HESITATION WHEN ACCELERATING?
This is the most common problem that people encounter with a carburetor, and many people try to tune it away with jets and rods, but that isn't the right way to do it. Jets and rods control the air-to-fuel mixture of the Primary and Secondary Systems. A hesitation needs to be tuned with the Transition Circuit of the carburetor, and it is actually pretty easy to do.
Tune this LAST. If you have problems with your Primary or Secondary Circuits, tune those before the Transition Circuit.
Tune this LAST. If you have problems with your Primary or Secondary Circuits, tune those before the Transition Circuit.
FIRST, LUBE IT UP!
The accelerator pump is the only part of an Edelbrock carburetor that should be lubricated. It won't hurt to remove the accelerator pump, inspect the spring and seal, and lubricate the seal while reinstalling it.
WHY DOES BOGGING HAPPEN?
This usually comes from a lean condition that is the result of (lighter) air being able to travel faster than (heavier) fuel into the carburetor. When you press the gas pedal down hard, there is a large rush of air that enters the carburetor, but it takes a few seconds for the fuel to catch up. This is where the Transition Circuit and accelerator pump come into play, and that is where you should start to tune it.
The accelerator pump is an independent system that provides more fuel when the flow through the carburetor is greatly increasing to compensate for the air/fuel velocity differences. Forget about gasoline for a second and think about states of matter. Solids, liquids, and gases. Remember gas here is not referring to the fuel gasoline. Air, which is a gas, travels faster than fuel, which is a liquid. Gasoline is still a liquid when it exits the accelerator pump nozzle and enters the carburetor throat.
If you still don't understand the heavy fuel / light air thing, think about it like this. It is easier for you to blow air out of your mouth than spit phlegm out of your mouth? Right? That is what is going in your carburetor with air and fuel instead of air and spit.
The accelerator pump is an independent system that provides more fuel when the flow through the carburetor is greatly increasing to compensate for the air/fuel velocity differences. Forget about gasoline for a second and think about states of matter. Solids, liquids, and gases. Remember gas here is not referring to the fuel gasoline. Air, which is a gas, travels faster than fuel, which is a liquid. Gasoline is still a liquid when it exits the accelerator pump nozzle and enters the carburetor throat.
If you still don't understand the heavy fuel / light air thing, think about it like this. It is easier for you to blow air out of your mouth than spit phlegm out of your mouth? Right? That is what is going in your carburetor with air and fuel instead of air and spit.
ADJUST THE ACCELERATOR PUMP LINKAGE
There is a built-in adjustment on the accelerator pump linkage that can deliver a larger (or smaller) amount of fuel. The stock location is usually the middle hole on the accelerator pump linkage, shown in blue in the picture below. Carburetors greater than 750 CFM come with the link in the top (richest) hole. To have more fuel delivery (richen the transition circuit) move the accelerator pump link to the hole closest to the carburetor body, shown in green. To lean out the transition circuit, move the accelerator pump linkage to the hole furthest from the carburetor body, shown in red.
BOG STILL NOT GONE FROM SWITCHING THE LINK POSITION?
If changing the link positions does not smooth out the hesitation, there are other options. If you are comfortable bending the accelerator pump linkage, then bend the rod so that the accelerator pump linkage starts moving as soon as the throttle is pressed. If you are unsure how to bend the linkage, engage the throttle and watch how the throttle cable engages and moves the accelerator pump link. The idea is to bend the link so that the pump moves earlier from the same throttle movement.
THINK ABOUT STEP-UP SPRINGS
The step-up springs are not part of the Transition Circuit. Remember that the accelerator pump is a part of the Transition Circuit that delivers extra fuel with a large press of the gas pedal to smooth out a hesitation as the carburetor reacts to try to keep up with the increase in throttle. Coincidentally, if the engine isn't working very hard, and suddenly the throttle is pressed enough, the metering rods transition from cruise mode to power mode. The step-up springs DO control when your metering rods switch from cruise (leaner) mode to power (richer) mode. If your power mode is not kicking in fast enough, you may think it is a hesitation when really the engine isn't getting enough fuel early enough.
The Metering Rods and step-up springs are part of the Primary Main System while the accelerator pump and nozzles are part of the Transition System! The two systems can work together, but you have to tune them separately. There is a difference between a hesitation (bog) or a starving engine (engine falling flat at a certain RPM).
The Metering Rods and step-up springs are part of the Primary Main System while the accelerator pump and nozzles are part of the Transition System! The two systems can work together, but you have to tune them separately. There is a difference between a hesitation (bog) or a starving engine (engine falling flat at a certain RPM).
A bog or hesitation is from the Transition Circuit.
Falling flat is from running the secondary or the metering rod power mode too lean.
Falling flat is from running the secondary or the metering rod power mode too lean.
The step-up springs and accelerator pump work independently, but many times they both "start working" at the same time. Don't confuse them just because they often "work together." Tune them independently.
STEP-UP SPRINGS - DOES YOUR ENGINE HAVE LOW VACUUM?
As long as you understand the differences I just explained, sometimes you can't ignore one and just tune the other. The step-up springs control the vacuum level when the primary metering rods change from their leaner cruise step to their richer power mode. If you have a radical cam and low vacuum readings because of that, then swapping the step-up springs could help tune away a "hesitation." The term "low vacuum" is a general term because it depends on a lot of things besides just your engine. If you don't know anything about vacuum levels, you could start with the step-up spring with the lowest vacuum level (this is the blue one = 3" Hg) and see if that improves it. From there you can move up a size until you are getting the response that you want. If you have "higher" vaccum levels, this won't make as much of a difference as you might think. The springs range from 3" Hg to 8" Hg, and most carburetors come with a 5" Hg spring.
Lower Rated Step-Up Spring = Metering Rod Power Mode (Rich Step) Happens Earlier (Gets Richer Faster)
Higher Rated Step-Up Spring = Metering Rod Power Mode (Rich Step) Happens Later (Gets Richer Slower)
Higher Rated Step-Up Spring = Metering Rod Power Mode (Rich Step) Happens Later (Gets Richer Slower)
This isn't a perfect analogy, but if you still have trouble with this concept and understand distributors, think of your step-up springs and how they affect your carburetor in the same way that your distributor curve affects your overall timing. If your distributor is advancing sooner, your engine will reach your total timing faster. A weaker step-up spring lets your carburetor reach the richer power mode of your metering rod sooner. You are "advancing" (or retarding) when the carburetor supplies more fuel with the step-up springs. Some distributors use springs for a similar purpose, so the analogy isn't too far off.
INSTALL LARGER ACCELERATOR PUMP NOZZLES
Edelbrock part number 1475 comes with 0.024,” 0.033,” and 0.043” nozzle sizes. Depending on where the accelerator pump linkage is set, a larger nozzle size will deliver the same amount of fuel, however it will be delivered faster due to the larger nozzle size. Just remember that the accelerator pump linkage controls the STROKE of the accelerator pump which corresponds to a volume (amount) of fuel, and the nozzles change the RATE at which the fuel is delivered. Different size nozzles basically change the shape of the pump curve, but the volume of fuel delivered will be the same, just faster with a larger nozzle.
There are other nozzle sizes besides the ones in the 1475 kit. These other nozzles are in between the sizes in the kit, and these "in-between" sizes do come from the factory in different model carburetors. These aren't available separately, so if you are set on finding those sizes you have to scavenge them off another carburetor.
There are other nozzle sizes besides the ones in the 1475 kit. These other nozzles are in between the sizes in the kit, and these "in-between" sizes do come from the factory in different model carburetors. These aren't available separately, so if you are set on finding those sizes you have to scavenge them off another carburetor.
USE A “Stronger” AccElerator Pump Assembly
This is an option if you have a carb that is below 750 CFMs. The stock accelerator pump for carburetors less than 750 CFMS is Edelbrock part number 1470. Instead, you can use Edelbrock part number 1468 which has a stronger spring. See the picture below. The 1468 is the assembly used in 800 CFM carbs, so this isn't an option to upgrade on those carburetors because the part is already in there.
"Stronger" here means that the 1468 will deliver fuel faster because the spring is stiffer.
"Stronger" here means that the 1468 will deliver fuel faster because the spring is stiffer.
NOTHING IS WORKING! I NEED A MIRACLE!
There is one more thing you can do before you start drilling out passages and nozzles, and since I want to cover EVERYTHING:
In the picture below, remove part number 37 from the carburetor. That part is the Pump Discharge Weight. With the weight removed, there won't be much resistance to the fuel flow from the accelerator pump. This may be a solution for you, but if you have to resort to this you probably have something else out of whack.
In the picture below, remove part number 37 from the carburetor. That part is the Pump Discharge Weight. With the weight removed, there won't be much resistance to the fuel flow from the accelerator pump. This may be a solution for you, but if you have to resort to this you probably have something else out of whack.
IS IT GETTING WORSE AS YOU TUNE THE PUMP RICHER?
It is unlikely, but possible to have a "rich bog," where instead of not having enough fuel during the transition, there is too much fuel. If this is the case, you are probably running the carburetor too rich overall or your Transition System is tuned way too rich. Make sure that your primary and secondary systems aren't too rich, and then lean out the accelerator pump and possibly move to a higher rated step-up spring.
STILL BOGGIN'?
If you have tried all these things and there is little to no improvement, make sure that what you have is really a hesitation. If you look closer you will probably discover your tuning problems lie elsewhere. Once you figure out if it is the primary or secondaries, you may have to undo all the changes that you did to the Transition Circuit.
CHECK TO SEE IF YOU HAVE OLDER PARTS IN YOUR CARB
I don't know how important this is, but I have noticed that older carburetors have different parts than what is in the newer rebuild kits. It seems like these older parts came in carburetors from 10-15 years ago. Both of the major parts that are different are in the transition circuit. The accelerator pump gaskets in the new rebuild kits have an extra vacuum cutout that used to be solid. The other difference is the color of the accelerator pump cup. The new one is blue compared to the old one which is orange. If I take apart a carburetor, it's usually because it is having problems. If I find the orange cup, I rebuild it before putting it back together because I can assume it's an older carburetor and probably needs a rebuild anyway.
If you have found out for yourself that switching to these newer parts has helped, let me know when you bought your carburetor, or when it was last rebuilt and I will update this page to get a better time frame of when the parts started coming from the factory that way.
If you have found out for yourself that switching to these newer parts has helped, let me know when you bought your carburetor, or when it was last rebuilt and I will update this page to get a better time frame of when the parts started coming from the factory that way.