Ignition Timing With Modern Fuels

Discussion in 'Vintage Classics' started by Wheelnut, Apr 2, 2022.

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  1. Wheelnut

    Wheelnut Well-Known Member

    Oct 12, 2019
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    About to reset the ignition timing on my 1951 Tiger 100 engine and it occurred to me that the handbook recommended 37 degrees advance may not be appropriate for the crap fuel we are forced to fill up with these days. Ethanol free Shell Ultimate isn't available up north, so the so-called protection grade 5% ethanol premium is the best option.

    Anyone tried more/less advance to cope with modern fuel? Apparently ethanol burns slower so that would imply more advance than standard. On the other hand, I've heard that these old 4-stroke engines prefer a couple of degrees less advance with modern fuel. All of which has left me somewhat confused.

    What's the accepted wisdom here?
     
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  2. speedrattle

    speedrattle Senior Member

    Feb 19, 2021
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    #2 speedrattle, Apr 2, 2022
    Last edited: Apr 2, 2022
    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.
     
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  3. Wheelnut

    Wheelnut Well-Known Member

    Oct 12, 2019
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    I like your way of thinking and it all makes sense to me.

    The motor is running standard cams and compression (only about 8:1). After rebuilding the engine and gearbox I've been running it in for 500 miles, gradually building up load and revs. However, the magneto was set to only 30 degrees advance, which I couldn't alter until now because I've only just succeeded in freeing the ATD from its taper on the magneto (extractor bolt had a stripped thread). With that done, I can now alter the timing and was thinking that 35 degrees would be a good starting point.

    Although running sweetly enough at 30 degrees advance, it is definitely down on power, so hopefully a bit more advance should give it more go.

    I'll try it at about 35 degrees advance and listen out for any pre-ignition. I know how that sounds as I used to run a Rocket Gold Star with manual advance/retard and a very high compression. That needed a touch of retard on a long incline or it would begin to knock from detonation. Wish I still had that bike!
     
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  4. speedrattle

    speedrattle Senior Member

    Feb 19, 2021
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    #4 speedrattle, Apr 3, 2022
    Last edited: Apr 3, 2022
    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.
     
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  5. Wheelnut

    Wheelnut Well-Known Member

    Oct 12, 2019
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    Well, a plan is one thing. Putting it into practice is quite another.

    For one thing, the K2F Lucas mag doesn't exactly lend itself to simple or accurate setup with its taper drive and lack of provision for fine adjustment. Couple that with 70 years of wear and tear and it was rather more of a challenge than expected. In fact, the plan failed miserably.

    Got the timing disc set up with a pointer for TDC and a pencil at the ready to lock the auto advance to full advance. Then I checked the points gap to make sure it was 12 thou..

    That's where it all started to go pear shaped. With one set of points at 12 thou the other one was showing 18 thou, so the best compromise I could achieve was 10 thou on one lobe and 16 thou on the other.

    As a result, trying to get the timing anything like accurate with a points gap variance of 6 thou proved to be an impossibility. To achieve an average of 35 degrees at full advance I ended up with 25 degrees advance on the timing side and 45 degrees on the drive side. Not much else I could do. So the spark for one piston is too advanced and for the other piston it's way too retarded. No wonder the engine feels well down on power and it's just as well that I haven't pushed the engine too hard.

    I guess the root of the problem is excessive wear to the points cam ring for the magneto and the only way to get the timing consistent for both cylinders to fit a new cam ring. Either that, or convert to electronic ignition. That might well be a better option as would give a proper advance curve. But it does go against the grain to get rid of the magneto.

    Should have guessed that the magneto was knackered bearing in mind that everything from the oil pump and the big ends to the primary drive and gearbox appears to have been assembled by a drunk monkey, so it all had to be rebuilt again properly by me. Apparently, the bike had been fully restored by 'an expert' according to the seller. This expert managed to omit the nut that holds on the crank timing gear, but even worse, he left out the spring in the oil pressure relief valve, so it had almost zero oil pressure and so the big ends went almost immediately. And the early T100 doesn't have shell bearings that can easily be replaced, but that's another story.

    The bike looked the part and appeared to have been nicely restored, but I've had to rebuild absolutely everything. Even the engine plates were wrong as everything was out of line, so I had to make new ones together with the correct spacers to make the T100 engine fit properly into the A10 frame. It's certainly been a 'character-building' exercise but I reckon I'm almost there now.

    Here's the bike:

    Tribsa1.jpeg
     
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  6. Wheelnut

    Wheelnut Well-Known Member

    Oct 12, 2019
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    On the plus side, the bike started first kick and still ticks over nicely despite the dodgy ignition timing.

    On the minus side, new magneto points cam rings appear to be unobtainable.
     
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  7. speedrattle

    speedrattle Senior Member

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    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
     
  8. nickjaxe

    nickjaxe Active Member

    Sep 2, 2020
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    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.
     
  9. speedrattle

    speedrattle Senior Member

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  10. Iron

    Iron Elite Member

    Dec 29, 2021
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