A70 ignition timing

[DAVE BRADY]

Hi,

Does anyone know if the A70 ignition timing is different to the A65?

Dave.

[JulianS]

The 1972 data book gives 37 - 39 degrees for the A70, slightly different to the A65 figure of 34 - 36 degrees.

[DAVE BRADY]

Thanks Julian,

I will tweak it a bit and see how it goes.  I have used A65 and it is not too bad but with the long stroke I tried A10 but too retarded.

Dave.

[Servodyne]

The 1972 data book gives 37 - 39 degrees for the A70, slightly different to the A65 figure of 34 - 36 degrees.

Hi Julian,
Any chance that you could post the A70 engine data?
Cheers Jim

[DAVE BRADY]

Hi Jim,

If it is the page that Julian put in his reply then I just dragged the image on to my desktop.

Dave.

[JulianS]

This might help.

[JulianS]

Next 3 pages. Dont think they add anything.

[JulianS]

Last 2 pages.

[Servodyne]

Julian, thanks for the complete set of data.
Cheers
Jim

[DAVE BRADY]

Hi,

Just out of interest I asked SRM what they would suggest and they say 36º to allow for modern fuels which burn more quickly. I have set my 750 at 38º and will get out on it for ride and then maybe try 36º.
It does seem to kick back a bit at 38º but sounds really crisp.

Dave.

[DAVE BRADY]

Hi,

As said before I set the timing to 38º.  Hopefully it was accurate but a lot of kick back trying to start. Too much in fact so retarded it a bit until it was easier to start.  Perhaps a couple of degrees as I did it by eye by just moving the ign. back plate a bit.  Easier starting and it really went well.  Pulling from low revs to 60+ smoothly and quite briskly.  A bit of a kick when the power came in at about 3500 rpm or 60mph.  Only a single carb so not gut wrenching but quite satisfying and all of this on about 1/4 throttle.  I backed off at 75mph as it was a 60mph road. 
The carb fitted is a 32mm Mk2 Conc. which might be bit big considering the 750 Triumph Tiger ran a 30mm.  I am going to try a 30mm Mikuni to see if that increases the low to mid range as that is the most useful.
I know that this bike has topped 110mph with ease but I am not in to mega revs and low down grunt is far more enjoyable and useful on todays roads.

Dave.

[Servodyne]

Hi Dave
Mine runs quite happily at 34 deg, which I thought was correct for an A70 until this post. I may now up it a bit to see if it makes any difference. Mine should be able to tolerate more advance as I'm running with an A10 crank which gives me 20 thou less compression.
Interesting that you've got a thunderbolt head fitted, any particular reason why?
Jim 

[DAVE BRADY]

Hi Jim,

The 38º was from the BSA data that Julian posted but it did kick back quite violently.  SRM suggest  32º for the 650 and 35º for an alloy head A10 as they recommend 2º less advance for modern fuels. So RR and SR would normally be 37º?.  I am thinking that the stroke of the A70 is only 1mm more than the A10 then the same timing may work but I did try 35º and it made the bike a bit flat.  I will check my 'by eye' setting and see what it is.  I am also using the racing type rockers with the curved surface that actuates the valves so I may be getting a bit more lift and this may have a bearing on the amount of advance that works best for my engine with one carb.  SRM also suggest 36º for T140 which would mean 38º if not allowing for modern fuels.
The bike is originally a '69 Thunderbolt owned and originally converted by Stan Pollock.  He ran the bike for many miles and years with the 32mm single carb.  It has an A70 crank rather than a modified A10 and currently runs with T140 pistons so compression is about  8.5:1.

Dave.

[Ian C]

Thanks for posting this very interesting subject Dave..

[Servodyne]

Hi Dave
I'm also using the Devimead rockers but only on the inlets as I haven't found any for the exhaust. I'm also using A70 pistons which cost me an arm and half a leg from Bantam John, which by my calculations will give me 9:1 compression with an A10 crank, if the 9.5:1 ratio is correct for an A70.
I've just compared the alternator rotor keyway positions between my damaged A70 crank and an A65 one, and they appear to be bang on the same, so the increase in advance may have been accommodated by the location of the timing pointer on the primary chaincase cover.
I remember Stan from years back when he was campaigning modified A65s in sidecar racing to good effect. I seem to remember him favouring Thunderbolt heads for racing, but I could be wrong, it's a long time ago.
Jim