REFLECTOR: Engine backfiring

[Dave Dent]

It looks like you are getting raw fuel into your exhaust system. The JR 
system fires on compression as well as exhaust. So any un-burnt fuel in the 
exhaust will ignite if still raw. So a leaking intake valve could do this. 
Do a compression check and look for a leak in the intake.  Listen for it 
during the test at your intake.  Have someone do the test holding the blade 
while you listen at the throttle body for air. An intake leak is harder to 
detect.  Fuel can seal a very small leak yet showing a high 70 plus over 80 
leak down test.
Also sometimes if the mag ignition is right at 25 degrees and the EI systems 
fires right at the very same time it will have a big bang effect, it's 
called.  I found this out when I had a dual EI system. I had to off set one 
system by five tenths of degree to prevent it.  Sounded like a backfire but 
it wasn't.
Just some thoughts
Dave

----- Original Message ----- 
From: "Dave Philipsen" <velocity at davebiz.com>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Tuesday, August 15, 2006 8:38 PM
Subject: REFLECTOR: Engine backfiring


> Here's a question perhaps for some of the engine experts:  I've got a
> STD FG with IO360-A1A engine.
> It has one standard magneto and one Jeff Rose electronic ignition.  The
> engine and oil cooling are not a
> problem even with OATs in the low 90s. But, I have a problem (it's not a
> huge problem) with backfiring
> during taxi *after* I land.  Taxiing before takeoff seems to be ok.  The
> backfiring happens maybe once
> every 30 seconds or so.  The engine also seems to run a little rough at
> this time.  I've tried running lean,
> running rich, running on just the magneto, running on just the
> electronic ignition.  None of these seems to
> prevent the backfiring during taxi.  The engine never seems to have
> problems before or during takeoff or
> during cruise or descent...just taxiing back after landing.
>
> I'm starting to think it's a vapor lock problem or something since the
> fuel flow is way down and the fuel is
> sitting in the lines longer and heating up.  I had the cowling off the
> other day and was checking the routing
> of the fuel injector lines and they don't appear to be routed near the
> exhaust manifolds or anything like that.
> I was thinking of maybe experimenting with some insulating material
> around the individual injector lines as
> well as the main fuel line.  Any ideas?
>
> -- 
> Dave Philipsen
> STDFG
> N171SP
>
>
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