But wouldn't the fact that sven is running the 4.0 head and a hotter cam, both of which flow much better than the 4.2 head... be enough to justify using at least 155 mains, or maybe 152's? And a 55 idles at the least?
Shawn set me up pretty darn good. I have headers, an HEI, and what's sounds like a stock to possibly a very mild cam in it.
Im currently running, 60 idle jets, 150 mains (they were 145's originally, then 155's), 160 air correctors, and F7 emulsion tubes (were F5's)
Absolutely purrs at idle... everyone thinks I have a 304 under the hood, pulls like a train all the way up to 3000rpm. The 4.2 just doesn't really flow after that... but I imagine the 4.0 head would keep keep flowing well beyond that, and require more fuel than the 4.2 would.
The Clifford manifold with LARGE plenum and ports, the 4.0 head flowing much better than the 4.2, larger camshaft with more duration, all of this adds up to slow velocity at idle and low low speed. (slugish)
He is running .45mm idle jets and can't get it to idle on the idle circuit of the carburetor. (less than 1/2 turn in AND ZERO vacuum at the "S" ported vacuum source). By increasing the volume of fuel at idle (.55mm or .60mm) you are suppling enough fuel for the engine to run on. With the throttle plates above the enrichening hole, it is using that fuel to idle on and then he has a flat spot as when the throttle plates open, it does not have any enrichening holes to gain that extra fuel that is needed on accelaration.
BOOM... flat spot. (Speed screw 1/2 turn in MAXIMUM... Change the jets to find the Len Best Idle around 1 1/2 turns out.)
As far as the "extra" fuel from the main jets 1.55mm is a large jet. If you want to, try a 1.50mm main jet and see how much performance you may loose. We all tune with different goals in mind, like most power without black smoke out the rear, good power and without studder or hesitation, or just the leanest you can get without the lean problems, burnt valves etc.
This is when a better way to measure (wide band Lamda) performance can help guide you to better, burnable mixtures without going to "pig rich" as they say. I have been pretty lucky useing the seat of the pants method for main air and fuel adjustments. I have been on a few dynos that can control, record, and mearsure power verses air fuel ratios. Again, 1.55mm mains along with the .60mm idles is allot of fuel. Your F7's and 1.60 - 1.70mm air jets are great.
Hope this helps define my ajenda when it comes to lean vs rich main circuit jetting.