Finding the right Jet Size and Accelerator Nozzle Size - Holley Carb 

Glad you're documenting your carb adventures - that's some black magic that I'll need to learn once mine is up & running. Looks like you're getting closer.

Andy gorgeous car. I stumbled across your channel I like how you talk through step-by-step. And explain everything. I am actually going through the same steps with my 302. I will continue to watch your progress like I said before your car is gorgeous

You nailed the exhaust note! Car sounds great.

great content andy keep it up

Hey Andy, If you can find a 2 stage power valve it will allow you to run leaner and not experience lean / flat spots. My 85 5.0GT used a 2 stage to solve that problem. I implemented the same tactic in my 66 coupe 302 w/ 650 DP holley. Better economy, the plugs stay clean and smooth throttle response.

You may already know this, but the main jets in the primary are supposed to be sized for cruise at constant rpm. The best way to do this is to NOT pick an AFR that you want to run at. The best way is to let the engine tell you what is best. The way to do this is to run a vacuum hose from a FULL vacuum source on your carb or manifold, to inside your car so you can read it while you are driving. The object is to find the primary jets that give you maximum vacuum reading at a constant speed of around 60 mph. Note this maximum vacuum reading. That's your ideal primary jet. The lean spot you are experiencing is likely being caused by the power valve. The rating on your power valve is too low. The job of the power valve is to enrich the mixture when you press on the gas from a steady speed. This is exactly the area you need more fuel. The power valve should be rated to a minimum of 2 in-hg below your constant speed vacuum you noted before. If your vacuum at cruise is 12 in-hg, then 12-2=10, so you would need a power valve rated as close to 10 as possible without going over 10. That would be a power valve rated at 9.5 in-hg. I hope that helps. By the way, based on your build, your carburetor is absolutely NOT over sized. It just needs tuned.

I bought a QF Slayer 600 and the stock squirter was a .018. I bought a .025 and a .028 with a cam kit and see what happens. I was getting the bog big time off idle, love your car, man...

You really want to feel a difference play with front pump cams, try a green and it'll wake up the hit

This carb tweaking adventure is going to make for a great video series. Maybe some power valve tweaks too? Anyway, thanks for sharing 👍

Hi Andy another quality video. I have a built 347 in my 65 fastback. Originally I had a 750 double pumper and the engine was never happy and idled so rich it burned your eyes. Finally after a couple years I dropped down to a 670 street avenger vacuum secondaries and it was like magic. The engine loved it however I've continue to tweak it. It came with .065 main jets but my plugs showed a rich condition, I have no AFR gauge. Anyway I dropped jet size to .063 and it didn't like it so I put the 65s back in. I may even try .067s later. My squirter size is .031. Anyway it's back to how it came. I have no lean bog and it runs great. I think the plugs looked rich because too much idling in the garage during periodic start ups. AFR gauge is next on my list. I use to have a lean bog but solved it tweaking the transition circuit with more air.

Learn about how the nozzle size affects the power band going from the transition circuit to the main circuit in your beast.

You need EFI 😁 It's been something I've been debating trying out.

A 600 is fine for a mild 302 331 a 750 is more ideal for a 331 or 347 but still a little bit to much if you want maximum efficiency, a 650 would would be about perfect but 750s are easier to come by. You should be able to tune that 750 to work almost perfectly with that engine setup, alot of people tend to over carb there cars, I wouldnt go any bigger if anything you could go smaller but like I said the 750 should work perfectly once you tune it.

Holley carbs are great for racing applications ifvyouvuse a dyno you have a lot ofvwayscto fine tune a holley the higher priced models have changable air bleeds power valves sone even have changable emulsion tube jets endless tuning adjustments. A hetter carb for the street us edelbrock avsc2 much easierti dial in (most of the time good roght out ofbthe box) keepa adjustments gives in most cases better fuel milage. I didnt catch what size engine you have most tend to put too big ofca carh on. Cu in x rpm x vloumetric efficiency sock is about .85 modified engine .90 highly modified 100%then divide by 3456

While the carb size may or may not be ideal... I'm no expert... being bigger actually makes it run leaner, not richer, so that shouldn't be why it's running richer than you want on cruise. For the pump shot, you can also change the pump cam as well as which screw hole you use, and you've still got more nozzles you could try too.

Andy you need to change the power valve.

68 main jets will make it perfect

which powervalves do you have in the car when shooting this videio ? and which sizes do you have in the box extra?drive on cruisespeed and put a litlle more throtlle in so to get 5mph more speed and see with a vacuum gauge wat it reads . if the stumble lean spot is around 10 vacuum you need a 9.5 or 10 inch powervalve . this solved my exact tesame problem. when cruising you have vacuum but when you put a litlle more throtlle in vacuum almost is gone and so the jets cant pull fuel enough and so the powervalve has to come open so the extra fuel can go thru the pvcr's (located behind powervalve) and give the engine more fuel at this low vacuum signal

so, did you ever go back to adjusting the accel pump nozzle? In your video it looked like it snapped into 16, but back to good. Im dealing with the same thing and am thinking of going to 37 on my nozzles based on your video. I have a lean condition that seems to happen longer than a second.

Step up in power valve size like from a 6.5 to an 8.5.

Did you try a different pump cam first? Sounds like you need a higher number powervalve. What is your AFR at WOT? 1 change at a time

Are both jets supposed to work at the same time? For a 2 bbl? Or is one a idle jet and the other a main jet.

What is your combonation?

Why not contact thunderhead 289 and try his carb cheat

Next time take the butterfly plate off

Why does everyone have to play horrible music for good videos???