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Re: [T3] Crabby Carbs
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
--
*******************************
Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA
*******************************
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