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Fuel Systems

Carb Class: Bog Issues Continued

Carb Class: Bog Issues Continued

Q: In your previous explanation of the accelerator pump system one item you did not address is shimming the pump arm spring.  Is this a good idea or not?

Answer: The pump arm spring (technically the pump over-ride spring) provides a cushion when the pump system is operated.  Since liquid will not compress, it is impractical to have a straight mechanical lever between the throttle shaft unless it were constructed of a material that has some give.  So, at wide open throttle the spring on the pump arm compresses and ultimately continues to push the operating arm on the pump housing for a continuous fuel discharge.  You are correct in a couple of areas.  First, some early high performance carburetors actually did use a shorter and thicker spring than today’s typical spring.  Second, there are a number of people who install shims between the pump arm and the spring to in effect put more preload on the spring.  The shims increase the overall length and to get back to the original installed height the spring must be compressed. 

Does it work?  The jury is still out on that and while we have not seen any definitive data to confirm it. We’ve been told this procedure can reduce or eliminate a bog or hesitation without resorting to a larger pump nozzle.  We think this type of modification might work on applications that have a fairly large pump nozzle. We have had reports of this helping on applications using “Oxygenated” fuel.  Something to think about for anybody using reformulated pump gas in their hot rod.

Q: When I make my vacuum secondary open mechanically even with the “bog” it still runs quicker and faster then just leaving it stock what’s the deal?

Answer: Vacuum secondary carburetor can be a mixed bag.  On the one hand they are extremely forgiving and regulate the air flow through the carburetor based on engine demand.  Unfortunately, it’s difficult to find the optimum balance over a broad RPM band.  In a lot of Stock and Super Stock class cars that still require the original vacuum secondary carburetor they often make the secondaries open mechanically or have the secondary spring so light that the vacuum diaphragm “window shades” the secondary side.  Bear in mind, in most cases these engines are under carbureted because all the internal modifications so this just compensates for a carburetor that is too small. In a lot of bracket and street/strip cars the carburetor is more appropriately sized or many are too big.  In a condition where the car runs faster by forcing the secondaries open mechanically; our guess is the secondary diaphragm spring is too heavy and does not allow the secondaries to open completely.  In some situations the engine simply does not produce enough air velocity through the primary venturi to open the secondaries all the way with a spring that does not produce a bog off the line.  In those cases start looking for “upstream” restrictions in the vacuum passage.  Older carburetors had a check ball in this passage and conventional wisdom was to simply take out the ball.  That definitely enhances the signal and a stiffer spring might be needed.  Newer carburetors often have a drilled restriction where the check ball formerly resided.  Removing this restriction has basically the same effect as removing the check ball allowing for a more aggressive opening rate.  

We attempted to solve some of these problems with our adjustable secondary housing. 

We use a tapered screw to change the area of the restriction (making it larger or smaller) to tailor the restriction to the application rather than having to rely on a spring.  The combination of changing springs and the additional adjustment feature makes the secondary operation almost infinitely adjustable.  Changing the tapered screw’s position changes both the timing (when the secondaries begin opening) and the rate (how fast the secondaries open).  This component may not be legal in some class racing associations (check the governing rules) but this certainly is a worthy investment for bracket racers.