[fastekibeast]

Hey all!

I just bought a Quick fuel 750 cfm drag series carburetor stock jetting (primary #72 secondary #83, #45 power valve. Is here anybody who can roughly estimate what size power valve, primary jets and secondary jets do I need when my camshaft is Bullet 292/292-06R @050 259/259 LSA 106 engine is 400 + .030 over, MSD 6AL-2 box with factory distributor Pertronix unit inside.
Thanks for advance
Janne

[i82much]

My experience with carburetors is that you should follow the following tuning procedure:

1) set the carb on a bench, swap jets, air bleeds, idle and power valve restrictions at random until your urge to tinker is satisfied.

2) return the carburetor to factory calibration.

3) install carburetor on vehicle and never touch it again.

Works like a charm!

[PunchT37]

Originally Posted by i82much

My experience with carburetors is that you should follow the following tuning procedure:

1) set the carb on a bench, swap jets, air bleeds, idle and power valve restrictions at random until your urge to tinker is satisfied.

2) return the carburetor to factory calibration.

3) install carburetor on vehicle and never touch it again.

Works like a charm!

Lol

[n20ta2]

I wouldn't change anything until you can get it running and tune from there. You will just waste your time and money on parts you may not use .

[NeighborsComplaint]

You should be good to go. If you find it loading up on you, you may have to go to a 3.5 power valve. That's a big camshaft you're running so check your vacuum at idle, divide by 2 and round up to the nearest power valve size.

[fastekibeast]

Originally Posted by i82much

My experience with carburetors is that you should follow the following tuning procedure:

1) set the carb on a bench, swap jets, air bleeds, idle and power valve restrictions at random until your urge to tinker is satisfied.

2) return the carburetor to factory calibration.

3) install carburetor on vehicle and never touch it again.

Works like a charm!

🤣🤣🤣👍 Definitely best in this week!!

[i82much]

Originally Posted by fastekibeast

🤣🤣🤣👍 Definitely best in this week!!

FYI Quick Fuel has great tech support. They will email you a copy of the factory calibration for your carb, makes life easy. That way if you decide to tweak, you are starting from a known good calibration.

[PunchT37]

Remember, the power valve doesn`t richen the idle circuit.

[Tom Vaught]

I ran a 255/260/106 for several years and for the most part it was able to be driven on the street but ate gas like a fiend with the 4.33 gears I had in the rear.

I ran a basically out of the box 850 DP carb (#4781) with very minor changes to the carb.
No power valve in the rear, 50cc accelerator pump in the front, 30cc accelerator pump in
the rear (swapped parts front to rear). Drove that car all over the east side of Michigan.
Took it it several PMD Car Shows, to the GM Tech Center, to the Packard Proving Ground show.

The key is getting the air bleed correct for the proper signal to the fuel side. Same concept, 850 carb OR 750 CARB!!!!!!!

Have fun. As was posted, do not be surprised if the calibration from Marvin B (Quick Fuel) requires very little change. Drive it first. With a GOOD Air/Fuel meter in both Manifold pipes.

Tom V.

I HAVE PUT MORE CARBS, THAT WERE MESSED WITH BY PEOPLE WITH LITTLE CARB KNOWLEDGE back to stock calibrations,
than you can imagine. And they later ran fine for years.

[chuckies76ta]

I found removing the bowels and making sure the gaskets are not stuck to the metering blocks. You can use non-stick gaskets which are pretty good. I also put a very light coat of Vaseline on the gaskets when new or used. Another thing I noticed after years of running my quickfuel carb was the accelerator pump diaphragms were very hard and needed replacement. They now have them so they are ethanol compatible. Although I don't run any ethanol in the engine.

[fastekibeast]

Originally Posted by PunchT37

Remember, the power valve doesn`t richen the idle circuit.

Oh! 😳 I thought it will, but this is a good news.


I ran a 255/260/106 for several years and for the most part it was able to be driven on the street but ate gas like a fiend with the 4.33 gears I had in the rear.

This one is bad! Gasoline here in Finland is about 6.60$ per gallon, but fortunately I got 3.55 with TKO-600 transmission.

[Tom Vaught]

With your TKO-600 trans you will be able to find the "Happy Spot" in rpm for each trans gear with that camshaft. It really is not that big of a camshaft. It will talk to you at idle though. 64 GTO and I could reach inside the car and hit the starter and the engine would fire right up with that camshaft when the engine was warm. Never touched the gas.

Tom V.

[Formulajones]

Originally Posted by PunchT37

Remember, the power valve doesn`t richen the idle circuit.

Correct, very common misconception.

Power valve will not dump fuel until the transfer slot is open triggering the main circuit. So as long as your throttle blades are adjusted properly at idle, you will only be running on the idle circuit no matter what your vacuum power valve is.

[Cliff R]

Couple of things.

Right to start with NO ONE can tell you anything about how to set that carb up without ALL the information. Engine specs, including thte compression ratio, heads, head flow, intake, exhaust, etc. Transmission and converter/stall speed or stick shift? Gear ratio? Vehicle, weight, suspension, tires used.

This thread should also be over in the "race" section because a 400 with that much cam is going to need a LOT of compression to make much beyond 5" vacuum at normal idle speed, and WAY too much cam for 99.9 percent of the 400's that would be built for pump gas/street use.

As far as tuning on something you just bought, initially I wouldn't touch it with a 200' pole.

Install the carb, make sure you have adequate fuel delivery for the power level, and do some testing. Evaluate the carb in all areas, then go back and make changes to those specific areas that need help. You may not have to touch it, but in my experience (I've built a few carbs here and there) it's rare to have anything come right out of the box and be spot-on in all areas with a smaller CID engine with that much cam in it........FWIW......

[NeighborsComplaint]

Originally Posted by Formulajones

Correct, very common misconception.

Power valve will not dump fuel until the transfer slot is open triggering the main circuit. So as long as your throttle blades are adjusted properly at idle, you will only be running on the idle circuit no matter what your vacuum power valve is.

Take your power valve out sometime and let me know how the car idles. In theory, it shouldn't matter but if your car has a long duration cam and low vacuum and you are overlapping the idle circuit to the off-idle circuit or cruise circuit, it will flood.

[Formulajones]

Originally Posted by NeighborsComplaint

Take your power valve out sometime and let me know how the car idles.

That's not exactly how it works. A leak is a different thing entirely.



The power valve needs a venturi affect (assentially what a carb is) to pull fuel from the float bowl through the power valve circuit. That circuit DOES NOT become active until 2 things happen.......the transfer slot is exposed enough to enable the main circuit, and the vacuum drops low enough to open the power valve. Period!! BOTH of those things have to happen for the power valve to become part of the fuel curve.

So in other words, if the vacuum is low enough for the power valve to open, but the throttle blades are not open enough to start the main circuit, (hence idle condtions) then you get no fuel from the power valve.

Or if the throttle blades are open enough to enable the main circuit, but the vacuum is too high for the power valve to open, (light cruise for instance) then you get no fuel from the power valve circuit.


This power valve flooding an engine at idle has been a long time misconception that has unfortunately grown since the internet and lead people down the wrong path of carb tuning. Those that experience this issue obviously either have their transition slot too far exposed, or had a leaky power valve with a cracked diaphram or gasket problem and just blamed the power valve vacuum reading.

The correct way to tune these things is to have the power valve fairly close to idle vacuum. A couple inches is fine. It doesn't have to be half of the vacuum reading that is so commonly thrown around. All that does is cause lean stumbles right off idle and light tip in drivability problems, and then people get all crazy with pump shot trying to cover it.

[Tom Vaught]

A lot of miss-information out there from people who really do not understand the Holley Circuits in a carb ACTUALLY works.
It become obviously in posts but for the most part I stay out of that issue.

I also recognize people who have a CLEAR understanding of the circuits of a Holley Carb (or any carb work) and recognize those people.

Formulajones does have a clear understanding of how they work and you can take his posts to the bank.

The other "Carb Experts" posts I tend to ignore as they have already shown in print that they do not have a real UNDERSTANDING of the carb circuits and WHEN they operate.

You can take SHAKER455's posts and Cliff Ruggles, and Jon Hargroves posts to the bank
and I add Formulajones name to that list.

It is embarrassing to read some of the other posts.

Tom V.

[PunchT37]

Originally Posted by Formulajones

Correct, very common misconception.

Power valve will not dump fuel until the transfer slot is open triggering the main circuit. So as long as your throttle blades are adjusted properly at idle, you will only be running on the idle circuit no matter what your vacuum power valve is.

I used to run a high overlap cam with a 1000 HP. It had 6.5 idle vac in park and 5.5 in drive. The carb had a 6.5 power valve in front. Blocked in back. No problems with loading up at idle.

This thing had a lean spot deeper into the primary circuit. So, I started the power circuit ,earlier , before the secondaries. Problem solved.

[Cliff R]

"Loading up" at idle typically happens when you have so much idle fuel to the mixture screws that you loose control. This happens easily with big cams as having a lot of transfer slot exposed when the throttle plates are open quite a ways supplies uncontrolled idle fuel to the engine. This is why idle bypass air helps as it adds air so you can turn the idle speed screw back some and expose less transfer slot. Once you get control of the idle fuel and show sensitivity with the mixture screws you are now able to lean things up enough to keep from flooding the engine with fuel causing it to "load up".

Another way engines "load up" at idle is that the throttle plates are simply open too far exposing most of or all of the transfer slot and engaging the main system pulling some fuel from the boosters. This is often called "nozzle drip". It's not a common problem with Holley square bore carburetors but can be a HUGE problem with Q-jets and TQ's as they have very small primaries and engage the main system at very low throttle openings.

Another way Holley's load up is a leak at the PV. Either at the gasket or a tear in it. Either way puts uncontrolled fuel directly to the intake.

Here's another thing I see here that will cause an engine to load up at idle and it is very difficult to diagnose. Any leak on the pressure side, especially a very small leak will allow the fuel in the bowl to rise slowly. The scenario created here is that the engine runs and drives fine until you stop at a light or let it sit and idle for a minute or two. If you have a leak at the N/S assembly and the rate of fuel being used is slower than the leak the level of fuel in the bowl begins to rise. When it gets high enough fuel starts dribbling in from the boosters. The engine starts idling rough, blowing black smoke, loading up and will eventually flood out and die unless you hit the throttle to clean it up or drive the vehicle. Next time you stop it does it again if you sit there long enough..........

[Holeshot71]

Originally Posted by Cliff R

"Loading up"

Here's another thing I see here that will cause an engine to load up at idle and it is very difficult to diagnose. Any leak on the pressure side, especially a very small leak will allow the fuel in the bowl to rise slowly. The scenario created here is that the engine runs and drives fine until you stop at a light or let it sit and idle for a minute or two. If you have a leak at the N/S assembly and the rate of fuel being used is slower than the leak the level of fuel in the bowl begins to rise. When it gets high enough fuel starts dribbling in from the boosters. The engine starts idling rough, blowing black smoke, loading up and will eventually flood out and die unless you hit the throttle to clean it up or drive the vehicle. Next time you stop it does it again if you sit there long enough..........

I’ve had this happen before with a 4150 DP. The plastic float on the primary side was taking on gas and causing to the needle to come off the seat.