Ignition Timing With Modern Fuels

ignition timing is one of those very interesting subjects where theory is inferior to testing.

in my opinion.

the fuel in your intake charge requires a finite time to burn. because of geometric considerations of rod length/crank angle and other stuff, the optimum point for maximum pressure of motors like we're running is going to be somewhere in the vicinity of 14 ATDC. there are really interesting reasons why this varies a little, but its close to that.

fuels have different chemistries. they burn at different rates, they have different inherent resistances to detonation, the mixture that you introduce has an effect on cylinder temperature and pressure, and your compression ratio, camshaft configuration, piston crown profile, elevation, ambient air density, temperature, and rpm all affect this.

what is certain is that if you can, you want to select a firing point that will achieve maximum pressure at 14 ATDC, which means somewhere around 34 to 38 BTDC, for an engine that will not detonate. at that tuning. your motor will be most efficient. if the fuel has a low octane rating, like all of ours do these days, or if the engine has very good breathing or a high compression ratio, or some other special condition, you will have to retard the spark some from the original recommendation.

here's the thing. im assuming youre running original-style pistons and compression ratio? if so, 37 degrees (stock) will be as far advanced as your motor willl likely tolerate, and it might detonate at that figure with modern lower-octane fuel. you will have to test to find out, but its easy.

make sure youre running new spark plugs with the recommended heat range. i dont know what plugs yuor machine uses, but later riumphs used champion N3. you can substitute an NGK B9ES for that. set the valves, make sure youre running correct carburation, and put in fuel of the kind you plan on using. set your ignition timing to the stock 37 degrees, take the machine out and warm it up.

ride he bike, and from a mid range rpm -- NOT LOW RPM-- open the throttle fully to load the engine, and listen. a triumph experiencing hard detonation will knock, loudly. it will sound like a little demon is hammering on the inside of the combustion chamber, bcause there is one. it will destroy your engine if you run it that way for long

a correctly-tuned motor will always detonate if you try to ride up a hill at lugging rpm with the throttle wide open. dont do that. downshift on hills instead.

if you hear the sharp rattle of detonation, reset your ignition time several degrees retarded, maybe 35. repeat. that might be enough. if you dont hear detonation, and you might not, then the 37 is okay. i dont know what your CR is, but the lower the CR, the less sensitive the motor is to detonation.

when you do this listen thing, pull over and take out your plugs. look at the top of the ceramic insulator with a hand lens in good light. if you see little gray balls of metal on the ceramic, then youre looking at pieces of aluminum that was supposed to be still on your piston. even if you didnt hear detonation over your exhaust, the little pieces of metal are an indicator that your spark is too early. retard it.

because 1951 is long gone, there is no right answer that you can read from a book. if you dont have time to do any testing, set your timing at 34 BTDC and call it good. make sure you check for overheating, but that should be okay. if you get good fuel and use only that, 37 might be fine forever. if you ride only at high elevations, you might use 38, but any more than that will likely run you into diminishing returns.

this is all eay to do. you just have to set aside a few hours on a weekend. i do all my ignition timing with a stop watch, measured times over a set distance, with an assistant. i do carburation the same way. i dont use a book, i just give the engine what it wants. you can too.

 

thats a plan. 30 BTDC is way retarded unless youre running high compression and a weird ignition. stock, you should advance it.

i try never to sell a bike. ive only sold two-- one was a 1976 honda 360 i wish i still had, and the other was a 74 trident i sold to someone who wanted to fix it after i blew it up.

anyway, on spark, there's two ways to do it. you can keep advancing spark until you get no improvement and stop there, like youve done with your BSA, or you can test with a stopwatch.

in my opinion, the stopwatch method is the best one. find a place out in the sticks somewhere with a straight ection of road, uphill is better than flat.

mark out about 1000 feet of road, some place where an observer can see the ends clearly. i am very lucky beause i have a 2500-foot airstrip available to me to tune on.

place your observer at the end and then go back and do timed runs from the start point to the end. i start the run at the beginning at exactly 30 mph, and drop my arm to signal my kid to start the stopwatch. then i run WFO to the end mark and he clicks the watch. repeat three times, average.

doing this i get a time, maybe 10 or 12 seconds, for whatever the distance is that day. then i change the timing and do it again.

the fastest speed over the measured distance is the best possible ignition timing for that motor, that fuel, and that day. period, end. make sure you dont have any detonation while youre doing it. if you hear the knocking, then youre limited by fuel and have to march to that drummer rather than maximum power. retard the spark until you dont hear detonation and live with that setting.

i cheat, because i drill my cylinder headsout for twin plugs, and that allows the motor to run harder without detonation. on my 1972 T120, i run 9.5 to 1 on 87 octane at 30 BTDC and 118 mph in the standing mile. i have n LSR machine that i run at 11.75 to 1, also on 30 BTDC, at 131 or so in the standing mile, but i run 112 octane fuel in it.

 

you didnt say it was a tribsa. thats a nice looking machine.

im out of my depth on lucas magnetos i run a fairbanks morse in an ARD on one mschine, but thats different.

have you condidered a morris/joe hunt magneto to replace the lucas? you can buy them new and parts are easy

 

On my 2 stroke BSA Bantam....I had to go from 16.5deg BTC to 15BTC to get the engine acceptable when running on 5% ethanol.

Not used it with E10 but I am expecting to have to drop down further.

This ethanol fuel burns at a different speed be it 5% or 10% so I believe.

All a total PITA.

 

if you havent read cameron's stuff you should

https://www.cycleworld.com/story/bl...linder-bore-stroke-affect-engine-performance/

on anything. he's been wrtiting for some 30 years that i know about and is an interesting man

 

I've had this article by Kevin Cameron bookmarked for ages. I refer to it every so often to remind me about what an engine is doing.

As the ethanol content is causing the engine to run leaner it is getting hotter. It's no help just looking at the spark plugs any more (as the Haynes manuals' pictures) so I re-read this and re-learn.

https://vintagebikemagazine.com/technical-articles/engines-and-heat-by-kevin-cameron/