How does a carb work?

[pflaming]

Been playing abit with the carb. It has a small pump which is activated by the foot pedal, and it has a vacuum advance, and it has a choke 'butterfly'. Now it seems to me that fuel must be drawn into the carb by something besides the foot pedal pump or one would have to keep pumping.

So...the foot pedal must determine the amount of fuel that is available and that determines the RMP's. BUT what then brings in the fuel / fumes / vapor? Since the small pump injects fuel, directly, it then is VERY important not to pump that pedal too often if one wants any kind of gas mileage, is that a fair observation?

Finally, when a carb is rebuilt, what is new: gaskets, jets, and cleaning?

[stevenelle]

Others on this forum can explain it better, but essentially, the air mixed with fuel is drawn from the carburtor into the manifold and then to the cylinder by engine vaccum. Vaccum is created when each piston is pulled downward during the intake stroke. The throttle (butterfly) valve at the bottom of the carburator determines how much air / fuel mixture is drawn down and thus the speed of the engine. The accelerator pump is merely used to get a bit more fuel into the system quicky for acceleration or to help start a cold engine. A rebuild kit is mostly a bunch of gaskets along with a few other parts such as the needle valve and maybe a new float. Rebuilding involves the careful disassembly of the carb and a thorough cleaning of every metal part, esepcially the ports. That's a layman explanation; others can explain with more detail and add things I've left off.

[Tony WestOZ]

Basically as air is drawn through the carbie it creates a low pressure atmosphere. Because of this low pressure, fuel in the fuel bowl which is at normal pressure is drawn into the flowing air by either the idle circuit or the main jet depending on the engine rpm.

When you put your foot on the pedal and the butterfly opens the air flow in the carb increases (resulting in lower air pressure) it starts to draw more fuel through the main jet.

This however can take a little time to happen so the pump is there to add extra fuel for initial increase in rpm.

[greg g]

You need to read up on the venturi effect as it applies to fluid dynamics, The fluid here being air. The foot pump you speaak of is a brief enrichining squirt that keeps the engine from bogging down while the air fule mix adjusts to the increas in air going in. There is also an intermediate pump inside the carb that reacts to different levels of vacuum in the intake manifold.

So the venturi in the carb ( a narrowing of the air inlet) speeds up the air flowing in and causes a low pressure area in the throat of the carb. The main jet in the bottom of the float bowl allows fuel to be drawn into the air stream in reaction to that low pressure area. The throttle plate determins the amount of air ad the air determins the amount of fuel. It is easiest to visualize this with an old perfume sprayer with the bulb feeding a tube which blows air across an opening, in the reservoir holding the perfume. The air flow pulls the perfume out and breaks it up into a mist.

The intermediat power valve in the carb controls a needle thar opens an auxiliary jet. Normally this needle is kept seated in the jet closing it. as you step on the gas you lower the vacuum in the intake maifold. There is apassage in the carb that feeds this vacuum to a piston, holding it down. When the vacuum drops a spring pulls the piston up bringing the needle with it

sliding it out of the jet and taking over the enrichment chore that the accelerator pump started. As the vacuum returns in the intake, the spring pulls the needle back down closing this jet. As the car runs down the highway it operated only on the main jet.

The idle screw adjusts the amout if air that mixes with the idle jet. This i located below the throttle plate, when the trottle opens the idle port is blocked and removed from the equation.

As far as a rebuild kit goes they contain, gaskets, new needle and seat, float bowl level gauge, maybe a couple check ball and retainers. So yes its basically a cleaning and instalation of new gaskets and adjustments of linkages. The Carter B1B is probably one of the easiest carbs there is to rebuild.

[TodFitch]

As I understand it, the accelerator pump is there because when the throttle plate is rapidly opened the manifold vacuum drops quickly. That causes fuel to condense out of the fuel/air mixture in the intake manifold. Without the extra gas from the accelerator pump the mixture would be too lean for a moment until the manifold liquid/vapor ratio re-stabilizes.

The reverse happens when you rapidly take your foot off the accelerator: The intake pressure falls rapidly and any liquid gas in the manifold or in the pores of the manifold casting vaporize and cause a rich mixture for a short period of time. This can cause back firing, especially if there is a leak on the exhaust manifold that lets in air to burn with the excess gas.

Of course I am not a fluidics engineer, so I probably have this all wrong.

[Tony WestOZ]

You need to read up on the venturi effect as it applies to fluid dynamics, The fluid here being air. The foot pump you speaak of is a brief enrichining squirt that keeps the engine from bogging down while the air fule mix adjusts to the increas in air going in. There is also an intermediate pump inside the carb that reacts to different levels of vacuum in the intake manifold.

So the venturi in the carb ( a narrowing of the air inlet) speeds up the air flowing in and causes a low pressure area in the throat of the carb. The main jet in the bottom of the float bowl allows fuel to be drawn into the air stream in reaction to that low pressure area. The throttle plate determins the amount of air ad the air determins the amount of fuel. It is easiest to visualize this with an old perfume sprayer with the bulb feeding a tube which blows air across an opening, in the reservoir holding the perfume. The air flow pulls the perfume out and breaks it up into a mist.

The intermediat power valve in the carb controls a needle thar opens an auxiliary jet. Normally this needle is kept seated in the jet closing it. as you step on the gas you lower the vacuum in the intake maifold. There is apassage in the carb that feeds this vacuum to a piston, holding it down. When the vacuum drops a spring pulls the piston up bringing the needle with it

sliding it out of the jet and taking over the enrichment chore that the accelerator pump started. As the vacuum returns in the intake, the spring pulls the needle back down closing this jet. As the car runs down the highway it operated only on the main jet.

The idle screw adjusts the amout if air that mixes with the idle jet. This i located below the throttle plate, when the trottle opens the idle port is blocked and removed from the equation.

As far as a rebuild kit goes they contain, gaskets, new needle and seat, float bowl level gauge, maybe a couple check ball and retainers. So yes its basically a cleaning and instalation of new gaskets and adjustments of linkages. The Carter B1B is probably one of the easiest carbs there is to rebuild.

Was not going to throw the bloke straight into fluid dynamics but a good description.

Though I don`t think its quite a power valve in the old B&B. More just a check valve.

[pflaming]

Reread the posts from a prior question. Always helpful.

I've got two identical carbs. I am going to CAREFULLY disassemble one, CLEAN it and see if I can get my engine going. Tried this afternoon, no luck, good spark at the plugs, timing light indicates the timing is correct, new points, new plugs, new wires, new coil, OLD CARB.

I have LOTS of GAS in the carb and the plugs are wet! Something should have happened, not even a backfire. Back fired several days ago with starting fluid.

This is a bit frustrating but I find it incredibly fascinating so I take the setbacks in stride and it will happen.

[austinsailor]

If your timing is 180 degrees off you'll have these symptoms.

To check without any equipment or special help, take your plugs out. Turn your motor over by hand, holding a thumb or finger over the #1 plug hole. When It's pushing air past your thumb it's going up on the compression stroke. When the air stops you'll be near top dead center. Pop the distributor cap and the roter should be pointing to the wire that went to #1 cylinder. If it's pointing the other way you're 180 out.

[pflaming]

Austin: rechecked, my head has a brass screw on cylinder #6. Put a long small screwdriver in that hole and that cylinder was up! Rotor is pointing at wire #1.

Disassembled one carb this evening. DIRTY!!! Didn't remove the ball down in the 'hole'. It is held in with a spring 'clip'. Maybe should do that tomorrow. Where is the second ball?

Gaskets are good, drying them out tonight and will 'plane' the top of the carb tomorrow for a tighter fit. Reread a bunch of threads on leaky carbs and float adjustments, now know more for tomorrow.

Here's hoping!.

[greg g]

Paul there should be 2 balls, they react to the movement of the accelerator pump. when you step on the gas pedal, it forces the pump bellows down pushing one ball out of the way and allowing the fule to be pushed into the carb. When you let off on the gas the bellows moves the other way sucking the other ball in and allowing fule from the float bowl to recharge the acc pump reservoir.

Go to the carb doctor, or the carburetor king websites they have explided views and step by step instructions for rebuildidng the carb.

http://oldcarmanualproject.com/manuals/Carbs/Carter/BB-downdraft/1939up/index.htm

Figure 32 and 33 32 is under the slotted screw 33 is under the clip or cage in the pump chamber.

Edited December 9, 2009 by greg g

[Jim Gaspard]

When # 1 or #6 are TDC, shouldn't rotor be pointing to 7 o'clock?

pull number one plug and bump the engine over with your thumb over the hole (disconnect the coil) when you fell compressing building against your thumb, rotate the engine over by hand till you reach TDC, rotor should be pointing at 7 oclock. Greg G

You can use plug wire #6 or #1 as they both will both strobe every second crankshaft revoloution. But if you want to really brighten the timing light connect the probe to the coil wire. This will cause the timimg light to read both #1 and #6 as well as the rest of the plugs. But the strobe will only indicate #1 and #6 but be much brighter. Don Coatney

The crankshaft goes around twice while the camshaft and distributor shaft go around once. So the first time around, No. 1 fires. The second time around, it's No. 6. Donald Smith

Number 1 and number 6 are both at top dead center at the same time. Don Coatney

1. You may still be 180 degrees off 2. If not then recheck to make sure your wires are in correct order.

[austinsailor]

>Austin: rechecked, my head has a brass screw on cylinder #6.

>Put a long small screwdriver in that hole and that cylinder was up!

>Rotor is pointing at wire #1.

What Jim is telling you is that both #1 and #6 are going to be at the top. The problem is, one is at the end of the compression stroke, one is at the end of exhaust stroke and you don't know which is which.

You could use your thumb over that hole and do the same thing, but then the rotor should point to the wire going to #6. It'll be hard to turn it over, though, unless you remove the rest of the plugs.

Gene

[pflaming]

Thanks to all. Will recheck everything and reassemble the carb. It was so dirty that has to be a contributing factor. I have a low compression engine but I have got it to backfire twice so it should run, even if poorly.

The carb pictures were helpful. Will NOT take out the rivits, in case they may not be reusable. All else I got correct.

[desoto1939]

I never remove the venturi tube. You need the specila carter tool to do this and since this is a very very thin brass tube you can bend the tube if you try to take it out with a pair of needle nose pliers.

If you can find a good old carb rebuilding specilaist they can put the carb in a specila ultrasonic cleaner and this gets to all of the inner opening and really cleans out the unit for you. I had this done on one of my older carbs and they got a great deal of junk out of the carb.

Also as a safety point put a plastic fuel filter on the line just before it goes into the carb. This way it helps catch any crud before going into the carb. This will help to keep the carb cleaner. Also you can see if you have crud in the gas line that might be coming from the tank this is cheap precautionary insurance.

rich Hartung

[pflaming]

When I 'tuned' the engine several weeks ago, my mechanic friend said the carb needed to be rebuilt. So I rebuilt it and installed it then had to do some catch up work for my very patient wife. I lifted a 7" x 7" x 86" beam on Thursday while cleaning an area. That beam weighed 175#. My back is still very, very tender. Got to be more careful.

Well this morning got back to the truck and I connected the gas line and pressed the starter: after a few revolutions, WOW, it started. Then I did some trimming on the carb, idle, rich/lean valve and it really runs smooth. I turned it off had a cup of coffee and went back to start it. It starts with no choke, no foot pedal, just turn on the key and press the starter button. I was really pleased.

[Tom Skinner]

Great! Good Work.

Now you are a Carburator Doctor - as some in the industry call themselves and charge exorbidant rates for services. L.O.L.

I hope your back feels better, next time use a helper, or let those timbers be where they may be.

[Don G 1947]

Greg,

Thanks for that great description of how the carb works. I knew the basics, but you did a great job of filling in some of the blanks.

Truly a Zen Master!!

Don

[greg g]

Thanks Don, now that you know the basics of a variable velocity, fixed jet carb do you want to learn about the workings of a Constant velocity, variable jet carb like an SU??

Different way to skin the same cat.....

[Don G 1947]

Well Greg, sounds interesting, but I will probably never use that information. The "intermediate power valve" was the mystery to me. Knew I needed it, but had no idea what it did or how it worked. Now I know.

When you know the why and how of an item it makes it a lot easier to trouble shoot and repair. Otherwise you are just doing it by the pictures and little things that could be important can escape you.

Don

[pflaming]

Question: since rebuilding my engine really runs smooth and I changed nothing else. Does a carb constantly fine tune as the engine's rpms change.

[1flipdog]

Why hurt your back??? Let your:) wife lift it next time.