[DeTomaso] Spark advance

[Will Kooiman]

And...

The next is the transfer slot. The transfer slot smooths the transition
between the low speed idle jets and the main metering circuit.

The transfer slot is not adjustable in standard Holley carburetors.  There's
a new Holley that might work differently, but I think it just incorporates
all of the features of the tuner Holleys.  It looks like the best Holley
yet.

-----Original Message-----
From: detomaso-bounces at realbig.com [mailto:detomaso-bounces at realbig.com] On
Behalf Of John Taphorn
Sent: Friday, September 23, 2011 8:29 PM
To: Thomas Tornblom; detomaso at realbig.com
Subject: Re: [DeTomaso] Spark advance

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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