[dana44]

You will need a wrench that fits the nut on top of the float adjustment (that has the big screw on top of it, top front and far rear of the carb if dual adjustment).

Loosen the screw, fit the wrench to the nut, right turn lowers the float, left turn raises it. Fuel will leak out of the screw because there is fuel pressure below it.

On the side of the carb is another screw, located about one inch from the top edge of the fuel bowl. It is a plug that allows visual access to the fuel level. You can initially determine if it is high or low by removing it when the engine is off. If the level is correct, a small drip will start to come out, not a pouring, and if too high, the metering tubes will continually flood rich the engine, if too low it will lean out a bit.

Depending on what you think when you take the plug off, too high or too low initial assist adjustment, start the car, remove the plug again, and fuel should not flow out of the hole, but there should be just the most tiny of fuel showing at the bottom of the threads, shaking of the engine might get one drip to run out but not enough to puddle or flow.

Sorry, no pictures.

Next step is to check your vacuum to make sure your power valve is the right size. Vacuum, divide by 2 and add 2 gives the correct power valve. So 16 inches of vacuum at idle is 8 plus 2 is a 10.5 power valve.

[1968NewportCustom]

dana44, on Dec 5 2008, 02:47 PM, said:

You will need a wrench that fits the nut on top of the float adjustment (that has the big screw on top of it, top front and far rear of the carb if dual adjustment).

Loosen the screw, fit the wrench to the nut, right turn lowers the float, left turn raises it. Fuel will leak out of the screw because there is fuel pressure below it.

On the side of the carb is another screw, located about one inch from the top edge of the fuel bowl. It is a plug that allows visual access to the fuel level. You can initially determine if it is high or low by removing it when the engine is off. If the level is correct, a small drip will start to come out, not a pouring, and if too high, the metering tubes will continually flood rich the engine, if too low it will lean out a bit.

Depending on what you think when you take the plug off, too high or too low initial assist adjustment, start the car, remove the plug again, and fuel should not flow out of the hole, but there should be just the most tiny of fuel showing at the bottom of the threads, shaking of the engine might get one drip to run out but not enough to puddle or flow.

Sorry, no pictures.

Next step is to check your vacuum to make sure your power valve is the right size. Vacuum, divide by 2 and add 2 gives the correct power valve. So 16 inches of vacuum at idle is 8 plus 2 is a 10.5 power valve.

Serious? So if my carb has a 6.5 power valve... it would work better if I switched it if the case was it had 16" vacuum?

[Volunteer]

Well, if your 'cruising' vacuum is 16 inches and you press quickly, though not necessarily all the way, on the accelerator, a 10.5 power valve will open at or near that particular vacuum level while a 6.5 will not open until (the manifold vacuum) drops that low.
Generally, an engine with a lumpier cam will lose the vacuum sooner and the (6.5) P.V. is necessary to keep the engine from getting too much fuel before the actual demand is there.
Dana44 explained it well and his formula makes sense. :)

[dana44]

What happens if the power valve is too low a setting, the engine will start dropping and vacuum will diminish prior to the power valve kicking in and giving it a little gas to keep from dying and stumbling. Quite often the Holley blows the power valve because of backfire, thus blowing the gasket in the power valve, then stumbling becomes common because flooding can occur.

[1968NewportCustom]

dana44, on Mar 1 2009, 08:57 PM, said:

What happens if the power valve is too low a setting, the engine will start dropping and vacuum will diminish prior to the power valve kicking in and giving it a little gas to keep from dying and stumbling. Quite often the Holley blows the power valve because of backfire, thus blowing the gasket in the power valve, then stumbling becomes common because flooding can occur.

Ok, so it sounds like even an engine with stock cam would do better with a higher vacuum power valve. Why would Holley ship 6.5 power valves... oh cause they will break more often and you would have to buy more power valves from them? Rebuilding my first carb and it is easy enough to get to the power valve whilst off the car but don't want to have to replace while it is under my hood. <_< Need to go find the carb rebuild forum maybe, cause I just purchased a used 4160 Holley and it did not have a pump discharge needle or ball weight or check valve and I became confused. :(