I was reflecting on carburetor tuning the other day. I realized by talking with other folks how many people don’t really understand the basics of getting a Holley to work on an engine. Holley’s are an easy carb to tune in my humble opinion. I’ve never had much trouble getting them to do what I wanted them to do. I’ve talked to people who have become so frustrated that they pulled the Holley off of the car and installed a carb from another manufacturer on it that probably killed some of their potential performance.
To those of you who know how to tune a Holley you can stop reading this part now. If you don’t understand how a two or three circuit Dominator works then you might want to get back on the trolley when I get to that part.
The first place I start is the power valve. In a 4150 and Dominator, these carbs have a power valve in both sides, primary and secondary. 4160 Holley’s only use a power valve on the primary side because the secondary side is controlled with a jet plate.
The first thing you need to do is get a real-world reading on your manifold vacuum. If your car has a camshaft with lots of long timing chances are it idles pretty rough and has low manifold vacuum. Holley’s come from the factory with 65 power valves in them. Some I’ve seen have 85’s in them. This is Holley’s vacuum classification for their opening vacuum. An 85 power valve opens at 8.5 inches of mercury vacuum. A 65 opens at 6.5 inches. Attach a dampened manifold vacuum gauge to good vacuum source, preferably on the carb base somewhere. Taking it off of a manifold runner is asking a lot of the gauge to sort out reversion pulses, especially if the engine has a lot of valve overlap. So let’s say for goofs your car idles with about 5 inches of manifold vacuum like my Olds does. That ain’t much manifold vacuum folks! If you have a 65 power valve or even a 45 power valve in the primary side it will be open during idle and will dump excess raw fuel into the manifold besides what the carb’s idle circuits are allowing.
Holley says to read your manifold vacuum and divide your gauge reading by two to get your power valve number. Well my Olds idles with about 5 inches of vacuum. 5/2=2.5. So I need a 25 power valve. Once you get this right the car will start to really clean up on its idle. If you don’t have such low vacuum you still need to size the power valve to whatever your engine is making for manifold vacuum. Doesn’t matter what the hell power valve is in the carb right now you need to know what your engine’s manifold vacuum is. Find out. Change the power valves in both sides of the carb. You’ll have to take the float bowls off. Unless you have a stash of Holley float and jet block gaskets just go buy a rebuild kit. Yeah they’re spendy, get over it.
Okay so now we have the power valves squared away don’t we? We have that part of the parts baselined. You wont’ have to wonder about the power valves now. If you are using a carb with cathedral float bowls with the external float adjustments, take off the sight plug screws and check the float level with the engine running warmed up and idling. You should have fuel right to the bottom of the hole or maybe even dribbling out of it a bit. The higher your fuel float bowl level is the faster your main system start-up is. I’ll get to this in a minute. The external floats on the Holley carbs are adjusted by putting a BFS Big F_ _king Screwdriver in the screwslot and loosening the lock nut which is actually the float level adjuster. Don’t take the screw completely out of the adjuster there’s a little gasket under it that might get lost if you do. Besides it doesn’t need to come off to adjust the float. You can even do this with the engine running. If you turn the adjuster nut clockwise I think it raises the float level. Turning the nut ccw lowers it. Turn it in the direction your float bowl fuel level tells you to. Once you get it where you want it tighten the big screw. Now that’s done. Now you can move onto the idle mixture screws. These are probably one of the most misunderstood and most “knob-dicked” adjustment on any Holley Carb. The proper procedure is to run them in until they “GENTLY” seat. They screw in CW. I have a special screw driver that has a white stripe painted on it on one side of the blade so I can count the turns. Back the idle screw out one and a half turns from seated. Do that with both of them. Start the engined and see how it likes that. If you still have your vacuum gauge hooked up see where it’s idling at. Let’s say you engine has the capability of making 14 inches of manifold vacuum. Your gauge is telling you it’s only idling around 12 inches. Open (unscrew) each idle screw a quarter of a turn and see if the idle vacuum increases. If it does keep unscrewing the idle screw until it levels out. It shouldn’t require more than about 2 to 2 1/2 turns out from seated to give you the highest vacuum reading. You can also use an engine tach to do this if you don’t have a vacuum gauge. Now you will probably have the engine idling higher than what you like. This is normal. The next thing to do is to unscrew the curb idle screw until the motor slows down again.
You are accomplishing a couple of things by doing this. First of all you are insuring that the idle circuit is rich enough. Second by turning the curb idle screw out you are allowing the primary butterflies to close off the idle transfer slots. You don’t want the slots opened up and metering fuel along with the idle feed orifices under the slots! Too much idle fuel will load the plugs up bad.
Now we come to idle circuit 101. Holley jet blocks use a couple of teeny little orifices called idle feed restriction orifices, on each side of the bottom on the jet block. Dominators with two and three circuits use IFRO (Idle feed restriction orifices) on the secondary sides as well. The fuel from the float bowls is mixed with air coming from the idle air bleeds in the top of the carb air horn. These are those cute little brass screws next to the bowl vents. There are usually two sets of bleeds. The small ones are the high-speed bleeds. Both of these bleed jets are calibrated to work with the specific sized IFRO and main feed system. That’s why until lately Holley has made them NON REMOVABLE. Now some of their specialized carbs have removable bleeds and adjustable or removable IFRO’s too. Cool! The IFRO allows fuel mixed with air from the idle air bleed to pass down to the idle orifice underneath the transfer slots. This is what the engine idles on. That’s all. It’s not supposed to be getting fuel from any place else on the carb except right there. If there is fuel coming out of the transfer slot orifices then it’s not gonna run right. This comes from the throttle butterflies screwed too far open. When the curb idle screw is turned CW it raises the throttle blade away from the transfer slot. As this happens it allows vacuum to apply itself to the transfer fuel orifice. Not good. Transfer slots are there to help the engine transfer from an idle situation to main system start-up without falling all over itself and going way lean before the main system meters fuel thru the boosters. That’s all the transfer slots are there for.
Cars with big cams like lots of idle mixture screw opening. If you have so much camshaft overlap that you can’t make the car idle without dieing you can drill a small hole (under 1/8″ of an inch) at the leading edge of the primary butterflies. This will give you some idle control back. Do this as a last-ditch measure please. Make sure the carb is sealed down to the manifold base good too. A leaking base plate can create all sorts of tuning problems. Just spray a little Brakleen around the base of the carb and the manifold runners and see if you get a difference in idle speed. You can also use WD-40 but it’s messy. starting fluid can also be used but it’s explosive so be careful if you use this stuff please!
Take the car out and drive it and see if it’s happy now. If it isn’t then there are lots more you can do to it! Holley gives you this option, lots more than most carbs nowadays. If you are using a double pumper (4150) and the car is stumbling when you give it part throttle chances are good that A. the accelerator pump is worn out, B. it’s just out of adjustment, C. it needs to be bigger.
To check the accelerator pump move the throttle just a little. You should see fuel squirt out of the primary shooter. If it doesn’t then either the pump is shot or it’s out of adjustment. Try adjusting it before you condemn it. The adjuster is on the end of the arm that actuates the pump. To increase its’ stroke you need to unscrew the nut on the arm that allows it contact the pump diaphragm arm. Holley says to crank the carb wide open with the throttle back as far as it will go. You should be able to still have some accelerator pump linkage travel left. Albeit not much but it shouldn’t be bound up. If it is you must take out some of the pump arm adjustment you just gave it. It this results in the pump shot going away again then the pump diaphragm is shot and needs replacement. Okay now we replaced it and it’s adjusted correctly. You take the car down to the track, bolt on yer slicks and have a go at the old quarter-mile You launch the car and it launches great then falls down for a second. What is happening here is that the car has run out of pump shot. Probably needs either one or two things. A. needs a pump cam adjustment, or needs a smaller shooter installed. Here’s the deal, smaller shooters don’t empty the accelerator pump well as fast as the larger ones do.Try going down to a 24 shooter and see if that fixes the problem. If the car blubbers initially then picks up it might need a pump cam adjustment. There’s a screw that holds the pump cam locked onto the linkage. On double pumpers and Dominators there’s two cams one on the primary and one on the secondary. Leave the secondary pump cam alone now. Try moving the primary cam either forward so that it contacts the pump discharge lever sooner. This should increase initial pump discharge.
If your car is running a single plane manifold with a large plenum under it, or a tunnel ram you should probably swap your 30cc pumps out to the 50cc models. Large plenum manifolds and tunnel rams dig lots of pump shot. They need it because the large plenum area eats up the pump shot in sheer volume and thru fuel being detoured onto the manifold walls. The conversion kits consist of a larger pump well, and different diaphragm. A new beefier pump arm from the linkage and some longer screws. Neat thing about the Holley accelerator pumps is that the diaphragm also serves as its own gasket.
On tuning Holley 8082 Dominators: First of all think of the Dominators as just two really big two barrel carbs joined at the head like a Siamese twin okay? That’s all they really are is a pair of big two barrels cast into one big carb. Dominators nowadays come in a couple of flavors aside from their flow ratings. You can get them in two or three circuit models. The 8082 Dominator is a 1050 CFM two-circuit carb with soft, progressive mechanical linkage, annular boosters.
Annular what? Annular boosters allow better atomization and earlier main system start-up than standard down leg boosters. Here’s that old “main system start-up phrase again.” What this means is that it takes a finite amount of time to pull fuel out of the float bowls thru the main jets, up the emulsion tubes mix the air from the high-speed bleeds and give to the boosters. Once main system start-up in motion then the carb is running off the boosters and the power valves when under a load. When cruising the power valves are closed because they are “feeling” high manifold vacuum to keep them closed. When the throttle is opened up the manifold vacuum drops and the power valves open up and supply much more fuel. Some people remove the secondary power valve. Holley even makes a plug for this. However leave it in there if you can. It’s there for a reason. If the Holley engineers didn’t think it worked good then it wouldn’t be there. If you must pull the power valve out then you have to jet up the secondary size AT LEAST 10 sizes! Yep 10+.
Everything I said about the IFRO and transfer slots and throttle adjustments is applicable to Dominators as well. Just remember on a Dominator you have to repeat everything you did to the primary side to the secondary side as well. It also has a curb idle screw on it, idle mixture screws in the jet blocks and transfer slots in the throttle bores as well. Power valve sizing is exactly the same on Dominators as it is on 4150s, 4160’s. If you happen to look at the jet sizes in a Dominator don’t freak out they are lots bigger. Supposed to be. If you don’t believe it then check out the data base lines for 4500 Dominators in the Holley catalog under the tech section. The Domintors come with 50cc accelerator pumps. I even think the shooters from a 4150 will work, don’t quote me on that one though. Tuning and adjustment of those pumps are the same as the 4150 & 60’s. The main thing I want to stress on the Dominators are that they are a race carburetor. They won’t run as clean as say a 770 cfm Street Avenger. The IFRO is larger so the idle circuits are a bit richer. I think this is done because race cars usually run with open exhaust headers. When your headers are open it leans the fuel system down due to increased exhaust scavenging. Holley compensates for this by running the IFRO’s fatter so the entire idle circuit runs crisp with open headers because it’s stoichiometric Air fuel ratio is now once again on the money with open exhaust. When you cap the exhaust system back up it richens an already rich idle circuit causing the car to load up a tad. If your car is a tad weak in its ignition system installing one of these things on it will drown it big time. You need a good MSD distributor on it with one of their big fat blue HVCII coils on it. That’ll solve the problem.
Some of the latest Domintors have adjustable IFROs and replaceable idle air and high-speed bleeds. You really have to know what you’re doing to juggle these combinations around. Best way to find out is to try it. I suggest keeping a log of all the changes you make to the air bleeds and IFRO’s, main jets, shooter sizes, color of pump cams you’re using, what hole their set on, your idle mixture adjustment. Get a Holley catalog too. You need to be able to look up your carb and see what it came with from the factory so you can re-baseline it if you have to.
My advice to you is try tuning your carb. If you can’t make it work with any of my methods then maybe you should just tear it apart and rebuild it? If you want to get to know your Holley carb intimately then rebuild it. Don’t try to do it on the engine, you WILL drop something hard into the motor. Take it off the car and onto a table that doesn’t have all sort of crap on it. Make sure you have some compressed air handy. and some different sized screw drivers and a large 15/16″ end wrench for the power valves. I think they’re 15/16″ or maybe 1 inch? Anyway a good adjustable wrench will work if you don’t have any end wrenches that big. You should have a couple of spray cans of Berryman’s Chemtool or some sort of spray carb cleaner. Something like an Exacto knife for scraping gasket material off and lots of paper towels. Be patient and don’t force anything or get in a hurry. Take pictures with your digital camera if you are worried about remembering how it goes back together. Make sure the carb is slick as a whistle when you put it together. Blow out all the orifices on the carb body and jet blocks(s) when you have it apart. Clean carbon out of the throttle bores and orifices in the throttle body base. Use all the gaskets the kit gives you. Use Holley repair kits too. Other people’s kits are shit.
There is still a place for carburetors on hot rods and don’t let anyone tell you any different. Take a look at Sonny Leonard’s 864 cubic inch engines. They aren’t running fuel injection on them.