Backfiring on landing

[egp8111]

my 912UL powered KFIV backfires (and I smell fuel) when I pull the power to idle for decent and landing. If I keep a little power on it doesn't backfire (or at least not as much). It runs very well the rest of the time and will happily idle at 1500 rpm. I don't have a return fuel line ( installing one asap) but my KFIII didn't have one either and it never backfired. Thoughts are maybe the windmilling prop is pulling too much fuel into the engine making it run rich. Anyone have any experience with this?

Thanks
Skip
NC

[desertdave]

Sounds like it may be running rich. Here is what I would look at in this order.
1) Weigh your floats and see if one or more are sinking
2) Check your float levels
3) Check your float needles and seats
4) Check your fuel pump pressure
5) Consider rebuilding the carbs

I have been thru the fuel smell thing and readjusting the float levels fixed my problem.

[Delta Whisky]

Any chance you are hearing afterfire vs. backfire?

[egp8111]

Any chance you are hearing afterfire vs. backfire?

Aftert fire? It's possible. What would that be trying to tell me ? The engine does miss or cough, it just makes a pop or three. If I keep a little power on it doesn't do it. I weighed the floats and they were within limits. My next set is to pull the carbs and check the float arms and needle valve and install a return line. Hopefully something will turn up.
Thanks
Skip

[atosrider]

do you have the carb vent lines plugged into the air filters, or tucked behind the fuel bowl spring clips?
in earlier versions of rotax lore we were told to keep the vent lines away from potential suction forces from fast flowing air;
the newer version is to punch out the dimple on the rotax labelled k&n filters and insert the vent line ...
during a recent carb balancing session with the local rotax shop, it was suggested I get with the program and plug them into the air filters, I questioned the suction power but was told any drips are better there than dropping onto your exhaust;
shortly thereafter I routed the vent lines into the air filters, and the 912UL ran rough and upon smelling gas you shut it down, wipe up the gas stains and tuck the vent lines back behind the fuel bowl spring clips ... and resume normal operations ...

[egp8111]

I have the vent lines tucked in behind the float bowl clips. It has the carb heat single air filter set up. Thanks

[jiott]

I have been running the carb vent lines into the back of the K&N air filters for 10 years (1200 hours) now on my 912 uls. No rough running or gas smells. Think about it; its exactly the same way you would hook up the vent lines if you had the Rotax airbox installed.

[Delta Whisky]

Afterfire is most often a result of incomplete ignition issues. That would normally reduce the list of potential culprits to timing, plugs or wires. Assuming your issue started fairly recently, I'd start with the easiest and cheapest: clean and gapped plugs at less than 5k Ohms. Timing on a Rotax shouldn't be an issue and you don't want to think about a module problem ($$$).

As to the venting question - what jiott said. Mine have been vented to the air filter there for over 200 hours now, no problems.

[egp8111]

Afterfire is most often a result of incomplete ignition issues. That would normally reduce the list of potential culprits to timing, plugs or wires. Assuming your issue started fairly recently, I'd start with the easiest and cheapest: clean and gapped plugs at less than 5k Ohms. Timing on a Rotax shouldn't be an issue and you don't want to think about a module problem ($$$).

As to the venting question - what jiott said. Mine have been vented to the air filter there for over 200 hours now, no problems.

I have the carb heat with the single air filter on the front of the valve box and scat hose running back to the carbs.. Where would you vent the lines with this setup? I can't find anything in the plans