Holley tuning help

It is a good idea to check out the carb, make sure everything is clean, adjusted properly and it has the OEM jets and power valve to establish a baseline. You can look it up using the part # - possibly 4777. BUT.....that is the wrong carb for the application! Double pump carbs tend to run on the rich side for higher performance engines. But if there aren't enough cubic inches, converter, gearing, etc. to to get a good vacuum signal, it's not going to behave properly. Holley's can work well OOTB if they are used on a combo close to their designed application. I would bet that it is a lean bog unless the pump cam and squirters are wayyyyy too big. Either way, the carb won't be easy to "bandaid" tune to the heavy truck.
I also suggest a 600-650 vacuum secondary carb.

Not sure on jet sizes or size of the squirters or the power valve. I haven't had this apart yet.

I would try bumping up the timing a little more before messing with the carb.

Sounds like you need to tear it apart and see what you got first.

I adjusted the mixture screws and it is a little bit better now, but the bog is still there and now I have a little hesitation on the top end.

My

First off....for a late model low compression engine with a stock cam in a heavy vehicle... a double pumper and air-gap intake is not the way to go. You want torque to get you off the line. That intake is rated 1500-6500 for a reason. You're hurting off the line torque and wasting fuel with an air gap. Plus ...double pumper carbs are meant to sit on top of a race engine with generally low vaccum signal and lumpy cams. Theyre jetted richer and also idle richer = wasted fuel.

A regular Performer intake (NOT RPM or Air Gap RPM) and vacuum secondary carb is all you need for that setup and will provide more torque at lower rpm. If its a new intake and you can swap it for a performer....do it. Go with a 600 vac sec holley or Edelbrock 625/650 AVS carb and you'll be much happier.

Of course.... theres always a Performer intake and Demonsizzler prepped Thermoquad! Small primaries = Gas mileage and Gobs of low rpm torque + Large vacuum secondaries for when your foot gets happy!

I knew before hand that it wasnt the perfect combination for my setup. I only used this carb and intake setup because I already had them laying around as extra parts. Anything is better than the stock 2 barrel setup that was on it as that needed a complete rebuild. It was running so rich that your eyes would burn standing next to it and you could basically watch the gas gauge go down. The truck runs 100 times better than it did. The only problem is if I try to get on it quick it will bog down bad until I get moving. I just want to try to get this fine tuned for now until I get my 340 finished to put in it.

Sounds like a fuel delivery hole from idle to main metering. Even though you said its squirting fuel doesnt mean the pump shot is timed right or its enough. Ive felt cars hesitate...even backfire with fuel delivery holes but they generally dont outright stall like yours.

1. Make sure your float levels are set correctly before you continue to eliminate that as a contributor.

2. Adjust the accel pump circuit timing. On the side of the primary throttle lever check the screw position for the accel pump cam. Its usually in hole 1 from the factory. If so...remove the screw and then move the plastic cam a little so the position 2 hole in the cam lines up with the #2 hole in the throttle lever and screw it down. There may be no threads in the plastic cam #2 hole if its never been used bcus the screw cuts the threads the first time so you should hold the cam in place from behind while tightening the screw. (be careful not to lose the screw as it can fall in the exhaust crossover port...hmmm...how do i know that?)

3. Once the cam is set to #2 position you're going to need to re-adjust the pump lever arm clearance. Make sure to do any curb idle speed adjustments first so its at the desired idle speed. Then with 2 wrenches adjust the accel pump lever to actuator bolt to pump arm contact gap.

If after moving the cam to position 2 theres space between the bolt bottom and accel pump lever then loosen the nut and bolt to make the assy longer until it just contacts the pump lever arm. I go for .005 spacing between the lever and bolt bottom and then loosen the nut just a hair more for nearly zero gap. Theres a little side to side play in the lever arm so if u move it back and forth from underneath it shud just lightly scrape the bottom of the bolt.

If after moving the cam to position 2 its too tight for the .005 feeler gauge...then tighten the nut and bolt together to make the length shorter until you get the .005 gauge between the pump arm and bolt bottom then back the nut off just a hair so so the bolt just scrapes the pump arm.

4. If the stumble is gone you're done. If its mostly gone...go to step 5.

5. Accel pump shooter. If the cam adjustment doesnt help...or not enough... you may need a larger primary shooter. Stock for a 650 DP is probably a 25 or 28. If its a 25 then go to a 28 or 31. Obviously the smallest you can get away with to eliminate stumble will be better for gas mileage and spark plug life. If the stock is a 28...then try a 31. Get the type with extended shooter tubes if possible. I wouldn't recommend trying to drill a stock shooter unless you have set of numbered micro drill bits with precision tolerances and even then....at those sizes you risk breaking a bit...and then ...you have no shooter at all.

The above changes should eliminate the launch stumble.

If not....you may need a different cam profile and might have to buy a Holley plastic cam kit and experiment with different cams.

You really can't go much more on initial timing...but....Since you're already running 14deg initial and 34 total I wouldnt go much more. You risk pinging with todays garbage pump fuel and you dont want to risk piston damage due to pre-ignition. I would at least check the vacuum advance using a hand vacuum pump/gauge tester if you have one or you could try putting a timing light on it and checking the timing with the vacuum advance line removed and the advance port at the carb plugged. Check timing at idle then 1000 1500 2000 rpm. Shut down.... reconnect the vac advance hose and re-test noting advance at same rpm to confirm its advancing. - -

Let us know how you make out.

I think the real issue has been pointed out that the air gap intake and double pumper are not a good match for the engine and application. That being said, I think you will have to richen up the transition circuit to cover up the hole between the accelerator pump shot and transition into the main circuit. Usually enlarging the idle restriction and maybe the power valve restriction in the metering plate by a few thousandth should help.

I have a billit metering block with the screw-in restrictions. I used it to tune may carb, then used that as a guide for modifying the stock metering block.
This was after making all the standard tuning adjustments.

I tried the larger squirters with limited success. To start, jet that puppy bigger 2 to 4 sizes per jet (primary and secondary) if the problem gets less, you need more fuel from the accelerator pumps. I believe the 650 comes with 31 nozzles standard. Going bigger doesn't help as the 30 cc pumps don't have enough volume per stroke to spray the fuel in as it is needed. I put dual 50 cc pumps on mine with 31 nozzles and I used the brown pump cam, turned it so it was about to push the accelerator pump lever, marked it and drilled my own hole. Hesitation went away for me.
You need more fuel, the manifold is fairly big for that combo, it has no exhaust crossover to heat anything so it needs more fuel.
This is all assuming the engine is happy and the carb is as well.

I ran a 650 double pumper on a .040 over 360 in my 4x4 in 80`s,it had 12-1 jahns pistons,strip dominator manifold,13/4 headers,2.02 int valves.First thing I thought was maybe a little over carbed on 318.But reading thru replys they are good with fix tips.Mine was 75 white sweptline express with keystones and 12/33 15 tires.Twisted yokes on transfer case pulling wrecker and mobile home out of mud with both burried.I did not lack for power.....lol

The 650dp comes with 28 squirters F&R.

having been down this road before putting a hirise dual plane and vac secondary 600 on a previous stock 2bbl with 2.76 gear.
It killed the off idle and bottom end.

assuming the carb is ok..my cure was alot of time experimenting on initial timing the idle mixture and timing curve. getting a clean idle with alot of initial and lighter spring for quicker advance was the fix but you have to deal with pump gas and pinging issues also if your to agressive.

need to limit total advance also since alot of initial and the lite springs will put total to high.

more gear would help if you are running "hiway" gears now.

Havnt had a chance to work on the truck for a while but I found out that the miss and hesitation up top was from when I found out that I forgot to hook up the vacuum advance back up and then after hooking it up I checked the timing again and the total was at 52 degrees. Guess Ill have to lower the initial a little to get it back down.