Topic: Odd carb/pod "pfft" when key is put to on position

[mickvain]

I ha e had this odd thing happen twice now that I have noticed. Both times after a long ride, have stopped, ran an errand, gotten back, put key in ignition, turned to on position and then heard an audible "pfft" noise and seen a small white puff of smoke, vapor (I really don't know) exit the pod filter on the number 1 carburetor.
Bike is a 75 CB400F. It is a bit befuddling. Any ideas as to what I should look for?

[Frostyboy]

Do you have the original points ignition or electronic? I've noticed some electronic units will fire a coil off when switching on. My 550 has the electronic ign from David Silver & I've heard the "pfft" on the odd occasion when I switch on. I've never worried really, I think it just depends where the crank finally comes to rest after switching the ignition off.
It only ever happens as you say when restarting after a short time after stopping. I put it down to there being still just enough fuel being present to cause a lil explosion. It's a waste spark system, so it'll be firing the plugs on the cyl that's on compression & the cyl where both valves are rocking. Hence the puff out of the inlet on the companion cyl.

Edit: The more I think about what I just said, it's more likely the plug firing on the cylinder that is on compression if blown ever so slightly back down the bore which results in another cylinder blowing back out the inlet.

[Terry in Australia]

Yep, common in bikes with electronic ignitions. Nothing to worry about. Cheers, Terry.

[mickvain]

Thanks all. Cheers!

[andyvclifford]

I just installed the pamco ignition and noticed this for the first time today and was wondering the same thing!

[pamcopete]

It is normal for any ignition to fire the coil when you turn the ignition OFF with the engine not running. This will happen if the engine has stopped with one set of points closed or in the case of an electronic ignition, with one of the driver transistors on. Turning the switch OFF is the same as opening a closed set of points.

However, if you are experiencing this phenomena when you turn the switch ON, then it can be an indicator of dirty switch contacts or other malfunction of the switch, known as "contact bounce", where the switch initially does turn on, but then immediately turns off and then back on as you rotate the key. Might be time to measure the resistance of the switch contacts and / or clean the contacts. Another test would be to stop the engine with the kill switch before turning off the ignition switch, and then turn the ignition switch on first before the kill switch the next time you start the engine to see if the same "pffft" happens.