Air filter popped out in flight

[lr172]

Originally Posted by Bill Boyd

There is no front-fire. There is backfire, and there is after-fire.

Choose one, you must.

agreed. I was trying to be nice as maybe he was referring to something else and creating a learning moment for him

[lr172]

Originally Posted by rzbill

Larry,
Before I learned and practiced the mixture sweep method of hot starting a Bendix injected Lycoming, I got a "pop" or explosion in the intake on a couple of occasions due to igniting fuel in the intake of an overly flooded engine. The filter looked exactly like the OPs image. Replaced it once, bent it back once.

Never had an another issue since using mixture sweeping.

That would be called a backfire and excluding valve timing issues, usually only occur at low RPMs, mostly during start attempts.

[Monkey]

Backfire on start up. Caused by a lean start. Prime for 1-2 more seconds. I've replaced dozens of these on customers RV's in 28 years. P/N 33-2060

[Draker]

You know, until this thread, it did not occur to me that the "eruption" happened on the ground during startup rather than in the air. I did a hot start that day, and was experimenting with a different than usual hot start technique, recommended by another RV'er. My usual hot start, which works but sometimes takes a bit of cranking:

• Do not prime with boost pump
• Throttle: Crack to 1/4"
• Mixture: Idle cutoff
• Engage starter
• Mixture: Move slowly to rich as engine starts

The "alternate" method:

• Do not prime with boost pump
• Throttle: In
• Mixture: Idle cutoff
• Engage starter
• Mixture: Move to rich as engine starts
• Throttle: Move to cracked then adjust

The alternate method did start quicker than usual, but perhaps poor technique might have caused the airbox event.

EDIT: To address the theories that perhaps something shifted in my airbox to "squeeze" the filter out: This doesn't seem likely to me. I built my airbox opening much thicker than Van's design, essentially molding extra layers of fiberglass around an actual filter, plus a thick flange at the top. The filter fit is exact and immovable.

[Bdh55]

This doesn’t have anything to do with the original question, and I can’t tell if it’s the lighting or something else but I don’t see any oil on that air filter photo. K&N filters require their red oil to function properly.
Brent

[Bavafa]

Originally Posted by Draker

You know, until this thread, it did not occur to me that the "eruption" happened on the ground during startup rather than in the air. I did a hot start that day, and was experimenting with a different than usual hot start technique, recommended by another RV'er. My usual hot start, which works but sometimes takes a bit of cranking:

• Do not prime with boost pump
• Throttle: Crack to 1/4"
• Mixture: Idle cutoff
• Engage starter
• Mixture: Move slowly to rich as engine starts

The "alternate" method:

• Do not prime with boost pump
• Throttle: In
• Mixture: Idle cutoff
• Engage starter
• Mixture: Move to rich as engine starts
• Throttle: Move to cracked then adjust

The alternate method did start quicker than usual, but perhaps poor technique might have caused the airbox event.

EDIT: To address the theories that perhaps something shifted in my airbox to "squeeze" the filter out: This doesn't seem likely to me. I built my airbox opening much thicker than Van's design, essentially molding extra layers of fiberglass around an actual filter, plus a thick flange at the top. The filter fit is exact and immovable.

Ryan,
Give this method a shot, slightly modified from your second method. It works best for me when other methods are not successful.

• Do not prime with boost pump
• Throttle: In all the way
• Mixture: Idle cutoff
• Engage starter
• Throttle: Move back semi slowly to cracked till the engine begins to start
• Mixture: Move to rich slowly
  

The idea is that there is a fine point that the air/fuel mixture will be just right and this is to find that pint. Once the engine begins to run, feed it some fuel slowly.

[Draker]

Originally Posted by Bdh55

This doesn’t have anything to do with the original question, and I can’t tell if it’s the lighting or something else but I don’t see any oil on that air filter photo. K&N filters require their red oil to function properly.
Brent

Thanks, Brent. The filter was thoroughly oiled 1 month ago. The pictures probably aren't at a great angle to show that.

[wnplt]

That's from a backfire. Ive replaced a few that looked identical to that. I now no longer hot start with the throttle open more than a half inch. The butterfly in the fuel servo will prevent the backfire that blows out or deforms the filter. Have not had one since and that was years ago.

[Draker]

You guys are great. I think we're ready for some corrective action, here.

1. No more experimenting with hot start technique. I'm going to switch back to what I know works.

2. Filter security. I deviated from Van's plan a bit on the airbox, so it's up to me to better secure the filter. I re-did the left side "air dam" in front of the #2 cylinder to include a hold-down tab for the air filter. This should help keep the thing in place should I ever backfire like this again:

[DFly]

First post to this forum, we have an RV7A freshly out of its 40 hour fly off that experienced this same problem in the HOT weather in Texas. After refueling and restarting, we experienced a backfire during our hot start up. When we applied power on the takeoff roll after a normal runup, the engine did not develop appropriate power and we aborted the take off. Upon inspection, our airfilter - attached in a very similar way to those shown here - was actually displaced.

Again about 10 hours later (also in very hot weather), we experienced a small backfire after a hot start. So we shut down and inspected; the filter was deformed as in Draker's post.

Now chasing the cause of these backfires (haven't experienced them in the cooler weather as we flew to Oshkosh from California) but know that any backfire with this horizontal induction filter will signal shutdown and inspection of the filter. One is on order to replace the current part - and they are not cheap!!! Going to try that hot start with Throttle only open half like wnplt and see if that helps!