modifying a boyer ignition for less total advance

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hey folks

i recently bought a copy of jim schmidt's Norton Race Manual to see what it would say about my 70 commando, and discovered this intriguing bit about shortening the spark curve on boyer ignitions:

modifying a boyer ignition for less total advance


now this is useful to me not because i'm going to try it on the norton, but because i have a triumph with a problem it might solve. that machine has good breathing, a 9.5 to 1 compression, and a dual-spark plug cylinder head. i've tested it extensively with a stopwatch, and the best fully advanced timing is at 30 BTDC, down 8 degrees from triumph's standard setting of 38 BTDC. since i run a boyer, retarding the high rpm advance means i also retard the low rpm advance by the same amount, and so my triumph is a bit soft on the bottom end.

schmidt suggests that i can run a resistor between the two wires leading from the stator to the ignition module, which he says will retard the total advance available from the unit. then when you re-time the full advance, the result is that the low speed setting is advanced by the amount that you have shortened the total curve.

i haven't tried this yet. has anybody done it here? if you did, what were the results?
 
This has always been my complaint about e-ignition systems that do not duplicate the OEM points/AAU advance curve. The engine will lose some low/mid grunt. Advance should be all in by 3000 RPM.
 
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....paging @dynodave what say ye?
First 10 words are wrong...
Ever tried to unscramble an egg and put it back in the shell then incubate it to make a chicken?
I could tell you the truth about the conceptual and verbal incorrectness of Jim's words but "GOD" worship makes me the bad guy.
Therefore the scientific and conceptual principles seem irrelevant and an uphill battle.
I would need students with at least 2 brain cells, a clean slate and IN PERSON probably a full hour in front of a black board.
I hate to type and would require too much effort.....for what?
 
It was of interest to me, as I am reasonably sure my main bearing failure was caused by too much advance
 
I always thought coming out of turns sluggish means somebody isn't keeping the motor wound up tight enough going through the turns and may need to adjust their riding style some.

Using a Tri-Spark Firebox Pro or similar would be a better option for modifying the advance curve.
 
well mine will kick back very very occaisionally on 31 , it did once begin to kick back quite a lot so I strobed it and got reading of 34or maybe 36 degrees. If yours never kicks back, then I would assume it was less than 31 and unlikely to be a cause of main bearing failure.
 
on my triumph the twin plugs prevent any detonation, even running at the stock 38 advance. no kickback, no problems. at 30 btdc, it runs happily on 87 octane.

i've emailed ernie, so we'll see what he might have to say. i will report back.
 
Be interesting to see... my inclination with an extensively modified engine would be to set itup on a dyno for max power and then pull the stator back a tad to be on the safe side
 
well mine will kick back very very occaisionally on 31 , it did once begin to kick back quite a lot so I strobed it and got reading of 34or maybe 36 degrees. If yours never kicks back, then I would assume it was less than 31 and unlikely to be a cause of main bearing failure.
I believe the issue is that Boyers will advance further than the set advance figure at higher revs. So it can be ok at kicking speed (thus no kick back) but too advanced at higher engine rpm, IIRC.
 
yes this was true of the analogue not sure about the digital... a quick blip upto 5and half or more would not indicate a still advancing advance curve..The official recomendation was I think for only 3.5 or 4 and that certainly might cause over advance on full shout ,but it would stillkick back
 
Combustion conditions rely on four factors which are in balance - comp.ratio, fuel octane ratio, ignition timing and jetting. Leaning-off the jetting usually has the same effect as increasing the ignition advance. You should probably be able to get enough adjustment to get good low range power by changing the taper on the needles. I don't use Amal needles - only Mikuni, even in Amal carbs.
 
I always thought coming out of turns sluggish means somebody isn't keeping the motor wound up tight enough going through the turns and may need to adjust their riding style some.

Using a Tri-Spark Firebox Pro or similar would be a better option for modifying the advance curve.

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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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.
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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