Hitchcocks Forum

I'm working on a G that came to me in bits and boxes, you know the story, and I now have it finally back up and running. I have set the ignition timing at 3/8 in before TDC full advance, I just guess this is about where it should be, haven't read up on it at all, but when I retard the ignition it cuts out, I would have assumed it should still run at full retard. I'm also getting a bit of spitback through the carb with flame so I'm thinking of retarding the inlet cam one tooth. This thing is fitted with a 75mm Toyota Corolla piston so probably anything goes here. Any thoughts

Hi Norm,
Try 7/16" BTDC full advance and I am with you on retarding the inlet cam 1 tooth to try and fix spit back with the odd flame - it worked for me recently on a 612 I built. Regards, Paul.

Thanks Paul, that seems to have fixed it

When was the mag last rebuilt?


Weak old capacitor, failing insulation on the windings, cracked pick-up, plug gap too large (try 12thou) poorly dressed points...

Spit back through the carb is more likely to be weak on the throttle slide than cam timing related, retarding the cam 18 degrees is a HUGH jump...

Retarding or advancing a cam by 18 degrees [1 tooth] does sound like a lot, but in Bullet and probably G type engines, it doesn't make much difference and I have seen the results on the dyno on several occasions, where a rise or fall of anywhere between 0.5 - 2 bhp out of around 40bhp from the race engines was about all that could be detected. The 612 which I retarded the inlet cam on had been faltering on tick over, then spitting a small flame back to the carb and stalling. Fitting a 'silencer' would fix it, but the owner wanted a straight through, open pipe. With the cam retarded - which was done initially to lower the compression pressure a little to try and make the thing easier for the owner to kick start - a much steadier tick over resulted and the spitting back was gone. It would seem that, due to the nature of the exhaust, a big bore, straight pipe, that some of the still very hot exhaust charge was hanging around on the 'overlap' and lighting the incoming charge coming in through the opening inlet valve. Retarding the inlet cam put an extra 18 degrees between these two events and provided a cure. The performance was not noticeably affected. Here are some videos, the first one with the cam timings 'on the dots', the next with the inlet cam retarded by 1 tooth is a long test run in the Welsh mountains:
https://www.youtube.com/watch?v=owIQcjBfDAQ
https://www.youtube.com/watch?v=WlAh_uFB9hI
Glad it seemed to work for you anyway, Norm.
Cheers, Paul.

Some numbers I seem to remember from laboratory stuff -
Spark discharge in dry air is 10kV per centimetre or 25kV per inch. But that's at 1 atmosphere. at e.g 8:1 or 8atm the distance becomes 8 times less or you need 8 times as many volts
An ignition coil can or should produce 30,000 Volts. Do the back of the envelop thing and it becomes 25kV so it can jump 1/8-in at 8atm. But it's not dry air, it's fuel mix which has a higher density, less gap then. Hence 25-thou for spark plug (20-30). Make the gap too small and you don't get good ignition, it's about the flame path.

Also seem to remember 16-thou for contact breaker or you get pitting and spark erosion. 400VDC can jump 16-thou, the cap and primary side of the coil (LC circuit) develops some serious volts at the points, oops.

And sorry but I recently gave my last points condenser away, probably 25µF (25-MFD). Some digital multi-meters have a capacitance meter function to test them, anything over 16µF should work.

This thing must have shaken itself to bits when it was running, the flywheels were so far out of round when I put them in the lathe, drive side wheel was really bad.

Norm?

you are back...YAY!

Missed your posts!

Sorry Scaly, just passing by, trying to get the last few bikes finished that I had promised I would over the last few years