Dual Plug timing questions

[x1Buellist]

So I finally have gotten back to try and fine tune everything after the rebuilding of my /5. It has dual plugs with Dyna ignition. The engine bogs down. When I look at the timing at idle I see no marks. If I rev it up to 3-4k I get a dot that just comes into view at the higher rpm. I don't want to rev it much higher than that since it is a new engine build. Everything I read refers to letters. I see no letters, just the dot. It does have a lightened flywheel so I wonder if the timing marks have been modified. Would that be normal when lightening a flywheel? So, I see I have the palctic ring with the set screw by the ignition but I don't want to do anything until I have a good concept in my mind of what I am shooting for. Any hints, observations, links to help?

[Major Softie]

Take a flashlight and look at what you have on the flywheel without doing the whole timing light process. If you don't have any of the stock markings, then you have to start trying to figure out what the marks are that you do have.

If you have nothing but a single dot, my first guess would be that it would be TDC, but you'd have to check it and see.

[Deleted User 62]

It sounds to me like your timing is way too retarded, the dot should be centered in the hole at a little over 3000 rpm. That would explain the "missing" S marks too. I assume you didn't static time it before starting? If not, disconnect the ground wire, pull the plugs and the front cover and turn the engine over with the crankshaft, clockwise as viewed from the front, while looking in the timing hole with a flashlight. I bet you'll find the "S" mark.

[x1Buellist]

Thanks for the tips. I didn't set the static timing. The engine was started at the shop when it was rebuilt. I guess I made the false assumption they would have done that. My receipt (from 1985) shows the Dyna ignition was purchased at the time the engine was rebuilt so I assumed they installed that also. To much assuming So the dual plug bit. Any idea how to offset the timing to account for that.

[Garnet]

Snowbum covers it here: http://bmwmotorcycletech.info/dualplugging.htm

There are two schools of thought on the subject. The theory is that two flame fronts make the combustion process happen faster so less full advance is needed. Some believe that the rpm that full advance is reached should be raised as well.

The other big debate is weather the the static or idle timing should be at the stock 9 degree mark or also be retarded the same amount of the full advance.

More reading here: http://mailer.fsu.edu/~jroche/road_rod.html

[Duane Ausherman]

I strongly suggest that you take time out to learn about ignition timing. You have a special set-up now and you better know how it works.

As a former owner of a BMW dealership, never assume that any motorcycle shop has any clue as to what they are doing. Don't fall for the idea that a motorcycle is a motorcycle. You need a shop that specializes in airheads and has proven that they are fair and competent.

[Roy Gavin]

According to folks who have tested a few airheads on a dyno the optimum timing advance varies from by about 6 degrees, somewhere between 30/36 degrees on a single plug motor, and around3/4 degrees less on a twin plug one.

And at 26 degrees there isn't much to see in the timing hole.

One simple way of getting close to the mark is to adjust the timing while the motor is ticking over for the fastest tickover, then retard it until it just starts to slow down.

Then find a good long steep hill and do a few test runs a few degrees either side of that.

Or take it to a dyno specialist and get him to set it for you - chances are his initial setting will be done the way I suggest.

Boyer and most other after market ignition suppliers make a system with a twin plug curve, and the Boyer Micropower system which comes complete with twin dual coils, leads, and plug caps for not a lot more than the price others charge for the coils ,leads and caps alone works particularly well for me.

It only has one curve, but as that seems to be the right one it is the only one you will need.

Some people have the skills and knowledge to alter the bob weights and pins etc to change the curve on a stock unit, but as I needed the coils leads and caps it was a no brainer to fit the complete system for around $50 more.

And it has to be said that most of the folks claiming the skill seem totally unaware that the optimum timing can vary by 6 degrees from bike to bike.

[Frog]

I have a dual plugged slash five with dyna coils...though I run mine on a dyna booster triggered by the stock points.

On the stock single plugged bike, you would set the timing such that you would see the S in the window at idle and the F would appear at full advance (3,000 rpm). If I remember correctly, full advance was 34 degrees before top dead center.

Like others have pointed out, the dual plugged bike should be retarded (spark later) by about 6 degrees. The important goal is to get the full advance to be at about 28 degrees before top dead center. Just remember that these are general numbers, that may be fine tuned based on your specific conditions, but they represent the concept well.

So, the easy way to achieve the desired timing retard is to widen the slots in the points backing plate such that the whole assemble can be turned back the 6 degrees (making idle close to the OT mark). This makes the bike work correctly at full advance, but some argue that at idle, it prefers the old setting (S mark). I had done this initially and it ran fine.

If you would like to retard the full advance, but maintain near stock idle timing, Ted Porter sells little bushings to limit the advance mechanism. That is what I am currently running and all is well. I have no idea how your ignition is triggered, or advanced, so am not sure if this would work.

Hope this helps you understand the concept.

BTW, I also agree with all the previous posts and hope I was not too redundant.

[x1Buellist]

Thanks for all the info. I have tomorrow off so I will be re-reading everything tonight and working on it tomorrow. I'll let you all know what I find.

[mattcfish]

You actually want to run stock timing at idle but have full advance at 28 degrees. The reason people retard the idle is because they don't have adjustable curves on their ignitions. The Dyna ignition won't allow a shorter curve on it's own, the mechanical advance would need to be modified.
Read this article for more info...
http://mailer.fsu.edu/~jroche/road_rod.html
"Much discussion has occurred about how to time a double plugged BMW Twin. I believe that total combus tion is more efficient and completed quicker in a double plug combustion chamber. In other works "its all over a little sooner". This "little sooner" begins to add up as revs increase so that by the time top RPM is reached the engine is running with too much advance. Using a degree wheel set your full advance timing at 33 degrees BTDC on a single plug engine and 28 degrees on a double plug engine if your revs are to be 8200 or less. Its still not that simple however, please remember that when adjusting your advance curve you still want to be at near stock timing in the lower RPM range. So many people make this basic mistake - they just retard the timing and are baffled at the power loss down low. Think instead of reducing the timing range by five degr ees - not retarding the whole system by 5 degrees. And of equal importance when double plugging - move the RPM point where full advance occurs way up as mentioned above . Forget little springs - just lop off the metal tip of your centrifical advance arm s by 1/2" to 5/8" - then fine tune with springs if needed. Keep in mind that 2.04 mm measured at flywheel rim face equals 1 degree of crank movement."



If you only have a dot to go by , you need to determine what the dot means. On a stock flywheel it's full advance.
Once you know what the dot is, you can use an adjustable timing light to time it anyway you like.