Re: Backfire

["T. A. Dapper" <tdapper@value.net>]

I suggest that you get your VW on the road, run the engine up to about
3500 rpm in second gear, leave the clutch engaged and back off of the
throttle - then measure to see if there is any excess fuel in the
exhaust system.  This test works best with carburetor systems.

Tim Dapper


Jim Adney wrote:

> On 20 Jul 98 at 15:34, type3@vwtype3.org wrote:
>
> > I don't know enough to be commenting personally, but I happened to
> > show this to a friend of mine (mechanic of 10 years) and this is his
>
> > 2 cents...
> >
> > :: By the way...
> >
> >   Cars DO backfire from exhaust leaks, anyone who can tune an engine
>
> > well enough to burn every last bit of fuel before it enters the
> > exhaust should be applying for the position of GOD!!  they would
> > probably get it LOL    On a car, its absolutely NOT realistic to
> > expect that clean and efficient of a burn. Thats teh reason that
> > cars have thigns like air injection systems, catalytic converters
> > etc...  The EPA wants them clean, but even they understand it cant
> > all be done in teh engine, some of its is finished in teh exhaust
> > and lots of it is NEVER finished. There is most likely a leak if its
>
> > backfiring in teh exhaust, but a really poorly running engine can
> > also do it.. The exhaust is definately MOST likely :)
>
> Your mechanic is as entitled to his opinion as I am to mine, but if
> you look at the ppm of unburned hydrocarbons that the EPA expects,
> even in a 67 or 68 T3, then you will see that there is not enough
> there to burn, let alone explode.  For every combustible gas, there
> is an explosive range; for gasoline it is rather narrow.
>
> The exhaust leak hypothesis assumes that the leak will allow oxygen
> to leak into the exhaust system, mix with the unburned gasoline, and
> explode.  This ignores the fact that the exhaust is under postiive
> pressure and will almost never draw air INTO itself.  Without this
> mechanism, the hypothesis looses strength rapidly.  It also ignores
> the fact that there is as much unburned oxygen in the exhaust as
> there is gasoline, so the addition of extra gas is unnecessary.
>
> I stick with my opinion (labeled as such.)
>
> Jim
> -
> *******************************
> Jim Adney, jadney@vwtype3.org
> Madison, Wisconsin, USA
> *******************************