No it's not uncle buck pulling up its Austin in his bus!!! Why does my bus back fire when I turn the ignition off? Leads, plugs, cap, condenser, timing, all good. It's a 2.0 type 4 running twin 40 weber Ida's. Any ideas or could it be I have missed something obvious. Other wise it's great to drive so long as I don't have to stop any ware!!! Cheers Austin
Every time or only after a longer run and engine up to full temp??? Do 40s have any sort of fuel cut off attached to ignition?? Like solexes
Mine was doing that spring on weber was missing leaving choke open and engine racing when ignition was turned off its still drawing air/fuel mix in
Last time I drove it, I was driving it for about 20 mins, when I pulled up and switched the ignition off it was louder than a shotgun!! This is probably the shortest kind if journey I would do on it. Bit sure about any cut off or anything like that.
when you shut the ignition off after idling you'll probably have a bit of unburnt fuel vapour in the exhaust...assuming the engine and components are up to temperature, it may be hot enough to ignite.....but it wouldn't ignite unless there's a source of oxygen....which suggests you also have a slight exhaust leak at the gasket, which wouldn't necessarily show up when you're driving....Soon as you shut down, the exhaust starts to cool and contract...introducing the oxygen....sometimes it can happen several seconds after you've switched off...I had an old Austin 1300 which used to do this, after a decent run....
Hmmm the last time I set the timing was when my exhaust was shagged. It was totally ko'd being belief, I tried to patch it up but it was no good. I have since put a new back box on but not adjusted the timing again. Both gaskets are sealed up thoroughly. Do I need to do the timing again?
Won't hurt. The timing shouldn't have changed though. I could do the same trick if I gave it a rev and switched off while the revs are up. That was with my old exhaust which was leaking like a sieve, but the new one doesn't do it, so I'm with the above who've suggested an air leak. I would have thought it would pop and bang a bit on overrun as well though.
Got a feeling it's not the exhaust, I changed the header and back box for brand new items and used fire gum to seal it. Overrun sounds familiar, is this to do with the carbs or timing?
Air leak. I had the same but I couldn't trace it. Solved the back fire by letting it idle for a couple of minutes once we'd stopped. Getting my timing right and getting rid of the popping on the overrun helped too. My guess was inlet manifolds to head air leak. I changed everything, drilled out and fitted new studs which I can put a high torque on, new exhaust gaskets etc but I never did find which one was the issue. Great fun having a backfire in the middle of a forest with a cliff behind though!
Carbon in the combustion chambers glowing red hot causing preignition? Idling for a couple of minutes allows the coke to cool. I've had that before, put into gear and "stall" the engine as you switch off. The condition just went away in the end. Tony
Make sure your timing is right 7.5 btdc Check it with the correct tickover when warm, then check again after a run. Make sure all your throttle linkages are spot on
