Odd starting...

[englishmaninwales]

Very occasionally when hot restarting the engine fires, sounds odd with some carb induction noise and exhaust smoke out of the air box....almost as if it starts and runs backwards ......if that is possible. Only runs like this briefly, as I've switched off, and then is difficult to start. Then restarts properly when I've left it for 5-10 minutes.
What's going on and is it possible to stop it?
Thanks
Malcolm

[Chancer]

Yes, its running backwards, well kicking backwards actually it usually only fires once.

Caused by the ignition advance, it can break cam followers on a X/flow pushrod engine where the inlet valve (large head) tries to open against the explosion.

After a couple of follower breakages which didnt manifest themselves until the dry sump pump was toast I had to use a latching relay push button start system, a bit like a hand craked propellor aircraft, the plugs firing only after the engine was cranking.

I had a X/flow that i built, the compressions were so good that sometimes just stroking the battery lead against the terminal with the ignition on (like opening the points) would create a spark that would cause the engine to start or run backwards according to where it had stopped. Some vintage engines could be started this way, you would turn them on the crank to TDC and then retard the ignition on the steering Wheel control until the points opened and the engine would often fire up.

You also risk a carb/air filter fire.

Editted, mine did it also when hot, it was because the cranking was very slow due to the higher hot compression, either upgrade the starter/battery so it always cranks fast enough to overcome the statis advance that is opposing it, retard the ignition which will cost you power, do the push to latch ignition circuit which also is a good anti theft device, or sledgehammer to crack a nut, a fully mapped programmable ignition system.

Double editted, you said "does it briefly as I switch off" that is as I described, the engine is stopped just before TDC with the points closed (coil charged) one cylinder full of charge ready for the power stroke, switching off the ingintion collapses the field in the coil creating a spark, the engine will kick backwards and may turn for a couple of r?volutions Under the inertia.

[englishmaninwales]

Thanks, so maybe I'll start by just reducing the amount of advance a little, and see if that eliminates it. I did wonder about the risk of a fire in the air box, though

[Mick6186]

I found that timing too far advanced will cause the engine to 'kick back' and may break the locating flange of the starter motor, several times if you don't realise what the problem is!!
Mick

[englishmaninwales]

Taking a couple of degrees off the advance seems to have done the trick, without any noticeable loss of performance.
Thank you.
Malcolm