Mike’s Carburetor


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We have a ton of technical information for just about any carburetor and new information is being added on a daily basis. Check back again.

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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!

Rochester 2 Jet Hesitation

When you step on the throttle, the engine seems to bog down and in some cases die.
A bad distributor advance can cause a hesitation when stepping on the gas, but we aren’t addressing that here.
Stepping on the throttle is where the accelerator pump circuit comes into play. When the pump moves up (let up on throttle), the accelerator pump well is filled with fuel. Some get the fuel by filling over the top of the well through a slot on the side, others use a intake at the bottom of the well which is shut off with a check ball.
2 Jet Accelerator Pump
In most 2 jets the small aluminum check ball goes in the pump well. In a few 2 jets the large aluminum check ball goes in the pump well. At any rate aluminum is used because it is light and easy for the fuel to lift up.
With the engine off, pump the throttle while looking down the carburetor throat. You should see 2 strong squirts coming out of the venturi. If not, then you have a blockage somewhere.
The small holes in the venturi can be blocked with ethanol residue. run thin wire down the holes.
Check the gasket under the venturi, they don’t always fit well and have to be trimmed. If they are too big then the venturi won’t site flat.
Put fluid in the pump well and press the accelerator pump down. Fluid should come out of the main discharge.
The main discharge should have a check ball, spring, then a T to hold it all in. This is where the bigger stainless steel check ball resides.

Blow air through the passage way at the bottom of the accelerator pump well. Air should come out of the main discharge.
Is your accelerator pump cup getting stuck in the well? Most wells are tapered so that the pump gets tighter as it goes down. If ethanol has damaged the pump cup then the cup may be swollen.
Does your accelerator pump have a delayer spring (spring over the stem) and on non marine a return spring below it?

VW Fuel Injection

By the mid 1960s, mechanical fuel injection had been successfully employed in Formula One and sports car endurance racing for more than a decade. But manufacturers were just beginning to install it on production cars. Volkswagen introduced it first in 1967 on what the company referred to as the Type 3: the notchback, squareback, and fastback models. It was a Bosch system called D-Jetronic. Volkswagen would not introduce fuel injection to the rest of its line-up until 1975, when they switched to the Bosch L-Jetronic system for their Beetle, Super Beetle, and Type 2, better known as the Bus.

Bosch had developed a direct injection system for gasoline engines as early as 1952, for the Goliath GP 700, a misnomer if there ever was one. The Goliath was a tiny car only 160 inches long, and production numbers were low. The VW Type 3 was the first production model built in sizable numbers to sport mechanical fuel injection on gasoline engines.

These were simple systems, but all systems break down over time. And forty years is a pretty long time. Today, parts are increasingly difficult to find, especially for the earliest D-Jetronic system. Even finding mechanics experienced with these early systems can be a challenge. Some owners prefer to go the DIY route. And some convert their engines to carburetion instead. There are useful manuals out there for the DIY-ers. One that is a little pricey but very helpful is Bosch Fuel Injection and Engine Management, by Charles O. Probst.

Several aftermarket companies make replacement “bolt on” fuel injection systems, but installation is typically far more complicated than merely bolting them on. They may require some drilling, fuel line modification, and perhaps welding of the O2 sensor. You have to be a pretty ambitious DIYer to tackle this job. And the initial cost of these systems can also be prohibitive. Some of these do come in kits, however, which help to simplify the process. For the truly ambitious DIYer, you can always build a system from scratch. But that will involve not only the new fuel line, but also the pressure regulator, electric pump, throttle body and its sensor, a computer, and of course, the injectors. And anyone ambitious enough to consider going this route has probably already replaced the Bosch system.

There are many paths you can take to put your VW back on the road, some far more elaborate than others. A lot of it is simply deciding what is best for you, your car, and your pocket book.

Subaru Class Action

A lawsuit has been filed in the United States District Court in New Jersey against Subaru of America, Inc. (“Subaru”) and Fuji Heavy Industries, Ltd.

What is the suit about?

The suit alleges that certain Subaru vehicles suffer from a design defect that causes them to consume excessive amounts of engine oil. The lawsuit seeks class action certification for people who currently own and lease the affected vehicles, as well as for people who previously purchased or leased the affected vehicles. The suit alleges that Subaru violated certain consumer statutes and breached certain warranties to its customers.

Subaru denies the suit’s claims, maintaining that it has not violated any warranties, statutes or laws, and maintains that the vehicles in question are not defective.

What remedies are available?

Unless they exclude themselves from the class action settlement by June 13, 2016, eligible Subaru purchasers/lessees are eligible for the following remedies if the court approves the proposed settlement:

  • Cash reimbursements for expenses not previously reimbursed for vehicle oil consumption tests;
  • Cash reimbursement for expenses not previously reimbursed for vehicle repairs to address oil consumption;
  • Cash reimbursement for certain engine oil purchases, to the extent not previously reimbursed;
  • Extended Power train Limited Warranty to cover authorized repairs to correct excess oil consumption (up to the earlier of 8 years or 100,000 miles);
  • Cash reimbursement for rental car expenses and towing fees for the period of time the vehicle was being serviced for excess oil consumption.

What vehicles are involved?

Remedies are only available for people who bought or leased one of the following vehicles within the continental United States (including Alaska):

Automatic/CVT Transmission Manual Transmission
2011-2014 Forester (below VIN *529004) 2011-2015 Forester (below VIN *543624)
2012-2013 Impreza 4-door (below VIN *033336) 2012-2015 Impreza (below VIN *270253)
2012-2013 Impreza 5-door wagon (below VIN *886714) 2013-2015 Crosstrek (below VIN *2702874)
2013 Crosstrek (below VIN *856139) 2013-2014 Legacy (all)
2013 Legacy (below VIN *048086) 2013-2014 Outback (all)
2013 Outback (below VIN *321435)

For more information, including the proposed settlement agreement, the claim form, important dates, and answers to frequently asked questions, visit the lawsuit settlement page at http://www.oilconsumption.settlementclass.com/index.html.

Autolite 1 Barrel Accelerator Pump Action

Accelerating Pump System

Smooth acceleration requires a momentary increase in the supply of fuel. The air flow through the carburetor responds almost immediately to any increase in carburetor throttle valve opening. The fuel within the metering passages will lag momentarily in its response to the pressure difference created by this increased air flow. This lag in fuel response will cause a temporary leanness in the fuel air mixture that results in a hesitation in engine acceleration. A mechanically operated accelerating pump system supplies added fuel to provide a richer fuel air mixture for this brief period of time.

The accelerating pump, located on the side of the lower body assembly, is actuated by linkage connected to the throttle shaft. When the throttle is opened on acceleration, the diaphragm forces fuel from the accelerating pump chamber into the discharge channel. The inlet ball check closes to prevent a reverse flow of fuel. Fuel under pressure forces the discharge ball check and the weight off its seat, allowing fuel to pass up to the discharge nozzle. The fuel is sprayed from the nozzle into the air stream above the main venturi.

When the throttle plate is closed on deceleration, a return spring forces the diaphragm back, drawing fuel through the inlet channel. The inlet ball check opens, allowing fuel to pass into the chamber while the discharge ball check closes, preventing entry of air.

Watch a video about how the accelerator pump works:

Holley 1940 Float

The Holley 1940 carburetor originally used a Nitropyl type of float and because they tend to absorb fuel over time, we recommend that they be replaced at each carburetor rebuild. Once a Nitrophyl float has absorbed fuel it will become too heavy and allow more fuel to enter the bowl than what is needed, causing a flooding effect.

1940 Float

The only way to test the Nitrohyl float is to weigh it. The 1940 float should be 12.0 grams. If you don’t have a scale, then replace it.

The Nitrophyl float can be identified by the black color and it looks like it is made of a plastic material.

The Nitrophyl float has been replaced by a brass type. The brass float is tested by heating up a pan of water just prior to boiling, immerse the float, look for any bubbles.
1940 Float