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I have a Question that I hope someone can help me with, my rebirth of my R80/7 1978 model is coming close to completion, the issue I have is ignition timing as the bike when I got it had all the original ignition system replace with a Boyer Bransden electronic unit, the issue I have is the original system if I am correct had full advance at 3000rpm, the manual for the kit on the bike states full advance at 4000rpm.

Why the change in the full advance setting I find odd unless there is something here I am missing as in a different advance curve programmed in to the module.

Thanks in Advance.

Does your ignition still use the stock centifugal advance mechanism?

I believe that all BB ignition systems, certainly since the Mk3 contemporary with the airhead, do away with the mechanical advance.

The answer to the question is that the original machanical system is crude in the extreme and debateably reaches full advance far too soon. The advance curve achieved by electronic advance curves is said to be far better tailored to the requirements of the engine... whether it makes any real difference or not is open to discussion. In any case, it shoudn't be seen as a problem.

Rob

But the difference in 3000 versus 4000 RPM? How does the engine/timing deal with that?? My limited knowledge says that the reason for the advance is to keep the firing point of the plugs far enough ahead of the piston speed so that the flame front at firing still has time to burn. But with the system having a stop in advance at 3000 (or maybe 4000), the piston continues to move faster and faster but yet the advance curve doesn't change anymore. We ride our bikes mostly above the 3000 point which suggests that the ignition is firing in an ever increasing retarded situation.

Kurt

gspd wrote: ↑Wed Sep 28, 2022 10:11 pm Does your ignition still use the stock centifugal advance mechanism?

No that's all gone as part of the kit it is replaced with a magnetic pickup trigger unit, I have all the original documentation for the kit.

Kurt in S.A. wrote: ↑Thu Sep 29, 2022 6:39 am But the difference in 3000 versus 4000 RPM? How does the engine/timing deal with that?? My limited knowledge says that the reason for the advance is to keep the firing point of the plugs far enough ahead of the piston speed so that the flame front at firing still has time to burn. But with the system having a stop in advance at 3000 (or maybe 4000), the piston continues to move faster and faster but yet the advance curve doesn't change anymore. We ride our bikes mostly above the 3000 point which suggests that the ignition is firing in an ever increasing retarded situation.

Kurt

That's not a bad assesment... except that it's very much more complex than that. The ideal ignition point is affected by a large number of factors, not just engine speed. These include mixture, throttle opening, engine temperature, inlet air temperature, load to name just a few. You also need to factor in whether you are looking for maximum power output, fuel efficiency or tractability. The mechanical advance is very much a stab in the dark but a fixed, electronic advance curve based just on revs is not a lot better. Modern engines with monitoring of all of the above factors and constantly adjusted mixture/advance do a substantially better job.... but still aren't spot on... and fitting such a system to an airhead would be exceedingly complicated.

You have to balance all of that against just how important the exact ignition point is. Yes, an electronic ignition advance can make an improvement if the curve is carefully calculated but only by a few percent and it's questionable whether the improvement is worth the cost of the new system. You pays yer money and takes yer choice.

Rob

Rob

Rob Frankham wrote: ↑Thu Sep 29, 2022 4:51 am I believe that all BB ignition systems, certainly since the Mk3 contemporary with the airhead, do away with the mechanical advance.

The answer to the question is that the original mechanical system is crude in the extreme and debateably reaches full advance far too soon. The advance curve achieved by electronic advance curves is said to be far better tailored to the requirements of the engine... whether it makes any real difference or not is open to discussion. In any case, it shoudn't be seen as a problem.

Rob

Hi Rob, Thanks for your response and I have in fact spoken this week to someone here in NZ who used to sell the kits before he retired, seems that what you are saying is spot on and in fact with broken and worn springs on the original kits caused in some cases detonation and you don't want that, he has said set it up as per Boyer suggested and it should be fine.

What he told me is that this system has a what is known a a fixed pre programmed advance curve and not to worry about it, basicly it works off RPM and adjusts the advance accordingly.

Modern motors take and I know this as this is what I do at the Polytechnic is take info from throttle position, RPM, O2 sensors in exhaust, Knock sensor, engine load and now gear selected, and this is far more complex than you can imagine.

My worry with this kit was that full advance as in the Boyer kit stating 400RPM over 3000RPM was a miss print but it seems not to be the case.

Cheers.