Auto advance /retard mechanism

[oldjexy]

Does anybody know what the correct adv/ret mech is fitted to an 1965 AJS 18 short stroke .
I have a 10 degree on my bike and the only reference I can find seem to indicate that 15 degree were fitted to other models.
When I set it up to fully advanced, 38 deg that makes fully retarded 18 deg which backfires me into the top of the garaged when attempting start.

[Duncan]

I don't know the answer to your question or how the units are marked but would 10 degrees on the unit run off the cam at half engine speed equate to 20 degrees at the piston?
There is in one of the red books (i don't have them to have at the moment) settings for coil ignition singles that gives details for setting the timing with the bobweights closed or wedged open, I'm not sure if the corresponding degrees are mentioned as well to give you a guide to what a standard long stroke setup is.

[oldjexy]

I have a mate with a 1959 Norton ES2. His adv/ret mech is marked up 15 deg. The timing setup states fully advanced 38 deg, fully retarded or static as 8 deg. The adv/ret mech drives from the exhaust cam and is a 2:1 ratio, so on the camshaft it is 30 deg, mine is only 20 deg.
I'm trying determine if this is the problem with my set up.

[Groily]

Couldn't see any mention of degrees when I scoured those sources Duncan. I'm not sure the info is actually out there in an easily-accessible published source.
As a matter of reference, for long-strokes fitted with magnetos, the ATD had 11-13° (22°-26° crankshaft) while the widest range of movement of any Lucas ATD designed for a magneto was 16°-18° (on Vincent Comets, some Ariel singles and on a good few twins which perversely probably never needed it).

Lucas coil ign ATDs clearly came in several sorts from the tiny 5° movement on ones designed for use on Energy Transfer systems, and then a fairly common 12° one (Triumphs maybe?). Seems there was also a 10° version, and obviously there's a 15° version per the ES2. If one of those were to hand, it could be the answer.

I wonder idly if the bobweights could be carefully ground down a bit to increase the arc of movement? They'll all be the same basic unit almost for sure, bar the 'stops' and perhaps the springs, so that could be a workaround if a 15° one wasn't available maybe?
I also wonder how these short-stroke motors might run these days on a tad less max advance, say 36°ish? I've found a degree or two less than 'book' has been good on modern fuels on some engines but no idea in this case!

[Duncan]

Found it!
http://archives.jampot.dk/book/Workshop ... models.pdf
Page 28 - Setting the ignition timing
It refers to 34 degrees advanced and 19 degrees retarded so a range of 15 degrees for the single cylinder heavyweight model,

[SPRIDDLER]

I've never had coil ign, only mags so I may be talking out of my decompressor but are the bobweights returning fully to the 'at rest' retarded position?

[Groily]

Found it!
http://archives.jampot.dk/book/Workshop ... models.pdf
Page 28 - Setting the ignition timing
It refers to 34 degrees advanced and 19 degrees retarded so a range of 15 degrees for the single cylinder heavyweight model,

Well spotted Duncan! But . . . that speaks to 15° of variation on the CRANK, which to me looks like a mere 7.5° on the ATD at half engine speed on the inlet cam? On the engine in question we'd then have 38° and 23° and the garage rafters would still beckon!
I reckon an ES2-style 15° ATD giving 30° on the crank might be the solution, but would a short-stroke tick over and pick up nicely from a starting point of 8°BTDC I wonder? What's needed is a part number and spec for the version of the ATD used on those later engines I reckon, and that info is proving elusive.
Why is nothing simple?? And are the workshop instructions actually correct??

[Reynard24]

According to page 53 of my copy of Service and Overhaul Manual for the AJS & Matchless Single Motorcycles by F Neill, the maximum advance is 38 degrees (10.98mm) for the the ‘64 & ‘65 short-stroke singles with the ignition unit fully advanced. The 34 degree figure is for the 350 singles.
This information is also on page 102 of the manual that Duncan posted the link for earlier in the thread.
I have a spare ignition unit (A/R module) for a 1965 short-stroke 500 single and this is marked 10 degrees on the back plate.
I have used the 38 degree (or rather the 10.98mm when using the rod down the plughole method) on two 500 short-stroke singles and had no problems with kickbacks.

[Groily]

"I have used the 38 degree (or rather the 10.98mm when using the rod down the plughole method) on two 500 short-stroke singles and had no problems with kickbacks."

Yes, there was no confusion (as far as I understood it) about the 38°BTDC full advance setting for a short stroke - the thing that's caused the pause for thought is the degrees on the atd.
Out of curiosity, I checked yesterday with a friend who has a short-stroke M18, and he reckons he has a 10° one too, same as you. It starts easily first or second kick. I have started it myself many times and it also doesn't kick back.
Which makes me wonder if the initial setting in this case was too advanced possibly, or, as Spriddler wondered, if the unit was sticking.