914 ULS backfiring

[jrevens]

...
I think the point about switching off with the choke out is a valid point and is something he will try today after his flight to see if the mixture is too lean for any reason...

Amer,

My understanding is that backfiring is caused by excessive un-burned fuel in the exhaust system that ignites. I would think that “too lean” wouldn’t be the problem. Maybe excessive carbon build-up in the exhaust system. That could be the result of a too-rich mixture. I agree with the spark plug change for some reason also being a possibility.

Thanks, Jim & Dave for the feedback... I’ve been doing it (shutting off the fuel valve after shut-down) as you suggested Dave, & that is how I was analyzing it also. For shut-down I use the “2500 rpm, then retard throttle while simultaneously turning off both ignitions” technique. It seems to work very well.

[Esser]

My lawn tractor will always backfire unless I pull the choke when I shut it down. If it is rich, there is too much fuel and it doesn't ignite. Maybe a bandaid solution but pre-detonation ect is usually cause by too lean of a mixture that auto ignites from something hot as an ignition source. That is similar to what they are experiencing, so I would say either starve the engine for fuel (lean it out to the point where it wont auto ignite) or enrichen it so that it won't auto ignite. Either way you have the perfect fuel air mixture right now that is causing auto ignition so you need to change the mixture one way or the other. At least that's the way my amateur view sees it. You may also have carbon deposits in your exhaust that are glowing hot when you shut down that is your ignition source but it may even be the hot turbo.

I'm really surprised on everyone's view about fuel starvation for shutting down an engine. The only time I would care about that is on an injected system where the fuel lines are dead ends, ran dry, and had to be primed again(On my EFI set up all my injectors are on a loop so this isn't an issue). For most of my carbureted things such as my motorcycle, I routinely turn off the fuel petcock and let it starve of fuel so that unstable auto gas isn't stored in the carb bowls and make a mess if I don't ride it often.


Also, people with vapour lock issues on starting should be starving their engine of fuel on shutdown in my opinion. The rotax runs so hot that when you shut down and close your fuel valve, your still hot engine heats under the cowl when there is no air flowing through it that it causes fuel vapour in the lines. This increases the pressure in the lines. With no where for the vapour to go, the increased pressure pushes fuel into your carbs and you have a flooded condition which is why you end up have hard starting problems. If you starved your engine of fuel on shut down, when you went to start again you could open your valve and fill your bowls with fresh cool-ish fuel. This trapped heat under the cowl is why you see lots of rotax guys open their oil doors on their cowls to let out excess heat.


Take all this with a grain of salt. I am in no way a professional about these these things. Just my personal observations that are worth as much as free advice is worth.

[jrevens]

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Also, people with vapour lock issues on starting should be starving their engine of fuel on shutdown in my opinion. The rotax runs so hot that when you shut down and close your fuel valve, your still hot engine heats under the cowl when there is no air flowing through it that it causes fuel vapour in the lines. This increases the pressure in the lines. With no where for the vapour to go, the increased pressure pushes fuel into your carbs and you have a flooded condition which is why you end up have hard starting problems. If you starved your engine of fuel on shut down, when you went to start again you could open your valve and fill your bowls with fresh cool-ish fuel. This trapped heat under the cowl is why you see lots of rotax guys open their oil doors on their cowls to let out excess heat. ...

That's the reason for the fuel return line that Rotax now recommends... you'll have no excess pressure in the lines, and you'll have more cool, fresh fuel circulating when running, right up to the distribution point where the line branches off to each carburetor.
Not just Rotax guys, but many airplane owners (of RVs for example) open their oil doors for the very same reason.

[jiott]

I open my oil door not because of vapor lock, but because I don't want to fry my expensive ignition modules.