[Bonger6]

I have lots of info and questions so hopefully you enjoy the read and can give me some pointers!!!

First off, here's the motor:
350 4 bolt block decked/line-honed and bored to .030 = 355cu in
Forged crank .010/.010, Forged 1 piece oil pump drive, Melling HV pump and pick-up, ARP rods, ARP bolts, Forged Pistons, .125 pop tops
11:1 compression
Cloyes Tru Roller Timing Set
COMP CAMS EX286C Cam .284/.480,
Vortec Heads pocket ported and bowl hawged, Dual springs: 1.94 intake valve/1.50 exhaust valve, Lift at both valves .484in, Lobe Center 109.0, Overlap 64
SpeedPro Hydraulic Lifters
Edelbrock Performer Intake
Hedman Headers

It previously had an Edelbrock 600cfm 1406 performer carb which I ousted in favor of a freshly rebuilt Holley 4777 650 Double pumper. After much screwing around I finally got it running as I had issues with floats/needles and squirters.

Now that it is on and running I've got the idle mixtures/speeds set/etc. However, I'm thinking that the builder put in jets that are too large........at least for the secondaries based on the description I'm about to give!

I took it out and it seem to cruise fine and slowly accelerate fine as well. However, a short burst of W.O.T causes it to nearly stall, hesitate, and then pick-up only AFTER I start backing off the pedal. I can't see outside to tell if its blasting black smoke under a load but it does if you floor it quick in park....... but from this description I think the second shot is choking the motor.

At idle I have around 12-13" of vacuum pressure, going up to around 20-22 if you bring it up and hold @ 2000rpm (or close, I couldn't see the tach) from the engine bay.

It was initially tuned/timed/etc to the edelbrock carb so should I advance the timing slightly to incorporate the added size of the new carb?
***Also, I do not know the size of jets or power valve that are currently in the carb either. I've emailed the builder to save myself a little time before pulling everything apart and actually checking the numbers but does anyone have any GUESS on what sizes would likely work best in this set-up???

It does have an automatic which I know are not ideal for Mech secondaries but it is geared @ 3.73 and has a 2400 stall converter.

*****THANKS IN ADVANCE for any info or direction! It'll save me having to call my mechanic friend over to mock me......and if you need more info just ask and I"ll try my best to accomodate

[56GM]

first off check the numbers on the power valve don't quote me but i believe if you diviide the amount of vacuum you see at idle it should give you the size of the power valve you should be running. Secondly if the plugs are a little dark after running them for a little bit then Holley says to start by jumping down two sizes (i.e 76 to 74). If your having issues at wot I'd say check your power valve, accelerator pump and accelerator pump. If you haven't seen these yet watch em. Hope this helps

http://www.youtube.com/watch?v=mE_xT...eature=related
http://www.youtube.com/watch?v=k-Nx5...eature=related

[454C10]

http://www.bob2000.com/carb.htm

[66GMC]

The first thing I gotta say is "Here we go again!" LOL
Expect a large number of theoretical "fixes".

But I'l bite.

You're not describing a "bog" situation which leads me to believe that your "transition" phase (idle to WOT) is working quite well. A puff of black smoke is par for the course with a double-pumper as it's fed a LOT of fuel to cover the lean hole caused by standing all 4 butterflies on their end when you mash the pedal.
You might play with pump cams and shooters to clean that up just a little.

Power valve operation is likely fine, too ... unless you've got clouds of black smoke or surging under load (i.e. pulling a hill in top gear under part throttle.)

The first thing I'd be looking at, if it were mine, would be fuel delivery. It sounds to me that you're running out of fuel.

1.) Check the pressure and volume coming from the pump and float levels, first of all. (Basic of the basics)
If its a mechanical fuel pump, it should pump in long, steady streams, and should fill a pop bottle in about 3 strokes max while cranking with the coil wire off.
2.) I'll assume that you know how to determine and/or adjust the float level externally using the sight plug holes and the externally adjustable needle and seats, right?

3.) If they are fine, the filter(s) are fine ... then it's time to pull the bowls off and do a visual inspection. You said freshly rebuilt, so everything SHOULD be clean as a whistle in there. Have a spare set of gaskets handy, just in case ... and make sure they are the CORRECT ones for that carb.
Some of the aftermarket kits (i.e. NAPA Echlin, Blue Streak HyGrade) will contain several different gaskets, and it's the responsibility of the carb rebuilder to select and install the right ones.

4.) Go to Holley.com and download the numerical list. It will give you a "baseline" idea of the components that were originally installed in the carb when it was new. (jets, squirters, power valve etc) It will also give you the part number of the correct Holley "renew" kit and maybe even those bowl and metering plate gaskets that you could buy seperately.
You shouldn't need to stray very far from the factory jet sizing. Most carbs are jetted for sea level, and I didn't take note of your location ... but you decrease the recommended jet for higher altitudes. Is it one size for every 2,000 ft? That info is also on the Holley site. That double-pumper IS designed and calibrated for 1/4-mile racing as well, so I wouldn't be expecting to see great fuel mleage. iF this vehicle is a daily driver that sees a whole lot of highway ... you might even want to entertain the idea of looking for a vacuum secondary "fuel economy" type carb instead. Some of which use a two-stage power valve.

In any case ... good luck, and let us know what you've checked and what you've tried.