Chapter 19 - Small Tuning Woes
> Brandon Weiss (Detective brandon to you) Wrote:
> Holy moly. I've never heard a more clearly defined pipe hit than that.
> It sounded like your dropped a gear!
> Absolutely nuts, can't wait to see some onboard footage, with a gps
> please! (To help understand the acceleration you're feeling)
Ha, you're almost in luck! I was planning to make a detailed post about this anyway, so you get the long description before the vid, sorry! It's still rendering anyway.
The gearing is set to top out around 45-50mph. Acceleration was adequate for 50cc, and the test run was ever-so-slightly uphill, not much wind. Pulled hard and then hit a wall at 42mph after about 12 seconds, and started four-stroking/misfiring. This made me suspect main jet was too big, since it should have a few more MPH in it (80 in a PHBG 19, set for my 6000' elevation - stock for the KTM setup is 85).
Since this was just a quick test run, I didn't take the time to swap jets - a wise choice, as I had to climb a ~10% grade, 500' hill to get back home. It was no problem cruising up at 35mph, and it had a little more acceleration left, but the speed limit is 25mph, so I just went fast enough to lock the clutch.
But my assumption of being too rich got a challenge here: things got a little hot on the ride up, about 230F water temp - so maybe a little lean? Pulled the plug, this hill is like a mile long, so should have left a good jetting indicator - it came out almost perfectly clean, almost no carbon deposits at all... huh.
I had also swapped in the Tomar clutch, set at about 10k, because I was struggling to get the jetting in the ballpark with the stock clutch at ~8000rpm on some quick acceleration tests in from the of house a couple days before. It acted like it wanted more fuel at the 8k clutch slip, but then it started 4-stroking at lower-than-normal speed once I revved into the power band. So I figured I would just get it to run on the pipe only with the higher clutch stall and have an easier time jetting it.
I've really been scratching my head hard, though, especially after that long hill climb on the way back from shooting this video. One last symptom: kill button wouldn't stop the bike from idling, until I went WOT while holding the button and it would finally die.
Turns out, there may be a simple answer to all this. KTM used a BR8ECM plug as stock (which I didn't realize at first), and when I got the motor it had a B8HS in there, so I threw in the same before the video and hill climb (and for that initial test run in the previous vid). Some of you probably already see the issue, but I didn't have the head off with the plug installed until I realized the issue. Oops!
Here's how the B8HS looks in the head:
...versus the BR8ECM:
I'm pretty sure I did the same stupid thing on my Peugeot a while back, and had the same annoying tuning headaches. Finally realized I had an "H" instead of an "E," and things got instantly better after swapping. So, hopefully the same thing happens here, and I get better performance. I'm kinda certain I must have been preigniting, too, because of the mostly ineffective nature of the kill button... might have had some carbon buildup on the end of the plug hole that was lighting off at idle?