Dual Plugging

[140008]

Have a 1983 R100 Dual Pluged18K.When I first checked the timing it was off,so I set it to "S" not knowing that it should(?) be advanced to close to OT.Bike ran very good 35mpg w/a sidecar.Researched and reset the timing as per Snow Bum.Idle went up,readjusted,used vac gauge,Have hesitation at take off,poor starting,and can't seem to balance right.A new idle jet is mentioned.Question; Can I go back to the "S" timing,what are the pros and cons,is there any problems with that? Somewhere in Alaska!

 

[toooldtocare]

In Sept and Oct 2011 of Airmail Oak did an article on dual plugging. The cut and past below are the paragraphs that deal with timing. I do not know if this will help or not, but it may. You may want to see if you can get those copies and read them for yourself. Wayne

 

[20774]

With a dual plug bike, the best timing set up will be to be at the S-mark at idle (this is best for starting the bike) and have full advance come in around 27-29 degrees BTDC. Since the flame front will be quicker with the dual plugs, you need to wait a little bit before firing the plugs.

If you don't restrict the point of full advance (ie, still fires at 30-34 BTDC), you'll be firing the mixture too soon and the explosion will take place while the piston is still coming up. If the firing is too far advanced, you can get detonation which is hammering on the piston. That can cause some serious damage.

 

[140008]

Now I'm confused Kurt.The "S" timing is stock timing,nondual pluged, and you say to use that.Then you say to restrict (retard,delay) timing or preignition/damage may occure. That would be moving timing closer to the OT mark.So is the "S" timing OK or should I play with the OT timing and tuning somemore?

 

[toooldtocare]

Confused? Me too, I trust Kurt's knowledge.

My bike is dual plugged, done by San Jose BMW with Dyna III. They retarded the timing when it was installed 12k ago (1985) and it runs fine. I installed the same setup on a friends R90 two years ago and also retarded the timing, it runs fine. I would play with your bike starting at OT and advance it to the point it runs the best with no pinging. Wayne

 

[20774]

In general, you need to delay the spark because of the faster burn of the charge in the chamber. But, one has to be careful down around OT. If you delay the spark too much to ATDC, then you get no power from that and you push very hot gases out the exhaust. Plus, you need a reasonable timing point when you trying to start the bike...this is a somewhat critical period. Typically, though the bike will start with quite a variation on timing around the S-mark as we've seen. So, if the general trend is to delay the spark at all regimes, that means it needs to be delayed at the F-mark. From what I've read, just rotating the entire timing so that full advance is achieved in the high 20's of degrees results in the static point being near 0 degrees. Plenty of opinions on doing this an alternate ways. One possibility is to slip a bushing onto the post that the advance bob weights hit...this stops the timing advance but leaves the static timing alone. There's been much discussion between Ted Porter, Tom Cutter, Snowbum, and Oak on the Airlist.

 

[140008]

I havs no F mark.Rotating,first comes the Z them 8 teeth later comes the S then two teeth later comes the OT.I believe that the Z is the full advance mark.I have a Hall unit,don't know about weights.I think I'll fiddle with the timing btwn the "S" and the OT.How about the dual plug Hall unit from Motorrad Elektrik,any experence with that unit?

 

[beemerguru]

Put the "S" near the bottom of the timing hole and go for a ride. That retards it just enough.

 

[20774]

The timing hole is about 16mm in diameter and 2mm equals about 1 degree. Putting the S-mark at the bottom of the hole results in probably 4 degrees of retarding...probably not bad. Yup, Go Ride!!

 

[140008]

Thank You very much,exactly the info I wanted,especily the degree incriments.Me thinks I was retarding the timing too far.I see the light!

 

[8ninety8]

dual plug timing curve

Having just installed a 28 year old dual plugging kit, accel/black andrews, both of which are said to fail or not be compatable when re-reading certain articles from way back, my observation is that a '74R90S runs like it's outacontrol when cracked WO in second gear with dual plugs.

This advance curve at stock has 25 degrees of full advance, 9 degrees BTDC at idle. 34 BTDC wide open.

So, when set to recommended 2 degrees ATDC at idle, at full advance for dual plugs it runs at 23 degress BTDC. Why would I limit the advance any more than 23 degrees as per some suggestions as I re-read articles concerning this? Could go to TDC at idle, and full advance would only be 25 BTDC wide open.

It seems to run like never before, idles great, tears off down the road, only thing slightly weird is it seems to back down slower when throttle is closed.

Guess if it's running, check the plugs once in awhile, see what's going on in there, and ride it until the old amp fails, and get a Dyna?

 

[mneblett]

I havs no F mark.Rotating,first comes the Z them 8 teeth later comes the S then two teeth later comes the OT.I believe that the Z is the full advance mark.I have a Hall unit,don't know about weights.I think I'll fiddle with the timing btwn the "S" and the OT.How about the dual plug Hall unit from Motorrad Elektrik,any experence with that unit?

F=Z = full advance -- BMW changed the letter somewhere along the years ...

In general, you need to delay the spark because of the faster burn of the charge in the chamber. But, one has to be careful down around OT. If you delay the spark too much to ATDC, then you get no power from that and you push very hot gases out the exhaust. Plus, you need a reasonable timing point when you trying to start the bike...this is a somewhat critical period. Typically, though the bike will start with quite a variation on timing around the S-mark as we've seen. So, if the general trend is to delay the spark at all regimes, that means it needs to be delayed at the F-mark. From what I've read, just rotating the entire timing so that full advance is achieved in the high 20's of degrees results in the static point being near 0 degrees.

Put the "S" near the bottom of the timing hole and go for a ride. That retards it just enough.

The timing hole is about 16mm in diameter and 2mm equals about 1 degree. Putting the S-mark at the bottom of the hole results in probably 4 degrees of retarding...probably not bad. Yup, Go Ride!!

So, when set to recommended 2 degrees ATDC at idle, at full advance for dual plugs it runs at 23 degress BTDC. Why would I limit the advance any more than 23 degrees as per some suggestions as I re-read articles concerning this? Could go to TDC at idle, and full advance would only be 25 BTDC wide open.

Kurt, et al., have it right. For someone new to this game, I summarize it a bit simpler: set the timing with the primary criteria being correct timing for *that* bike at full advance.

Full advance is the parameter which can/will determine whether you damage the motor, so you want to control the full advance point as the best way to protect the motor. The idle timing will naturally result from whatever the full advance timing is (i.e., set initial static timing at/near OT, and after starting the motor check to see where full advance occurs -- adjust full advance to where you want to start, then see whether that it too far or too little retarded). The static timing will be whatever it is after full advance is set.

What you are trying to do is balance retarding the full advance far enough to prevent pre-ignition, but not retarding it so far that the motor becomes difficult to start.

Because engine damage is to be avoided, focus on the full advance, experimenting until you are at the pre-ignition threshold -- then back off a degree or two to provide some margin -- and maybe another degree or two if you are doing this in cool air and you plan on driving in hot air next summer. The problem is that everyone wants a "set it to "X" degrees" rule, but every bike is different, so experimenting is required. The guidelines above are good starting points.

*Ideally* you would have a way of effectively "shortening" the advance curve so you could set the idle timing to S, and full advance to the ~28-29 degrees which is typically ideal. I believe one of the vendors is at least in beta (if not already selling) an ignition module which will provide such a dedicated "dual plug" advance curve.

Unless/until this hits the market and you have the $$$ to put one in, the "retard just enough, but not too much" approach, while a bit crude, will get the job done.

 

[8ninety8]

It's getting too late

in the season to any reliable road testing, however, this bike on single plugs would ping quite easily when put on a load under 4K. The few rides so far, no matter how or when I pull the throttle brings no stumble, no rattle, it just goes like never before. I may advance the timing a degree or two next spring and adjust around it. I had a K100 and a 100RS, and they were completely different from this ride. I suspect the higher comp. ratio likes a later rather than sooner ignition with the faster flame.

 

[140008]

Hey brewmierter is that one up on the idle jet or main jet. 49 below Alaska!