[T3] Crabby Carbs
B Fye
bfye at canyonville.net
Wed Sep 25 19:22:46 PDT 2013
W7ac is awesome in hot weather under load
Brian Fye
> On Sep 25, 2013, at 7:20 PM, "Keith Park" <topnotch at nycap.rr.com> wrote:
>
> Yea, that's what ive used for years. They are called 7902's now if I recall
> right and the Russian version are inked in black instead of green.
>
> Keith
>
>
> Top Notch Restorations
> topnotch at nycap.rr.com
> http://www.a383ina68.addr.com/radiorest/main.htm
> 71 Squareback "Hothe"
> 65 Notchback "El Baja Rojo"
> 65 Squareback "Eggcrate"
> 87 golf "Winterat"
> 93 RX7 "Redstur"
>
>
> -----Original Message-----
> From: type3-vwtype3.org-bounces at lists.vwtype3.org
> [mailto:type3-vwtype3.org-bounces at lists.vwtype3.org] On Behalf Of B Fye
> Sent: Wednesday, September 25, 2013 10:16 PM
> To: type3 at vwtype3.org
> Subject: Re: [T3] Crabby Carbs
>
> I like the w8ac when the weather goes cooler on my 1600s
>
> Brian Fye
>
>> On Sep 25, 2013, at 5:42 PM, "Keith Park" <topnotch at nycap.rr.com> wrote:
>>
>> Very interesting Jim,
>> I pulled the Brazillain Bosches out (I think they are called 7902 today)
> and
>> the insulators were quite white and clean, the inside of the plug was
> black
>> with carbon otherwise but not crusty or anything, they only have about
> 1500
>> miles on them and 18 miles to work and back since it was really bad.
>>
>> I put the new Russian Boshes in there, Ill see how it runs with them
>> tomorrow for comparison.
>>
>> Thanks
>>
>> Keith
>>
>>
>> Top Notch Restorations
>> topnotch at nycap.rr.com
>> http://www.a383ina68.addr.com/radiorest/main.htm
>> 71 Squareback "Hothe"
>> 65 Notchback "El Baja Rojo"
>> 65 Squareback "Eggcrate"
>> 87 golf "Winterat"
>> 93 RX7 "Redstur"
>>
>>
>> -----Original Message-----
>> From: type3-vwtype3.org-bounces at lists.vwtype3.org
>> [mailto:type3-vwtype3.org-bounces at lists.vwtype3.org] On Behalf Of Jim
> Adney
>> Sent: Wednesday, September 25, 2013 10:15 AM
>> To: type3 at vwtype3.org
>> Subject: 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 at vwtype3.org
>> Madison, Wisconsin, USA
>> *******************************
>>
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