So, I decided to change carbs on my engine and what a difference! I was running a Q-Jet and thought it was loading up at idle so I bought a used 1405 Edelbrock on Ebay. After much tuning I have it running pretty dang good. I don't really rough house her to much but do enjoy banging the gears occasionally. I was having an issue at about 4500 rpm's with the truck sputtering between 2nd and 3rd. I rejetted to 98's in the back and 101's in the front and it fixed that except it idled way to rich. So I put in a bigger metering rod for idle and left it alone on the tip and still good. I was puttering around today and decided to romp on it. I was in first and I brought it up pretty high, I wasn't looking at the tach but it did that same crap from first to second, when I did look I was close to 5,000 rpm's. I don't plan on changing at 5 to many times but it bugs me that I can't......lol.
According to carb calculations I should only need a 580 cfm carburetor.

360 standard bore
Elgin 963-P cam
.514 lift and 300 deg duration
Offenhauser intake
long tube headers
600 cfm carb.

 

Not knowing what springs your using have you considered valve float.

 

Don't think so......It feels more like a fuel issue, like it's running out of gas momentarily. I had this issue once before with a sbc, but I had that engine set up pretty hot with high flow heads etc. It was a 750 cfm Eddy and it just couldn't keep up. I went to a double pumper and cured it. Don't need anything like that for this street only vehicle.

The springs are what came with the kit. I took them to a machine shop to set the height for me and do just a little port work. Don't think ignition is breaking down either, running a petronix with flame thrower coil and 8mm MSD wires.

 

It did it during the shift, or just before?

 

It is during/after.....just for a split second.....

 

If you rev the engine up to 5K sitting still, and pop off the throttle, same as when you shift, does it do it?

Key indicator is the high vacuum as you snap off the throttle. Either your idle jets are open too far (rich) or you have a minor vacuum leak somewhere. Brake power booster one-way-valve (the part there the vacuum hose connects to on the booster), something.

Matter of fact, that power booster one-way-valve would do this - you're on it, you have low vacuum, the vacuum in the booster bleeds off. Snap off the throttle, the booster takes all the available vacuum to "fill" it again (empty, really), and the throttle plates are closed while you have a short almost completely open vacuum leak.

Ever notice the brakes get hard at weird times?

 

I have a check valve on my booster......need to check that though........I don't think it would be the idle circuits as I don't have to open them very far 2.5 turns to get it to idle well.....

 

I think I found the problem and Krewat you were correct in it being a minor vacuum leak. My pcv valve is hooked into that little breather on top of the valve cover. It has wallowed out the grommet enough I'm guessing it was sucking in air at higher rpm's. I have ordered an aftermarket chrome one and a new pcv valve as well. I have never changed it and it is cheap enough.....Anyway I wrapped a little electricians tape around it, enough so that I have to push it in and viola, no more miss.......

 

Some times it's the little things that bite us in the butt.

 

Interesting - nothing to do much with my theory, but hey, I'll take the credit

 

OK Donald. lol