A7 Plunger Timing

[bsarider]

Could anyone tell me the correct BTDC ignition timing in degrees for a 1949 longstroke A7 plunger I know the book says 5/16" with the advance weights wedged fully open but how does that relate in degrees BTDC.
    I have seen the formula on the site but am unable to work it out without going for a degree in nuclear science. Thanks Alan

[MG]

Not having a suitable table for the longstroke A7 to look it up, I could calculate it for you.
I would just need the values for stroke (82mm I think???) and conrod length (eye centre to eye centre).

Cheers, Markus

***edit***

values in mm would be highly appreciated  

[bsarider]

Marcus, Not having the engine apart I would not have the conrod lengths so hopefully someone else on the forum may have that information that they will post.The stroke is shown in the haynes manual as 84 mm. Alan

[MG]

Alan,

I thought the longstroke A7 had a bore of 62mm originally?
If so, it would have a stroke of 82mm, resulting in a capacity of 495.13cc.

A stroke of 84mm would give a capacity of 507.20cc, which is not correct.

Perhaps someone here knows exactly and can also tell us the conrod length in order to calculate the timing figure (I set up a little Excel-spreadsheet some time ago for that, so that's an easy one then).

Cheers, Markus

[a101960]

bsarider

If you want to calculate degrees BTDC here is a handy calculator dedicated to this function. However 5/16th equates to 32 degrees BTDC.

John

http://john.rushworth.com/Pics/Piston/index.html

[BSA_54A10]

I always check using a degree wheel.
You can glue a magnet to the back of a plastic or aluminium timing disc and pop it onto the end of the cush drive nut.

[bsarider]

In the  booklet BSA Instruction Manual A series in the technical data it gives the A7
stroke as 72.6mm  and the bore as 66mm and gives the cc as 497 but it doesnt specify "Longstroke" anywhere. The plot thickens I am sure someone will know. Alan

[bsarider]

http://www.bsawiki.co.uk/index.php?title=Chapter03 this is also shown in service sheet
701 which I havent found yet. Got this information from Dragonfly Motorcycles. Alan

[MG]

So yours being a 1949 model (engine numbers ZA7 101 - ZA7 7000) would have a bore of 62mm and a stroke of 82mm, like I thought, resulting in 495cc capacity.

Ignition setting, following the link you posted, would be 3/8" BTDC, so being timed a little earlier than on the short stroke models, which makes senses regarding the higher piston speed of the longstroke engine.

But still we'd need the conrod length in order to get the correct crank angle. 

[terryk]

I have long stroke conrods in the shed I will measure them for you ASAP.

[bsarider]

TerryK from down under has sent me the details of the length between centres of the A7 longstroke plunger which is exactly 6.5".
Also talking to SRM they kindly gave me the information that with modern fuels the timing should be set at 5/16" or 35degrees BTDC on the  longstroke A7.I will give it a try when my Magneto returnes. Alan

[MG]

Alan,

there's something wrong with the data from SRM.

Considering a stroke of 82mm and a conrod length of 6.5" (165.1mm), 5/16" would be approx. 32.7 deg, 3/8" approx. 35.7 deg.

[bsarider]

MG it seems I keep getting conflicting data could I put it to the forum where would they time the engine 3/8"as it says in the BSA data or 5/16" as SRM have indicated which does not work out to 35 degrees. I havent received my mag back yet so there is still time to alter the degrees . What about 34 degrees and alter my points in or out
when I try the bike?

[a101960]

there's something wrong with the data from SRM.

Considering a stroke of 82mm and a conrod length of 6.5" (165.1mm), 5/16" would be approx. 32.7 deg, 3/8" approx. 35.7 deg.

Spot on MG. SRM have got it wrong. The figures quoted by you are much nearer the mark. Bsarider if you set the timing to 35 degrees you will be to far advanced.

John

[Brian]

Just to clear up a small point here, the standard longstroke A7 runs 5/16" advance but the Star Twin has 3/8".