914 ULS backfiring

[amerkarim]

Hi All,

I have a friend who is having problems with his 914 ULS backfiring in his Kitfox 6 when shutting down. The engine is quite new, about 100 hours now, and a replacement of his old identical 914 ULS engine that was replaced at 1000 hours. It has been backfiring ever since the first change of the spark plugs at about 25 hours. The engine runs perfectly, starts easy and runs smooth.

The backfire occurs on shutdown. The engine is idled to about 1800 for a few minutes, then the mags are switched off one at a time. Once the prop stops turning there is a delay of about 6 seconds, then there is a loud backfire from the muffler every time. The fuel is always fresh and high octane - Shell V power 98 Ron unleaded. The plugs are genuine Rotax plugs and are put in with the recommended heat paste.

The plugs have been changed a few times, the carb needles have been adjusted from rich to lean, and the throttle has tried in a range from 2200 to 1400 during shutdown, but the backfire is consistently there. The idle screw is set to idle at 1400 when pulling the throttle fully out.

The only way to avoid the backfire, is to starve the engine of fuel by closing the valve and switching off the pumps, and letting it shut down on its own.

Could this backfiring be related to the mixture settings? Or an air leak in the exhaust? If it was still happening with needles set at full rich, surely it cant bee that the mixture is too lean? Any tips or guides on what to check would be appreciated.

P.s. I have the same engine and settings, and both my carb needles are leaned out same as his, and I get smooth lots of power and no backfire on shutting down.

Many thanks in advance

Amer

[Shadowrider]

Just curious why he replaced his 914 at 1000 hours?

[Esser]

My gut feeling is that when you shut off the ignition, the engine is still spinning and pulling in a fuel air mixture that ignites from the hot exhaust. Is his EGTs higher than your when idling after a flight? Have you tried pulling the choke to shut down the engine?

Is there a reason why you wouldn't want to shut down by just shutting off the fuel?

[jrevens]

Hi Josh,

I don't think pulling the so-called "choke" will have any desired effect on engine shutdown.

I also have a question for the group (forgive me Amer... I don't mean to hijack your thread)... How do others feel about shutting off the fuel during or after shut-down? I can see no real good reason to do that unless you had a problem like a carburetor float valve not shutting off, or something like that.

[Esser]

Sorry John, I had poorly worded that. What I meant is if you pull the choke and shut down does it do it still. I was just curious if a much richer mixture would stop the afterfire.

[jrevens]

That's ok... my fault, Josh. I see what you were getting at now. Seems like a richer mixture would probably add "more fuel to the fire" and make it worse, if you know what I mean, but hey... it sounds like an unusual problem anyway and you might as well experiment a little. Who knows?

[jiott]

He may have lots of deposits in his muffler/exhaust system that continue to glow a few seconds after shutdown and ignite the fuel/air afterrun. Just a thought.

John, I have operated for 4.5 years, 600 hours and never shut my fuel valve off after the flight was over. I have left it ON for maybe 3-4 weeks during the winter bad weather in between flights. Never had a problem or a float bowl overflow. I did this because years ago someone said its better to keep the whole carb wet with fuel when using Mogas to avoid letting it dry out and varnish deposits may want to form, and seals/gaskets need to stay wet. Note: this is not an excuse to keeps months old Mogas in the tank, because it can go bad. I don't remember who suggested this or how knowledgeable they were. A few weeks ago I mentioned this on the Rotax-Owner.com forum in answer to the same question you asked, and I got criticized by several of the gurus on that forum who said the fuel should always be shut off at the end of the flight. Its too dangerous in case of a needle that doesn't seat and overflows the bowl out the vent tubes. They didn't think my reasoning was valid or worth the risk. So go figure your own opinion. Actually I see their point (even though I have had no trouble for a long time) and have changed my mind and am now shutting off my fuel.

[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

[jiott]

John, regarding the other part of your question, I definitely don't like shutting off the fuel to kill the engine. I was forced to do this several times when I had an ignition "P" lead break and couldn't shut off the engine normally. Starving the engine of fuel causes a rough, hesitating shutdown which seems to me may be hard on the gearbox.

[jiott]

Amer, a couple more thoughts: How do the present set of spark plugs look? Are they a nice light brown color with no deposits? If there are black deposits, they could glow and cause dieseling after shutdown; could also indicate a mixture/carb problem.

Also, I would never recommend running the engine at idle for a few minutes before shutdown. Idling will cool the engine down and quickly cause carbon deposits to form on the plugs. You want to shut the engine off when still nice and hot. I use the method suggested by.....can't remember: When done taxiing immediately rev it up to about 2500 rpm, then pull the throttle clear back to idle, as you do this shut off one ignition, count 2 seconds, then shut off the other ignition. It stops smoothly and immediately.

[amerkarim]

Hi Jim

The plugs when taken out are black and sooty as is normal for the Rotax I think. They have been like this for us all.

We have 3 Kitfoxes in our flying group. 2 x Rotax 914, 1 x 912 100hp.

I think we have got into the habit of letting the 914's run at about 1800 for a few minutes to let the turbo spin down, then reducing to 1600 then switching off one mag then 2 secs later the last mag.

As I said my engine is the same as his, same settings for fuel and mixture, same fuel and same shutdown procedure and no backfire. However, I am just at 22 hours from new, so I haven't changed my first set of plugs yet. Maybe I will have the same set of strange plugs in that Garry had? Hope this backfire thing is not contagious?

I will pass on the message to Garry about shutting down as you have described, and I will do the same as well from now, and then give you some feedback on if this helped with the backfire.

Thanks

[Dave S]

Regarding Johns two questions

1) Shutting down the engine by shutting off the fuel valve I would reserve only for a non standard circumstance such as busted p lead or observed fuel leak firewall forward.

2) Shutting the fuel valve off after engine shutdown is something I do as a matter of a normal procedure to prevent a leak in the hangar should the carb decide to leak and to exercise the shutoff valve to assure the fuel valve seals are working and to assure that the fuel shutoff valve will work in an emergency situation. I have observed rental aircraft where nobody ever shut off the fuel valves in the entire fleet and three quarters of them were frozen and totally incapable of being rotated not so good if you really, really need to shut off the fuel flow.

[jiott]

Black and sooty sure is not normal for all of us flying the 912us/s. I don't know about the 914 but it sure doesn't seem right to me. I'll bet it is too much idling.

[Ronin]

Also, I would never recommend running the engine at idle for a few minutes before shutdown. Idling will cool the engine down and quickly cause carbon deposits to form on the plugs. You want to shut the engine off when still nice and hot. I use the method suggested by.....can't remember: When done taxiing immediately rev it up to about 2500 rpm, then pull the throttle clear back to idle, as you do this shut off one ignition, count 2 seconds, then shut off the other ignition. It stops smoothly and immediately.
A 914 needs to spool the turbo down and get some cooler oil into it, you absolutely need to idle it a couple minutes before shutdown.

About 900 hrs behind 914's, never shut off the fuel, and always killed both ignitions after two minutes idle. Never had any shutdown dieseling or backfiring, just a smooth shutdown with no gearbox rattling.

I would look at the last thing changed (the plugs)or altered.

[Clark in AZ]

Don't know if it's been suggested, but I would put the old plugs back in ( if you have them) and see if you get the same result. It seems that the problem started with the plug change.

Clark

[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.

[amerkarim]

Hi All,

After trying a lot of things, managed to solve the backfire by putting some exhaust paste sealant around all the joints of the exhaust system.

It must have been an air leak into the exhaust pipe. Hope it lasts now.

Happy landings

Amer

[DesertFox4]

Thanks for the report Amer. Glad you discovered the issue and resolved it.