening the throttle. Run 189 in blue was with the throttle opened somewhat rapidly, run 190 in red was with the throttle gradually rolled on, and run 191 in green was with the throttle snapped open as fast as I could. The motor bogged, then recovered, which is why both the HP and torque curves on that run start above zero; it confuses the dyno computer.

[scottly]

Pulled the carb off the bike today to replace the original #35 pilot. In the 2nd pic, with the jet removed, you can see a gap between the bottom of the washer and the tube, which simply dropped from gravity. I suspect that when I took the carb apart last summer to check the settings the washer fell out and I never noticed it.

[scottly]

I tested my new pipe today. The graph shows the new pipe compared with the best previous run:
 

[J6G1Z]

Congratulations you passed the 30HP level.    This illustrates how much a free flowing exhaust can release more power.

Have you noticed any change in MPG, or range on one tank of fuel?

Please keep up the good work & documentation.

J.

[scottly]

I've tested three different exhausts now. The first, that resulted in the 2+ HP increase over the stock pipe had primary tubes 26" long, measured from the sealing surface at the head, around the outside of the bends. The second used the straight lengths from the stock outer pipes to extend the length to 34". The third had 22" pipes. The 22" pipe had a slight advantage over the 26" from about 3700-5100 RPM, while the 26" had a slight advantage from about 5600-6700. The 34" pipe had a noticeable drop from about 4300-5100, but interestingly all three pipes showed the same peak torque.

[scottly]

This is the next pipe up for testing:
(J, you asked about MPG; all I can say is the dyno tests of the 3 previous pipes appear to have consumed over 1 gallon of gas. )
 

[scottly]

After testing a few different configurations of pipes, I finally settled on a design that seemed to be the best compromise of usable power band and weight. As far as being a "tuned" exhaust, I started it up without the muffler to heat up the header paint, and it rattled my ear drums! I did a couple of dyno runs with the stock muffler removed from the stock header, and it was no where near as loud as just blipping the throttle with this pipe. It's even loud when shutting the throttle; it sounded a bit like the scene at the beginning of "The World's Fastest Indian" when Burt wakes up his neighbors.
 

[scottly]

As Chuck has raised the question of pipe size, I thought I should update this thread.
I did try an XR 500 pipe, adapted to the FT, and found it caused a loss of both torque and HP compared to any combination of primary pipe lengths with an ID of 1.270". The XR pipe dyno run is 223, plotted in green.