1979 BMW 100RT

[Robert Padisak]

Hello all, and i hope you can help.

I inherited a 1979 BMW 100 Rt that was in a Barn for 16 Yrs,, I finaly have it running, but i am wondering if anyone knows why this bike has twin front brake rotators and twin spark plugs in each head,,

 

the bike is fair to poor bond,, Runs well but need more work..

Any help with info greatly appreciated.

 

Sir Robert J Padisak

Mansfield Ohio

419-370-9006

[Jake]

Welcome. Not trying to send you to those crazy Advrider guys, but this thread has some discussion on your bike, as well as some links that you mind find helpful.

 

Get her all fixed up and send us a pic!

 

 

[tallman]

Twin plugs most likely aftermarket mod (San JOse).

 

Twin front discs available then.

My 1980 had twin front single rear.

 

Go here, for Airheads. .

 

Welcome, best wishes.

[mneblett]

AFAIK, all of the R100RTs from the beginning ('79 in US; '78 in Yurup) have had dual front discs. IIRC, some of the early R80RT's had single discs.

 

Dual plugging is something that has been around for more than 30 years, with lots of different folks offering to do it. The original single plug cylinder head design has the plug tip offset from the center of the combustion chamber. These 979cc engines have big bores (approaching 4" across), which can be a long way for the flame front to travel to get to the other side of the cylinder before self-ignition begins, particularly when the engine is rotating slowly and is running hot.

 

The idea behind dual plugs is to improve the flame propagation (particularly at lower engine speeds) by igniting the mixture from upper *and* lower locations at the same time, so that combustion occurs throughout the combustion chamber faster, i.e., before uncontrolled ignition occurs. It also doesn't hurt that dual-plugging requires two dual-tower coils, and modern coils tend to deliver a stronger spark.

 

Is dual-plugging a good idea? Well, BMW started factory dual-plugged the last of the oilheads (R1150's) and all of the R1200 hexhead/camhead engines, in part to address the lean combustion/surging problems of the single-plug oilheads.

 

On the airhead, getting a dual-plug engine to run well requires altering the ignition timing -- they require *far* less ignition advance at low engine speeds. The problem is that the way folks first started to do it was to simply rotate the ignition points plate or hall sensor canister to fire closer to TDC, but that left the timing too far retarded at full advance (i.e., at higher engine speeds), leaving the engine feeling a bit "flat." Later approaches retarded timing at idle but still allowed full advance. Another common mistake was to radially change the size of the carb jets -- a dual plugged engine doesn't need any more or less fuel/air than a single-plug engine. My dual-plugged R100RT (heads sent to Tom Cutter for modification) ran great.

 

Two other things to mention:

 

Due to the thickness of the head castings, either the lower plugs need to be shorter reach than the uppers (and then cannot be mistakenly swapped without risk of interference/damage), or the head needs to be built-up to provide enough depth to allow use of a standard-reach plug (the building-up approach also gives much greater thread length, minimizing the potential for stripping out the few threads available in an un-modified head).

 

Also, there are two primary ways to dual-plug: drill a 14mm plug hole or drill a 12mm plug hole in the lower position. The 14mm plug is the same size as the upper plug, and at least it used to be you were more likely to find this "standard" plug at an auto parts store in an emergency. I don't believe that is still the case. Two problems with the 14mm plug: easier to install the wrong plug in the wrong place, and the 14 mm hole removes a lot material pretty darn close to the valve seats. The 12mm approach eliminates both of these concerns.

 

Bottom line: You need to inspect your bike's arrangements to be sure you know *exactly* what lower plug set-up you have.

[Robert Padisak]

Thanks for replying.. No the twin Plug setup is factory set up.. I know from some literature.. Just wondering if it adds any value. I am into this for 2K and wondering if i should bother with future repairs.

 

 

[mneblett]

???

 

I've never heard of *any* publicly-available airhead being dual-plugged from the factory -- doesn't mean it didn't happen, but I've never heard even a whisper in that direction. I'd love to see any pics or literature you have (including how the ignition points and coils were set up to address a dual-plug bike's needs).

 

As noted above, some dealers (San Jose BMW, for example) offered dual-plugging, and would be willing to dual-plug a bike before customer delivery. But a ex-factory bike with four plugs? I have to say I'm a bit skeptical.

[lkchris]

Thanks for replying.. No the twin Plug setup is factory set up.. I know from some literature.. Just wondering if it adds any value. I am into this for 2K and wondering if i should bother with future repairs.

 

You are simply mistaken. There was never ever a dual plug ignition system for Airheads from the BMW factory. Period.

 

And yes, every R100RT produced came off the assembly line with dual front discs.

 

As regards dual plugging, a history lesson is in order.

 

Back in the mid-1970s when unleaded fuel was phased in in the USA, there was an infrastructure problem. There were no gas pumps with electronic buttons for selecting which fuel grade you wanted. Rather a different grade meant a different pump, and most stations only had two pumps or multiples of two pumps, i.e., one for regular and one for premium. "Mid grade" did not exist. For the first few years after the introduction of unleaded, many stations simply dropped premium and sold unleaded regular and leaded regular.

 

Since all Airheads at the time required premium fuel, their owners had a problem and dual plugging was originally introduced to permit these bikes to run on low-octane, i.e. regular gasoline. By 1981 BMW finally reacted and lowered the compression ratio of all Airheads. In addition fuel station infrastructure caught up and the availability of premium fuel (now unleaded) returned.

 

Now, those that had invested in tooling and expertise in performing dual plugging suddenly found the reason for dual plugging Airheads superseded and proceeded to "invent" all kinds of new reasons why any Airhead should be dual-plugged, even the low-compression models. Most are spurious and today there is simply no need to have a dual-plugged Airhead. IMHO they are more trouble than they are worth and I'd never buy one.

 

As regards Germany, none of this happened there and there was never a shortage of premium fuel and all German Airheads continued to be made with high compression engines. BMW's response for the USA market was lower compression and NEVER dual plugging.

[mneblett]

Since all Airheads at the time required premium fuel, their owners had a problem and dual plugging was originally introduced to permit these bikes to run on low-octane, i.e. regular gasoline. By 1981 BMW finally reacted and lowered the compression ratio of all Airheads.

+1. The lower octane the rating of the fuel, the easier it ignites. Thus, the 9.5:1, squish-band head, large piston R100RT motors were more susceptible to low octane fuel self-ignition in a remote region of the combustion chamber before the flame front from the single plug could get there.