Impco makes a wide variety of mixers to suit just about any engine size imaginable. Impco's Model 425 mixer is used quite extensively on carbureted propane vehicles. This is probably due to the fact that propane is very commonly used on fleet vehicles which typically have large engines requiring this mixer. As a result, it is relatively easy to find in junk yards.

The main factor that controls the fuel mixture in an Impco mixer is the shape of the gas valve. Impco designed their gas valves such that the valve profile provides a richer fuel mixture as air flow increases. This is mentioned in Jay Storer's book "Economy or Performance Propane Fuel Conversions for Automotive Engines (1986-01-24)" but he didn't discuss it in any detail. Impco describes their gas valves in their carburetor theory as follows:

Mixtures between idle and full-load conditions are controlled by the gas metering valve shape. The gas metering valve is shaped to produce lean mixtures at light loads and increasingly rich mixtures at heavier loads and higher engine speeds.

The shape of the gas valve is designed for optimum mixtures for the mid-size engine between the largest and the smallest cubic inch displacement upon which the carburetor will be installed.

Impco's technical department confirmed that their gas valves do not necessarily keep a stoichiometric fuel/air ratio across the entire flow range. The airflow remains constant with each air/gas valve assembly based on the lift of the assembly at different load points. The difference in the assemblies is the gas valve portion that is cone shaped. Each gas valve has it's own unique contour and length thus resulting in different gas flow at the same air flow or lift point of the diaphragm assembly. In other words, the flow curves are fairly linear and the rate at which the fuel mixture becomes richer with increasing airflow is different for each gas valve.


Propane Carburetor Fuel Mixtures

Impco has the following explanation for fuel mixture adjustments:

There are three carburetor adjustments. The idle speed adjustment controls the degree of opening of the butterfly valve at idle, the same as the adjustment on a gasoline carburetor. The idle mixture adjustment controls the mixtures at idle and slightly above, while the power mixture adjustment controls full power/high RPM mixtures.

Light load or cruise mixtures are controlled by the contour of the gas metering valve. These are not adjustable at the carburetor. Power mixture adjustment must be set under full load. This adjustment has no effect at idle or in the light-load range. A CO meter or exhaust analyzer is highly desirable for making the power mixture adjustment. If an exhaust analyzer of the Wheatstone Bridge type must be used, care should be exercised to limit exhaust pressure into the instrument as it can give a false rich reading. The analyzer must be kept reasonably dry with a water trap as well. An infra-red CO analyzer is a more accurate instrument; however most of these operate on 110 AC voltage and require an inverter to 12 volts if they are to be used in a moving vehicle.

Proper CO reading for the power adjustment depends on the type of vehicle and and its use. A heavy-duty truck operating under continuous full-load conditions should be adjusted no leaner than 1.5% to 2.0% CO for safety. A light-duty vehicle can operate safely as lean as 1.0% to 1.5% CO, since it will not be operating "flat out" for long periods of time. The slightly richer mixture for trucks will help avoid detonation, without being rich enough to elevate exhaust gas temperature unreasonably.

Extremely lean mixtures during acceleration, possibly caused by a restricted balance line from the carburetor to the converter, or lean mixtures under load, place a high voltage requirement on the ignition system which may cause missing or crossfire with resultant backfires.

Maximum richness of optimum performance is close to 3.0% CO. Horsepower will begin to drop as the mixture is richened beyond this point. there is not much change in power from 1.0% CO to 3.0% CO.

If the engine does fall off in power with a power mixture leaner than 3.5% CO, it is a sign of poor fuel distribution in the air stream to the intake manifold. In an eight-cylinder engine, for example, four may be rich and four lean. Mileage would be poor.

It is unclear why Impco recommends using a slightly richer fuel mixture to help avoid detonation for trucks. Propane responds to rich fuel mixtures opposite to gasoline. Full-throttle rich mixtures in gasoline engines allow atomized liquid gasoline to evaporate in the combustion chamber. Because propane is already vaporized before it reaches the combustion chamber, there is no way for it to provide internal cooling from the latent heat of evaporation.


Idle Fuel Mixture

Once the engine is fully warm, set the idle mixture to provide the highest and steadiest idle speed at the vehicle's recommended idle speed.

Part-Throttle Fuel Mixture

The only practical way of changing the part-throttle fuel mixture is to change the profile of the gas valve. Normally, this is done to improve fuel economy with leaner fuel mixtures. For most of Impco's mixers, lean gas valves are available. Some mixers also use spacers or shims to lean the fuel mixture.

Power Fuel Mixture

  • Since this only really makes a difference at full throttle, your vehicle can run with any setting of the power mixture until you can go to a dynamometer or a emissions-testing shop.
  • Alternatively, you can take your vehicle to a drag-strip for test & tune day and, while starting with a lean mixture, richen the mixture until the 1/4 mile times stop decreasing. Keep the power setting on the lean side of that which give the fastest 1/4 mile time.
  • The power mixture does have a small effect at cruising speeds. For the best fuel economy, you may want to try turning the screw in (clockwise) to lean the fuel mixture. If you find that this lowers your full-throttle power unacceptably, turn it out (counter-clockwise) until it becomes satisfactory.


Model 425 Fuel Mixtures

While I was driving both of my 425 mixer-equipped propane vehicles, it seemed to me that the fuel economy for both cars was lower than I was expecting. I was expecting propane gas mileage to be somewhere in the neighborhood of 75%-80% of gasoline gas mileage. The driving I was doing at the time was highway commuting with steady speeds of 60 mph.

It seemed to me that my first propane vehicle, a 1978 Chrysler New Yorker with the 440 engine, would be cruising with a richer than necessary mixture just because its airflow would be at the upper end of the range for the 425 mixer. I believe that the 425 mixer with the stock gas valve is better sized for the 350 CID engine. Interestingly, I found that supplying cooler fuel to second propane vehicle, a '77 Pontiac Parisienne, by throttling back on the water to the evaporator did not help and actually hurt my fuel economy.

The 425 mixer may have been designed with a 350 CID engine in mind under normal conditions. This would explain why my 440 CID engine with the same mixer (and gas valve) had higher than expected fuel consumption because it was getting a richer mixture than necessary due to the profile of the gas valve.

There were four gas valves originally available for the Impco 425 mixer. The AV1-16 is the standard valve that comes with the 425 mixer. The AV1-1637 valve supplies a rich mixture and is used with the EC1 system, which is now obsolete. The AV1-1644 valve supplies a lean mixture and is used with 370 CID engines and above. The AV1-1651 gas valve is required for the feed back carburetor.

Some people have also experimented with different valve profiles but there is very little information available about this. Water flow to the evaporator has been discussed in any detail only with regard to converter temperature. Impco's service manual is not overly helpful nor is Impco. Impco does not provide specific profile information beyond the above explanation, as they feel that this is proprietary intellectual information.

The 425 mixer has adjustability at both idle and full flow. The AV1-16 is fairly linear in the flow curve. The AV1-1637 is also linear but is richer throughout its flow curve than the AV1-16. As far as use with the EC1 system, Impco’s technical department does NOT recommend this option. The EC1 was quite complicated to adjust and was used to provide a lean cruise mixture. This was achieved mechanically via the use of vacuum source on the atmospheric side of the regulator. The AV1-1644 is also linear but is leaner throughout its flow curve than the AV1-16. The AV1-1651 is richer throughout it's flow curve, relying upon a processor to evaluate the air/fuel ratio via an oxygen sensor signal and thus adjusting the output from the regulator by varying the atmospheric pressure through a fuel control valve. Although Impco did not say it, I believe that the AV1-1651 valve must maintain a constant ratio of fuel to air so that the feedback pressure regulator in the converter could regulate the fuel mixture by adjusting the fuel pressure (and thus density) supplied to the mixer.

Although it does not state this in the Impco catalogue, the 425 mixer will bolt directly to most all Holley 4 barrel throttle bodies without the need of an adapter. You can order a mixer with the standard or feedback air/gas valves already installed. As the EC1 valve is obsolete and as you would would only want to install a feedback valve in a feedback system, the only real option available for changing gas valves is to install the AV1-1644 lean valve in the standard mixer.

Even if you were to find an AV1-1637 valve, there is no reason to install it because engines respond to richer gaseous fuels opposite to the way they respond to liquid fuels. Richer mixtures burn slower and result in burned exhaust valves. They also cause the engine to be more susceptible to detonation or pinging due to a hotter than necessary exhaust valve.


Impco Carburetor Flow Ratings

There is a characteristic pressure drop for every flow across the mixer. At a flow of 460 CFM, the 425 mixer will have a pressure drop of 1.5” Hg. At higher flows, the pressure drop will be higher and at lower flows, the pressure drop will be lower. However, the important thing is that the gas valve is fully open at 460 CFM.

The 1.5" Hg is manifold vacuum that is very close to wide-open throttle on any engine and 460 CFM is the maximum air that the 425 can flow at that manifold depression. Since the air/gas valve is variable, if the engine does not require 460 CFM of air, the air/gas valve will not open all the way. If the application requires more then 460 CFM's of air, then the air/gas valve will be all the way open and the engine will begin to starve for fuel above 460 CFM and the 425 would not be the carburetor of choice.

According to Impco, because these are Air / Gas valve assemblies, the amount of air and fuel are both controlled by the movement of the diaphragm. The capacity of any carburetor is defined by its ability to flow air. The maximum amount of air that the 425 can flow on a normally aspirated engine is 460 CFM. Generally you can adjust the high flow fuel adjustment to be very rich even when the air/gas valve is at the full open position unless the regulator being used has also reached it's full flow capacity. Even if you have a 600 CID engine capable of 10,000 RPM and if you had the 425 carburetor on it you would only be able to flow a maximum of 460 CFM's of air, thus starving the engine for air

I disagree somewhat on this point, as a 600 CID engine capable of 10,000 RPM will flow more than 460 CFM with a 425 mixer. My disagreement is that that engine will continue to draw more air with increasing RPM but the pressure drop across the mixer will become larger than the rated 1.5"Hg for the air flow. The engine will become starved for air because the density of air filling the cylinders will become less due to the increasing pressure drop. That is, the large pressure drop across the mixer will dramatically reduce the volumetric efficiency of the engine. The more important problem will be that the fuel mixture will be uncontrolled at excessive airflows because the gas valve reaches the limit of its travel at 460 CFM.


Gas Valve Comparison

When we say that the fuel mixture is governed by the shape of the gas valve profile, it can be hard to visualize the difference between the different gas valves. Have a look at the following photos to see the difference between the standard gas valve and the lean gas valve:

The two gas valves side by side. The AV1-16 standard gas valve is the one marked "19" and has the black hydrin diaphragm. The AV1-1644-2 lean gas valve is marked "4X4" and has the yellow silicone diaphragm. The blackness on the standard gas valve is due to propane residuals from over 90,000 miles of service.

Model 425 Mixer Gas Valve Comparison

The standard gas valve up close:

Model 425 Mixer - Standard Gas Valve

The lean gas valve up close:

Model 425 Mixer - Lean Gas Valve

The gas valves on Impco mixers are extremely easy to change. The steps are detailed on our gas valve upgrade project.