From: John De Armond
Subject: Re: Generator Backfires Violently when shut off
Date: Tue, 11 Apr 2000 03:09:07 EDT
Newsgroups: rec.outdoors.rv-travel

Vogt wrote:
> 
> My youth pastor recently picked up a big Pace Arrow and we have been
> working on swapping the generator from his old '73 Escapade.  The
> history is unknown but the dang thing has already exploded 2 exhaust
> systems in the 4 or so years he has had it.  It is a Onan model 4.0CCK
> (4 kW).  It is much more "open" than today's Onans with all the linkage,
> etc pretty much exposed and not in a nice little green box.  Anyway
> after holding down the "off" side of the spring loaded rocker switch,
> usually after the motor has completely stopped, which takes a while, it
> will let out with a big boom.

Mine does that.  When you kill the ignition, perfectly combustible
(assuming your carb's adjusted correctly) mixture is pumped into the
exhaust.  A glowing hunk of carbon or anything else that adds a
little energy to the mix causes it to explode.  Can scare you out of
a year's growth if you're behind the RV!

I'd not advise tinkering with the carb or ignition setting, as it
will impact the normal operation too.  The problem is most likely
deposits buildup which would require disassembling the engine and
exhaust.  What I've found to be 100% effective in stopping the
backfiring is when shutting the engine down, after it coasts down
enough that it can't restart but is still turning, release the kill
switch.  This will allow the engine to weakly fire a couple of
strokes and purge the combustible mix from near the engine.  I've
yet to have a single backfire since I've been following this
procedure.  Another thing I've found to be effective whenever I have
time to do it is to fully unload the generator and let it idle for
awhile.  This lets everything cool off.