Preignition

[idie]

Definitely 7/16 inch not 7/8 inch my mistake.

[Greenfield]

Taking one step at a time, by the look of things, I need to retard ignition by about 1/4". By my reckoning the magdyno pinion needs to be rotated on its taper. I have removed the magdyno pinion nut and I need an extractor to ease the pinion free. Can someone confirm that the extractor is 61-1903 before I buy? The inside thread looks about 5/8".
Once the pinion is free I suspect the fun will start getting everything into position and tightening the pinion nut without disturbing the set up. Happy days!

[martinh10]

It's a 5/8" 20 tpi thread, the same as the the cylinder head bolts (in the crankcase). If you have a spare/scrap cylinder head bolt just tap a thread for a 7/16" BSF or whit bolt in it and you've got a mag pinion puller - see photo.

[Mike Farmer]

 

Re tightening without disturbing positioning.

I set it in position then press hard to stabilise it. Then get a suitably sized socket put over the spindle against the sprocket and give a "vigorous" tap with a hammer. This will set it on the taper. Then with care it wont move as you tighten things up.

Mike

[Greenfield]

Fortunately a local B31 owner heard my cry for help and kindly lent me a puller.
Once the timing had been set, I started it up and it was awful. Although the carburation was pretty close before altering the timing, perhaps slightly on the weak side, it was now way out and running very weak, so much so that the exhaust blued in just a few minutes. I rechecked the timing and found it slightly retarded, so its now been set correctly. It still runs weak. The pilot screw is fully in and the engine will not pick up from idle on full retard; it spits out of the air intake and cuts out. The positive thing is a short trial run with the advance set around middle, I've now got rid of the suspected preignition.
Having stripped the carb and removed the jet block to make sure that everything was thoroughly clean, I would now recommend to myself that I fit a slightly bigger pilot jet. But that's not possible with a 289. I'm fairly sure that air isn't leaking at the manifold and the throttle slide was renewed not long ago.

[Pete C]

Apart from the ignition timing, I think a 289 is too large for a B21, standard would be 276 1 inch bore or a 376 monobloc.
Trying to diagnose problems when the initial setup is non standard makes it more difficult.   

[idie]

He has a B33 500cc isn't a b21 a pre-war 250cc

[Greenfield]

Yes its a B33. Everything about the carb is as to specs apart from the fact I've raised the throttle needle a notch. I've just taken the carb off yet again and probed every orifice with fuse wire to make sure everything's clear. Having started up and warmed it up, the pilot circuit really needs to be a bit richer. I hadn't appreciated how closely linked the timing and carburation were. Running weak will also contribute to preignition.
What has crossed my mind is how much air still enters the idling mix when the airscrew is fully in, and if the screw is not seating properly, could additional air get through?

[Spaceman]

Normally when sorting out ignition timing and carburation problems on a bike, the last thing to worry about is the tickover and the pilot mixture. In general, the pilot circuit will have no effect on how the bike runs after the throttle has been opened slightly. Clearly, in this case, there are more fundamental problems with the way the engine is running and it might well be a case of having to go back to basics including, possibly, a top-end overhaul, a check of the valve timing and a thorough check of the magneto and the ignition timing. As said, worrying about the pilot mixture would be bottom of my list!

[Greenfield]

Thanks Spaceman. That's what I'm thinking. I'll take it out for a decent run and see how it goes. Yesterday on my one mile circuit it seemed to pull ok at a range of revs. So as long as I can rely on it not to stop and I can reliably start it, I'll live with it for the time being. Anything major can wait for 'a rainy day'.
At least the clutch, gearbox, brakes, speedo etc are all performing now.

[Pete C]

Apart from the ignition timing, I think a 289 is too large for a B21, standard would be 276 1 inch bore or a 376 monobloc.
Trying to diagnose problems when the initial setup is non standard makes it more difficult.   

Sorry I got confused by a previous post and thought it was a B31 (my B21 was a typo!  )