914 ULS backfiring

[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]

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