Backfire (kaboom)

BlueDubPirate

My B&S Vanguard 23hp Vtwin backfires through the muffler on shut down. It goes like this...take a nice drive through the neighborhood. Pull into the garage, turn off the key and as soon as the motor stops, Kaboom!

I love it, but the rest of the house doesn't. Any thoughts on the cause or cure?

Thank you in advance.

BGW

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crash12888

Does your motor idle when stopped or shut down when you let off of the pedal?

crash12888

Just reread where you turn off key to stop so it idles when you park?
Is your engine new enough to have a fuel shutoff solenoid on the carb?
Are you able to drop the RPMS at idle any?
When you stop try letting it run at idle for a few seconds to clear extra fuel?

BlueDubPirate

Hey, yes it is a key start and I've already adjusted the idle RPM's down a bit since installing the engine. It does idle nicely when stopped. The motor is brand new (Vanguard Vtwin 23hp). I'll try letting it idle a little longer before shutting it off.

crash12888

If it is brand new it should have come with a fuel shutoff solenoid on the carburetor. when you turn off the key it closes and shuts off fuel to the intake. look for it (has wire to it) and see if it is wired to close at shutdown or maybe sticking open?

slonomo

It should have the "anti backfire solenoid". It was either removed, or it was not wired up correctly. You can tune the carb to resolve that issue too. Idling does help.

Attached Images

anti backfire solenoid.jpg (100.6 KB, 0 views)

chevydmax6

I would look for a exhaust leak where the exhaust mounts tk the motor

Fairtax4me

Good ole backfire briggs!
Dunno how many of these I’ve dealt with and it’s almost always carb adjustment. Like these other guys said if it has the fuel shut-off solenoid make sure that’s working properly. If it doesn’t have that try adjusting the idle mixture needle a bit, and try adjusting the high speed mixture just a hair more rich if it has that.
When it’s running at higher rpm, if the mixture is lean it’s building up enough heat in the head and the exhaust to ignite the still raw air/fuel mixture that’s going into the exhaust system after you kill the spark. Those last few revolutions before the engine stops turning it’s still pulling fuel through the carb. When it finally stops turning these engines almost always turn backwards 1/4-1/2 turn and that can suck a small amount of clean air back into the muffler. Clean (unburned) oxygen plus hot A/F mixture that got blown out of the cylinders makes a Big Bang.