Author Topic: A70 ignition timing  (Read 658 times)

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DAVE BRADY

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Re: A70 ignition timing
« Reply #15 on: 10 October, 2020, 11:28:01 »
Hi Jim,

I wonder if the pointer was in the righthand hole it would give 38?  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

Servodyne

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Re: A70 ignition timing
« Reply #16 on: 10 October, 2020, 12:51:18 »
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

DAVE BRADY

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Re: A70 ignition timing
« Reply #17 on: 10 October, 2020, 19:39:46 »
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 32 to 38 in 2 increments.  It would be so much easier if the rotors had these as standard.
That is tomorrows job then.

Dave.

DAVE BRADY

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Re: A70 ignition timing
« Reply #18 on: 11 October, 2020, 11:15:15 »
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 36 and 38 on the rotor and I have set my timing to 36. 
I will go for a ride tomorrow to see how it feels and perhaps strobe it if my strobe light still works.

Dave.

DAVE BRADY

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Re: A70 ignition timing
« Reply #19 on: 12 October, 2020, 11:27:55 »
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.

Servodyne

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Re: A70 ignition timing
« Reply #20 on: 12 October, 2020, 18:52:41 »
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

DAVE BRADY

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Re: A70 ignition timing
« Reply #21 on: 12 October, 2020, 19:21:25 »
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.

DAVE BRADY

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Re: A70 ignition timing
« Reply #22 on: 14 October, 2020, 13:11:03 »
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 34 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.

DAVE BRADY

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Re: A70 ignition timing
« Reply #23 on: 15 October, 2020, 15:20:44 »
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.
« Last Edit: 15 October, 2020, 15:26:10 by DAVE BRADY »

Servodyne

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Re: A70 ignition timing
« Reply #24 on: 15 October, 2020, 20:21:57 »
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

DAVE BRADY

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Re: A70 ignition timing
« Reply #25 on: 15 October, 2020, 20:51:47 »
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.

Servodyne

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Re: A70 ignition timing
« Reply #26 on: 15 October, 2020, 22:05:40 »
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

DAVE BRADY

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Re: A70 ignition timing
« Reply #27 on: 15 October, 2020, 22:35:40 »
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 36.
When I get/fit a Pazon I am going to try 38 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.

DAVE BRADY

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Re: A70 ignition timing
« Reply #28 on: 20 October, 2020, 09:08:42 »
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 38 is on the rotor and in relation to the timing pointer.  A 38 pointer would be where Servodyne has his positioned.  But having said that, the difference between 36 and 38 is very small on the rotor hence we use huge external timing discs to get accuracy.

Dave.

DAVE BRADY

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Re: A70 ignition timing
« Reply #29 on: 26 October, 2020, 15:41:53 »
The next instalment,

Bought a Pazon Sure Fire and fitted it yesterday.  Static timed using a degree disc to the 38 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 38 of advance specified for the A70 engine.
I cannot wait to eat the pudding.

Dave.