Followup: re electronic ignitions for BMW airheads

A week or two back, I'd asked about aftermarket all-electronic
ignitions for airhead BMWs, particularly those with ignition
canisters.

At that time, the mechanical advance was screwing up big time
and I'd been assured they couldn't be repaired. I was looking
at all electronic ignitions with electronic advance as an alternative
to replacing the very pricey BMW mechanical unit.

I finally managed to rehabilitate the original mechanical advance.
While BMW only sells the complete Hall cannister at ~ $440
a pop, Motobins in the U.K. sells springs for a couple bucks
each.

For '79 on BMWs, electronic ignition options seem to be
Omega (requires stock alternator) or Boyer (points canisters
only, points can would fit all '79 on bikes).

I decided that since I'm thinking of dual plugging the bike
anyway, electronic advance would be of minimal benefit
as the advance would be greatly reduced by dual plugs.
With the mechanical advance working OK, I'll live with
it for now and think hard about a dual plugging project.

Thanks to all for good ideas.

 

As you say, dual plugging allows the bike to run higher compression
and/or lower octane gas as there is faster flame propagation.

Because of the reduced time needed for flame propagation, people
often recommend either retarding the timing a few degrees
and better yet, limiting the advance mechanism. This is sometimes
accomplished by adding a few layers of heat shrink tubing to
the stop post to limit the throw of the weights.

In any case, it would seem that with better flame propagation, trick
ignition timing perhaps becomes less of a win than it would be
with a single plugged engine. This is per some reading on various
airhead sites.

 

Motobins is great. I got new out of production Koni rear shocks
from them, not available here. I've talked to many guys over
the years and the general consensus is that dual plugs were
and are helpful only if you are suffering from pinging due to
the lower octane gas than the combustion chamber was designed
for. I'm not sure what you mean by the electronic advance being
greatly reduced. The retarding and advancing is adjustable like the
mechanical device by rotating the plate.

 

Seems like that would wear pretty quickly. I'm sure the airhead gurus
have tackled that one by now but I think some guys probably
created more problems than they solved. I don't know why low
RPM performance was an issue unless perhaps they spend a lot of
time in city traffic. I would look at the info for your particular year
and model. BMW long ago addressed the octane issue
with their combustion chamber design. My '78 /7 heads had no
problem with pinging, nor did/does my '84 RS ones and I use
low octane gas.

 

It's not particularly subject to wear. It makes contact with a
nylon pad to act as a stop when the unit reaches full advance.
No turning action or friction.

The GS is speced to run 91 octane. I used to run 89 but went
back to 91 when the problems with the advance unit started.

A GS can spend lots of time crawling up dirt trails in the hot
sun, so it's a different breed of cat than a bike dedicated to
highway use. It does fine on highways too, but I want to be
able to crawl along in first for long periods of time on hot
summer days. I'd also like to not have to worry about finding
crap gas at single pump stations way out in the boonies.

 

I thought it wasn't at full advance. I never had a can type unit but
the metal posts were the limit of the weights' travel. If you mean
putting heat shrink on the posts that the weights hit I don't see
why it wouldn't wear through. Aren't the weights hitting the plastic
repeatedly? Seems like a metal sleeve could be fitted. One thing
to watch for is the dreaded cam shaft end nut kaput gemachen.

A lot of guys prefer the GS and never even leave the road. Nice all
around machine.

 

I meant putting heat shrink on the posts which limit the
travel of the weights. Dunno about "hitting" the weights
repeatedly. This would be caused by rapidly cycling
the engine between 1000 and 3500 RPM ?

People who have made this mod report no problems. A
metal sleeve could be fitted but the metal that it fits on isn't
round, so the machining might be messy.

Cam shaft end nut kaput gemachen isn't an issue
on canister models. The canister is driven by an
arrangement almost like a U joint that engages with
a slot in the end of the cam. No nut to worry about.
worry about. From what I understand, the points end
of the camshaft could be a problem area on earlier
bikes.

 

Yes, every time the engine revs up. The posts limit the advance
travel so the weights do hit them each time.

Mine were small and round so it is a different animal altogether
then.

Yes, they broke off rather easily. Over torquing was the usual
MO but in my case it was under torquing and it loosened, causing
the weights to actually bend the dinky shaft. It broke when I had
it almost perfectly straightened. Enter the Boyer system that
didn't use the weights. It was cheaper than a cam shaft plus install.