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Carburetor Tuning    

Jim D. Moore - July 03, 2014 01:56 PM

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Referring to the Holley website, we find that this is an 800 CFM double pumper with 1-11/16 inch throttle bores along with a primary venturi of 1-3/8 inch and a secondary venturi of 1-7/16 inches. It was born with #70 primary and #85 secondary jets. It also had a 6.5-inch power valve on the primary side only and came with .031 accelerator pump nozzles. This also tells us it was manufactured on the 68th day of either ’67/’77/’87 or ’97. This one was 1977.

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Some carbs have brass orifices pressed into these holes. By using small strands of wire, you can drastically alter the idle circuitry and bring the idle screws “back to life”.

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Adding small wires to the idle feed restrictions (if needed) in the metering block is the most common way to bring a carb within range. The orifice marked in red is the feed to the curb idle discharge port. The orifice marked in blue feeds the idle transfer slot and can be resized to correct light off idle “tip-in” issues and light cruise mixtures. Lead pellets can be tapped in place and drilled as necessary.

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QUESTION: I recently bought a “nearly new” Holley 750 Double Pumper for the 383 in my ’57 Chevy. It’s got a TH350 transmission with a 2,200 stall converter and 3.55 gears. My problem is that I don’t have the money to purchase a wide band tuning tool right now.  Some buddies say it’s too big, will run terrible and bog since it has mechanical secondaries. Others say it might work if I install smaller jets.  I want to make this carb work … but is it even possible?

ANSWER:Don’t worry. While having a wide band oxygen sensor can be handy, it can be just as frustrating to people who try to chase a “magical” air/fuel ratio without paying attention to what the engine wants/needs. By design, a carburetor has a wide range of automatic tuning ability since it relies solely on atmospheric pressure/pressure differentials to operate. So if it’s calibrated correctly for a 350, you can bolt it on a 454 of similar characteristics and it will do well there too without big changes (if any) in jetting. Same with altitude, it takes very little change (if any) to make one operate at high altitudes.

Your 750 double pumper purchase will prove to be an excellent choice. First, because a 383 can easily utilize it, and second, it’s about the most universal carburetor ever made for performance. It can easily be made to run well on just about anything from mild to wild. Begin by retrieving the complete “LIST” number on the choke airhorn. Go straight to Holley’s website and make sure it has the jetting and power valves that came in it originally. Plus you’ll find a ton of other great info there.

http://www.holley.com/TechService/Library.asp

 

The original Holley double pumpers were calibrated for engines with pretty strong cams that had a weaker vacuum signal at idle. Since Holley has no way of knowing what you’re going to bolt it on, they make sure it works decent on everything. Installed on a mild combo with a strong vacuum signal, it will run richer at low speeds and idle because the stronger vacuum is allowing more fuel to flow. Remember, it’s all pressure differential. The newer “Street HPs” are a little leaner and are excellent street carbs out of the box.

Check the float levels. Everything depends on the fuel level being steady. Now, make sure the base timing/curve is correct. No carb can correct ignition issues. More initial provides much better response, but make sure total timing stays in line, 15° initial and 36° total (no vacuum) are good starting points.

On most engines, adjust the idle stop screw(s) to show .020-inch of the idle transfer slots. That will make things very close. A big cam that requires a high idle speed often has the slots uncovered too far which adds a ton of fuel and makes the mixture screws ineffective. Some later carbs have a separate idle air screw under the air cleaner stud to allow more air to enter.

The idle circuit feeds the engine until airflow increases to the point where fuel flows through the main metering circuit and boosters. It also continues to contribute as engine speed increases to some degree. Usually, if your idle screws are responsive, you’re in good shape. If your transfer slots are set right and the screws have no effect, we’ll need to lean the idle circuit. Start with the fuel orifice and use a small strand of wire to restrict it. Pull a strand of copper wire (or any kind) out of a piece of electrical wire. Make the restrictor about ¼-inch to 3/16-inch long and put a small bend on the end. Insert it into the two referenced holes with the bent end back towards you. Lay the gasket in place and carefully reinstall the block so the wires don’t fall out.

I’ve also used lead shotgun shell pellets tapped into the corresponding orifice in the main body, which can be drilled to whatever you want. If you go too big, just dig it out and install another pellet. Adjust idle screws for best idle. Tuning to the highest vacuum reading will often be slightly lean, so give it what it needs to be smooth. The screws should be adjusted equally approximately 1½ turns out from seated.

Every combo is different, but somewhere around 1,800 to 2,000 rpm, main fuel enrichment is beginning through the main jets. If it’s lean it will have a “missing/skipping” feel to it. If rich, it will be a little lazy feeling, but will run OK. Just lean main jets ‘till it cruises well with no surging, etc. 

Next, pay attention to the Power Valve opening setting. Find a nice long gradual hill and drive into it at 50 mph or so in high gear. Holding a steady speed without more throttle opening, the power valve should smoothly open and allow you to maintain speed. If it opens too late, you’ll get a flat spot; too early and it will get a little “blubbery”.  You want it to pick up with no lag under light throttle acceleration. Generally if you have eight to nine inches of idle vacuum (in gear with an auto) you will need a 4½-inch to 5½-inch Power Valve. Higher vacuum will allow a 6½-inch or even a 8½-inch one. A higher number means the PV activates earlier.

The carb has Power Valve Channel Restrictors (PVCRs) to add/control fuel under full power. If you’re staying close to factory jets, the PVCRs should have you fine under power. Since it’s impossible to accurately “read” a plug for wide open throttle mixture with a lot of idle time, select the cylinder that’s easiest to get to and install a new plug. Now make a hard acceleration run from a 30 mph roll to 70 mph or so.

Shut the engine down (don’t lock the column!) and coast to a safe spot on the side of the road. Remove the plug to “read” the mixture. I usually swap jets right there and make another test. If the plug was “lean” you can re-use that plug. If it was rich, install another new plug so that it’s possible to see the changes. Once you’ve got it correct, install all the other plugs and you should be good!

When you have all the big stuff handled, it’s time to adjust the accelerator pumps, which is done by adjusting the lever screw/locknut to remove all play in the linkage. Start out with play and “loosen” the locknut to make it longer – barely remove play, don’t add preload. Ensure that when you fully open the carb, the pump is not bottomed out. It should still move .010-inch to .030-inch.

The pump squirters are numbered with respect to their orifice size. Sometimes they are different on the front and rear of the carb. If so, you can swap them to test what works best and then either buy a new correct one, drill them larger if needed or epoxy the holes closed and re-drill smaller. Be careful to make sure it directs fuel to the proper area in carb throat.

Obviously there’s a lot more to maximizing a Holley carb to its full potential, but these basic steps will net you a responsive, tire smoking street machine! If you want to go further, please refer to these excellent, detailed articles by one of our other feature writers, Hib Halverson.

Click HERE and HERE.

  

In between his day job and writing technical how-to stories, Jim Moore likes long-distance jaunts with the top-down on his nine-second, street-driven ’67 Corvette. He is an ASE Certified Master Technician and Machinist and also has numerous OEM certifications. Jim is no stranger to horsepower and enjoys sharing his 30-plus years of torque-filled knowledge with veteran and freshmen engine builders. If you’ve got a performance question, feel free to email Jim at autoeditorial@amosautomotive.com



 

 

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