Crank rotor scribed lines - are they TDC?

B+Bogus said:

I've found that the timing marks on the chaincase are quite often up to 5 degrees out, so checking manually is essential, IMHO

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See Fast Eddie, 300 minutes out.

errr, I agree with y’all about checking the on board degree marker for sure. Jus’ tryin’ to put some perspective to this, there is zero point in chasing the last minute of a degree. That’s all. 300 minutes out is probably something best corrected however.

I have tried playing around with ign timing on the dyno before. Basically, on old a Brit stuff, I have frequently found that you gotta go 4-5 degrees either way before you can detect that anything is happening. OTOH I once experimented (read: I set my ign timing wrong, rushing, etc!) and discovered that at 10 degrees retarded power drops off a cliff ... and heat does the opposite !!

A degree or two either side of nominal is really going to have zero effect on a normal road going Commando, because A) unless the bike is 100% standard, running on points and using 1970’s 5 star full-lead fuel then no one can really say what nominal actually is and B) the up-to-the-minute 1940s technology jus’ don‘t care !

The best learning on this I had was with a 988cc hot-rod Beezumph triple motor. It had a Boyer with wasted spark and 4 volt coils. On the dyno it seemed to be flattening out at a lower rpm and lower BHP than anticipated. Myself and Neil Beadling hypothesised weak sparks as the cause. I fitted a very expensive fully programmable ign that used a dedicated spark, allowing the use of 12 volt coils, so I fitted very high output Dyna coils which gave a spark I cudda welded with!

Back on the dyno, with no other changes, I saw an immediate 5bhp increase and a very healthy improved curve. I rubbed my hands expecting more to come as I carefully dialled in the programmable ignition. So, imagine my disappointment when after an entire day on the dyno playing with endless permutations of advance, retard, advance curve, etc I saw no meaningful improvement on that initial gain !

Moral of the story, the big, fat ‘could weld with it’ spark from high output coils gave the 5bhp increase. Fiddling several degrees either side of ’standard’ and playing around with advance curves made zero measurable difference to power or torque output or curves.

All good fun tho !

In a 1968 race engine build article Mr Degens gave a four degree 'window' in which the engine ran: 'well enough'
All the precision he needed!!