[DeTomaso] Spark advance

[Thomas Tornblom]

I guess I should have mentioned that I have EFI, a standard Ford Mustang 
5.0 EFI, but with upgraded injectors, MAF, and a TwEECer so that I can 
adjust everything in software.

The real issue may of course be the same, running lean.

I may have to go open loop at low rpm.

Thomas

On 2011-09-24 03:28, 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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-- 
Real life:   Thomas Törnblom             Email:	   thomas at hax.se
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