Mk 3 Primary noises

Norton Dave said:

I pulled down the primary and found the sprag blown up .It appears to be the old problem boya ignition and when you require power to start it drains the amps and the bike will backfire .

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The Boyer may have caused the backfire, but the sprag is usually damaged because the overload backfire device doesn't slip when that happens, after all, the backfire device was originally intended to protect the starter drive from points ignition backfires.

http://www.oldbritts.com/e_start_backfire.html

Norton Dave said:

I think that a new type of electronic ignition with a lower runing amp draw is the only answer .

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It's not the amps, it's low voltage (especially during electric starter operation where the system voltage can drop to around 9-10V) which causes the old Boyer Micro MkIII boxes to go to full advance during starting. The Boyer Micro MkIV ignition is better as it can operate at a lower voltage, however there are other EI options such as Tri-Spark or Pazon etc.

Norton Dave said:

Any one know a cheap source for sprag.

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No-but if you find one, be sure to tell us.

dynodave said:

Since MKIII are not typically race bikes you will find the clutch slips (21T sprocket 4th gear) at a much lower RPM, closer to where the peak torque is. Mine will not slip in 3rd.
My MKIII clutch research stopped at that point, pursuing starter research instead.

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But then, the "MkIII clutch" is really no different to any other 1973-on* Commando clutch.

*(Actually Nov '72-on, from engine 212278)

Les
Yes the MKIII clutch is not different from other bronze clutches. I just did not own other 850s at the time.

Boyer MKIII does not go "full advance" but rather a wide range of random advance and retard. It's the hyper advanced trigger that will kill the starter mechanism.

dynodave said:

Boyer MKIII does not go "full advance" but rather a wide range of random advance and retard. It's the hyper advanced trigger that will kill the starter mechanism.

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Yes, a poor description, I should perhaps have said "over-advances".

(Dave, I've enabled your BBCode setting)

I've had terrible problems with my Mk3 clutch slipping over the years. I'm 240lbs and 6 feet tall and when in the uK I used to ride like the proverbial Hobot...;-) I tried everyting in stack height etc, to try and get a non-slipping light bronze clutch, but could not stop the slip. (I use teh ES so no belt drive - yet).

Norman White could never get the clutch to slip wjhen he rode it - but he's a only a jockey build, as all good racers are.

Now in Canada I've recently changed to Barnett friction plates and I lapped the steel plates on wet and dry paper. So far clutch does not slip - here's hoping!!

I'vbe been trying 160mm ATF and it seems a little noisy but not outrageous. I may go to half and half ATF and 50 weight Morris and up to 170 mm. I'll let you know hos it goes.

Suspect the primary tensioner needs a rebuild....;-(

L