[T3] Cylinder Performance

[Jim Adney]

On 17 Jun 2011 at 18:00, Adriel Rowley wrote:

> Pulled number three and was fouled: black as coal.  Wire wheeled it,
> and reinstalled.

Cleaning a spark plug with a wire brush will leave metal tracks on 
the insulator which will often allow the spark to go to ground along 
the surface of the insulator rather than jumping the gap. You might 
want to replace that plug now, or swap it with a different cylinder 
to see if the problem moves.

> Checked four, and was clean as it was when I put it in less than 25
> miles ago (unknown hours though).  Therefor, figured number three must
> be getting too much fuel,

Black plugs usually mean that the cylinder is not firing at all. This 
would be due to lack of spark, lack of fuel, or lack of compression. 
Check the resistance of all 4 SP wires.

> checked number three injector beside four.  Got a bit of a stream
> rather than number four's fog.  So, swapped to another injector but
> did not test, ASSuming it was fine.  However, with engine running,
> still is not quite right.  Pulled the number three spark plug wire
> from the distributor cap, I get about half the response as I do from
> the others.  I do notice the spark is brighter than before, but hard
> to be sure in bright light.  I am now thinking after all the sitting
> with gasoline the injector did in its life, might need cleaning.  So
> sending out a set to be tested and cleaned, since I have heard this
> strong recommendation many times but thought all was fine.  Lesson
> learned: prevent rather than fix.

In 40 years, I don't think I've ever had to replace an injector due 
to a bad spray pattern, only due to leaking gas externally. If you're 
having spray pattern problems, it makes me think that you've either 
got dirt getting in there or water that's causing rust. Make sure 
your overflow hose is good, or repaired, and change your fuel filter.

Your injectors should all be BOSCH with yellow bodies and part number 
ending in 007.

> My question is how much of an affect does variations in spray (already
> know what streaming does) have on cylinder performance, i.e.
> variations of fine atomization. 

I've never seen the spray pattern be very important. Since the spray 
just goes onto the intake valve, the combustion chamber never sees 
that pattern. My guess is that the pattern might have more to do with 
cold starts in cold weather but becomes irrelevant once the intake 
valve warms up.

> Also, what about spark plug gap?  I am using the disk gapper, so
> thinking I could be in the range of 28 to 30 thousandths, and do not
> know if this is something to be aware of. 

Try reducing the gap a bit, to more like .024". Sometimes the larger 
gaps make it more likely that the spark will "leak" out somewhere 
else before it gets to the plug.

-- 
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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