Size Your Jets
To access the jets, the float...
To access the jets, the float bowls need to be removed. Many people simply remove the metering block(s) of the carburetor altogether since power valve changes and new gaskets are often installed at the same time.
Jet size changes can be called for if the engine is determined to be running too rich or too lean. An air/fuel meter used during a dyno session concluded that our engine was running in a too lean condition at over 14:1 air/fuel ratio at wide open throttle. We knew that besides being a dangerous condition, more power could result with a jet change on the secondary side of the carburetor.
Back on the bench we had a...
Back on the bench we had a wide selection of jet sizes. Our car was running 73s for the primary and secondary. We went to a 76 and then a 79 for the secondary, trying both and gaining a little each time.
We use a special tool that...
We use a special tool that allows us to change jets quickly and easily, but a proper sized screwdriver will do the trick if you're careful. When we finally settled on an 82 jet, our mixture was much better at 12.8 to 13:1 air fuel ratio. We also picked up 11 horsepower and 12 lb-ft of torque in the process.
While other type of carburetors...
While other type of carburetors use metering rods to add additional fuel under moderate to heavy loads, this type of carburetor uses a power valve. Power valves can also be changed to adjust how much and when the additional fuel is added. Power valves used to be susceptible to blow out because of backfire but now a check valve prevents this problem in modern carburetors. Consult manufacturer recommendations when making power valve selections.
Carb upgrade? Don't forget the timing
Here's the car's single-feed...
Here's the car's single-feed 600cfm carburetor in place. While the car ran fine, we suspected a little bit more airflow wouldn't hurt this engine with its top-notch AFR cylinder heads and lumpy Comp roller cam.
Once you've got your engine running right with any carburetor issues ironed out, is it possible to wring more performance out of your engine by going to a larger carb size? We wanted to find out so we took our car, which was running fine with a 600cfm Holley four-barrel on it and decided to step the carburetor size up a bit. The 351W engine has AFR aluminum cylinder heads on it, headers, and a reasonably rowdy roller cam. We felt fairly confident that the engine had enough oomph to absorb and successfully utilize some extra induction capacity. We wanted to see what differences might emerge when comparing a Holley 4160 600cfm single-feed carburetor with vacuum secondaries to a 4150 650cfm double-pumper with a mechanical secondary.
The new 650 drops right into...
The new 650 drops right into place over the Edelbrock Performer 351W dual-plane intake. Most everything attaches in the same fashion as the 600. The flange nuts should be tightened in an "X" pattern so as not to damage the carb's throttle body. The only change that really needed to be made was to make up a new fuel line for the 650's dual-feed fuel inlet system.
After testing the new carburetor...
After testing the new carburetor at the same timing setting, we bumped the timing up on the engine to get it dialed in with the new carburetor. We advanced the timing to 14 degrees initial and 34 total and put in some 91-octane gas. Overall, the new carb and increased timing made our test session a success as we picked up well over 20 horsepower at the tire. The combination of the swap and setting the timing at 34 degrees total advance got us right up to the 320-rear-wheel-horsepower mark with 320.2 at 5,400 rpm-an improvement of 15.9 horsepower on top of the carburetor swap.
We visited Superior Automotive...
We visited Superior Automotive in Anaheim, California, for baselining our car with the 600cfm Holley. Our result was a respectable 298.7 horsepower at the tire at 5,200 rpm. Timing was set at 10 degrees initial with 30 degrees all in at 3,200 rpm.
Here are the dyno results with the new carb and the same timing. A simple carburetor swap to the slightly larger 650 got us to well over 300 horsepower with a solid 304.3 horsepower at the tire at 5,300 rpm. Although the difference in peak-to-peak horsepower is only 5.6 hp, the curve is better overall. For example, at 4,000 rpm the difference is 7.6 more horsepower with the 650cfm carb. The bigger carb made a nice improvement on torque. The peak improvement in torque output ended up being 9.1 lb-ft, or 344.2 lb-ft with the 650 carb as opposed to the 335.1 lb-ft reading we got with the 600cfm carb.