Noise when turning ignition off

[DomP]

I've mentioned this before but not found out what causes it, every now and then when I turn the ignition on then off I get an airy pop type noise from around the seat/air filter area.  Any idea what's causing it?

[taysidedragon]

It could be some kind of spit back through the carbs and air filter. Usually caused by timing or fuelling being a bit out. Another cause of the sound could be air leaks around the intake manifolds?

[Athame57]

I've had that myself, also when I turn the fuel tap back on. Maybe the bike has farted?

[DomP]

That's kind of what it sounds like😄

[McCabe-Thiele (Ted)]

Could it be that turning the ignition on then off causes a spark that ignited any fuel vapour in the cylinder head?

 I would expect this to reduce to zero with repeated on/ off as the petrol vapour is burnt off.

[DomP]

That does sound like what it could be, is it normal or suggest anything Ted?

[McCabe-Thiele (Ted)]

That does sound like what it could be, is it normal or suggest anything Ted?

I have only encounted this type of noise decades ago on a highy tuned A series engine that had a racing camshaft fitted with intake trumpets.
As has been mentioned is the timing correct likewise is it idling smoothly on balanced carbs?

Is the air intake system/filter  suitably enclosed to surpress what might be a normal occurrence?

[DomP]

There's definitely a few of those questions I've not addressed yet Ted, timing I believe was spot on when I refitted the Dyna s ignition.   As for balancing and air filter cover, they are a work in progress

[K2-K6]

There's definitely a few of those questions I've not addressed yet Ted, timing I believe was spot on when I refitted the Dyna s ignition.   As for balancing and air filter cover, they are a work in progress

Thats potentially the origin of it. Some system routinely discharge on shutdown which can fire anything fuel vapour-eee thats laying around in air intakes.

Likely that one of the cylinders has stopped with intake valve open to facilitate a route to vapours from one of the plugs.

A friends Ducati 916 routinely does the same.

[taysidedragon]

I've just realised that you get this without running the engine. 🙄 I was tired!
I would look at the starter solenoid. It's in the area you hear the sound and is powered on and off by the ignition.

[deltarider]

The starter solenoid was also my guess.

[K2-K6]

Solenoid doesn't switch with ignition at all.

Only active to pull load across the two large terminal when you "call" it with starter button. Not used otherwise.

In any, most, start system, those two high load terminal remain live irrespective of ignition key, being direct from battery positive and not through additional loom routing.

[K2-K6]

To test for OP, you can first switch the engine cut out to OFF as that interupts the coils, then turn the key first on then off to see if it still has symptom.

[Laverda Dave]

I've had similar symptoms with the Triton. Turn the key to off and it would pop back through the carbs. Having bell mouths on the carbs meant I could actually see it spit back through them. It is definitely the ignition timing causing the problem and despite setting the timing as per the manual it is not correct. The reason is probably as a result of the high compression, change of combustion shape etc.

[deltarider]

Solenoid doesn't switch with ignition at all.

Must be the regulator then I hear, when I switch the key ON or OFF.
I seem to remeber I have read somewhere on the international forum Dyna ignitions can cause this popping, but for what reason?