Motorcraft 4300 Power Piston

The Motorcraft 4300, 4 barrel carburetor power piston is used to supply extra fuel when powering up.

Motorcraft 4300Motorcraft 4300

The power piston is made of brass (at least the piston itself is) and fits into the float bowl top. See #26 in the diagram. The purpose of the power piston is to supply extra fuel when you are in the power mode. At idle and low speeds, vacuum is at the highest level and pulls the power piston up and off of the power jet (closing it). The power jet is located in the bottom of the float bowl. As you gain power, vacuum starts to drop and the spring on the power piston rod forces the power piston down and pushes the pin on the power jet open. This allows more fuel to feed into the carburetor throat.

It is very important that the power piston works smoothly. When you push on the spring and pin end, the piston should move into the top, then when you let go it should snap back. The power piston is moved by vacuum alone, so it needs to work without any binding whatsoever.

You will need to remove the power valve from the well in order to give it a good cleaning. Buff the brass piston using a wire wheel. Polish the well with crocus cloth to make it smooth. Lubricate with Silicon Spray Lubricant.

Gas Tank Fuel Gauge

For most original gas tanks, the tank gauge unit is located in the gas tank.

Tank Gauge UnitThe tank contains a float that is attached to a resistance device similar to that used in an electric oil pressure sender. See the resistance unit in the illustration. The travel of the float up, or down, causes the resistance to vary, depending on the depth of the fuel in the gas tank.

Most gauges are designed to read empty when one or two gallons of gas is left. This allows the driver to get gas when the gauge indicates empty before running out of gas.

When the float drops, current flow to ground through the bimetal metal coil will be less, because it must travel through the more resistance wire. This cools the bimetal haripin and pulls it together. When the tank if filled the contact slides up, cutting out resistance and current flow increases. This heats the hairpin and the ends separate, causing the needle to move toward the full mark.

Gas Gauge



Tips on Removing Frozen Parts

Over the years I have learned the hard way how to remove screws & bolts that are frozen. Using the techniques below I seldom need any specialty tools for removing screw, bolts & nuts.

Here are some ideas that might help you:

Removing the Small Screws on the Choke, or Throttle Shaft

Take your time removing these screws. It will pay off big time. These screws are usually mushroomed on the threaded end. Using a dremmel tool, grind the threaded end flush with the shaft. Using a screw driver that fits the screw head well, twist the screw counter-clockwise. It is sometimes a good idea to turn the screw back in, then out again. This helps flush out rust particles. Don’t turn too hard, or the screw will break.

Not coming out? Put a block of some kind on the threaded end to prevent the shaft from bending, then hit the screwdriver with a hammer while you try to turn the screw. This will sometimes break the screw loose.

Still not coming out? Using a butane torch heat up the area around the outside of the screw. Don’t apply too much pressure, or you risk bending the shaft.

Did you break the screw? You will have to drill & tap. Using a drill bit (and a good one), just shy of the screw diameter, drill out the screw. If you do this carefully you will be able to remove the screw without damaging the threads. The trick here is to have a good set of small drill bits.

After inserting the new screws, you can mushroom the end, or as I do use thread locker.

Frozen Shafts

The best way to un stick frozen shafts is to heat the area outside of the shaft and tapping on the end of the shaft. The shaft will almost always come loose. If it doesn’t then it is most likely beyond repair.

Frozen Screws, Bolts or Nuts

When a screw is damaged to where a screw driver won’t hold, use a drift punch and hit it a few times with a hammer. This will cause the screw slot to shrink hopefully enough to allow the screwdriver to work. The banging will often times jar the screw.

For bolts use the same drift punch technique to see if the bolt comes loose. Avoid using 12 point sockets. A 6 point will give you a better grip. When a bolt start to loosen, then tightens up, stop. Try moving the bolt back and forth to help loosen it up.

Still frozen? Apply heat around the outside of the screw, bolt, or nut.

Always use good tools and the correct tool on nuts & bolts. That does not include a cresent wrench.

Nut or Bolt Has a Stripped Head

Use a flat file to file the flat parts and remove the mushroom edges. From there try a smaller socket or wrench. Sometimes moving from a US wrench to a metric wrench will do the trick, otherwise a vise grip will be in order. Again applying heat will probably help the nut, or bolt move easier.

Stromberg WW Worn Throttle Shafts

Stromberg Worn Throttle Shafts

Stromberg WW carburetors are notorious  for having worn throttle shafts. Unfortunately unlike most carburetors, the WW wears out the shaft instead of the throttle body.  As I mentioned, most carburetors wear the throttle body and while not a simple process, re-bushing the throttle body on these carburetors is very doable as long as you have the required tools, like reamers and good drill bits. We used to be able to get new throttle shafts to replace the worn shafts, but they haven’t been produced for several years now and finding a used donor carburetor is very difficult unless you happen to find one that has been sitting on the shelf for several years.

To our rescue is someone that I have consulted with many times in the past, “The Old Carburetor Doctor”. Jeff, the owner, has a fix for this problem, which he does when rebuilding the Stromberg WW. He tells me that he will repair your throttle body without a complete rebuild if that is what you want. He just asks that the throttle body be stripped down completely.

Here is what is says about this problem.

Stromberg WWs are no problem. Yes, the shafts are always worn, but we have developed a fix:  I plug the (usually) free right-hand end of the shaft bore with a welch plug, and I rebush the left-hand (throttle lever) end with an extended bushing which allows the un-worn part of the shaft outside of the casting to be supported by new bushing material.  Just look at a WW and you will see how it accommodates this fix; the factory should have done it that way in the first place!

To have your WW rebuilt, you can call Jeff at 800-945-2272. Please do not call Jeff for technical advice. That isn’t what he does. He is a carburetor restoration expert. Leave your technical questions here on this site. Jeff does not sell parts of any kind.