Hello,
I was hoping that one of you carb experts can help me out with an off idle stumble. This question is not related to my Chevelle, but rather to the 1979 LUV that is used at the local drag races.

Here are the specs to the truck.

Model 20 axle with 4.11, spool, Moser axles.
Turbo 350 Transmission, B&M Shift improver kit, TCI 3500 Stall.
The engine is a 385ci, with Keith Black Hypereutectic pistons. Since the head change, compression is down to about 9:1.
Heads are Trick Flow Twisted Wedge.
I run a Herbert Hydraulic Roller cam with solid roller lifters lashed and .06
Specs are as follows. Duration @ .050 224/234 Lift @ .050 .499/.502
Intake is now an old Weiand 7546 which I believe they have dubbed Xccelerator now.
Carb is a Model 3310 4160 750cfm Holley
Run 29 X 10 Slicks
Best ET 12.39 106mph (cool day)

Okay, I race at the old airfield in Bitburg Germany. Elevation is about 1181 feet above sea level. Towed up there Saturday morning and the weather was hot. Got it off the truck and ran it through inspection. Lined up and launched and ran a 12.6. 60 Foot time in the 1.8 range at 103 mph There has been a slight hesitation at launch forever and I have been changing Shooters like a champ trying to get rid of it. If you have the hood open and you chop the throttle you can't feel it, but if you sit in the car and mash the gas you get it, and it is present during launch. I have to mention that my buddies and I wrench on the truck and my brother drives. Since we put a cage in, I don't fit inside.
Anyways. we came up there with an old Edelbrock Torker intake. I pretty much think that is a stock replacement and have never liked it. We ran that intake Saturday and changed intakes Saturday night at the track to the Weiand. Problem was still there with the other intake. Thinking that the initial shot was too lean I changed the shooter from a 28, to a 31, to a 35, then to a 37, and to a 42. I went back to the 35 because that one seemed to work best a minimizing the off-idle stumble. But without making any other changes, just the intake, the mph went up to 105, so I knew that was a good decision and the ET came down to 12.5.

Jetting started off with 78 jets,a pulled the plugs, too rich, went down to 76's and then eventually to 73's. I still may go leaner. Through all of these changes the stumble was still there, just minimized.

Timing was played with too and I found that it like about 12 degress initial best.

I think the problem lies within the carb. I was just thinking about it and I think I should have adjusted the idle mixture screws with the vehicle in gear, to make sure that it isn't too lean with a load. I haven't played with the secondary springs at all. I know Holley tends to put a pretty stiff spring in there, but the truck doesn't seem to lag, or get a kick in the butt with the springs opening late. I know I am jumping around here, but I am trying to explain the situation the best I can. I hope one of you have experienced the same and can help me out.

I haven't messed with the pump cam at all, still the stock orange cam. Idle (not in gear) vacuum is about 10-11 inches. Power valve in the carb is still a 6.5 inch. Jets 73, Shooter 35. Seconday springs are stock so I assume hard.

Thank you,
Alex

First make sure your idle slot is squared, alot of time i find people open the primaries to much to compensate for idle ,and then you get a lean hesitation! try opening the rear butterflies a bit more and the primaries a bit less. this usually helps alot !! if that doesnt work try a bigger discharge nozzle, dont be afraid ,if the engine needs it ,it needs it.

Thanks for the tip on the secondaries. I will check that out. Concerning the primaries, I drilled a 3/32 hole in the butterflies to ensure that they were closed as much as possible at idle. Initially I thought I had them too far open.

Does anyone thing that the holes drilled in the primary butterflies will lean it out too much and possibly be the cause of the off idle stumble??

Alex, to me it looks like you don't have enough intitial timing. 224/234 is pretty stout and will want more initial I would think. I'm thinking around 16 -18*. If you don't have enough initial, no matter what you do to that carb, the hesitation will never go away.

Also, I found that if your too lean on the primaries, you will also get a stumble thats hard to tune out

Vince,

Thanks for your advice. Now, I tried more initial timing but then resorted to the old method of no gun, I turn the distributor and my brother mashes the pedal to get the best response. Then I put the light on it and found the initial to be at 12.
Concerning the jets. I stepped up the 78's and then went back down because the truck still had the hesitation and had a black smoke trail all the way down the 1/4 mile. Checked the plugs and they were black. Therefore, I started going back down. I forgot what the initial jet size was (70 I think) and the plugs were snow white, that's why I jumped up on the jets so drastically initially to keep from burning a hole in the crappy KB Hypereutectic pistons.
Most of what you have suggested I have tried, therefore I am thinking it is the damn holes in the primaries that may be too large.
Going to set up a different carb I have (A 4150 3310) and give her a try and see if it is a carb problem or a timing problem.

I won't count timing because of the crap mallory distributor. Going to try to change that out too and see if it makes a difference.