preignition? don't know what to call it....

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Jan 4, 2007
so i had this kind of backfire/pop through the carb sometimes and even caused a little kick back on the kickstart. anyway, i can avoid it by kicking through once or twice with the ignition off and clearing any left over petrol left in the chamber...can someone explain this to me? is it easy to rectify or should I just keep my current ritual of clearing the cylinder first then starting?
pre ignition

Hi Commando 74, that sounds like the ignition may be too far advanced,try retarding the ignition a little,better yet, do a full timing check with a strobe. Good luck. James
What type of ignition system is fitted? Points or electronic (if electronic, which type)?

Is the exhaust system the type with the balance pipe?
It does sound advanced, but before you adjust anything, do you have a Boyer ? This has been dealt with a lot here and low battery volts will give you too much advance.

It could just be that the couple of kicks are enough to put some life back into the capacitor.

The idea of unused petrol in the cylinders is not one that i've come across.

It may be that a battery charge up will do the trick.
Strangely enough I have recently started to experience the same thing after fitting a Tri-Spark sequential digital ignition.

I had an idea that some kind of flash-over of unburnt fuel was occurring between the cylinders through the balance pipe, the Tri-Spark only fires one cylinder at a time, unlike many electronic ignition wasted spark systems.

This never happened with the Boyer unit that was fitted before.
Curiouser and curiouser !

Not something I remember with points.

Looking forward to hearing the outcome :)
I have always been a little kick shy with the Boyer. Once in a while it fires the strong amp spark to the wrong side or kick back using other devious means. So I turn in the key push it though slow than with gusto. Spark ,fuel, it don't care were the valves are.
i knew I left out something's a Boyer and the battery is a brand new 12v 14amp sealed lead acid VARTA.
I reckon the problem is fuel vapour condensing out in the combustion chamber while the motor is swtched off. On starting the mix is too rich to burn quickly and as the exhaust opens it ignites. Usually gives puff of smoke or a backfire, and sometimes a light kick back.

Could be caused by many things, perhaps a leaking petcock or modern fuel.

Just a theory.

well thanks for the info....haven't checked the timing yet, but will do so and report the meantime will keep clearing the chamber as before.
Hi Commando 74,

For what it's worth, I had similar sounding problems a while back starting my old girl.

She would kickback and pop and wheeze while starting but once running she ran fine.

I checked the timing on the Boyer with a timing light and it was sitting at about 27 or 28 deg. @ 5000 r.p.m.

Once setting it to 31 deg. @ 5000 she now starts 1st. or 2nd. kick most times and never kicks back, unless the Battery is getting low.

As soon as she refuses to comply I whack a charge into her overnight and away she goes.

This is the only time I have trouble starting her, as once the Battery gets down to 9 volts the black box shuts down.

Good Luck Mike
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