By joining you will help ensure that we can continue to provide this service
JOIN HERE!
Dual points for R69S (or any twin with magneto ignition)
- schrader7032
- Posts: 9061
- Joined: Fri Oct 27, 2006 3:00 am
- Location: San Antonio, TX
- Has thanked: 3 times
- Been thanked: 36 times
Re: Dual points for R69S (or any twin with magneto ignition)
I thought we were talking about the stock Pentacomm plate...one set of points.
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.
- jwonder
- Posts: 784
- Joined: Wed Nov 23, 2016 7:50 pm
- Location: Long Island, New York
- Has thanked: 3 times
- Been thanked: 8 times
- Contact:
Re: Dual points for R69S (or any twin with magneto ignition)
Dual points are very positive for 8 cylinder motors to increase the dwell time. New engines (and Cosworth race engines, etc) with a 180 degree crank typically fire two plugs at a time like ours do. Engineers found that firing the plug on the exhaust stroke promotes better evacuation of the combustion chamber allowing for a better intake charge.
The dwell time of our opposed twins (180 degree crank) is huge, actually overkill. If you are having problems with not having a hot spark I would look at magnetizing the magneto, a new coil and all the other things Vech mentioned.
For a great way to figure out dwell time check out this article from Denso: https://www.denso-am.eu/news/deneur21_0 ... -charge-up
I LOVE the pentacomm plate as you can really adjust it perfectly, and easily; BUT, it does its work by varying the points gap, hence affecting the dwell. Again, the dwell in our opposed twins are so much overkill I am not worried by the small amount you may lose with the Pentacomm plate.
Vice President, Vintage BMW Motorcycle Owners
2022 BMW Friend Of the Marque
Long Island, New York
-
- Posts: 72
- Joined: Sat Nov 28, 2020 11:40 am
- Location: Toms River, New Jersey
- Has thanked: 6 times
- Been thanked: 4 times
- Contact:
Re: Dual points for R69S (or any twin with magneto ignition)
- vechorik1373
- Posts: 268
- Joined: Fri Oct 27, 2006 3:00 am
- Been thanked: 26 times
Re: Dual points for R69S (or any twin with magneto ignition)
Yes, same procedure. The only thing a pentacomm plate does, is it forces you to change the point gap, via use of an allen wrench, to adjust the timing. Unlike the original set up, here is no movable breaker plate that the points are mounted on.
With the Penticom plate, whatever point gap you end up with, be it too wide or too narrow (and there for changing the dwell out of the original specficiations) (the amount of time the points remain open, expressed in degrees) .
Which is precisely why I don't care for the Penticom plate.
Technical Adviser, Former owner, Bench Mark Works
662 312 2838 cell 9 am to 4pm CST PLEASE!
Re: Dual points for R69S (or any twin with magneto ignition)
Given that the inlet tract lengths and exhaust lengths are not equal ( though the exhaust length difference must be at least partially mitigated by the balance pipe), some timing differential may actually be desirable to get both pots running exactly right....
- schrader7032
- Posts: 9061
- Joined: Fri Oct 27, 2006 3:00 am
- Location: San Antonio, TX
- Has thanked: 3 times
- Been thanked: 36 times
Re: Dual points for R69S (or any twin with magneto ignition)
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.
Re: Dual points for R69S (or any twin with magneto ignition)
Update. Followed Vech’s instructions. Differential timing was off 3/16 to 1/4” so borderline according to BMW. One tap halved the distance between marks. Second tap went too far. Slight tap in opposite direction and now spot on. Bike started on first kick (although that’s pretty normal.) Waiting for some decent weather to road test. Thanks Vech. You’re the man.vechorik1373 wrote: ↑Sun Dec 24, 2023 9:28 amThanks Vech for detailed explanation. Can this be done with a Pentacomm plate?
Yes, same procedure. The only thing a pentacomm plate does, is it forces you to change the point gap, via use of an allen wrench, to adjust the timing. Unlike the original set up, here is no movable breaker plate that the points are mounted on.
With the Penticom plate, whatever point gap you end up with, be it too wide or too narrow (and there for changing the dwell out of the original specficiations) (the amount of time the points remain open, expressed in degrees) .
Which is precisely why I don't care for the Penticom plate.
Re: Dual points for R69S (or any twin with magneto ignition)
because the length of the inlet tract affects the filling pressure of the cylinder, and the exhaust length affects the back-pressure which in turn affects the exhausting of the spent charge, which in turn affects the induction of the fresh charge, each engine has an optimum length for both.
The resulting density of the charge in the cylinder has an effect on the speed of the flame-front post ignition, which dictates the amount of time required for complete combustion. The point at which the spark is delivered must correspond to the time required for the flame front to traverse the whole combustionchamber.
I am talking about very small differences here but, nonetheless they exist. The point that I was trying to make is that getting the timing exactly the same on both cylinders, to a figure which is probably a bit of a compromise should not be the prime objective, rather getting the bike to run as well as is possible.
Re: Dual points for R69S (or any twin with magneto ignition)
Running well is a very subjective meassure, depending on many variables and adjusting the timing difference is not a very exact procedure.
Getting a boxer engine well balanced is a good idea anyway. Starting with the crankshaft offset at as close to 180 degrees as possible. Equal combustion chambers. Equal valve spring heights. Balanced carburettors. And ignition timing difference as close as possible.
- schrader7032
- Posts: 9061
- Joined: Fri Oct 27, 2006 3:00 am
- Location: San Antonio, TX
- Has thanked: 3 times
- Been thanked: 36 times
Re: Dual points for R69S (or any twin with magneto ignition)
'78 R100/7 '69 R69S '52 R25/2
Fast. Neat. Average. Friendly. Good. Good.