Testing carburetors on an engine dyno can sometimes be misleading. We've all seen tests showing large-cfm carburetors making more power—even on a small engine that nobody in their right mind would run such a large carburetor on in the real world. On the street, idle quality and part throttle drivability make the difference between a car we drive, and one we wish we drove more. When we test different components on an engine dyno, we can do more than compare power at wide open throttle. Putting a load on an engine to simulate cruising down the road or up a hill can be enlightening.
We had been testing different intake manifolds on our 360 budget engine, and noticed that after a switch to an air-gap style manifold, that our trusty Holley 670 street avenger was having a hard time idling. With its isolated runners and true split-plenum design, the carburetor just could not provide enough fuel through the idle circuit. We had a combination of parts that just seemed to be out of the realm of what our carburetor was designed to work with, even though the parts seem to be pretty common these days. The intake was making the low-vacuum signal of our Comp Thumper cam wreak havoc with our tuning.
After we opened up our idle feed restrictor from .026 to .030-inch to cure not being able to get an A/F reading better than 15.5 at idle, we had a nice performing engine again. We now were able to get a steady idle mixture of 14.0. We did end up jetting the carburetor's main jets up a few sizes also, but this is pretty normal for the pump gas available to us where we were testing. As we went to full throttle, the A/F reading went as lean as 16.5 from 2,000 up to 3,500 rpm, and then richened up to 13.8 at the top end before the jetting change.
After swapping the factory jetting from 65/68 to 68/72, we were at a much nicer A/F reading of 13.5 at cruise, and the top end was at 12.5, which brought us up to where we needed to be.
We had to wonder how many guys drop in a performance cam, and then can't get things dialed in because they either can't or won't start drilling out carburetor circuits after shelling out their hard earned money for a new carburetor.
We ended up testing a few other Holley carburetors to see if it was an across the board issue, and to see how each performed from idle through cruise, and on to the higher rpm range. We decided to try a 4777 (650 Double Pumper) to see if it acted the same. The idle circuit in this carburetor was calibrated much richer, and required no modification. The 650 performed pretty well everywhere, but we did notice that the carburetor went extremely lean at part throttle again. On our little 360, the idle circuit worked well enough, but our part throttle mixture was again in the mid 15's. A jetting change from 67 to 69 on the primary side brought our midrange tuning in line at 13.8, while only changing the top end slightly to 12.7 from our previous 12.8.
1. First for our test was the Holley 3310 750 vacuum secondary carburetor. It has long been considered one of the best street-performance carburetors you can get.
2. We had been using a Holley 670 Street Avenger with good results prior to an intake change. The swap to an air-gap intake weakened the vacuum signal.
3. A Holley 650 Double Pumper is calibrated more for performance as opposed to street use. We have to agree, that it performed well on our street 360 regardless.
4. We finally got to try a new Street Demon carburetor with a polymer body.