Autolite 4100 Secondary Circuit

4100 SecondaryThe secondary throttle plates are operated by the secondary diaphragm using vacuum.

As the primary throttle valves are opened the vacuum at the venturi increases. When vacuum reaches a certain point the secondary operating diaphragm kicks in.

There is a check ball located in the secondary vacuum passage which controls the rate at which the secondary valves are opened. Note that this check ball is often missing and some carburetor people don’t think it is necessary. At minimum check to make sure if you have this check ball that it is loose. They do get corroded and stuck. When stuck, apply heat to the outside of the housing while tapping the carburetor on the bench. The ball will eventually fall out.

As the secondary plate begin to open fuel flows from the secondary fuel bowl through the secondary jets, up the main well tube where it is mixed with air and out the secondary venturi.

Be sure the secondary air bleed and the anti siphon air bleed is clear. Gas leaking into the secondary venturi while the secondary valves are closed could be caused by these tubes being plugged.

A warped 4100 main body could affect the secondary vacuum passage and gas can enter the venturi area from the fuel well.

Here is a question about a 1957:
On the 4100 Autolite carburetor that when the engine is off, you can move the secondary linkage real easy. When the engine is running the secondary linkage is hard to move. I put a paper clip on the link to test it to see if the secondary’s opened up and I drove it down the road and the paper clip never moved. Which, I take it, that the secondary’s are not opening,

Ford 4100 secondaries are vacuum-operated via a diaphragm. If the secondary positive closing tang (which is a flimsy nylon pin IIRC) is broken off, then yes they can be opened easily when the engine is off.
When the engine is running they may indeed be harder to move due to the increased low-pressure area below them; I can’t really say for certain but it seems logical.

If they are not opening at all when the car is warmed-up and the primaries are opened wide and held there, then he probably has a bad secondary diaphragm and/or a warped diaphragm cover.

The secondary diaphragm is mounted at the rear/back of the main body. It has a conical spring mounted in front of the diaphragm with the small end of the spring toward the cover.  You must take care when mounting the lever to the back side of the diaphragm or it will not open! I use contact cement when installing a new diaphragm, so the edges of the diaphragm stay aligned with the screw holes and the vacuum port.

The diaphragm gets it vacuum signal from a “small” brass tube mounted in the air horn that points downward into the main body primary opening and is close to the air cleaner Bolt. The vacuum signal then travels through the long drilled port in the air horn, across the Secondary opening to the top of the diaphragm housing. The air horn gasket can be installed incorrectly blocking the vacuum signal. Check the small brass tube for being clogged. If the Diaphragm has not been replaced, I would, because the get stiff when old.

Check for correct alignment of the Secondary Link. If it’s bent it will no work good with the vacuum signal If all above is good it has to work!

Electric Choke Conversion Kits

We recently started to add several electric choke conversion kits.

These kits include everything you need to convert your current hot air choke to an electric choke. Thermostat, wiring and step by step instructions.

Electric Choke Kit

Some of these conversion kits have been a very good replacement solution for some of the thermostats that are no longer produced. For example, some of the 2 Jet carburetors used a choke thermostat that is mounted in the intake manifold, called a divorced choke. These thermostats are no longer made, so an electric choke conversion is a great answer.

Along with the divorced choke conversions, we are also adding some choke kits we have not had available before. For example we now have an electric choke conversion for the Autolite 1100.

Some conversion kits are somewhat universal like the CU1445 which fits 70-78 small Block Chevy car and Chevy / GMC truck Quadrajet. Then some are more specific, like the CU1454 with fits 1970-72 Cadillac 472, 500.

These chokes use the 12v when the engine is running to heat up the thermostat and allow the choke to open. Eliminates the hot air tube and need for any choke vacuum. You do need the choke housing to be on the carburetor, but the choke piston will not be needed. Basically if you have been running a hot air choke and want to convert to electric, this kit will have everything you will need.

All electric thermostats are 12v and are tested to meet or exceed manufactures recommendations. Adjustment instructions are included.

We are excited about this new addition to our inventory. Please let us know if you have any questions.

Motorcraft 2100 Choke Pull-Down

Watch a video about how this choke pull-down works:

The choke pull down on the Motorcraft 2100 2 barrel carburetor is operated by vacuum. When the engine is off, the choke valve is completely closed. If the valve stays this way when started, the engine will not run due to lack of air.

When the engine is 1st started the vacuum is at the highest point and pulls the pull down, down all the way. This opens the choke valve enough to let some air in.

If the diaphram is bad the engine may die after starting and when cold.

Watch to see if the diaphragm pulls down when you 1st start the engine. If not, replace it. Also check the vacuum passage to make sure it isn’t blocked.

2100 Pull Down

2100 Pulldown

Holley 1920 Floods Coming to a Stop

After running my vehicle and I turn it off, gas floods over into the intake flooding the engine. Won’t start.

  • Could be fuel pressure – When the engine if shut off there should be some release of pressure. Make sure you have a vented gas tank. After running open the gas cap. Do you get a rush of air? If so, then you need to vent your gas tank. Some vent through the gas cap itself and some have a separate tube coming up from the gas tank. Make sure the tube is clear. Test your fuel pressure. Should be about 4 lbs or less, but compare with what your motor manual says.

Holley 1920 Flooding

  • Your gas may be percolating – The engine gets hot, you turn it off and the gas boils over, or evaporates out of the carburetor. Gas has a lower boiling point these days and especially in hot parts of the year, this may be a challenge. Experiment with different brands of gas. Try Startron ethanol additive in the gas. Make sure fuel lines are away from the manifold. Is your water running at a good temperature. While very hard to find, spacers will sometime help. Do you have a bowl vent on top of the carburetor and is it hooked up to a hose correctly? Check the bowl vent gap. Specifications are in the carburetor kit instruction sheet.

Holley 1920 Flooding

  • Some Holley 1920 carburetors have a hot idle compensator. The compensator opens a vent into the main body letting off some pressure when coming to an idle. If the idle compensator is bad the idle will be erratic. To test:
    • Remove the air cleaner with engine running.
    • Cover the hot idle port. If the idle smooths out there is something wrong with the idle compensator. Check the seal and/or replace.

Rochester Fuel Filter Installation

I get questions about how the fuel filter installs in the Rochester 1, 2 & 4 barrel carburetor so decided to add this information here.

The Rochesters that does use a filter in the inlet will normally use the bronze type fuel filter, or the 1″ paper filter. A few use the 2″ paper filter. The marine carburetor normally use a screen filter.

These 4 types of filter all install in the same way, but not necessarily the same fittings and gaskets.

Rochester 2 Jet filter assembly. Here is an illustration showing how the filter is installed on the 2G, 2GC, 2GV carburetors. This may not be true on the late 70’s carburetors.
#16 is the return spring.
#15 fuel filter. Usually bronze type.
#14 is a small gasket about the size of the filter and is used to keep the fuel from leaking around the filter. This is not necessary if you use our 1″ paper filter. This filter has a gasket on the end.
#13 is the fuel inlet gasket and there are a couple of sizes to choose from.

2 Jet Filter

Watch a video about the filter assembly.

Gas Dripping From Venturi

2 Jet Venturi

Gas dripping from the venturi can happen on most carburetors, but we will talk about the Mercarb and the Rochester 2 Jet, 2 barrel.

The only time gas should come out of the venturi is when you pump the gas throttle. That is when an extra squirt of gas is required from the accelerator pump circuit. Make sure the engine is off when looking down the carburetor bore.

If it happens at idle, or cruising along, then you have a problem. This can be caused by a vacuum leak, or something plugged in the idle circuit.

Remove the idle mixture screws and blow out the circuit. Remove the venturi and blow out the small holes on the top. You may have to run thin wire down the holes to get them clear.

Still have the problem?
You could be getting too much gas. Check the fuel pressure to see if it is 4.5 to 5 lbs. Also check the float level. On the Mercarb turn the top upside down and set the float level.

Another thing to try. Start the engine and bring up to temp. Rev up the RPM to 2500 or so and cover the air intake with a gloved hand or rag while leaving the at same 2500 or so position. The engine will begin to die, but just before it dies let it recover. Repeat this several times. What happens is fuel goes through air/vacuum circuits and visa versa. If the idle circuits are dirty this often flushes them out!

One other possibility is “heat soak”. This happens when the engine gets hot and you turn it off, or come to an idle. The fuel boils over, causing it to flow out just about anywhere it can. If this is happening, try other brands of gas. Without ethanol if possible. Ethanol is not our friend.




Motorcraft 4300 4 Barrel Float

Motorcraft 4300 Float

We have recently added the Motorcraft 4300, 4 barrel float to our catalog. This float has been out of production for several years and up to now very hard to find.

After over a year of development and testing, we now have this 4300 float available for sale.

The original Motorcraft 4300 float was a Nitrophyl type of float, which were solid floats but it is possible for these to absorb fuel over time. We recommend replacing this type of float anytime you run into them.

There were some aftermarket brass floats used, but we seldom if ever see these.

Our new 4300 float is made of a special plastic material and is designed to have the same weight and shape as the original float and is resistant to ethanol.

If you have a gram scale you can weight your old float to see if it can be re-used. 15 grams is the perfect weight for the 4300. a gram or two either way would be OK.

The 4300 was used on Ford applications from 1969-1973. In 1974 the 4350 was introduced.

It is easy to get the 4300 & the 4350 floats mixed up, but they have distinct differences. These two floats are not interchangeable.

The Motorcraft 4300 float will be discounted when you add it to a 4300 carburetor kit purchase, but is also available as a stand-a-lone purchase.


4300 float – 15 grams
4350 float – 14.5 grams

Hot Idle Compensator

The Hot Idle Compensator is used on some carburetors to offset enriching effects caused by percolation during hot operation. Generally used on vehicles with air conditioning. When the engine is running hot and then stopped, the fuel can boil or percolate and vaporize causing a rich condition at idle.

Hot Air Compensator

The compensator is mounted on top of the venturi, or behind the float bowl and at a extreme hot temperatures, the bi-metal thermostat bends and opens a passage that leads from the venturi to below the throttle valves. This adds just enough air to the fuel mixture to lean out the fuel giving you a smoother idle.

Be sure the compensator is closed when adjusting the idle mixture, otherwise you may be getting too much air introduced.


Weber Marine 4 Barrel Accelerator Pump

Accelerator Pump Adjustment
Accelerator Pump Lever AdjustmentA – Rich
B – Lean
C – Leaner

The Weber marine carburetor accelerator pump lever has 3 holes. The standard position is to have the rod in the 1st (A) hole. If you are experiencing a bogging down on acceleration try placing the rod in hole B, then hole C.

Fuel Injection Cleaning Service

If you’re experiencing a rough idle, a hesitation under acceleration, poor starting or declining gas mileage it’s possible your fuel injectors are dirty. You could try fuel additive cleaners but they’re marginally effective at best. The only way to return injectors to an as-new condition is by removing and cleaning them thoroughly.

Why clean injectors?

When the engine in your vehicle was designed, the manufacturer carefully optimized the combustion process for power and efficiency while minimizing emissions. A key element in that is the way gasoline is sprayed into the intake airstream. The particle size must be small enough to ensure complete atomization and the spray pattern must provide a uniform fuel:air mixture.

Over time gasoline leaves a tarnish on metal surfaces. This builds up and can reach a point where it partially blocks an orifice or affects how the injector opens and closes. When that happens the result is an uneven distribution of gasoline particles in the air stream, and a mixture that’s either too rich or too weak.

Dirty fuel has a similar effect if particles get through the filter to the injectors. Under light throttle the engine may run smoothly but apply a load and the mixture strength weakens and the engine stumbles.

Regardless of what made your injectors dirty, the only complete fix is thorough cleaning.

Not a DIY job

Let’s be clear – you’re not going to actually strip down the injectors. They’re full of tiny components that need automated equipment to assemble. That leaves your only cleaning option as trying to flow a solvent through the injector. As you can’t operate it once it’s off the vehicle there’s a risk of actually making a blockage worse.

Mike’s injector cleaning service

After logging receipt of your injectors we start with a thorough visual examination and then testing. This lets us identify any particular problems and shows if cleaning will be beneficial. If we determine that replacement is the only sensible option we will let you know quickly.

Assuming we go ahead with cleaning, we do this with specialized ultrasonic equipment. This uses high frequency energy to gently lift away material adhering to the inside and outside of the injector. It’s a kind of vibratory process that leaves internal components unharmed but results in an injector that’s as clean as the day it was manufactured.

Next, we replace the seals and filters before testing the injector again. This starts by checking for leaks, after which we measure the volume or flow rate and check the spray pattern. Finally, we verify electrical properties and ensure the injectors are balanced as a set.

Only when your injectors are back as close to new as possible will we call to ask for payment. We understand that having the injectors out means your car or truck is off the road. That’s why we strive to complete the whole process in 24 hours.

Please note that at this time we do not handle gasoline direct injection (GDI) fuel injectors. If you have a late model vehicle it’s quite possible that it uses this type of system. Please verify you have throttle body or manifold injection and not GDI before sending us your injectors!

Taking advantage of our service

You’ll need to remove your vehicle’s injectors. The details of how to do this vary by vehicle and engine, but it usually entails:

  • Depressurize the fuel system. (Remove the fuel pump relay or otherwise disconnect it, then crank the engine a few times. Alternatively, you may see a Schrader-type fitting on the steel fuel line. You can release the pressure through this, but be careful to prevent it spraying!)
  • Remove whatever covers the inlet manifold. (Often air ducts run over the top.)
  • Disconnect the wiring harness from each injector.
  • Remove the fuel pipe with injectors attached.
  • Remove each injector from the piping.
  • If possible, blow out any remaining drops of fuel before packaging them up and sending to us.

One last tip for when you get back your freshly-cleaned injectors: before installing them, replace your old fuel filter! That way they’ll run like new for many miles to come!