Mike’s Carburetor


Welcome to Mike’s blog. I hope you find my articles helpful as well as informative. I welcome comments on any of my articles. You might also want to sign up for my email notification so that you are notified any time a new article is added.

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.

For all of your carburetor needs, please visit Mike’s Carburetor Parts.

Thank you for visiting


Mobile Responsive Web Store

Six months ago our statistics showed that only 1% of our web site users were using mobile devices to view the site. This includes devices like smart phones and ipads. At the time I decided it wasn’t enough to get excited about, but recently we noticed that number had gone up to 20%. That is a huge jump in 6 months and then a customer called me letting me know he was having problems finishing an order on his smart phone. He didn’t use a computer, but could get things done on a smart phone. That is just the opposite of how I thought things would go.

This was an eye opener for me and without hesitation, I got my development staff started on changing the site and we recently launched a new site that is responsive to mobile devices. This means the screen adjusts to whatever device is being used. A smart phone will see it one way and a desk top computer will see it another way. The old version was non responsive and displayed the screen one way, no matter what device was being used and wasn’t all that useable on a phone.

So, let the mobile devices come. We are ready.

Do you use your mobile device to use the internet? I would like to hear from you about how you use it.

Mike’s Carburetor Parts


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.

Carter AFB Flooding Problem

Gas is leaking out of the main throttle shaft, or gas is coming out of the top vents, or you get black smoke while idling. All of these are indications of too much gas, or flooding.

Here is a list of possible causes in no order of importance.

afb needle & seat
Most float valves (needle) have a black Viton rubber tip on the end. The Viton tip needle may have been damaged when installing. Be sure not to put pressure on the needle when adjusting the float. A damaged Viton tip will allow too much fuel to enter the float bowl. Sometimes wiping off the Viton with mineral spirits to take any residue off will help.
Did you forget the gasket that goes behind the seat. Also make sure the old gasket was completely removed. See #22 in the illustration.
Check for cracks around the seat area. This would allow the fuel to bypass the needle & seat, so the fuel would never get shut off.
The float may be leaking causing it to sink. Heat up some water just prior to boiling and immerse the float. There should be no bubbles.
Gently move the float up and down. You should not feel any resistance or catching. A worn float pin, or improper installation of the float might cause this. If there is a metal clip that attaches the float to the needle, be sure it pulls the needle straight out. Move the clip around until it does. The float could be pulling the needle at an angle and it might cause it not to seal.

Check the venturi gasketmain discharges to make sure they are sitting flat and there there is no old gasket residue left under the new gaskets. See number 32 in the illustration.

Ethanol will leave residue behind and the small orifices of the venturi are subject to clogging. Use thin wire to clean out the small passages. In the past we would have said not to do this because you might make the openings bigger. This is still a possibility, but there isn’t any other way to get the passage cleaned. Carburetor cleaner and air pressure will not remove ethanol residue, so the wire is necessary. Just be careful not to enlarge the openings.

The fuel pump could be putting out too much pressure. New pumps are especially suspect. Test your fuel pump and compare with the specification in your motors manual. It would be somewhere between 4 & 7.

That should cover most problems.

If you had an experience with a flooding carburetor, it would be nice to hear what your solution was.




The B&B or Ball & Ball Accelerator Circuit Gone Bad

checkballdischarge b&bpump check inlet





Here is the question. When I accelerate the engine bogs down, or dies.
Assuming the electrical system is OK, because the distributor advance will act the same way, so check that out also.
The first thing to do is turn the engine off and look down the bore of the carburetor. Pump the throttle and you should see a squirt of gas come out of the main discharge. This is located just under where the carburetor top mounts (see #28).
No squirt, or weak squirt indicates a blockage in the accelerator pump circuit. Remove the aluminum plug where the main discharge is located. See #27 in the illustration. This plug is included with our Carter B&B carburetor kits. Remove and clean the main discharge jet #28. Be careful removing this jet. They get stuck and will break easily and if it doesn’t come out easily, then leave it in and clean it out the best you can. Run thin wire through the jet to make sure it is clear.
Your carburetor may have an intake check ball located at the bottom of the accelerator pump well with a retainer wire holding it in the hole. If you don’t have the hole, then a check ball is not necessary here. The action of this part is as follows: When the pump returns to the top, gravity pushes fuel up through the bottom hole and lifts the check ball up, allowing the pump well to fill up with fuel. You can add some fluid into the float bowl and watch to see if this does happen. If your carburetor kit includes an aluminum check ball, then this is where it resides. If you can’t get fluid through this hole, then it is plugged up. Try blowing air through it. When accelerating this check ball will close keeping fuel from going back to the float bowl. Seat this check ball (only if not aluminum) by tapping on it with a brass drift punch.
In the bottom of the float bowl close to the accelerator pump well will be another check ball inside a small column. The column will have either a cap screw, or an aluminum plug covering the check ball (#29 & #30). This is the discharge check ball. When pressing down the pump, fuel will force this check ball off the seat allowing fuel to reach the main discharge.
Test the 2 check balls this way. Hold down the discharge check ball with a brass drift punch. With the pump well full of fluid press down on the pump. You should feel some resistance. If you suspect this ball to leak under this test, tap on the check ball with a brass drift punch to seat it. Now press down on the pump (full of fluid) and see if the fuel lifts up the discharge check ball.

How To Measure A Part


Measurement Toolsby John Klyzek

How do you tell someone what size something is?

What diameter is that hole?

How long is that part?

What we are really doing is comparing the size of the item in question to some known or agreed upon standard.

What is the standard? Is there more than one kind?

In the old days a foot was the length of the Kings foot or a meter was a fraction of the distance from the north pole to the equator..

In the United States we use two basic systems commonly referred to as the English system or the Metric System.

The English system commonly uses the inch and foot  for measures of length.

The Metric system uses the meter and millimeter.

How long is a millimeter?  1/1000 of a meter. Mil refers to one thousandths of something.

For more precise measurements we divide a known standard length into smaller divisions.

Ever hear a mechanic or a machinist say “it’s bored 30 thou over” or “ bored 30 over”.

What does that mean?

Bored refers to a machining process called “boring”, but let’s concentrate on the “30 thou” part for now.

Thou is short for thousandths of something.

30 is short for the decimal equivalent of 30/1000 or .030

Most precision measurements in the USA are expressed in thousandths of an inch.

So when the guy says 30 thou in the USA he usually means .030 of an inch.

How big is that? Roughly 10 times the thickness of your hair.

One thou is 1/1000 or .001, 2 thou is 2/1000 or .002  and 500 thou is 500/1000 or .500 of something, could be an inch, meter or a potato.

Pick a unit of measure and tell me how many of those units it is.

Some of the time it is some number of units plus some fraction of a unit.

You could say the part is 2.500 inches or 2  1/2  inches long.

You could also express that same measurement in the Metric system.

How many millimeters is 2.5 inches?

Since there is 25.4 millimeters in an inch, the answer is 2.5 multiplied by 25.4, or 63.5  millimeters.

In the USA we are stuck using both systems for now.

When you express a dimension use the unit of measure that fits the situation.

If you talking to a farmer in Oklahoma about a cotton picker he is probably talking inches.

If you are talking to Europe or Asia they are probably talking Metric.

For clarity you can always say 2.500 inches or 2.50 mm or millimeters to be sure the recipient knows what unit of measure you mean.

Get your micrometer and calipers out and practice measuring something that has a known dimension, like a drill bit.

See if your measurement is accurate.

The accompanying chart is a shortcut method of comparing the same length, expressed 3 different ways.

This chart has fractions of an inch, decimal expressions of an inch and decimal expressions of a millimeter.

Let’s talk about 1/2  inch. Find that in the fractions and notice it’s also expressed as .500 of an inch in decimals and 12.7 mm in Metric.

All 3 of those are the same length just described different ways.

You need to drill a hole the size of a  linkage that goes in that hole.

I measure the linkage and find that it is .187 inch in diameter.

I only have a fractional drill bit set. What size drill will produce a .187 diameter hole?  3/16 of an inch is the correct answer.

This chart saves me time figuring out the correct drill.

There are dozens of kinds of charts like this and we will have them available on our website.


Fraction Decimal Millimeter Fraction Decimal Millimeter Fraction Decimal Millimeter
1 1.0000 25.4000 21/32 0.6562 16.6687 5/16 0.3125 7.9375
63/64 0.984 25.0031 41/64 0.6406 16.2719 19/64 0.2969 7.5406
31/32 0.9688 24.6062 5/8 0.6250 15.8750 9/32 0.2812 7.1437
61/64 0.9531 24.2094 39/64 0.6094 15.4781 17/64 0.2656 6.7469
15/16 0.9375 23.8125 19/32 0.5938 15.0813 1/4 0.2500 6.3500
59/64 0.9219 23.4156 37/64 0.5781 14.6844 15/64 0.2344 5.9531
29/32 0.9062 23.0187 9/16 0.5625 14.2875 7/32 0.2188 5.5563
57/64 0.8906 22.6219 35/64 0.5469 13.8906 13/64 0.2031 5.1594
7/8 0.8750 22.2250 17/32 0.5312 13.4937 3/16 0.1875 4.7625
55/64 0.8594 21.8281 33/64 0.5156 13.0969 11/64 0.1719 4.3656
27/32 0.8438 21.4312 1/2 0.5000 12.7000 5/32 0.1562 3.9688
53/64 0.8281 21.0344 31/64 0.4844 12.3031 9/64 0.1406 3.5719
13/16 0.8125 20.6375 15/32 0.4688 11.9062 1/8 0.1250 3.1750
51/64 0.7969 20.2406 29/64 0.4531 11.5094 7/64 0.1094 2.7781
25/32 0.7812 19.8438 7/16 0.4375 11.1125 3/32 0.0938 2.3812
49/64 0.7656 19.4469 27/64 0.4220 10.7188 5/64 0.0781 1.9844
3/4 0.7500 19.0500 13/32 0.4062 10.3187 1/16 0.0625 1.5875
47/64 0.7344 18.6531 25/64 0.3906 9.9219 3/64 0.0469 1.1906
23/32 0.7188 18.2563 3/8 0.3750 9.5250 1/32 0.0312 0.7937
45/64 0.7031 17.8594 23/64 0.3594 9.1281 1/64 0.0156 0.3969
11/16 0.6875 17.4625 11/32 0.3438 8.7312
43/64 0.6719 17.0656 21/64 0.3281 8.3344

Work on Carburetor While On Car

Not something I would do. Why?

Early in my career I was working on a Rochester Quadrajet on a V-8 Chevrolet Chevelle. I had rebuilt the carburetor but was having problems and I decided to save myself some time and take the top off of the carburetor and work on it while still on the car. Things worked OK until I dropped the check ball down the carburetor, which then rolled down the carburetor and into the intake. I wasn’t completely sure it actually ended up in the engine so I started the car. Ping! Ping! Ping!, I could hear the little check ball bouncing around. Check balls are stainless steal so they are pretty hard. Now what? Well, I had to remove the intake manifold and the head on one side of the engine. There the little devil was, embedded into one of the pistons.

I get the question asked about working on the carburetor while still on the car every so often and this story comes to mind every time.That was an expensive lesson I’ll never forget.

Bottom line. Always take the carburetor off of the car before taking it apart and when do other work around the carburetor, put a rag over the throat so nothing can drop inside.

Tri-Power Carburetors

Rochester Tri-Power carburetor kits. These are the 2G and 2GC carburetors. The 2GC is the carburetor in the middle and would have the automatic choke attached. The front and rear carburetor does not have a choke.Selecting kits for the tri-power carburetors is straight forward if you still have the tags attached to the top of the carburetor. Rochester did not stamp the carburetor number in the side of the carburetor until 1968.T ri-power carburetors use two different carburetor kits. One kit for the center and the other kit for the front and back carburetors. Each kit services one carburetor, so to do all three carburetors you will need three kits.Be careful because chances are you may not have the original carburetors. It is rare that we see a tri-power setup that has the correct carburetors (numbers matching). Here is an example of a tri-power setup. Notice that the front an rear
carburetors use the same carburetor kit, but the center carburetor uses a different kit.1959-61 Chevrolet V-8 A/T NO AIR

7013015 FRONT CARBURETOR Tri-Power Carburetor Kit K6121
7013017 REAR CARBURETOR Tri-Power Carburetor Kit K6121
7013016 CENTER CARBURETOR Tri-Power Carburetor Kit K636

What if you don’t have the carburetor tags?

You have to make the best guess and the suggested way is to take the carburetor apart and carefully compare the old parts with the parts we have illustrated in the photos, or videos.

Here are the typical carburetor kits used on the Chevrolet carburetors (prior to 1968)






Holley 1904 Problems

Here is a question I received from a customer recently.

I rebuilt my 1904 using kit #k4043, got new mixture screw and pump link. not having much carb experience I think I have some problems. I’ll list what I think is wrong.Will idle only if choke partially closed, did it before rebuild too Fuel dumping out of main metering port onto choke plate while idling Inline glass filter always full, used to be half full Vacumm hole in base of throttle body blocked, but intersects with hole in the bore
Idle mixture screw is ineffective whether screwed in or out. There is no dashpot lever spring. Carb doesn’t mount directly on the manifold, but on what appears to be a stack of gaskets, but everything lines up linkage-wise. I did the fix for the warped body and now it’s flat. In your video you mention the plastic ball in the main body sometimes not being there and not making a difference, I put mine in. The only thing I didn’t replace were the balls for the main metering body because the balls and weight were moving very freely. Any help would be greatly appreciated. Joe

The fact that the engine will only idle with the choke partially closed means that there is too much air due to a vacuum leak somewhere, or that it isn’t getting enough fuel. Since the problem was there before the rebuild, I would start by looking for a vacuum leak.

Spray carburetor cleaner around connections and if the idle smooths out, or the RPM changes, then you have located the leak. I would start with the non standard stack of gaskets. After that check all other connections such as the intake manifold. If the carburetor will mount with the standard 1 gasket, then I would take the other gaskets off.

Fuel dumping means that you are getting too much fuel in the float bowl. Did you seal around the inlet bolt with the Permatex anaerobic. If not, check out my instructions on how to  do that. Did you recently replace the fuel pump? A new fuel pump might have too much pressure. Test the fuel pump pressure and compare to the specification in your motor manual. I’m guessing somewhere around 5 – 6 lbs.

The hole in the base of the throttle body leads up to the vacuum port at the top of the carburetor. This is used for the distributor advance and needs to be open all the way to the intake vacuum. Having this covered up will cause problems. If you have to, make a hole in the gasket.

The missing spring from the dashpot is not the problem. The dashpot is used on automatic transmission type of cars and it holds the throttle open for a second or so when you let up on the gas, allowing for any left over gas mixture to be burned.

Weber Marine 4 Barrel Carburetor – Accelerator Pump Test


Weber Marine Accelerator Pump

 Weber Carburetor Main DischargeOne of the most common problems on almost any carburetor is when the accelerator pump circuit fails to deliver the correct amount of fuel. The accelerator circuit works, you guessed it, when accelerating. Throttling up requires an extra shot of fuel and the accelerator pump forces that extra fuel up through the main discharge and out the venturi, mixing with the fuel already being delivered to the engine.

(a) illustrates the main discharge hole. This hole will either have a ball and weight, or a ball and spring. Replace with the same combination. Check ball 1st, then either the spring, or the weight.

There are several things to think about when rebuilding your carburetor, especially around the accelerator pump circuit. Make sure the pump well is polished so that the pump cup glides smoothly and clean out all of the small passages in the venturi. Using thin wire is about the only way, since cleaners and compressed air are not going to remove all of the ethanol deposits left behind.

With everything clean and you are ready to assemble the carburetor, it is a good idea to test the accelerator pump circuit before closing up the carburetor.

Test your accelerator pump circuit as follows:

  • Fill the float bowl with liquid. We use mineral spirits.
  • The accelerator pump well should fill up with liquid.
  • Insert the pump and press down. Liquid should come out of the main discharge.
  • Replace the check ball.
  • Hold the check ball down with a brass drift punch (gently).
  • Again, press down on the pump. You should feel a slight resistance since the check ball is not allowing liquid up through the discharge hole. If no resistance, then gently tap the check ball and the drift punch in order to seat the check ball.
  • Install the venturi cluster and gasket.
  • Press down on the pump again. You should get a good squirt out of the venturi. If not, then the passages are plugged. Use thin wire to clean out the passages.

With the test above completed you can feel confident that the accelerator pump circuit is going to perform as it should.