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Bsa m20 magneto timing

Hi
Last couple of days I have been helping a buddy with his mid 50’s m20
Lots of bodges & incorrect stuff on his bike - all done in its past to keep it a runner
Most of the wrongs are now righted - but as we were finishing up today after sorting the carb & fitting a new correct cable - I noticed that when placed at full advance - the bike seemed to miss & splitter a bit
Timing looks good, points are gapped correctly, in good order, fresh plug installed
If the cam plate or cam plate stopper “stud” was worn - would excessive travel because of wear create this - the difference between fully retarded & full advance is very little - much less than on my own ride

Am I best to rip it out & check for wear and/or retime the motor
Br
Job

email (option): jonnyob1@googlemail.com

Re: Bsa m20 magneto timing

John if you timed it correctly at full advance, then that is where the optimum timing will remain. Any deviation in the system would only affect the amount of retard available.

Also when ticking over at low speed at full advance, will often result in a lumpy tick over, due to the same effect as when you get a kick back. Ron

email (option): ronpier@talk21.com

Re: Bsa m20 magneto timing

Yes, as Ron says ref. tick over running...The ignition should be set slightly retarded at the lever when ticking over...I wrote a piece for the Technical Section that details why that is so...Carburation settings are also something to check...Rich settings can cause lumpy running...Weak settings spitting back in the carb...
If you have a spark plug that you know is a good 'un try that as well...It's not unheard of for a new one to not be up to snuff...Don't forget to gap any new plug to .018" as well...Ian

email (option): ian@wright52.plus.com

Re: Bsa m20 magneto timing

Thanks guys
My own bike idles well even at full advance setting with a higher tick over - but I will normally leave it idle at a retarded setting

This one however - has a good idle at retarded setting & good idle at advanced too - but if I continue to move the lever to its full travel - it splutters & misses a bit

So yes - I have retarded & advanced settings - there is little between them in terms of lever travel - but if the lever is moved to the full extent of movement, the engine isn’t entirely happy even with throttle applied

It was further complicated by use of what seems to be a bsa magneto body - pick up away from motor & a Norton points end - cable entry at the left instead of the right side

Would anyone have a picture of the correct cam plate as used on a Norton mag - I’d like to compare it to a std bsa mo1 cam plate
If it continues to confuse or not be right, I’ll find the parts to put it right from my stash of parts here & convert the points side to a std bsa type
Carb is set up as best can be & a new plug gapped fitted also
Br
Job

email (option): jonnyob1@googlemail.com

Re: Bsa m20 magneto timing

John I don't think you can compare a Norton cam with a BSA. The cable is on the opposite side but it is still tight wire advance because a Norton mag has the opposite rotation to the BSA. Royal Enfield however have a slack wire advance, so with the cable at the front but will still have the same face cam as an M20. Ron

email (option): ronpier@talk21.com

Re: Bsa m20 magneto timing

The clockwise Norton cam is marked 'R' There are slots on each side for the A/R lifter...Movement should be the same whether left or right though. Norton went to slack-wire advance after 1948, but they changed the cam drive at the same time and the mag became anti-clock.

OLYMPUS-DIGITAL-CAMERA

Re: Bsa m20 magneto timing

I think it might be best to revert this mag to std set up Ron

There were a number of “mods” intended to keep it on the road which are now confusing issues

With std parts - I can help him better long term too

Br
Job

email (option): jonnyob1@googlemail.com

Re: Bsa m20 magneto timing

John from the point of view of running, it doesn't matter a jot if you have tight or slack wire advance. But for an anti clock mag you must have an "L" stamped on your cam plate, as seen here on my spare BSA/Triumph and Enfield mags. As per Rik's mag the clockwise mag needs an "R" cam plate.

I guess the Left and Right denotes what side of the plate the points are actually opening on. Ron

DSCF4260-2

email (option): ronpier@talk21.com

Re: Bsa m20 magneto timing

Magnetos built from pieces can be a problem, i have come across 3 different cam plate for these mags. First thing i do with an unknown mag is to check the magnetic flux timing. If right is when you turn the mag in its correct direction you can feel the magnetic flux. The points should just break on full advance just after full magnetic flux, dosnt make any differance if its slack wire or tight wire advance for this check as long as you have it on full advance. If you have got this then you are on the way to having a good magneto. But coil and condenser are not checked in this test.

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