[T3] Crabby Carbs

[Jim Adney]

On 24 Sep 2013 at 20:30, Keith Park wrote:

> I think the plugs may have gotten fouled... when I was restoring this car
> with the stock jets It would start running crappy rather quickly if all I
> did was move it around, unlike the FI 71 which just didn't care.  I put the
> 2X Richer jets in and It ran poorer than the 1X, then I drove the car down
> to the neighbor kids house several times without warming it up and that's
> really when things went downhill.

Here's my guess: You're running a bit rich at low speeds and loads. 
This allows carbon to build up on the SP tips because the SP ceramic 
never gets hot enough to clean up. It may be that the mixture is 
fine, but it's just that the engine never gets warmed up enough.

> What I don't understand though is how it can run so nicely at light throttle
> and have a perfect idle if the plugs are fouled and only have problems if Im
> stepping on it a bit... and it would start perfectly too, I always remember
> fouled plugs making an engine hard or impossible to start.

When our cars were new, gas had lots of tetraethyl lead. That stuff 
would accumulate on the SP ceramic, and everything else, after the 
gas burned. SPs that got enough lead deposits on them were "fouled." 
The lead deposits had the unusual property of becoming conductive at 
some temperature, so the engine would start, but would quit once it 
warmed up a bit. (BTDT) There was no cure other than bead blasting 
the SPs or replacing them.  

Lead deposit buildup occurred any time the engine was run mildly and 
also during warmup. Deposits of any kind get cleaned off slowly once 
the SP ceramic reaches what's called the "self cleaning temperature" 
(SCT.) SPs come in different temp ranges for different engines to try 
to assure that the SP insulator temperature is normally above the SCT 
and below the temperature where it's hot enough to ignite the 
mixture, the "preignition temperature" (PIT.)

So you want a SP that rises above the SCT quickly without ever 
reaching the PIT under any circumstances. SP Makers control the heat 
range of a SP by changing the length of the heat dissipation path 
from the hot end of the insulator to the cyl head, where most of the 
heat goes. The main heat transfer paths are thru the threads and the 
crush washer.

We don't have lead in our gas any more, but similar things happen 
with carbon buildup. Carbon deposits don't have the same temp 
dependence of conductivity as lead, but an insulator that has some 
carbon buildup just behind the ceramic tip might be a good insulator 
for "low" voltages, but if the spark voltage got higher, it might 
jump along the surface of the ceramic to the carbon deposits and 
short to ground instead of jumping across the SP gap.

If you manage to get more of the insulator above the SCT, you clean 
off more insulator, making it harder for the spark to short to 
ground.  

When you jump to wide open throttle (WOT) the combustion chamber 
pressures go way up, and this requires more spark voltage to jump the 
gap, so that's when a marginal carbon situation might turn into a 
"miss."

Gary Forsmo drove up here yesterday for me to look at his front end. 
He had been experiencing similar "miss" problems, but they went away 
once he got the engine up to speed and up to temperature. I suspect 
his problem was similar to yours.

Cars that foul their plugs frequently may need hotter heat range 
plugs. Cars that are run very hard may need cooler plugs to keep them 
from reaching the PIT. Modern plugs have a pretty wide range between 
the SCT and the PIT, so choosing the right plug is usually easy.

I've always used Bosch plugs, which seem perfect for our engines. 
When our cars were new, these were Bosch 145s, then they became 
W145T1, W145T1.1, W7A, W7AC (copper core), or W7AP (platinum.) Early 
Type 3s, like Keith's notch may have taken something different, I 
don't have any experience with that era Type 3.

If you're curious about heat ranges and self cleaning temperature, 
you can check out this site:

http://tinyurl.com/ksnbzyj

but the best explanation I've seen is in this very old booklet, which 
I bought in the early '70s from JC Whitney.

Bosch Electrical Systems for Automobiles
published by Interauto Book Co., Ltd., London, England, 1972
ISBN 0 903192 06 3
Adapted from "Service-Fibel fur die KFZ-Electrik"
published by Vogel-Verlag, Wurtzburg, 1971

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