914 ULS backfiring

[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.

[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]

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