Carb Class: Throttle Stop Tips

Carb Class: Throttle Stop Tips

Q: All the Carb Class questions to this point appear to relate to tuning for best performance.  Do you have any carburetor recommendations for the throttle stop classes?

A: A couple of us here at QFT run the .90 classes and there are a couple of suggestions we can provide that might help your throttle stop racing.  If your throttle stop is connected to the throttle linkage, the best and easiest change is to convert your throttle linkage over to 1 to 1.  The progressive linkage works great under normal conditions. However, during the throttle stop operation this requires the engine to operate with the primary throttle plates partially open.  With the throttle plates roughly in the same position during the throttle stop operation we feel there is a better balance between the primary and secondary side of the carburetor the instant the throttle goes back to wide open.  In other words roughly the same amount of air flow and more importantly the same accelerator pump volume.  Where we have asked people to make that change the report back is the car seems to be more consistent and predictable especially with weather changes.

Q: Any other tips for us throttle stop guys?

A: Right now there seems to be a big effort to increase MPH.  It is easy to get into a trap during this pursuit forgetting some basics of a good engine package.  One of the more distressing things we have observed is the use of larger carburetors in an effort to squeeze a little more MPH out of the car.  

A bigger carburetor will ultimately produce more horsepower and therefore a bit more trap speed.  Unfortunately, that can come at a price because the low and mid range power could be adversely effected.  With a bigger carburetor it is generally necessary to add more fuel to get the engine to come up on the converter.  This is usually achieved with larger pump nozzles and/or smaller high speed bleeds.  You may also have to go to a larger jet than your carburetor originally specified.  For a lot of reasons the shift point in these classes is now on time not RPM.  When the throttle stop comes off the engine has to pull from where the converter flashes on up through the RPMs until the car crosses the finish line.  A larger carburetor can create a problem if the converter flash point falls below the peak torque of the engine combination.  The engine has to labor through that RPM window and may cause bigger swings in E.T. than expected with weather changes.  

Q: I see the fuel bowls on your race carburetors have dual sight windows.  Is there anything I'm missing here besides checking the fuel level?

A: Actually, we put dual sight glass windows in for two reasons.  First, is so the fuel level can be checked from either side of the carburetor and secondly, it won't matter which way the carburetors are mounted on a 2 X 4 manifold the float level can still be seen.  An added benefit we hadn't even anticipated was the ability to detect carburetors that are installed on angle.  That doesn't sound too significant but it can make a real difference in the initial set up and ultimately how the car goes down the track.  With a single sight plug you either have to set the float level dry or to the bottom edge of the sight plug opening.  What that doesn't tell you is how high or low the liquid level might be on the opposite side of the bowl. We've had several customers tell us once they realize how far off their fuel levels were from side-to-side they realized they needed make some adjustments.  With the dual sight glass there is a much we feel there is a better balance between the primary and secondary side of the carburetor when the throttle goes back to wide open better understanding of how the fuel sits in the bowl and can adjust it accordingly.  Some have gone so far as to rework their manifold to achieve a more favorable orientation.  In drag racing every little tweak helps.