Running large jets with .070" main bleeds...

[firefox3]

I have a 17056208 truck carb. on my 78' Caprice.

Taking Cliff's advice in his book, I changed the .125" main bleeds to .070".
I first started with 72 jets and 43K rods.
With this combo, the car wouldn't hardly get out of it's own way as far as light to medium acceleration.
Next, I went to 41K rods and the same jets, a little better but I could still tell it was too lean.

After trying several other combos with 74 jets and various rods, I installed a pair of 77 jets and 45K rods.
This combo runs MUCH better than the earlier attempts with smaller jetting.

My question is why smaller jets don't seem to work for my combination?

I'm baffled how high-compression, high-horsepower engines from the 60's-70's could run off small jets even if they did have .050" air bleeds

The engine is a 356ci with GM vortec BOWTIE iron heads, flat-top forged pistons, Edelbrock RPM air-gap port matched to the heads, and a Crane hydraulic that's 212 degrees at .050".
Turbo 350 tranny and 3.08 gears.

Thank you...

[Cliff Ruggles]

The "K" metering rods do not have enough adjustment on the upper section, so with each combination that you used, the control of the part throttle A/F was limited to a range of .005".

We never use "K" series metering rods, as they don't provide enough control via the APT screw to give full control of the part throttle fuel from rich to lean.  With a full tapered metering rod, the APT will provide an adjustment range from the larger section (.044" for example) all the way down to the .026" tip size.....Cliff

[firefox3]

Thank you Cliff...I didn't know that.

I thought "K" rods were the rods of choice to use in the M4 series of carbs since so many came from the factory with them.

I set the APT screw so the rods were about halfway down into the jet at idle and never fooled with it again.

If you don't use "K" rods, then which rods are the rods of choice for a M4 series carb?

[Cliff Ruggles]

Using the "K" metering rods only provided .005" of adjustment even if you had used the APT feature.

Using a full tapered metering rod, like the ones we sell on the website, you will have full control of the part throttle A/F as they taper from .044" all the way down to the .026" tips.....Cliff