twin plug timing question.

[AnnapolisAirhead]

it is a 78 r100rs with a dana III ignistion with twin plugs. one dual plug coil per side.

On a side note, I have a similar setup. This summer, I had a problem while riding out to WV on some not so smooth roads. The black feeder wire to the right side coil was hanging on by a thread and the right cylinder wasn't working--I limped into the nearest town on one cylinder (but didn't have to) at 18-22mph.

A better arrangement is to cross the plug wires such that each cylinder is getting spark from both coils all the time (there is always a wasted spark on these engines anyway). Had I done that, I could have continued with both cylinders working (although not with two plugs per).

For what its worth, I have a setup that seems to cause a lot of controversy amongst the elders but runs like a clock for me. '83 R100, factory electronic ignition, dual plugged, Accel red coils, (single) Dyna Ignition booster. Came to me that way from the PO.

[PGlaves]

It has been a very long time but I do remember some of the details.

The ideal setup is to leave static timing stock - but to reduce full advance by 5 or 6 degrees. With a stock setup, using bushings to enlarge the diameter of the advance stop pins is the best method. With the fully electronic versions, who knows????

You accomplish the same full advance change by retarding the static timing by 6 degrees - which for many models is static timing at TDC. Not ideal at startup or idle, but not overadvanced in the power band.

The reason for the need to change timing is simple. Instead of igniting the fuel at one edge and the flame front needing to advance across to the other side, you now ignite two flame fronts that meet in the middle. So the travel time needed by the flame front(s) is reduced.

These parameters were dead-on in 1985, with the fuel then available. With today's alcohol adulterated fuels there may be distinct differences. But the principles are the same.

[jimmyivy1]

mine is set via snow bums article and some of oaks, it is set at the ot mark and runs like a scalded dog, however not really running out past 90mph(depending on speedo may be more like 100). it pulls very hard to 90 very similar to my 03 1150. not sure how this is but i like it! still would like to know why it doesn't want to run past 90 ish?? not that i ride a 30+ year old bike that fast.

[LOSTBOY]

Whatever method you use to retard the timing, you want total advance to be at 28-30 degrees BTDC. This will put the "F" mark (unless you have converted to an '81 on flywheel) just below the window. If the bike won't pull past 90 MPH-it should reach at least 120- with this timing, your problem is probably mechanical or caburetion.

[jimmyivy1]

lostboy,

the "f" mark, is it the ot mark or z marking? i have up dated this fly wheel, and trans. it will probably go to 120 it just doesn't want to. i have full adv at 2900-3k and the "z" is in the bottom of the window. i set the timing via snow bums (bmwtech..) article. i am running a dyna III ign. he said something about grinding(or bending posts) the holes on the advance unit to allow the bike to advance a bit more, however he said it was not necessary. could this be the reason?? the bike is jetted to the 77rs spec and in good repair.

[PGlaves]

lostboy,

the "f" mark, is it the ot mark or z marking? i have up dated this fly wheel, and trans. it will probably go to 120 it just doesn't want to. i have full adv at 2900-3k and the "z" is in the bottom of the window. i set the timing via snow bums (bmwtech..) article. i am running a dyna III ign. he said something about grinding(or bending posts) the holes on the advance unit to allow the bike to advance a bit more, however he said it was not necessary. could this be the reason?? the bike is jetted to the 77rs spec and in good repair.

F mark and Z mark are the same thing - mark for "full" advance

[LOSTBOY]

With the '81 on flywheel, "Z" is the full advance mark, as Paul noted. The only reason to increase the amount of advance by filing the imiter holes is so you could run the static or idle timing at OT and still get 28-30 at full advance.

Most bikes with dual-plugs run well by simply retarding the timing to reach that 28-30 at full advance with no further modification. Others need more help via limiter bushing(s) or modifications to the advance curve.