I'm curious as to what the current trend in spot on idle AFR is for the motorcycling community?
ive seen AF ratios vary between 17;1 and 14:1 at idle based on where in the pipe the O2 sensor was screwed in. mostly i set idle mixtures with a colortune rather than look for a chemically ideal AF number. i think my AF meter is precise but not accurate, so i use a stopwatch to find the best mixture and then tune the other circuits to whatever number is on the meter.
I'm just a babbling seat of the pants tuning hobbyist, but what the heck. Here's some more babble:
The reason oxygen sensor aka air/fuel ratio sensor position matters, is because the exhaust is not a steady stream, but a series of pulses. Between pulses, there are areas of low pressure. During the time periods when the exhaust valve is closed, atmospheric air can be pulled into the exhaust. The closer the sensor is to the outlet [or a crack in the pipe] the more outside air will be sucked in. The further from the exhaust valve that you install the sensor, the greater this effect and the greater the error in the sensor reading.
That information for shortening the advance curve CAME DIRECTLY FROM BOYER. I called them way back when I was racing complaining that with total advance set for best power - the low RPMs were sluggish coming out of the turns. They gave me the solution and it worked great.
Personal note - somehow I got the phone # of the person who originally invented and marketed the Boyer ignition. At first he was upset that I called him at home, then when he learned what I was doing he opened up and spilled the beans.
Dyno dave - Because of the wide advance of the Boyer ignition - setting high compresion race motors to the best HP of 28 degrees at redline means that its much too retarded at lower RPM - thats what I mean when I say "Boyer ignitions have a built in retard for low RPM".
Credit to Ken Canaga for telling me back then that Norton racebikes ran best with fixed advance (adjustable curves weren't available then). The bike with the modified Boyer is the monoshock shown below:
And on the track against monoshock ducs with the modified ignition:
It would be great to have that length of runway, and all the tools, jets, and so on right there to address the tune and retest immediately. Sweet
Good question Nigel - but one that has since been answered in the thread. Boyers were as you recollect a Triumph dealer , and when I fitted mine - one of the first ones , there was little or no mention of Norton in the instructions and I did think at the time it would be unlikely that the same advance curve would suit bth engines . But then it was xmas morning 1972 and I had other things on my mind. There was certainly no mention of 31 degrees at 6,000 rpm.
I am an industrial chemist. I would never set up the jetting on my bike by calculating from theoretical air-fuel ratios. Octane ratios of fuels depend on the amounts of branched and straight chain hydrocarbons. It is possible to reverse-calibrate an oxygen meter on a dyno, but you'd need to be a genius to do it from theory. The air-fuel ratios are irrelevant, what is important is the performance of the motor. If you are monitoring AF ratios on a dyno, the figure you get while running at constant revs probaly does not give you a basis to predict what you will get n terms of throttle response as you open the throttle. As you open the throttle the tapers on the needles move to compensate for the loss of vacuum. If you use a quicker taper, you are usually able to open the throttle quicker. But most guys do not need that.
