[DeTomaso] Spark advance

Asa Jay Laughton asajay at asajay.com
Sat Sep 24 01:13:15 EDT 2011


John,

Simplest explanation I've heard yet.

Asa Jay

Asa Jay Laughton, MSgt, USAFR, Retired
&  Shelley Marie
Spokane, WA
******************************
http://www.racingagainstautism.com
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On 9/23/2011 6:28 PM, John Taphorn wrote:
> All right, I'll continue to beat the drum
>
> At low speeds the engine is getting fuel metered through the low speed idle
> jets.  It is not until the throttle plate is meaningfully rotated does an
> adequate supply of air flow through the boost venturie to initiate flow
> through the main metering system.
>
> People jack around with their main jets to modify the mixture at highway
> speeds w/o realizing that the main circuit is generally NOT engaged at those
> RPMs with our big motors. Rather, they are operating on the low speed idle
> jets.
>
> I discovered this when I incorporated an AF gauge.  When the bucking was
> occurring at low speeds,  my fuel mixture was lean.  Bigger main jets did
> not have an effect; yet smaller slow speed idle jets brought the A/F mixture
> right in line ands the bucking went away.
>
> Of course, it helps to have an aftermarket carb with screw in slow speed
> idle air jets.
>
> The low speed idle jets affect mixture over a wide range of RPM.  You may be
> able to lean your main jets after the other jet change.
>
> JT
> ----- Original Message -----
> From: "Thomas Tornblom"<thomas at hax.se>
> To:<detomaso at realbig.com>
> Sent: Friday, September 23, 2011 1:16 PM
> Subject: [DeTomaso] Spark advance
>
>
>> I have a bucking issue while cruising at low load and low rpms,
>> 1000-1500 rpm.
>>
>> Looking at my spark tables I see that the advance varies from 24° to 36°
>> in this region, depending on load and rpm, and I suspect that this is at
>> least part of the problem.
>>
>> I checked an old log, and I see that I had 33.5° at 1000 rpm and 20%
>> load (VE). That seems a bit excessive to me.
>>
>> When I had the old flat tappet Crane cam (133052, 284H12), and open
>> chamber heads, I was using the stock 5.0 Mustang spark tables, which was
>> even more aggressive at low rpms, and it was silky smooth. I could
>> easily putter around town at 25 mph in 5:th.
>>
>> I don't remember if it behaved as good when I installed the closed
>> chamber 2V heads, but I know I had to retard timing at higher rpm as it
>> would ping even when cruising at low load. There were places that called
>> called for 40° of advance.
>>
>> After it ate a lobe in Italy, I built a new engine with a Crane 529801
>> hydraulic roller, and it has been bucking on and off since then.
>>
>> Does anyone have a spark table from a similar engine that might work as
>> a starting point?
>>
>> Or what would a reasonable spark advance be for around 1000-1500 rpm
>> light cruising?
>>
>> Cheers,
>> Thomas
>>
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