914 ULS backfiring

[amerkarim]

Thanks for all the replies Guys

A correction first on my part .. the engine was replaced at 1850 hours and the reason for that was rough running and some loss of compression and power. The engine had been on the plane for over 15 years and the plane had been operating in very dusty conditions in a desert, and I suspect that over the years some sand had got past the air cleaner over the years and done some damage internally. Also the engine had been running on a mix of avgas and unleaded fuel and so there was significant contamination inside as well. As he flies every week back and forward to work to a remote rural area, a decision was therefore made to change the engine and keep the old one for a potential overhaul in time, or spares.

Regarding the mixture, it doesn't seem over rich. When pulling and holding the choke out at 2200 RPM the engine speed increases by less that 600 RPM and the engine is smooth and stable. The idle mixture screws are set at 1 and a half turns as per factory settings and the carbs are both leaned to the last setting. Running fresh 98 Ron Shell V power unleaded.

Oil temp, EGTs and CHTs are right in the middle of the green zone and overall the engine runs cool and doesn't get hot, even when climbing. His plane is based at about 6,500 ft AMSL and flies at all altitudes from sea level to 11,000 AMSL. It backfires everywhere.

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.

Is there any reason one shouldn't starve the Rotax of fuel like in a normal plane engine? Switching off the pumps and closing the valve stops it in a minute or less and there is then no backfire. Can ay harm be done by doing it this way?

Will keep you updated on what we find.

Thanks again

[amerkarim]

Hi Jim,

Thats a good idea idea re the muffler...

I will have to check but I think the engine came new and complete from Rotax and so the muffler was new as well. Also strangely it didn't backfire when new until the first set of sparks plugs were changed at about 25 hours.

Strangely again, the first set of plugs removed look a bit strange and different from the regular plugs. They had to sets of electrodes facing each other, which is different from the regular Denso or NGK plugs normally used. I will try and find the part number.

If it is the old muffler that was reused, is there a way to clean it up or get the deposits out?

Any other ideas regarding the plugs and why after changing the plugs to the recommended ones, the backfiring would start?

Thanks

[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]

...
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.