Damon Kuhn 3/10/2010
Ok on to some tuning ….. we'll go from top to bottom …or as close as my rambling little brain can get … I'll try to keep the paragraphs a little shorter.
On top we have the choke (if present) vent tubes, float bowl vent on top of the bowl and air bleeds. I can't count the number of times I've had a car come in with a choke flopping around like a bad break dancer (I'm assuming there must be a good one). It doesn't matter what you do underneath if this is wrong – misadjustment causes bogging, flat spots, bad idle and a host of other "untraceable” problems. I like to set them hot with just a small amount of positive tension towards the open side. You can fine tune it by closing it up a bit when the engine is dead cold to make starting easier. Dead cold means completely cold on a cold day – on a summer day, it should not be completely closed- it will never open properly like that. With a little trial and error using this method, you can gain mileage and drivability. The reason I suggest this method over the common measurement method, is that if you use the standard method in a little too warm or cold a temperature, you will chase your tail endlessly. The leanest setting (furthest open cold) that allows easy starting is best.
The vents are next- they are there to maintain roughly the same pressure in the bowl as exists in the outside world, along with venting vapors and keeping the fuel from percolating when you stop a hot car. The reason I bring these up has to do with what was mentioned in the last installment about keeping the lid far enough above them- if you obstruct these tubes, havoc will ensue. The older carbs have vents (little arm with a rubber disc) on top of the float bowl which vent to the outside atmosphere- if your air cleaner holds this closed it can cause hot start problems among other things. The later carbs tie the vents to a charcoal canister- if it becomes clogged it can cause the same problem.
Next is the air bleeds – these are the little brass things with holes down in the barrel area. They can be pressed in or screw in removable (Holley). They contribute to richness of the mixture and main bleed startup. If they become plugged, they will cause an excessively rich mixture and the main circuit will come in too early. This is detrimental on the emission product and will severely affect performance and mileage. I will get into some simple modifications later, but want to mention now, that if they are screw in, it is not an invitation to experiment. These rarely need resizing and if played with by someone who fails to fully understand what they are doing, can really mess up an otherwise good carb. On a Holley the ones closest to the outside of the carb in each barrel are the idle bleeds, the inboard ones the mains (if there are three per barrel, the middle are the intermediate bleeds). If all else is well, but your idle is way too rich, one of the Possibilities is clogged idle bleeds-for a good idle and rich cruise / full power it could be the main bleeds. On an AFB/AVS they are the little tubes sticking up on the outboard side of the choke housing. On a Thermoquad they are buried in the bottom of the choke hosing, and Rochester has them in the primary area as well as beside the metering rod hangar. Simply spray them out good with carb cleaner (the little straw on the can is good for this). This is a very good seasonal maintenance routine – you may be surprised how much difference it can make.
The floats are next. A high setting will cause the mains circuit to come in early and can cause fuel to “spill over” during hard acceleration, cornering, and braking. If you have problems with the engine dying on hard braking or in the middle of a hard corner, your float setting may be high. High settings can also cause percolation and hard starting on hot shutdowns. Low settings will delay main system start up and can cause lean-outs (popping, cutout and backfiring) on hard cornering and braking. Factory setting recommendations are usually close, but for performance, some tweaking may help – I’ll cover this in the modification episode. One item of interest on Carters especially – there is a float drop setting called out – set this too low and the engine will try to die when you let out after hard acceleration. On a Holley loosen the screw on top of the bowl and adjust the nut up or down to get fuel to barely trickle out of the sight hole while the car is on a level surface. A good rough setting on most others is to invert the lid and adjust the floats to where the top of the float is level with bottom of the top plate casting.
Holley vacuum secondary setting- DO NOT make them mechanical. I’ll cover this more in mods, but in short there is no accelerator pump to cover up the hole caused when you open the secondaries mechanically. But you say “I wanna be able to feel the 4 barrel kick in”- NO YOU DON’T – all this means is you are feeling a lean spot, or high speed bog. The proper thing to do is experiment with springs, going lighter until you feel the aforementioned and then going back up one to eliminate it. Simply unscrew the cover (after purchasing a spring kit) and merrily start replacing and testing. While on this subject, check the rubber diaphragm and make sure it’s good- on the ones with external vacuum lines you can apply vacuum and see if it moves. On an AVS, there is a screw that tightens, loosens the air door spring – this works the same way as replacing springs on the Holleys. We’ll cover AFB’s in mods.
Accelerator pump – on a Holley ,these should be adjusted after the idle speed adjustment below to just touch the arm-the squirters should activate with the slightest movement of the throttle arm. Adjusting it too loose will cause a bog, too tight will cause the same because you are effectively shortening the stroke, and for crap sake, don’t put a bunch of washers in place of the spring – it’s there to keep from rupturing the diaphragm on quick throttle opening and help with shot timing. IF IT DIDN’T NEED TO BE THERE, they wouldn’t spend the money to put it there. On most of the rest, there are multiple holes or an arm to bend. Start with a midrange adjustment – it’ll usually work. On the bendable ones, shortening the arm increases stroke / volume and vice versa. On the AFB/AVS there are holes in the arm – closer is more shot, further out is less. If the car bogs when you stomp on it and then goes, give it more shot. If it goes, then bogs and goes again or blows a puff of black smoke and goes, lessen the shot.
The shooters – bigger is more shot, quicker and vice versa – if it bogs regardless of arm adjustment go bigger – if it goes, bogs and goes, go smaller. Usually, if the carb is properly sized these can be left alone.
Power valves and metering rods – this has to be my favorite. Everybody talks about how EFI is the way to go, it’s more accurate – while that may be true, PV’s and metering rods are a rudimentary form of the same controls. They are there to sense engine load and modify mixture accordingly. They allow a leaner overall setting and effect the whole fuel curve, allowing rapid throttle / load variations while maintaining a favorable mixture. For those of you with metering rods the rod controls the volume of the addition, the spring controls when (pretty much like the number on the power valve) Rods allow a volume change where a Holley requires drilling to accomplish the same. (see an advantage!)
You want a rod which has a small diameter (main or power setting) that gives best power and a large diameter part which allows a clean idle – we’ll cover that with jets.
The object is to size the power valve / metering rod spring to open under power and close in cruise / idle mode. Generally a setting 2-3 inches under your IN GEAR idle vacuum reading will work. On a drag car you have to be careful if your car begins gaining vacuum at the end of the run as it could rise above the PV / spring setting and go lean – too much of this and you get a new set of Pistons. Mount a vacuum gauge temporarily and monitor it towards the end of the run to see if it exceeds the idle setting – if it does or you encounter backfiring towards the top end, increase the number by one or two until you get a clean run. (this is assuming it’s carb induced, not ignition or valves)
On metering rods monitor them with help in gear at idle – they should stay down – cracking the throttle quickly and releasing should cause them to jump and return – this is a rough setting and may need road testing.
And then there’s the “I know what to do – block the PV and go up 6-10 jet sizes”.
BULLCRAP! If you have a drag only car with a radical cam and launch at 6000 rpm, maybe, but otherwise at the very least you are needlessly drowning your plugs / cylinder walls with excess fuel. On an automatic car, it can actually slow you down. HOW you say? As you sit at the tree against the converter, your engine begins to gain vacuum / lose load / require less fuel. If you have them blocked, you will leave way rich. That’s how. A properly sized primary PV can help you leave with a cleaner, more concise mixture – try it and look at your slips on the 60’ time-again – if they didn’t need it the Mfg’s wouldn’t spend the money. Emissions guys – this circuit allows a lean best setting.
Jets – Hey I’ll put real big ones in and make my carb bigger….nope Mr something for nuthin you’re making it richer. Jets control how much fuel per part of air. If you have to go more than 4 sizes bigger / smaller than stock -something is wrong. A good rough test is to count turns out on your idle mixture screws – on AFB/AVS more than 2 turns, you are too lean, less than 1 you’re too rich. Holleys, less than .75 too rich, more than 1½ lean. Thermoquad / Q-Jet, 1 for rich, 2½ for lean.
Idle mixture screws should be set for the highest smooth vacuum and then back out about 1/8 turn. If you don’t have a vacuum gauge use the highest smooth idle and back rich about 1/8th turn – richer on most carbs is counterclockwise.
Idle speed adjustment- another favorite especially on Holleys – there are adjustments for the front AND back barrels. Start with about 7/8 turn out on both. If you can’t get a decent idle without uncovering the transfer slot, look in the primary barrels for a little hole close to the bottom – this is the idle circuit – a small distance further up is a slot or another hole or series of holes (this is the transfer circuit) open the back barrels a little more and try again. Generally , if you have more than about 2 – 2½ turns from dead closed on the primary, you are too far open. Sometimes you can’t get it to idle anyway and need to drill some small holes in the primary blades (again assuming proper ignition settings). This will be covered in mods, but very rarely needs to be done if you have a properly sized carb.
A few parting notes- for those of you concerned with emissions – a very slight (1/4 turn adjustment) lean will help emissions as will a slightly low float setting. Another little trick is to set the idle speed slightly high – this helps with the dilution that occurs at low speeds and cleans up the idle.
Higher idle speeds without loading the engine against the converter will tend to keep a performance motor a little cooler /cleaner also. Properly sizing your carb is the single most important part of tenability. In general if you’re not sure, go smaller- this will limit all out top end, but helps response, takeoff, economy etc.
A carb that is too large will cause untunable bogging, muddy acceleration and general lack of response- this is caused by too low a pressure drop in the venturi to properly activate the main circuit- also to be discussed in mods…more later, hope this is not too basic….