Author Topic: A70 ignition timing  (Read 647 times)

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DAVE BRADY

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Re: A70 ignition timing
« Reply #30 on: 28 October, 2020, 13:45:05 »
The saga continues.

Out for a ride this morning whilst the weather was OK.  A bit cool but nice otherwise.  Not many bikes about but we managed to hold off a couple of Police bikes through some twisty bits.
So two up with the Pazon fitted.  It feels quite different so as well as no kick back when starting the engine seems to be more relaxed on pulling away but seemed to be quite progressive up to 60mph and responded well when the throttle was open up. 
The Boyer seems to give a bit more oomph from the start which is probably due to starting from a more advanced position but the kick back with 38 of advance was not acceptable.  Once up and running there did not seem to be a noticeable difference and this is born out by the graphs posted in this thread.  I would imagine both ignitions would be on a par at higher speeds.   The real test will be when we can get on some nice steep and twisty hills so that mid range performance can  be properly tested.

Dave.

DAVE BRADY

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Re: A70 ignition timing
« Reply #31 on: 06 November, 2020, 08:41:53 »
The next episode,

Chatting to the previous owner of my 750 A65 re timing he came up with this page from a late A65 manual. It obviously has reference to the A70 differences (the A75 is a miss print) and shows a rotor with a specific mark for the A70.
It is possible that these rotors were fitted to late A65s and the A70s,
Has anybody got one of these? It would be interesting to see the difference in the position of the timing mark on the raised bit that it sits on.

Dave.

JulianS

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Re: A70 ignition timing
« Reply #32 on: 06 November, 2020, 11:14:43 »
This service sheet will explain the 1 and 2 marks on the rotor.

DAVE BRADY

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Re: A70 ignition timing
« Reply #33 on: 06 November, 2020, 13:37:17 »
Hi Julian,

Another bit of a twist.  If the number 2 mark is for all engines except the R3 and the A70 timing advance is 38 as opposed to the 34 of the A65 there must be a difference somewhere to facilitate timing an A70.  If the rotors are all the same and the timing pointer is in the same place in the primary cover, where is this difference?  I suppose it could be the rotor keyway is machined differently but I think that is unlikely.
This Service Bulletin is dated about the same time as A70s were being built but no reference to them.
Maybe a later bulletin will refer to them.

Dave.

Servodyne

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Re: A70 ignition timing
« Reply #34 on: 06 November, 2020, 21:27:27 »
I've tried to find if there's a difference in the crank keyways but they appear to be exactly the same, in terms of relationship to the splines and the splines to the crank. The crank with the slightly blued shaft is A70 with the other one is a NOS late A65.
I haven't checked yet whether the primary chaincase is different to accommodate the 4 deg, but if it was, then surely it would have a different part No.

DAVE BRADY

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Re: A70 ignition timing
« Reply #35 on: 06 November, 2020, 22:07:16 »
Hi Jim,

I found something on youtube by Angus Campbell with a genuine A70 being timed and the primary cover looked the same as A65.  BSA could obviously specify rotors marked up for particular bikes but I cannot find a part number for the A70.  Did BSA produce a manual with the A70 variations covered or did they not bother for only 200+ bikes? 

Dave.

Ian C

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Re: A70 ignition timing
« Reply #36 on: 07 November, 2020, 13:40:40 »
I've only ever seen the attached for the differences Dave.

DAVE BRADY

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Re: A70 ignition timing
« Reply #37 on: 07 November, 2020, 14:13:36 »
Hi Ian,

Thanks for that.  I do have this info. and the data sheet that Julian posted but there is still a gap.  There are two possibilities to facilitate the 38.  Either the primary cover was drilled differently or there was a specific rotor with the timing mark at 38.
I can find no evidence of either and I am coming to the conclusion that, as the A70 was never likely to be a production model, BSA did not bother to publish a full manual.
From the data sheet I now know that the timing is 38, same as for the very similar bore and stroke Triumph 750, so I will use that.  At least with the Pazon it should be set and forget.

Dave.


 

JulianS

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Re: A70 ignition timing
« Reply #38 on: 07 November, 2020, 14:54:44 »
I do not think there was a rotor specific to the A70.

When the B50 timing recommendation was revised a service sheet showed how to mark the rotor with the new position.

Below is from Lucas dated 1978, even lists the A70, showing only one rotor for the whole range, 54202275, which will be the stronger welded item. Lucas rotor part numbers confuse me, sometimes the number listed is for rotor with its keepers, which is different to the cast in number on the rotor.

JulianS

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Re: A70 ignition timing
« Reply #39 on: 07 November, 2020, 15:03:31 »
Below is the only A70 specific service sheet I can find.

The mention of "Special tuning information..." is interesting but was it ever published?

DAVE BRADY

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Re: A70 ignition timing
« Reply #40 on: 07 November, 2020, 16:31:25 »
Thanks Julian,

The timing is probably the critical one as just about everything else is as A65L.  Thanks for your input.  All I need now is to be able to get out on the bike.

Dave.

Servodyne

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Re: A70 ignition timing
« Reply #41 on: 08 November, 2020, 09:26:01 »
Hi Ian,

I can find no evidence of either and I am coming to the conclusion that, as the A70 was never likely to be a production model, BSA did not bother to publish a full manual.
Dave.

Hi Dave
You could well be right with that statement. Thinking about it, the difference in the timing may only be in the timing cut out in the flywheel, which is specific to the A70. I remember Angus only timing his A70 that way and not strobing it to the marks in the Primary. If he had have done, he may well have found a 4deg discrepancy.
Jim

DAVE BRADY

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Re: A70 ignition timing
« Reply #42 on: 08 November, 2020, 09:32:30 »
Good morning Jim,

I think you said that you currently have an A10 crank fitted but did you use the A70 flywheel?
It will be interesting to see how the timing pin in your A70 primary relates to the rotor marks if you use the timing slot in the flywheel.

Dave.

Servodyne

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Re: A70 ignition timing
« Reply #43 on: 08 November, 2020, 09:59:13 »
Morning Dave,

Unfortunately I don't have the A70 flywheel, only a crank so I'm not able to verify if it was timed to 38 deg. Maybe someone should ask Angus to check his timing with a strobe.
The flywheel on my A70 is the A10 one turned down and lightened with the cut outs as per the late A65.

Jim

DAVE BRADY

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Re: A70 ignition timing
« Reply #44 on: 08 November, 2020, 15:19:17 »
Hi,

I have just put the question to Angus via his youtube video so we will have to wait and see what he says.

Dave.