Bobs Carburetor Shop

Recently I was drivingBobs Carbs through Oceanside California and a sign on a building caught my attention. “Bob’s Carb’s”. The 1st thing I thought was WOW, you don’t see many carburetor shops these days. I was curious about the business so I turned off and went inside.

Hi, My name is Mike from Mike’s Carburetor Parts. The gentleman says “Hey I buy parts from Mike’s Carburetor”. Well small world. His name is Mitch and he showed be around and told me a little bit about his family business.

Bob, Mitch’s father started the business around 1955, 1st operating a service station and eventually turning it into a carburetor shop, then a repair shop, which is what it is today. 

Bob’s Carbs has migrated to fuel injection, but they still do a fair amount of carburetor work on classic cars. In fact, they had a Ford falcon in the shop getting the carburetor rebuilt. Mitch’s father, Bob comes in and rebuilds the carburetors and he had a couple on the bench when I was there.

This is not your average shop either. I looked at their reviews and they are very high. Seems they take good care of their customers.


This is Mitch’s view from his desk and he wears shorts year around. What a life.

Carburetor Shop







Like I say, carburetor shops are few and far between and when I discover them, I love to hear the story behind the business. I’ll be in Oceanside again next winter and I will stop in again and say hello.

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.




Adjusting the Idle Mixture

The Carburetor Idle Circuit and Adjusting the Idle Mixture

The idle circuit is in effect only when idling. Once RPM increases above idle, the Idle mixture screws are no longer in play. If you have the idle RPM set too high, your idle mixture adjustment will be irrelevant. When you turn the idle mixture screw in all the way and the engine doesn’t change, you could have the idle RPM set too high. For example if it was at 1,000 RPM. On the other hand if the idle is normal, then you have a problem in the carburetor, or possibly a vacuum leak.

The idle mixture screws sets the mixture of fuel and air during idle RPM.

Single barrel carburetors will have one idle mixture screw while 2 barrel and 4 barrel usually have 2 idle mixture screws.
You can clean your idle mixture screws by buffing them off with a wire wheel.
Inspect the screw for grooves. Grooves are created when the screw is turned in too tight. Replace any damaged or bent screw.
Inspect the screw hole to make sure it is clear. When you blow through the hole you should get air inside the bore.

For Rochester Quadrajets pick up a idle mixture adjusting tool at most any part store. The tool bends which will make adjusting easier.

When assembling the carburetor turn the idle mixture screws in all the way, gently seat, then turn it out about 1 1/2 turns.

Bring the engine up to operating temperature.
Make sure the choke valve is completely open.
You may have to rev the engine slightly so that the fast idle cam moves to the idle position.
Adjust the idle to specification.

There are a couple of ways to adjust the idle mixture.

1. Using a vacuum meter
Hook the vacuum meter to one of the vacuum ports on the intake, or the carburetor.
Take turns with each idle mixture screw.
Turn each screw out a bit for a start (maybe 1 turn).
Turn each screw in 1/4 of a turn and wait for a second for the vacuum meter to catch up.
Do this until you get the smoothest idle and the vacuum meter stays steady.

2. By ear
Take turns with each idle mixture screw.
Turn the screw out 1 turn to start.
Turn the screw in 1/4 and wait for a second for the engine to catch up.
Keep doing this until the RPM starts to drop.
Turn the screw back 1/4 – 1/2 turn.

Rochester 2 Jet Fuel Percolation

Throttle Body Venting

I ran across some information about some of the 2 jets that deals with the percolation of gas after shutting off the vehicle. We hear about this problem a lot these days because gas now has a lower boiling point.

Percolation means that the gas is boiling resulting in a very strong fuel mixture. This can make a hard to start situation.

I’m not sure if this was done on every 2 Jet, but it wouldn’t be hard to figure out if you have this feature just by looking at the throttle body (float bowl side).

The purpose of throttle body venting is to give quicker hot engine starting after the engine had been shut down for a short period.

During extreme hot engine operation the fuel in the carburetor tends to boil and vaporize due to engine heat. I said extreme, but gas now has a lower boiling point and it doesn’t take a lot of heat for percolation to happen. Some of the fuel vapor tends to reach the carburetor bores and condense on the throttle valves and seep into the engine manifold. By venting the area just above the throttle valves, hot engine starting time can be reduced to a minimum, on applications where the carburetor is exposed to extreme engine heat.

 throttle vent

There are 2 methods used in venting the throttle bore area.

1. A special throttle body to bowl gasket is used. See figure A. This gasket has cut-out areas which vent fuel vapors from the carburetor bores just above the throttle valves.

2. The other type of venting is accomplished by drilled holes through the throttle body casting just above the throttle valves. See figure B. They serve the same purpose as the vented gasket.

The location of the vent holes are such that they will not disrupt engine idle or off idle operation. They are located above the throttle valves on the side opposite the mixture screws, in an area where the transfer from idle to main metering will not be affected.

Now don’t go out and cut holes to create the vent holes if you don’t now have them. That isn’t going to work. You will most likely create a vacuum leak.

Car Stalls When Putting in Gear

I was recently asked about a Thunderbird that dies when putting the transmission in drive.

This could be caused by a vacuum leak, or possibly it is starving for fuel. Knowing that this particular vehicle has a multitude of vacuum hoses going to the carburetor, I would go with a vacuum leak 1st.

Disconnect all of the vacuum lines from the carburetor and plug off the vacuum ports on the carburetor. If the problem goes away you know it’s a vacuum leak causing the problem. Connect the hoses, one at a time until the problem returns. Obviously if the problem returns when connecting one of the lines, you have found your problem. The hose may have a hole, or something it connects to is leaking.

The carburetor itself could be leaking vacuum. You can spray carburetor cleaner around the mounting plate and the throttle body. If the idle changes, or smooths out, then you found the problem.

If you rebuilt the carburetor check to make sure you installed all of the gaskets correctly. The wrong gasket could leave a passage open to air causing a vacuum leak.

If it is starving for fuel, then you have all sorts of things to look for. 1st, if the carburetor hasn’t been rebuilt, then it may just be dirty, clogging up a passage. The float valve could be sticking, not allowing enough fuel to flow in.

The fuel pump pressure could be too low. Test the fuel pump pressure with a fuel pump pressure tester. On a Thunderbird it is probably around 5-7 lbs, but always check your motors manual for the correct specification.

The float valve could be sticking closed not allowing enough fuel to enter.

The float could be adjusted incorrectly. Check the float level.

I’m sure there are several things I haven’t even thought about. Let me know if you have any suggestions. I would appreciate it.

Visit our technical section for more carburetor help.

Testing the Holley 94 Accelerator Circuit

When I’m rebuilding my carburetors, I will usually test the accelerator pump circuit before closing up the float bowl. The Holley 94 carburetor uses a check ball in the accelerator pump well and I want to make sure it doesn’t need seating.

1st, you need to know why the check ball is there. The accelerator pump well has two holes. One is used for the fuel intake (from the float bowl) and the other is for sending fuel to the main discharge and into the carburetor venturi. When the accelerator pump is on the up stroke (foot off the gas), fuel is pushed into the well via the check ball hole. The weight of the fuel entering the well will lift the check ball off of the hole, allowing fuel into the accelerator pump well. Now when you press on the gas pedal, we want the fuel to pump out to the main discharge and not back to where it came from, which would cause a hesitation from lack of fuel. That is the job of the check ball. It closes off the inlet hole keeping the fuel from back pedaling. When the check ball doesn’t seat into the hole completely fuel may bypass the check ball. We will test for that problem as well as some other things.


I use mineral spirits when I test because it doesn’t deteriorate rubber like gas and ethanol does, so the carburetor can sit on the shelf for awhile if I need it to. It also doesn’t stink up my shop like gas does. At this point the main jets are installed and the drain plugs are installed, nothing else. Using a turkey baster, I add enough to the float bowl to fill it about 1/4 full and I squirt some in the accelerator pump well.

For the 1st test I force the accelerator pump down the well and I watch to make sure some fluid comes out of the main discharge. That eliminates the possibility of the tube to the discharge being plugged.

The next thing I do is put my finger over the main discharge hole and again I force the accelerator pump down the well. This time I want to make sure I get a little pressure against the pump and that the fluid doesn’t escape past the check ball and back into the float bowl. If it does, I seat the check ball by tapping on a brass drift punch sitting on top of the check ball.

For the last test, I insert the check weight into the main discharge and using a brass drift punch, I hold the weight down. I again force the accelerator pump down the well to make sure fluid doesn’t come out of the discharge hole. The check weight should shut the hole off. The same tapping using a brass punch can be used here also.

If all of these tests pass, I can feel good knowing that I’m not going to have any accelerator pump problems once the carburetor is out the door.

This same procedure is illustrated in this video.

Holley 94 Carburetor Kit


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

Rochester B, BC 1 Barrel Carburetor Identification

Rochester B Carburetor

This carburetor is one of the hardest carburetors to identify when it comes to purchasing rebuild kits. Fortunately there are ways to find the original carburetor number, or worst case and with a little work, you can at least get the correct carburetor kit.

Identification. Rochester used a tag attached to one of the float bowl screws.

Some of the B type carburetors also had the number stamped on the face of the base (flange).

Some Rochester B carburetor were produced without tags. On these carburetors the last 4 digits were stamped on the bottom of the throttle body. The carburetor number was also stamped on part of the air horn gasket, but the chances of the original gasket being found is pretty low. Add 700 to the 4 digits.

When all else fails you can figure out which carburetor kit to buy by inspecting the accelerator pump. The older Rochester B carburetor used a flat stem pump and and is found here. The later Rochester B carburetor used a round stem pump and is found here.

Carter WCFB Carburetor Float Circuit

WCFB Float Circuit

The purpose of the float circuit  is to maintain an adequate supply of fuel at the proper level in the bowls for use by the low speed, high  speed, pump and choke circuits. Primary and secondary bowls are separated by a partition. The fuel line connections is above the secondary needle and seat. Fuel is supplied to the primary needle and seat through the passage in the bowl cover.
Setting the floats to specifications assures an adequate supply of fuel in the bowls for all operating conditions. Float adjustments must be made vertically (specified distance between bowl cover and bottom of floats) and laterally (sides of floats should clear the arms of gauge). Having the float bowl gasket on or off will be dictated by the carburetor kit instructions. Correct lateral adjustment is important. If the floats are misaligned, they may bind or drag against the inner walls of the bowl. Adjust by bending the float arms.
To avoid unnecessary bending, both floats should be reinstalled in their original positions and then adjusted.
The bowls are vented to the inside of the air horn and on certain models also to the atmosphere. Bowl vents are calibrated to provide proper air pressure above the fuel at all times. To assure a positive seal, always use a new bowl gasket when reassembling. An air leak at this point can result in poor mileage.

A connecting passage along the outside of the body effects a balance of the fuel levels and air pressures between the bowls.

Replacing Carburetor Expansion Plugs

Expansion plugs such as the one you might find on the choke housing will sometimes come loose, fall out, or perhaps you just needed to get inside for a good cleaning. The process explained here is for those types of passage ways that have no fuel such as the choke housing clean out plug.
Carburetor Expansion Plug

Remove dirt or burrs from the seat in the casting. Install the expansion plug with the    dome side outward.







Expansion Plug  Use a round flat end drift to expand the plug. Drive it inward until it is flat. If the plug is driven too far the plug will loosen and will need to be staked.








Expansion Plug This is a properly installed plug with the edge staked over.

Keep in mind that this type of plug is for those passage ways that do not have fuel flowing through it.

You can find clean out plug kits on Mike’s Carburetor Parts web site.