The power valve is higher than the jets and is the first to be uncovered

AcceleratorPump Lever AdjustmentDonot forget about the accelerator pump lever adjustment. For oval track use, thepump lever should be adjusted so there is no play in the pump linkage when hethrottle is closed. This will assure that there will be no lean stumble whenthe carburetor comes off idle. Tuning an accelerator pump for maximumperformance off the corner often involves reducing, rather than increasing, thepump volume and  washing machine spare parts drain pump  discharge rate.

Dragracing calls for a slightly different approach. For the hardest starting linelaunch with a foot brake, the pump lever override spring should be adjusted sothat fuel starts to discharge through the nozzle at an engine speed lower thanlaunch RPM. If a car leaves the starting line at 5000 RPM, the pump shot shouldbegin at 4700-4800; an 1800 RPM launch calls for the accelerator pump shotstarting at 1500. The key is to have no slop in the accelerator pump system atstarting line RPM, so that the pump shot is not "used up" below thatRPM. Although adjusting the accelerator pump as described will create a lot ofslop in the pump linkage at idle, (and may produce a stumble when driving inthe pits) a car will leave harder. Drag cars equipped with a stick shift ortransbrake, where starting line launch is accomplished with the carburetor wideopen, requires adjusting like an oval track application. FloatLevel AdjustmentWhenassembling a carburetor or reinstalling the fuel bowls, you need to adjust thefloat to be approximately 0.450" from the top of the bowl (in line withthe bowl screw bosses when the bowl is upside down). This "dry" settingis to get you in the ball park. Recheck the float level with the engine runningand the sight plugs out. Fuel should just wet the outside of the bowl as thefuel seeps out. On the secondary side, where the sight plug is lower, fuellevel should be slightly higher. On Demon carburetors, the large, patentedsight glass windows allow float level setting without having fuel escape fromthe float bowl. You can adjust the float level relative to the three cast-inmarks next to the float window. PowerValve TuningThepower valveýs function is to supply extra fuel for wide- open throttle, highload conditions. When manifold vacuum falls below the vacuum level stamped onthe power valve, the power valve opens and enriches the main circuit by aboutsix to ten jet sizes. This occurs under high engine loads such as fullthrottle. The power valve number should be at least 1.5-2.0 in/hg of vacuum under engine idle vacuum. If anengine produces 8.0-9.0 in/hgof vacuum, a 6.5 power valve would be a good first selection. Using a powervalve with a rating lower than this will delay the enrichment and can causehesitation.

However, on an oval track car with restricted carburetor rules,using a power valve with a lower rating can sometimes help performance comingoff corners. Any drag race carburetor with a secondary power valve must beturned sideways to avoid fuel starvation. The power valve is higher than the jets and is the first to be uncovered as fuel is pushed to the rear of thefloat bowl. Since there is no way to put an extension on the power valve, thecarburetor must be turned sideways to eliminate fuel starvation.Find out more about GM carburetor by visit miparts.com.