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While commuting 120 miles daily several years ago, I experimented with various power valve settings to see it made any difference to my fuel economy. Officially, Impco explains the fuel mixture provided by their mixer as being factory-set by the shape of the gas valve. The power valve only affects the fuel throttle fuel mixture.

However, I knew that restrictions in any fluid system (both gaseous and liquid) affect the pressure losses at all flow rates. Because the restriction at the gas outlet in the mixer is variable due to the movement of the gas valve, the gas valve's flow restriction is obviously highest at cruising flows and lowest at full throttle.

The power valve simply puts a fixed restriction in the mixer upstream of the gas valve. At low flows, its effect is small (compared with the gas valve's restriction) while at high flows its effect is large and the effect varies with the square of flow velocity. While not having a significant effect on the fuel mixture while cruising at highway speeds, it is an effect none-the-less.

I found that turning the power screw inwards to lean the fuel mixture did make a small improvement in my fuel economy. It also caused a small seat-of-the-pants drop in full throttle power. This fuel economy improvement suggested to me that the standard gas valve (AV1-16) is richer than necessary for my engine.

The only off-the-shelf option for leaning-out the fuel mixture on Model 425 mixer at this time is the lean gas valve (AV1-1644). Even though Impco recommends the lean gas valve for engines larger than 370 CID. I replaced the standard gas valve with the silicone diaphragm lean gas valve on my Chev 350. Upon my first drive with the new gas valve, I didn't notice any effect on normal drivability. However, I regularly monitor my fuel consumption and I saw an immediate improvement in fuel economy of over 10%.

With this valve's leaner fuel mixtures, it would be useful to readjust your power valve and vacuum advance.

Changing the gas valve in an Impco Model 425 mixer is extremely easy:

Impco Model 425 Carburetor
  1. Find a flat screwdriver and a sharp knife.
  2. Take the air cleaner lid off.
  3. Put the air cleaner wing nut in a safe place.
Old Model 425 gas valve removed
  1. Carefully unscrew the 4 small screws holding the air valve diaphragm cover in place.
  2. Put the 4 small screws in a safe place.
  3. Put the air valve spring in a safe place.
  4. Check for wear in the gas outlet orifice.
Lean Gas Valve installed in Model 425
  1. Remove the old gas valve's diaphragm from its cover. The diaphragm's gasket may be stuck to the to the cover so you will need to very carefully separate the two with the sharp knife.
  2. Clean off any old gasket from the cover and the mixer body.
  3. Place the new gas valve in the mixer.
  4. Be careful to line up the holes in the diaphragm with the holes in the mixer.
  5. Place the air valve spring in the center hole of the new gas valve.
  6. Replace the air valve cover and insert the screws back into their holes.
  7. Gradually tighten all of the screws in an X-pattern so that the cover is uniformly torqued down.
  8. Do NOT tighten the screws more than finger-tight.
  9. Replace the air cleaner lid and wing nut.