Backfiring T3

[jadney@vwtype3.org (Melissa & Jim)]

>Since rolling out my '71 fuel-injected squareback from it's resto last
>year I've had a little problem with backfiring.  I'm a little ignorant on
>the causes of backfiring, and I was hoping for a little help on this.  It
>typically happens in second or third gear when I am accelerating and
>abruptly let off on the gas pedal to stop or pause for something.  I
>didn't have this problem before; here is a list of possibly relevant
>things that were changed when it was garaged:
>
>1. New muffler system (stock, same as before) Can a loose fitting cause
>this?
>2. Position of fuel cut-off switch moved slightly. 
>3. New points, condenser, plugs.
>
>Is backfiring purely a timing problem, or can any of these changes
>contribute?  My plugs, I should say,  are typically black when I change
>them, I can't seem to get the timing right to avoid an over-rich fuel
>mixture. 

Doug,

Backfiring is the explosive combustion of exhaust gases in the exaust 
system.  It occurs when gas fails to burn in the cumbustion chamber and 
accumulates in the exhaust until it is ignited by the exiting hot gases of a 
cylinder which did manage to fire.  There is no need for fresh air to leak 
into the exhaust system since there is already plenty of oxygen there 
already (remember, some of the cylinders just pumped their mixture on 
through.)  The classsic cause of backfire is actually a too lean mixture.

Here's what I suggest: make sure your timing and valve adjustment is 
correct; timing to TDC, dwell to 45-52 degrees, and valves to .006".  If 
you're not sure what this means, stop right now and figure it out.  I 
recommend the Bentley manual.  Backfiring isn't caused by bad ignition 
timing unless it's WAY off.  I recommend genuine Bosch ignition parts.

Ignore the exhaust system; that's just a red herring.

I suspect you have a FI problem, possibly one of your own creation.  Go back 
to the throttle valve switch, the one you said you "moved slightly."  Read 
up on how to adjust it properly, then do it.  The idle contacts should JUST 
close each time you let go of the throttle.  You may have to rotate the 
switch just a bit extra to assure that they close even when you release the 
throttle slowly. Since you probably don't have one of the Bosch testers the 
easiest way to check the adjustment is with an ohmmeter.

Second, use the voltmeter to check the system voltage (say from the fusebox 
to ground) at "crusing" RPM with everything warmed up and all accessories 
off.  Your voltage regulator should regulate to 14.1-14.4 Volts.  You will 
get ~1/2 Volt more at the generator and ~1/2 Volt less at the fuse box.  If 
this voltage is low, the FI won't work properly and the engine will run 
rich.  In extreme cases this might cause your symptoms, although rich 
mixtures usually burn and don't accumulate in the exhaust.  If your voltage 
is low, replace the voltage regulator.  I strongly recommend a genuine 
Bosch, although one of my long term projects is to set up to make better 
ones myself.  (Don't anyone hold your breath.)

You should also make sure that you have connected the vacuum hose that is 
supposed to run from the intake air distributor to the pressure sensor.

If there is some chance that you or someone elso tampered with the fuel 
pressure regulator, you should find someone who has a gauge and can adjust 
it back to 30 psi (my recommendation for a 71.)

If your '71 is a very late one, you may have reversed two wires on the starter.
When you get in your car and turn the key on (NOT start), you should hear an 
immediate "click" (relays) followed about 1 second later by another "click" 
(relay.)  Do you hear both of these?  This is important--let me know.

If your plugs are black, as you say, then we know that you are running rich.
The only question is what is causing it.  I would concentrate on that first.

Did your "resto" include an engine rebuild?  If so, what was done?

The '71 was probably the best year for the D-Jetronix FI.  It is a very good 
system.  It will serve you well if cared for.

Jim
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       Melissa Kepner                                    Jim Adney
       jadney@vwtype3.org              jadney@vwtype3.org
                             Laura Kepner-Adney
                             Madison, Wisconsin
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