Holley Carbs
So ya got that big nasty 350 or 302 and want a great carb with endless tuning posiibilities? well I gotta say Holleys are great. A LOT of people talk bad about these carbs and to be honest thats cuz they simply dont know how to work on them. so lets walk through some set up and tuning
Idle
The best way to set the idle mixture is to lean the carb out until the vacuum just starts to drop, then richen the mixture by about 1/4 turn. If you have a bit of a stumble in very light, low speed operation, sometimes it helps to richen it up by another 1/4 turn.
If you are starting your adjustments fresh, begin at about 2 turns out. Depending upon if you have a regular carb or a smog carb, your adjustment can be lean when screwing in (normal) or rich going in (smog style). The way to tell the difference is the smog carb will have a little sticker on the metering block telling you it goes opposite of normal, and the smog carb will have blunt screws rather than the pointed ones on a regular carb. If you have a carb with a normal system where the mixture leans as you turn the screws in, then when you screw them in almost to a light seating, the engine should die.
On a regular, standard idle circuit Holley, turning the idle mixture screws all the way in should kill the engine. If your idle mixture screws do not respond to adjustments, you may have several different problems. The most common is using the wrong carburetor for the application. Smog carbs with reverse mixture screws (these are the ones with the little decals, telling you to turn the screws in for richer mixtures) only have a small band of adjustment built in. Smog carbs work well on stock engines only, and you can't expect them to work well on a modified engine, especially one with a lopey cam.
Another common cause of non-responsive idle mixture screws is having too much (more than .040 inch) of the idle transfer circuit exposed. The idle circuit allows a very small amount of finely metered fuel into the engine. By exposing the transfer circuit too much, the gross feed of fuel coming out of the transfer circuit overpowers the fine amount coming out of the idle ports. This is akin to adding a squirt gun's flow into a garden hose's flow...the fine amount of the squirt gun is so insignificant compared to the garden hose, that there is no way that adjusting the flow of the squirt gun will make any difference. There are several ways to fix this problem:
1. If your engine is worn, or has a big vacuum leak, you should fix those problems before trying to crutch your carb. Sometimes, with worn engines or one with a vacuum leak, the carb must be opened quite a bit just to get the engine to run.
2. Buy the right sized carb! If you're trying to use a 600 cfm carb on a lopey-cammed 460, then you must open the primaries too far into the transfer slots just to get enough air into the engine to get it to idle. A larger carb on this example would have a larger throttle plate, which would need to opened less to allow the engine to idle.
3. Open the secondaries a bit, and close the primaries a like amount. This will allow more air in, without exposing quite so much of either primary or secondary transfer slots.
4. If opening the secondaries doesn't work, then you may have to drill small holes (1 per plate) in the primary throttle plates to allow air in while the plates cover the transfer slots. This is a trial and error procedure, so start small, about 1/16 inch. You should not have to go much larger than 3/16 inch. Drill on the side opposite of the transfer slots, to help keep this added airflow away from the idle ports and transfer slots.
5. Check for a blown power valve. See the section on power valves below.
If you notice a lean surge or throttle tip-in stumble due to an excessively lean idle, and you can't get the idle richened up, you may have a clog in the primary OR secondary idle systems, or both. Most people don't know that there is an idle system on the secondary side of the four barrel Holley to prevent the fuel from getting stale. If the secondary idle system is clogged up, no amount of cleaning on the primaries will get the thing to idle correctly. You must clean BOTH the primary and secondary idle systems. And since those circuits are very small, small amounts of debris or even varnish from disuse will clog the tiny passages.
Main Jetting
Going down a flat road at a rate of about 45 mph or higher will give you a good indication of your main jet sizing. Shoot for between 400 mv to about 700 mv. Since carburetors are not as exact as computer controlled electronic fuel injection, keeping at the perfect 400 mv will be tough. You always want to go a bit rich, as excessively lean mixtures will cause damage to your engine, create pollution, and give you bad gas mileage and performance.
You can do this without a meter, but it's a bit tougher. Start with the factory jetting and proper float levels. Drive it around for a while, noting if the engine surges at highway speeds. Take a look at the plugs. If they are sooty, you may need to lean out the main jets. If the plugs are white, you may need to richen the main jetting. Start stepping the jets up or down, one step at a time, and drive the car around for a day or two. Do not make large changes on your jetting. You can't just throw any jets in there because you think it needs to go one way or the other. It takes time to get it right, and you must change jets one number at a time.
Secondary Jetting
This is a much more difficult task to tame, due to the acceleration most vehicles provide when the secondaries are open. When the car is acclerating hard, it is difficult to read any gauges, therefore, I can only recommend the trail and error, or dragstrip method to jet secondaries. Go by the seat of your pants or take the car to a dragstrip to work on the secondary jetting. If you, or a spotter, can see puffs of black smoke when the secondaries open, then it's too rich. That's about all the advice I can offer.
If your carburetor has the small metering plate inside the secondary bowl, you can change to a secondary metering block, with screw-in jets, using a secondary metering block conversion kit, which does not come with provision for a secondary power valve. Secondary power valves are really not necessary anyhow, and just add complexity and failure points.
For those of you sticking with the secondary metering plate inside the bowl, here is the assembly order of the gaskets, which can get somewhat confusing on the schematics. First down is the thick gasket covering the whole body surface. Next is the thin sheet steel plate. Then the thin paper gasket. Last is the metering plate with the calibrated passages.
Power Valve
Once you have the main jets set, it is time to play with power valves. Going up a slight incline at highway speeds, or accelerating slowly at highway speeds, you will notice the vacuum reading falling. As it falls, it will come to the opening point of your power valve. You can tell when the power valve opens because the meter will go lean for a while, then the valve opens, and the meter begins to show rich. You'll probably notice the power increase right when the power valve opens. I can tell exactly which power valve I have in my car by the vacuum reading when the lean misfire goes away under light loads. About 700 mv to 900 mv is a good reading for light loads with the power valve open.
If your PVCR's are too small, the meter won't go up high enough, and power will suffer because the engine is still too lean. You can drill out the PVCR's with a small drill bit in a fingertip drill bit holder. If the PVCR's are too big, you will see a big jump in the meter readings, and maybe a puff of black smoke. My Pinto's PVCR's were way too big, so I found a kit to let you install little Mikuni pilot jets into the metering block of your Holley to make the PVCR's smaller. The company went out of business after the owner passed away, so if anyone knows of someone still selling this kit, let me know!
Now that you have the power valve mixture correct, try some different power valves. I like a 10.5 in my Pinto, because it pulls about 14 inches of Mercury going down the road, and the 10.5 comes in pretty quickly when going up slight inclines.
Tip: Use a power valve that is about 2 inches of mercury below the LOWEST manifold vacuum reading you get on cruise and idle (in gear for automatics). If the power valve flutters open at idle, it can act as a pump, and push extra fuel into the main well, causing a drip from the booster venturis. If the power valve opens a lot while you're driving down the road, your gas mileage will suffer.
Idle
The best way to set the idle mixture is to lean the carb out until the vacuum just starts to drop, then richen the mixture by about 1/4 turn. If you have a bit of a stumble in very light, low speed operation, sometimes it helps to richen it up by another 1/4 turn.
If you are starting your adjustments fresh, begin at about 2 turns out. Depending upon if you have a regular carb or a smog carb, your adjustment can be lean when screwing in (normal) or rich going in (smog style). The way to tell the difference is the smog carb will have a little sticker on the metering block telling you it goes opposite of normal, and the smog carb will have blunt screws rather than the pointed ones on a regular carb. If you have a carb with a normal system where the mixture leans as you turn the screws in, then when you screw them in almost to a light seating, the engine should die.
On a regular, standard idle circuit Holley, turning the idle mixture screws all the way in should kill the engine. If your idle mixture screws do not respond to adjustments, you may have several different problems. The most common is using the wrong carburetor for the application. Smog carbs with reverse mixture screws (these are the ones with the little decals, telling you to turn the screws in for richer mixtures) only have a small band of adjustment built in. Smog carbs work well on stock engines only, and you can't expect them to work well on a modified engine, especially one with a lopey cam.
Another common cause of non-responsive idle mixture screws is having too much (more than .040 inch) of the idle transfer circuit exposed. The idle circuit allows a very small amount of finely metered fuel into the engine. By exposing the transfer circuit too much, the gross feed of fuel coming out of the transfer circuit overpowers the fine amount coming out of the idle ports. This is akin to adding a squirt gun's flow into a garden hose's flow...the fine amount of the squirt gun is so insignificant compared to the garden hose, that there is no way that adjusting the flow of the squirt gun will make any difference. There are several ways to fix this problem:
1. If your engine is worn, or has a big vacuum leak, you should fix those problems before trying to crutch your carb. Sometimes, with worn engines or one with a vacuum leak, the carb must be opened quite a bit just to get the engine to run.
2. Buy the right sized carb! If you're trying to use a 600 cfm carb on a lopey-cammed 460, then you must open the primaries too far into the transfer slots just to get enough air into the engine to get it to idle. A larger carb on this example would have a larger throttle plate, which would need to opened less to allow the engine to idle.
3. Open the secondaries a bit, and close the primaries a like amount. This will allow more air in, without exposing quite so much of either primary or secondary transfer slots.
4. If opening the secondaries doesn't work, then you may have to drill small holes (1 per plate) in the primary throttle plates to allow air in while the plates cover the transfer slots. This is a trial and error procedure, so start small, about 1/16 inch. You should not have to go much larger than 3/16 inch. Drill on the side opposite of the transfer slots, to help keep this added airflow away from the idle ports and transfer slots.
5. Check for a blown power valve. See the section on power valves below.
If you notice a lean surge or throttle tip-in stumble due to an excessively lean idle, and you can't get the idle richened up, you may have a clog in the primary OR secondary idle systems, or both. Most people don't know that there is an idle system on the secondary side of the four barrel Holley to prevent the fuel from getting stale. If the secondary idle system is clogged up, no amount of cleaning on the primaries will get the thing to idle correctly. You must clean BOTH the primary and secondary idle systems. And since those circuits are very small, small amounts of debris or even varnish from disuse will clog the tiny passages.
Main Jetting
Going down a flat road at a rate of about 45 mph or higher will give you a good indication of your main jet sizing. Shoot for between 400 mv to about 700 mv. Since carburetors are not as exact as computer controlled electronic fuel injection, keeping at the perfect 400 mv will be tough. You always want to go a bit rich, as excessively lean mixtures will cause damage to your engine, create pollution, and give you bad gas mileage and performance.
You can do this without a meter, but it's a bit tougher. Start with the factory jetting and proper float levels. Drive it around for a while, noting if the engine surges at highway speeds. Take a look at the plugs. If they are sooty, you may need to lean out the main jets. If the plugs are white, you may need to richen the main jetting. Start stepping the jets up or down, one step at a time, and drive the car around for a day or two. Do not make large changes on your jetting. You can't just throw any jets in there because you think it needs to go one way or the other. It takes time to get it right, and you must change jets one number at a time.
Secondary Jetting
This is a much more difficult task to tame, due to the acceleration most vehicles provide when the secondaries are open. When the car is acclerating hard, it is difficult to read any gauges, therefore, I can only recommend the trail and error, or dragstrip method to jet secondaries. Go by the seat of your pants or take the car to a dragstrip to work on the secondary jetting. If you, or a spotter, can see puffs of black smoke when the secondaries open, then it's too rich. That's about all the advice I can offer.
If your carburetor has the small metering plate inside the secondary bowl, you can change to a secondary metering block, with screw-in jets, using a secondary metering block conversion kit, which does not come with provision for a secondary power valve. Secondary power valves are really not necessary anyhow, and just add complexity and failure points.
For those of you sticking with the secondary metering plate inside the bowl, here is the assembly order of the gaskets, which can get somewhat confusing on the schematics. First down is the thick gasket covering the whole body surface. Next is the thin sheet steel plate. Then the thin paper gasket. Last is the metering plate with the calibrated passages.
Power Valve
Once you have the main jets set, it is time to play with power valves. Going up a slight incline at highway speeds, or accelerating slowly at highway speeds, you will notice the vacuum reading falling. As it falls, it will come to the opening point of your power valve. You can tell when the power valve opens because the meter will go lean for a while, then the valve opens, and the meter begins to show rich. You'll probably notice the power increase right when the power valve opens. I can tell exactly which power valve I have in my car by the vacuum reading when the lean misfire goes away under light loads. About 700 mv to 900 mv is a good reading for light loads with the power valve open.
If your PVCR's are too small, the meter won't go up high enough, and power will suffer because the engine is still too lean. You can drill out the PVCR's with a small drill bit in a fingertip drill bit holder. If the PVCR's are too big, you will see a big jump in the meter readings, and maybe a puff of black smoke. My Pinto's PVCR's were way too big, so I found a kit to let you install little Mikuni pilot jets into the metering block of your Holley to make the PVCR's smaller. The company went out of business after the owner passed away, so if anyone knows of someone still selling this kit, let me know!
Now that you have the power valve mixture correct, try some different power valves. I like a 10.5 in my Pinto, because it pulls about 14 inches of Mercury going down the road, and the 10.5 comes in pretty quickly when going up slight inclines.
Tip: Use a power valve that is about 2 inches of mercury below the LOWEST manifold vacuum reading you get on cruise and idle (in gear for automatics). If the power valve flutters open at idle, it can act as a pump, and push extra fuel into the main well, causing a drip from the booster venturis. If the power valve opens a lot while you're driving down the road, your gas mileage will suffer.
Comments
You may notice a bad power valve because you will not see the jump from lean to rich when your vacuum gauge gets to the power valve setting. You will see a rich mixture at cruise, when the power valve should be closed. You may also notice a badly BLOWN power valve as an overly rich condition at idle. This occurs only if the diaphragm has a big hole in it that lets fuel from the float bowl into the power valve vacuum passage and into the engine.
There are some hand held vacuum pumps and adapters out there to check power valves. I just use my mouth after making sure all the gas is dried off. If you suck a lot of air through the diaphragm, or it won't hold a vacuum, then replace it.
Accelerator Pump
You'll probably need the help of a spotter on this, because things happen fast when you punch the throttle from a dead stop. If your meter goes rich right away when you punch the throttle, then it leans out gradually, you need a smaller pump nozzle to make the fuel come out slower. If the meter doesn't go rich right away, but gets richer later, you may need more fuel earlier. A bigger nozzle is in order. Try some different cams, too, but be aware that bigger is not always better. If your car still has a lean stumble with the biggest cams and the biggest nozzles, you may have to go with the 50cc "Reo" pump diaphragm and the corresponding cam. You must use the big black cam on 50cc pumps to get the full shot of the 50cc pump! If you put that big old pump under there but don't move the lever any farther than before, what's the use?
Also, don't put the 50cc cam on small pump diaphragms! You'll break them! Most manifolds will require a 1/4 inch spacer under the carb to run the big pumps. So if you add a spacer, don't forget to check your hood clearance BEFORE you slam the hood!
Another thing to think about when you start getting into the big pump cams and nozzles is to use the hollow nozzle hold down screw. This is due to the fact that the little passage cut in the side of the internal threads may not be big enough when you're way up in shot size, over .040, as recommended by Holley.
Accelerator Pump arm adjustment: The accelerator pump arm must not be loose at idle. The .015 clearance is measured at wide open throttle (engine off!). This is to prevent damage to the pump arm and diaphragm. You may need to adjust the external spring one way or the other to get it tight at idle, yet still have the .015 clearance at WOT.
There are entire books on tuning these carbs but if ya got a question just ask. I've been using these carbs for around 15 years in street and strip applications and would love to help
Glad it was of some use to you :cool:
A flow chart for ya
It's a nice basic carb to work with.
It's only downfall for me is the side bowls. I wish Holley thought of giving one physical access from the top of the bowl.