No start and backfire when re-energized

[k_enn]

Its is a 1982, started within 10 seconds and ran good for the first ride of the year.  Second ride I noticed a little problem of a cut-out and backfire when the engine was still cold, but once the engine got up to temperature it was fine. Starting was not a problem on that ride.  

Just yesterday, the problem arose.  It would not start even with extensive attempts.  It was cranking strong, but the engine was nowhere near catching (not even a stutter).  After several attempts, I turned the key off.  A few seconds later, I turned the key back on and got a large exhaust side backfire (instead the common chuff).  The backfire was repeatable when I turned the key back on within a few seconds of a failed start attempt.

FWIW, the bike is a 1982, the battery was strong, and the coil was replace only 3 or 4 years ago with a Dyna.  Spark plug leads were repaced about 2 or 3 years ago.  Bean can and diode board are original.  

I am at a bit of a loss as to where to start the troubleshooting.  The fact that it backfired seems to indicate to me that it is getting gas to and through the cylinder.  The fact that it backfires when I energize the bike by turning on the key seems to indicate to me that it is getting electicity to spark plugs.  

I know the old addage, "if you think it is a fuel problem it is an electrical one, and if you think it is electrical it is probably fuel related."  I just don't where to start under these circumsances.  Where would you look first if you were troubleshooting this one?

k_enn

[montmil]

I would think it's getting fuel also as the not infrequent exhaust 'chuff' is likely excess fuel being ignited. Maybe.

Do you have fresh fuel in tank and carbs? Good fuel flow from the tank? Checked valve lash?

Fuel. Spark. Compression.

[Tony Smith]

First thing - has your timing slipped? Nothing like a retarded ignition to set you up for a really good backfire (except of course grossly advanced ignition which will do it too).
 
I am assuming you have done the usual things like checked the fuel, checked that you do not have water in the carbs etc.
 
i have a left field suggestion. A few weeks ago I was riding home and my bike cut dead. A few minutes later it started but was backfiring fearsomely and would not rev much passed 2,000rpm (which got me home). After doing the fuel, carb tests I started looking for more esoteric things as (as far as I was concerned then) the Snowbum test of ignition amplifier function (turn ignition off then on to get a "chuff" worked perfectly.  

I tested the beancan (you probably should too, all you need is an LED, a 9 volt battery and some alligator leads) and it tested perfectly, I was also able to see that the firing signal was about in the right place.

Armed with all that I fitted my spare coil (which were sourced from an outboard engine marine wrecker) thinking my coil had gone faulty.

The fault remained the same so working logically I replaced the ignition amplifier I had already "tested". Bike now runs like a waterbury watch with a new ignition amplifier that was the fault all along.

I suspect my amplifier had never been serviced in the life of the bike as there was zero heat transfer paste remaining, I have added pulling it off its heatsink and adding paste to the yearly task list.

I have to say that I think that the whole hall effect ignition system was a solution desperately looking for a problem.

All that added complication and we still have bob weight advance and retard. The Boyer Bransden fitted to my R100 in the mid 80s has never failed and has inbuilt advance control. I must admit that I was a bit suspect on the whole Boyer ignition thing and purchased a spare set of pick-up coils - they are stil lin the box they came in. If the Boyer ever fails it will take me about 10 minutes to revert to points. Admittedly the wife's R65/80 would be a tow job if her Boyer ignition failed, but that is because the entire points assembly including backing plate had to be removed form the 1979 beancan, but armed with my experience on the R100 I am not overly concerned.

If your problem turns out to be the amplifier, you will of course simply buy a new one because they are simply not that expensive, but it is irritating that BMW introduced two new electronic parts for absolutely no advantage over points.