Hi,
Does anyone know if the A70 ignition timing is different to the A65?
Dave.
The 1972 data book gives 37 - 39 degrees for the A70, slightly different to the A65 figure of 34 - 36 degrees.
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.
Quote from: JulianS on 04 Oct, 2020, 14:26
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
Hi Jim,
If it is the page that Julian put in his reply then I just dragged the image on to my desktop.
Dave.
This might help.
Next 3 pages. Dont think they add anything.
Last 2 pages.
Julian, thanks for the complete set of data.
Cheers
Jim
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.
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.
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
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.
Thanks for posting this very interesting subject Dave..
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
Hi Jim,
I wonder if the pointer was in the righthand hole it would give 38º? The manual says that this positions was for using an energy transfer ignition but there is no number given in the data. It is possible that the alternator rotor was marked differently but I doubt that there are any A70 specific rotors about unless someone with a genuine A70 can have a look or perhaps a T140 rotor as bore/stroke is very close to A70.
Dave
Hi Dave
I doubt if the rotor is different as this would make it a special part and not generic, and all the rotors that I have seen have the timing marks lining up with the keyway.
I suspect that the advance was incorporated into the timing indicator on the primary. You can clearly see a difference between my A65 Firebird and A70 in the pics.
Jim
Hi Jim,
I take it that the A70 is the bottom one. The shape of the casing looks a bit different but that could just be production variances. So it looks like the A70 pointer has been moved slightly to the right so in the direction of rotation. Does this not retard the setting rather than advance? Is the location of the holes slightly to the left to begin with and the middle location of the pin is fine tuning?
I think I am going to work out some degree markings on my rotor. As the mark is in line with the keyway and hence TDC it should be quite easy to measure some degree marks. I will do it on a spare rotor first and transpose it to the one fitted. probably 32º to 38º in 2º increments. It would be so much easier if the rotors had these as standard.
That is tomorrows job then.
Dave.
Hi,
This was not straight forward as I could not find my protractor so it was back to a degree disc and a wire pointer. So I now have marks for 36º and 38º on the rotor and I have set my timing to 36º.
I will go for a ride tomorrow to see how it feels and perhaps strobe it if my strobe light still works.
Dave.
Hi,
Went for ride yesterday two up and the bike ran fine but still a bit of kick back on starting so still a bit advanced. I do not think that my new and very hard to see marks are accurate enough.
Strobed today and very advanced so a bit more retard until it starts easier.
Dave.
Hi Dave
What ignition are you using? If you are using a Boyer or a Wassell, there's quite a bit less retard from the fully advanced position compared to the mechanical advance and retard. The attached chart is by Dave Comeau from the Access Norton forum and is for a Norton, but the values won't be far off a BSA A65.
I'm not sure what to do with mine yet. I've timed it to the timing marks which I thought were 34deg but I haven't actually measured it.
Jim
Hi,
I am using Boyer Mk3 on both of my 650 and the 750. On the various graphs I have seen it looks to be the smoothest.
I will start from scratch tomorrow and get it right.
Dave.
Hi,
So today I almost went back to basics. The A70 crank is different to the A65 and that and the flywheel are listed in the A70 specific parts supplement.
The A65 flywheel has the timing slot in it and sets the crank and pointer for 34º btdc. So logic dictates that the slot in the A70 flywheel would be machined to give the correct btdc position.
So I stick a screwdriver in to locate and centralise the slot and low and behold the timing is not that far out. The pointer is just to the right of the line on the rotor which would correspond with the position of the pointer on Servodyne's A70 image. Interestingly, there does not seem to be an A70 specific primary cover hence this modification.
On checking with the now independently powered strobe the timing looks spot on but still a bit of kick back when starting. This may disappear once the engine is up to full working temp.
A two up road test tomorrow if the weather is OK.
Dave.
Hi,
Had a two up run out today and the bike seemed fine. It is ever so slightly less advanced to what I had the other day but still pulls well right through the rev. range feeling best once it reaches about 55mph but that could be down to carburettor settings.
The graphs are interesting, if I am reading them correctly, in that only the Boyer Mk3 has a nice smooth advance although it starts from a slightly advanced position although the Pazon looks good and is said to be a development of the Boyer - perhaps next time I will get a Pazon.
The others may have better mapping further up the rev. range but look a bit of a mess at the bottom end.
Dave.
Hi Dave,
The Boyer is smooth but it only has a 20deg advance curve, which means if you time it at 36deg full advance, the 'retarded' position at start up will be 16deg btdc which could give you some kick back. I believe the Wassell units are pretty much the same.
The Boyer MD which I assume is the MkIV has a 32deg advance curve, so if you time it again at 36deg full advanced, the start up timing will be at 4deg btdc, so should be much easier to kick.
The 'ski' jump at the bottom end of the rev range on both the Boyer MD and the Tri spark is an anti-stall device which advances the ignition slightly to make the revs rise before the engine stalls.
Jim
Hi Jim,
I am thinking that I may change to Pazon Surefire. According to the graph on the Pazon site it looks like it has the better retard position and tick over stabilisation and is not too expensive. That will mean that I can go with the full advance and get better starting. It also works better with a slightly below par battery again helping easier starting.
I know there are always compromises but I think the Pazon seems the best.
I must admit I like the way the bike went with the full advance but do not want knackered ankles and knees from kick back.
What do you have on your A70?
At some point in the future it would be interesting to compare your A70 with my pretender with a single carb. It does seem to have almost seamless power delivery and, according to my pillion, is smooth at 70mph.
A real test will be loaded with camping gear and climbing an Alpine pass.
Dave.
Hi Dave
I'm running with a Boyer MkIII on my Firebird and a Boyer MD MkIV on the A70. I've strobed both to the timing marks, but only the Firebird gives a slight kick back occasionally, which now I've studied the graph makes sense.
Both Firebird and A70 are pretty smooth at 70mph, but the A70 has much more pulling power to gets there quicker even though it's geared higher, (21T compared with 20T on the 650).
Both cranks have been dynamically balanced by SRM.
I'm just wondering what the actual ignition timing has been set to on my A70, I think I'm going to have to set up a timing disc to get the actual value when the primary timing marks on are aligned. I've a funny feeling it may be the same as the Firebird at 34deg, in which case I may advance it 2 or 3 degrees.
I've never tried Pazon but all the reports that I've heard about them are good.
Jim
Hi,
I run a 21 gearbox sprocket on my 650 as well as the 750 so about 3500rpm at 60mph. If you have used the timing pointer on your A70 I would guess that it could be 36º.
When I get/fit a Pazon I am going to try 38º of advance again and see what it is like. I had the crank dynamically balanced by a chap near Rotherham. He had been involved in balancing racing and drag engines but still did a bit in retirement. Well worth the money.
Once the timing is all sorted I will try a 30mm Mikuni. The Triumph Tiger 750 ran a 30mm carb.
Dave.
Good morning,
Yesterday I took delivery of a Helix School Geometry Set so I now have a protractor. It still has a bigger radius than the rotor but I could get a reasonable idea of where 38º is on the rotor and in relation to the timing pointer. A 38º pointer would be where Servodyne has his positioned. But having said that, the difference between 36º and 38º is very small on the rotor hence we use huge external timing discs to get accuracy.
Dave.
The next instalment,
Bought a Pazon Sure Fire and fitted it yesterday. Static timed using a degree disc to the 38º full advance as per BSA data. I have made a mark on the rotor for strobing if necessary. The bike fired up with absolutely no hint of kickback and with good pick up on the throttle and once warm it ticked over reliably and slowly. I will take it out as soon as the weather permits. It looks like the wider range of advance/retard of the Pazon has solved the issue of too much advance on starting and should give the benefits of the 38º of advance specified for the A70 engine.
I cannot wait to eat the pudding.
Dave.
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 38º 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.
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.
This service sheet will explain the 1 and 2 marks on the rotor.
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 38º as opposed to the 34º 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.
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.
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.
I've only ever seen the attached for the differences Dave.
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 38º. Either the primary cover was drilled differently or there was a specific rotor with the timing mark at 38º.
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 38º, 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.
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.
Below is the only A70 specific service sheet I can find.
The mention of "Special tuning information..." is interesting but was it ever published?
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.
Quote from: DAVE BRADY on 07 Nov, 2020, 14:13
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
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.
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
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.
Hi,
Just had a reply from Angus Campbell. He relied on accurate static timing using the slot in the crank and careful points set up as the pointer and rotor marks cannot be used. For this reason he did not use a strobe.
So there we have it, a mystery solved perhaps.
Dave.
Hi Dave,
Well done, but it would still be nice to nail this once and for all by comparing the two, static and dynamic. By my reckoning there should be 2.6mm difference on the circumference of the rotor between the 34deg mark and the 38deg mark.
I haven't retimed mine yet but I will have a go when the weather bucks up.
Jim
Hi,
I will check the mark I put on my rotor using a degree disc tomorrow but 2.6mm sounds right.
Dave.
Hi,
Resurrecting an old post here but perhaps useful. We discussed the pointer in the primary cover and the mark on the rotor for setting the timing for the A70. There did not seem to be a definitive outcome based on info. at the time.
However, a more recent post resulted in JulianS posting a document showing a rotor with a number 2 on it so that the A70 was timed against this rather than moving the pointer. This was included earlier.
So anyone out there with an A70 have a rotor with this '2' on it?
Dave.