Super Rocket ignition timing

[robin smith]

I was recently in discussion with a well-known magneto restorer on the subject of Super Rocket ignition timing. He suggest 3/8 or even 7/16 BTDC would be possibly more appropriate with modern fuel, as apposed to the 5/16normally applied by myself.

I would be interested to know your views regarding this subject  

[STAR TWIN]

3/8" is the book setting. However, modern fuels are lower octane (they even changed the method of calculating octane ratings possibly to disguise the drop) so I would suggest 5/16" would be more appropriate. The one thing you should avoid is over-advancing the ignition timing. That way pinking and detonation lie. Don't know why you were advised 7/16". Seems quite wrong to me.

[JulianS]

I use 34 degrees BTDC on the most advanced cylinder with high comp pistons. Other cylinder is 33 degrees BTDC. Runs and starts fine on  that.

[DavidTilley]

Just timed my recently rebuilt engine at 3/8ths BTDC as per Haynes Manual but getting pinking when the engine is hot. I'm running high comp pistons and a spitfire cam. Any comments please? Cheers!

[JulianS]

A degree disc will give more accurate timing.

Expect some variation between the cylinders.

If you have timed it on the more retarded cylinder then the other could be too far advance and causing the problem.

[DavidTilley]

JulianS, thank you for your reply.
I'm puzzled though why the pinking only starts when the engine is hot.

[STAR TWIN]

See my previous advice regarding timing and present day petrol. Pinking will happen with a hot engine because the engine is hot... Pinking is pre-ignition of the petrol and that happens more easily in a hot engine than a cold one. Have you ever experienced running-on with an engine where it continues to run despite ignition being turned off? Same thing. Detonation is somewhat different. That is explosive burning of the fuel/air mixture instead of a controlled burn. It quickly destroys pistons.

[DavidTilley]

Thank you. Will try.