Author Topic: B40 ignition advance unit  (Read 361 times)

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chaz

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B40 ignition advance unit
« on: 02 March, 2019, 18:42:17 »
taking our latest project apart today we have come across a problem, the normal advancer on the cam shaft pinion , Im used to seeing with the bob weights system, this one has them brazed in place, its running Boyer ignition, would this be a modification to keep as it means we cant remove the pinion from the casing and therefore replace the bearing.

ANDY HIGHAM

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Re: B40 ignition advance unit
« Reply #1 on: 02 March, 2019, 20:54:48 »
Normally the Boter pick up rotor completely replaces the cam/ATU

berniej

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Re: B40 ignition advance unit
« Reply #2 on: 03 March, 2019, 09:13:33 »
What Andy said.... also, if it's just the weights brazed together I would have thought that you could still pull the plate from the shaft?

Does the BB unit have a model number? There seem to be a few that were made for single cylinder with side-points - might help to work out how the setup works (assuming it did!).

Cheers,
Bernie
'49 M21/B31 hybrid
'56 M21 combination
Bantam D7
C15 project
B40 Sportsman project

chaz

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Re: B40 ignition advance unit
« Reply #3 on: 03 March, 2019, 15:53:53 »
sorry, my mistake... thinking of the A50 which has Boyer. the B40 as far as I  know has no ignition with it apart from currently running points and back plate just removed from the engine, the rest is down sons dont remember coil or ignition box or any wiring for that matter

Rupert

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Re: B40 ignition advance unit
« Reply #4 on: 03 March, 2019, 17:11:47 »
Brazed up weights means the auto advance is junk. But you can still remove it from the end of the camshaft - put the centre bolt back in its hole but leave it loose, don't screw it in. Tap the head of the bolt sideways with a small hammer. This will free the auto advance from its taper fit in the end of the camshaft.
Rupert.

chaz

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Re: B40 ignition advance unit
« Reply #5 on: 04 March, 2019, 13:35:24 »
the auto advance is removed but original question. as the bike had a race number on, what would be the reason to fix the weights in place and is there a reason to keep

berniej

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Re: B40 ignition advance unit
« Reply #6 on: 04 March, 2019, 15:52:25 »
My guess would be that the timing was fixed at an advanced setting for running continually at high r.p.m.
I haven't come across that before but it's all I can think of given the reference to racing.

You would definitely want to revert to an auto advance system for road use - either mechanical or electronic.

Cheers,
Bernie
'49 M21/B31 hybrid
'56 M21 combination
Bantam D7
C15 project
B40 Sportsman project

chaz

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Re: B40 ignition advance unit
« Reply #7 on: 04 March, 2019, 18:33:32 »
thanks Bernie.
having talked to my son today.
from what I remember its got more teeth on 4th gear, has a #2 race plate, mx style rear tyre and road style front...  2 of the 3 wires from generator are joined, no electrics apart from ignition... Im thinking possible flat track racer, so top end revs a possibility.

 off to France at the weekend and nights this week so wont be able to check actual gearing until return.

berniej

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Re: B40 ignition advance unit
« Reply #8 on: 04 March, 2019, 18:57:06 »
The joining of the alternator wires sounds like a common mod to move from 6V to 12V electrics.

I'm doing the same with mine - feeding the output through a solid state 12V combined rectifier and regulator and connected to provide a -ve earth system.

Cheers,
Bernie
'49 M21/B31 hybrid
'56 M21 combination
Bantam D7
C15 project
B40 Sportsman project

chaz

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Re: B40 ignition advance unit
« Reply #9 on: 06 March, 2019, 17:42:48 »
B40 advance and retard unit question..
as it looks like we might need a replacement unit, can anyone advise as ive seen 5%, 10% and 12% units.

possibly looking to restore the bike back to flat track version (how it looks and looks to be set up ex US import), so will keep brazed set up for race displays and revert for road, anyone got a spare?

chaz

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Re: B40 ignition advance unit
« Reply #10 on: 14 March, 2019, 00:14:51 »
Still looking for an answer regarding 5, 10 or 12 degree unit? I've seen a few advertised but can't see any reference in the manuals.

Rupert

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Re: B40 ignition advance unit
« Reply #11 on: 15 March, 2019, 22:10:50 »
12 degree would be best for B40. Reason being the figure is doubled at the crankshaft, so say the fully advance spark position is given in the manual as 30 degrees (can't remember off hand what exactly it is for B40), 12degrees x2 is 24. Subtract that from 30 degrees and it gives you a fully retarded spark at 6 degrees BTDC which is fine.
The 10 degree auto advance timed at full advance would give a fully retarded spark at 10degree BTDC which won't produce as good a tickover.
The 5 degree auto advance is usually fitted to energy transfer systems.
Same reasoning is why bsa fitted a 15degree auto advance to the last B25 models. 37degree full advance position needs a wider advance range to give a good spark time for tickover.

chaz

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Re: B40 ignition advance unit
« Reply #12 on: 16 March, 2019, 09:07:27 »
thanks Rupert
great explanation.