modifying a boyer ignition for less total advance

dorset said:

interesting. i'd never heard of the firebox device. this old T120 has a 750 kit, some minor port work with OS intake valves, a mild cam with some extra lift and duration, the twin plug head, and 34mm VM mikunis. i've tuned the carbs with an AF meter and they're spot-on at idle and slide cutaway. i settled on 30 BTDC advance at an airstrip using timed runs, testing both ways at 2-degree increments to settle on 30. the problem is just at off-idle which otherwise could have been the slide cutaway.

it's possible the 34mm carbs are a bit big, although the head was ported for them. but i know the idle advance is certainly low, so i'm interested in correcting one thing at a time. i might go back to 32s if that doesn't fix it.

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I'm curious as to what the current trend in spot on idle AFR is for the motorcycling community?

I think a lot of people set their toys up too lean. Todays pump gas has a different stoichiometric value than it did 45 years go when a lot of documentation about tuning was written. Just blabbing. I know nothing.

chaztuna said:

If the issue is just off idle, check the idle feed hole under the slide. There are two holes, one just downstream from the slide is for closed throttle idle. The one under the slide is for off idle. See page 3 of the link below. They call it the "bypass"

http://www.vintagesnow.com/Carburetors_files/vmmanual.pdf

Make sure that passageway is not clogged

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no, the carbs are spotless and working well.this softness is not a bog, just a slightly vague throttle response.

but now you've made me curious about the carburation. i have a pair of 32mm round slide mikunis waiting for me to finish a track bike project, and i'm wondering whether the hesitation would go away with th ee smaller venturi. it wouldn't solve the advance issue, but it might cover it up.

chaztuna said:

If the issue is just off idle, check the idle feed hole under the slide. There are two holes, one just downstream from the slide is for closed throttle idle. The one under the slide is for off idle. See page 3 of the link below. They call it the "bypass"

http://www.vintagesnow.com/Carburetors_files/vmmanual.pdf

Make sure that passageway is not clogged

Click to expand...

Schwany said:

I'm curious as to what the current trend in spot on idle AFR is for the motorcycling community?

I think a lot of people set their toys up too lean. Todays pump gas has a different stoichiometric value than it did 45 years go when a lot of documentation about tuning was written. Just blabbing. I know nothing.

Click to expand...

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.

OP : no need for 'scientific and conceptual principles' or a blackboard.
Get a 50 cent resistor and see if it works.
Shouldn't take long..

A resistor will certainly lower the input impedance of the circuit. I had done this 20 years ago.
This will in effect reduce the size of the negative sine wave trigger with effect being to reduce the advance per RPM.

amazon tells them at ten for about US$3. in the mail today

dorset said:

no, the carbs are spotless and working well.this softness is not a bog, just a slightly vague throttle response.

but now you've made me curious about the carburation. i have a pair of 32mm round slide mikunis waiting for me to finish a track bike project, and i'm wondering whether the hesitation would go away with th ee smaller venturi. it wouldn't solve the advance issue, but it might cover it up.

Click to expand...

I'm just a babbling seat of the pants tuning hobbyist, but what the heck. Here's some more babble:

Vague (hesitant?) throttle response is usually not enough fuel (lean tip in), unless the exhaust note indicates a rich bog. Smaller carburetors would probably fix your issue at the expense of some top end. Not really an issue with a street bike. Arguably a Norton or Triumph motor built for racing should be able to handle twin 34mm carburetion without too much difficulty. Big carburetors do require closer attention to gear selection to keep the motor happy. Corner exists are going to suck if in too tall a gear.

Ludwig has a point. Give the resistor a shot.

BTW, I like the airstrip tuning method you use. I'd like to have one near me I could use. No load tuning in my driveway doesn't work that well.

dorset said:

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. snipped

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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:

Schwany said:

BTW, I like the airstrip tuning method you use. I'd like to have one near me I could use. No load tuning in my driveway doesn't work that well.

Click to expand...

i'm super lucky. its a private paved strip 2500 feet long with almost no users, owned by a recluse with a 750 ducati in his living room. i can hit 90 mph before i run out of stopping room. i set someone up alongside with a stopwatch and do 800-foot timed runs, average of three.

Arguably a Norton or Triumph motor built for racing should be able to handle twin 34mm carburetion without too much difficulty.

Click to expand...

i have a 650 land speed machine that takes 35mm FCRs. but it's heavily biased for top end.

jseng1 said:

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.

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no shit. old j schmidt gave nobody else any credit for that.

resistors on the way. can't seem to find any two-watt pieces in 680 ohms, though, but i got 1-ohm at 1%. seven bucks for 100 of em

straight from the horse's mouth by today's email:

Thank you for your inquiry,

This is a valid technique to give a reduction in the electronic advance range, useful for twin-plugging and racing engines.
Reducing the resistor value produces further reductions in advance range, 680 Ohms could be tried initially, 470 Ohms could also be tried.
Resistor Values down to 220 Ohms can produce a 12 Degree reduction in advance range, much lower resistances can make starting the engine difficult as the signal becomes too weak.

Kind Regards,
Boyer bransden.

I have run the resistor accross the boyer terminals of my Norton race motor for 3 seasons before moving on to a crank mounted system , During that time I had no problems with the motor in both 850 and 920 guise . If you fit the resistor you can see the reduction in advance using a strobe , so you need to retime accordingly, a word of warning though the smaller wattage resistors are very fragile on a lumpy motor such as the norton so I potted the resistor with epoxy and soldered on flexible tails to the connectors , a broken wire will advance the timing with disasterous consequences . As to the improvements , it did to me feel better coming out of corner s , but I do set the tickover at a min of 2000 rpm . ign timing set to 26 deg BTDC ( set on a dyno) , 37mm Keihin carbs and 84 RWHP.
Easy to do , easy to undo ,probably not worth it on a road bike.

jseng1 said:

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:

Click to expand...

Good show on the old Norton, It was exciting to watch! CH

dorset said:

i'm super lucky. its a private paved strip 2500 feet long with almost no users, owned by a recluse with a 750 ducati in his living room. i can hit 90 mph before i run out of stopping room. i set someone up alongside with a stopwatch and do 800-foot timed runs, average of three.

Click to expand...

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

Dorset - that info is so old there's no way I could possibly remember who to give credit to - for all I remember his name was Mr Boyer.

I think he said smaller watt resistors would work but the heavier ones were readily available and looked more reliable.

jseng1 said:

Dorset - that info is so old there's no way I could possibly remember who to give credit to - for all I remember his name was Mr Boyer.

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More likely to have been Ernie Bransden. Boyers (Boyer of Bromley) were the original marketing side of Boyer-Bransden so it's really a Bransden ignition.
"In 1969 Ernie Bransden designed and patented a miniature electronic ignition; one that was a fraction of the size and far more efficient than any of its competitors....

...The first commercially produced Boyer-Bransden electronic ignition was launched in 1969 for Boyer of Bromley, a then famous motorcycle dealership whose Triumph race team were experiencing trouble with the points on their race bikes..."

Fast Eddie said:

Why?

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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.

dorset said:

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.

Click to expand...

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.
The advance curve in a programmable ignition system is mainly determined by the rod length and the stroke. The combustion event is fixed time, but as the revs rise, the rockover time reduces, so you need more advance. Most advance curves are probably determined by trial and error. Calculating the optimum curve is difficult, if not impossible, With my bike, I accept the advance curve in the Boyer and jet to it. But I also adjust the static advance to suit the fuel octane rating and comp. ratio..

http://atlanticgreen.com/boyerexposed.htm


"Boyer Exposed
By David Comeau October 1989


My first Commando, bought new in July, 1970, was an unmitigated disaster. In September, 1987 I bought my second Norton Commando, a '72 Combat, and to insure some useful life for this motorcycle, I have been studying everything I can get my hands on. I've learned quite a lot in nineteen years, part of which is patience and skepticism, and part of which is the subject matter for this writing, that being ignition characteristics, primarily those of the Boyer.

The general consensus is that the factory Lucas centrifugal advance points system is bad, and the Boyer / Lucas Rita systems are absolutely great. I want to know why, for both cases, but I will sometimes refuse to accept "parroted" endorsements due to lack of factual substance. An example of this is the lack of any advance curve specifications for both the Rita or Boyer.

I understand that early 750 Commandos, Combats and low compression 850's with a Boyer installed will run down the road under their own power. I also accept the contention that the Boyer is likely the best 'consumer' product as of this writing. However, having had a substantial amount of ignition curve tuning experience to know of the power and driveability improvements that can be made with proper attention to detail, and what a pig a motor can be in the timing is too retarded, simple agreement with the concept that one advance curve fits all is too much 'Black Magic' from the 'Black Box' for me to accept."


Already on the job 19 years before this Oct 1989 article! If anyone had information back then ,,,no one was telling so I went on regardless.