For some, dancing around the carburetor like a trained performer is second nature. These tuning wizards wield a flat-blade screwdriver like a trusty sword. Before you know it, the changes are made and the car not only sounds healthier and runs cleaner, it rips. On the other hand, some look at the carburetor and avoid it like the plague. To be honest, it shouldn't be that way. Carburetors are simple to wrench on, and changes can be made in a matter of minutes. The good thing is those changes aren't final.
If you make the wrong change but have kept track of what was adjusted, you can always go back to square one.
While this may seem like a refresher course for the wrench-happy, remember that we were all rookies at one time. Learning the basics on carburetor function should start with jets and power valves. Not only are these swaps simple to complete, but gains can be had almost instantaneously. For jet changes, the carburetor can either run rich or lean depending on what size jets are installed. The higher the number, the more fuel will flow, and vice versa.
For the best view possible,...
For the best view possible, we set the Quick Fuel Technology Super Street Series 735-cfm carburetor on a small-block Edelbrock Performer intake manifold. Normally on a running engine, fuel would fill the outer bowls. We began on the primary side of the carburetor (facing radiator). Using a 5/16-inch box wrench we loosened and removed the lower corner bolt first. This allows for any remaining fuel in the bowls to drain out. Then we removed the rest of the surrounding bolts to take the bowl off.
For power valves, the same idea applies. For the street, power valves are a necessity since most street-thumping is handled at part throttle. The importance comes when the throttle is opened up. Additional fuel is needed for a safe air/fuel ratio. In wide-open throttle situations, vacuum in the intake manifold drops to almost zero. At this point the power valve will open and allow more fuel (along with main fuel jets) to the carburetor.
To illustrate these ideas, we grabbed up a Quick Fuel Technology (QFT) carburetor. While QFT also offers blow-through carburetors, E85, alcohol, and custom class carburetors, we opted for the Super Street Series 735-cfm to showcase our efforts. The methods for changing fuel jets and power valves are broad and apply to the rest of QFT's lineup of carburetors. We went through and pulled the bowls off the carburetor to demonstrate the proper method of swapping jets and power valves.
Quick Notes
What We Did
Detailed the swapping of fuel jets and power valves
Bottom Line
Jetting and power valves give you more power and efficiency
Cost (approx)
Jets: $4.89 a pair
Power Valves: $6.32 each
Carburetor (PN SS-735-VS): $519.73
With the bowl removed we could...
With the bowl removed we could access the jets. We were careful not to rip the nonstick gasket since we will be using this again once the jets are changed.
QFT's main fuel jets are manufactured...
QFT's main fuel jets are manufactured to a specific flow standard. This means that each main jet is flow-tested with a maximum 4.5-percent difference between jet numbers. Using a flat-blade screwdriver, we could remove the fuel jets. Typically, the larger-number jets will be found in the primaries and smaller-number ones in the secondaries.
The power valve is just behind...
The power valve is just behind the billet metering block. In order to remove it, we used a little elbow grease and were careful not to rip the gasket on the main body.
Holding the metering block,...
Holding the metering block, we used a 1-inch box wrench to remove the power valve and examine its number. QFT tests each valve by hand and sets them to their stamped value. The thicker molded diaphragm even adds protection against a backfire-induced rupture.
We threaded in our power valve...
We threaded in our power valve back into the metering block. Then we reinstalled our main fuel jets with the flat-blade screwdriver until snug. Before the aluminum fuel bowls can be installed, we paid close attention to mating up the squirter linkage.
Next we reinstalled all four...
Next we reinstalled all four of the carburetor bolts snuggly.
For the secondary fuel jets,...
For the secondary fuel jets, we followed the same procedure, loosening the lower corner bolt first to remove any fuel in the bowl.. We then removed the bowl to access the jets.
Since the secondaries do not...
Since the secondaries do not carry a power valve, it is only necessary to remove the bowls and swap out the jets for the tune. We reinstalled the jets, carefully placed the gasket, and put the fuel bowl back on.
We tightened down the four...
We tightened down the four outer bolts with a 5/16-inch wrench. While in reality nothing was changed, we hope this does give the new guys a bit of confidence to get out there and tune their own carburetor. This carburetor is ready to fuel virtually any motor combination in our arsenal.