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Im having some difficulty getting the ign timing correct - I need a simple method to check it, bike does not perform over 50mph. New carb, all lines renewed and tank is clean, new coil lead and condenser. I think the idea of setting of timing at full advance is throwing me off the right track ?

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Assuming the bike is manual advance? Are you sure your setting the mag to fully advanced position?


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Thanks Allan - It is an alternator model with distributor plus automatic advance and retard.

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Hi
You need to devise a way to lock the distributor cam in the full advance position
It depends on which type unit is fitted? Usually some suitable size washers placed under the screw that holds the cam in place can be made to hold the cam solid with the auto advance held fully against the spring pressure
I seem to remember that there was a static figure (retarded) given as well in the service sheets or manual ??

John

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Thanks John for your reply - I've looked for the static setting in the service sheets but I could not see anything like that. Maybe I'll contact BSAOC and see what they say. But meanwhile I'll work on your suggestion and see what happens - thanks again and stay safe.

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Does this help at all?

B31_33 with Alternator Timing Info.jpg

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Originally Posted by chaterlea25
.....You need to devise a way to lock the distributor cam in the full advance position........

But to answer the static timing question, it calculates out to be between 5 and 11 8 and 14 degrees BTDC for an engine that is 35 38 deg fully advanced.
I found this by looking up what distributor goes on a 1958-60 B33 B31 and finding the advance range in my Lucas parts book.

Distributor #40599A
Model #18D1
Advance range: 12-15 degrees
But here's the problem.... You need to know the actual range on your distributor so static timing can be set where the distributor will give the correct full advance as well. That's why BSA wants you to set your timing to a full advance setting. That will give you the best performance on the road but your idle quality may not be the best.

To further compound this problem, piston travel becomes very small relative to degrees of crank rotation as the piston approaches TDC. This makes accurate timing more difficult unless you use a dial indicator to measure piston position. The pencil method? Probably not.

And, to add one more problem to the pile, your advance unit is likely worn since B31's are sturdy and usually cover plenty of miles. It's advance range has likely grown longer from wear.

So say for example, you take this advice and set your static timing to 10 degrees, if your advance unit advances another 40 degrees because it's worn, your all in timing will be 50 degrees and you will be risking engine damage.

PS: I should have read the OP's post better. I mistakenly gave timing spec for a B33, not a B31. The B31 should be timed to 7/16" or 38 degrees fully advanced which makes retarded timing between 8 and 14 degrees. Apologies for any confusion caused.

Last edited by Stuart Kirk; 11/10/21 12:34 am. Reason: Posted in error
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follow the pics Ian posted .
the points cam shown in Ian c. pics
rotates clockwise
its the lump under the center screw
[Linked Image from i.ebayimg.com]
[Linked Image from i.ebayimg.com]
you can grab the top centrer of the points cam with two fingers and twist clocwise , or use a screwdriver or fingernail in the slot .
But it won't stay advanced on it own when the shaft is static .
to force it to stay advanced for static full advaced timing .
cut out one side of a small washer to made it loof like a C .
this washer can now be slid under the lossened center screw to help hold to cam in the full advance position .
use a small screwdriver in the top slot to lever to advance clockwise
and Tighten down the C Washer to lock it to full advance .
with the piston at 3/8" BTdc
the whole loosened distributor body can be rotated until the points snap a spark ( or lose continuity )
( or fag paper slips ) ... this is static full advace timing .
bolt down the dizzy , remove the C washer-stop and allow distributor to retard to its slack static position

If you have make a full timing mark , the bike can now be strobe timed to this mark . ( 2000 to 2500 rpm , don't have the exact spec)
If strobe timing is off , the whole distributor body only needs to be loosened and move clockwise to advance
Counterclockwise retard and and then clapped back down

this last pic shows part 425746 ... the points cam ,,, it lives on the mainshaft and under the points plate .
the cam has an stop arm that will show degrees of Advance stamped into it .
with the aid of weight and spings , the points cam can advance faster than the shaft
( until it hits its stop )

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i had this problem, the cam is pressed into the plate , on inspection it was moving

Last edited by PaulM; 11/13/21 10:50 pm.

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Thanks all appreciate the input and advice - stay safe!


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