The Stovebolt Site Forums The Shop Area Engine Shop Dual carb & Vacuum question on a 235

After having nothing but problems with my Rochester, I found a problem with my float getting stuck, after getting that sorted it's no longer leaking and running much better but still has a hesitation or a flat spot off idle. I have a 55' 235 with a Patricks mid cam, "848" head, Offy intake with two rochester bs. Solid core wires, #56 jets in each carb, fuel screws at 1-1/2 turns. My question is would having the vacuum for the distributor running to one carb and the other carb vacuum port blocked of affect the running condition? I can find no other causes to my flat spot...
Thanks!

See if the flat spot goes away with the choke partially closed. This might tell you if the carb is running too lean (not from the idle mixture screw, but from the jets).
Carl

No, that won't effect the running condition

A couple ideas;

Are you sure the vacuum advance is working? When you open the throttle do you see the distributor turn?

Accelerator pump(s) not working correctly. When you are at partial throttle and nail the peddle do you have a hesitation also?
If not it could be that there is too much wear/slack in the linkages to the accelerator pumps.

Watch the linkage while you open the throttle by hand, does the plunger move with the slightest opening or do you have it off idle before it starts to move.

Take the air filters off, engine not running and look in each carb. How far do you have to move the throttle plate before you see gas squirting in from the accelerator pumps.

If all else fails, richen the idle mixtures and see if that helps. Close each screw 1/4 turn and try it. If not fixed, another 1/4 and adjust idle speed and try again.

See this critter?

http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0663.htm

Item #9 in this picture. It must be free and smooth in its operation.
This is the part that will cause a hesitation just off idle under mild acceleration. It needs a vacuum signal to pull it off the seat and add fuel at a tipping point. That gives a slightly rich mixture for just a moment and keeps the hesitation away at the transition from idle to run. You now have two of these and...much less vacuum to move each one. One of the many joys of dual cab set up. It will work, but these must be free and smooth in the bore. Good news? They are in the cover and can be accessed easily. I have put them in my drill and polished with 600 grit paper to make certain they are smooth and free. Be sure they are straight and move easily when installed. Should help with that transition.

Good call Steve, I forgot about Rochester having the power valve.

I usually use W-1's for dual or tri setups.

We have solved MANY "hesitation from idle" issues by REDUCING the idle fuel (turning the mixture screw in).

Often, enthusiasts use a vacuum gauge, or set for the "fattest" idle which is too rich; and causes puddling in the intake. Cracking the throttle increases the air velocity in the intake, sweeping all the "puddles" or droplets into the cylinders creating an instantaneous over RICH condition and hesitation.

One screw at 1 1/2 turns is generally too much unless the engine has a radical cam.

Two screws at 1 1/2 turns is more than one screw at 1 1/2 turns.

Jon.

Jon,

Is is a bad practice to adjust the mixture to achieve maximum vacuum at idle?

It can be.

The vacuum gauge is a tool, and it definately has a purpose.

However, in virtually all cases, the highest vacuum will be obtained when the throttle plate(s) is(are) closed; as vacuum is measured below the plate(s). Depending on the design of the idle/off idle circuits of the specific carburetor in question; the highest vacuum setting may lead to a hesitation in the transition from the idle to the main circuit(s).

Jon.

The highest vacuum at idle-speed might be obtained with the throttle plate closed?

What about with a standard/stock Stovebolt (216/235/261) Rochester?
Or, dual W-1s?

Technically, the highest vacuum would occur when the throttle is closed at speed, but that is a transient value.

On most carbs won't allow an engine to idle with the butterfly totally closed because there isn't enough air flow to keep the engine running. A few designs have been built with complete idle bypass circuits.

At WOT there is virtually no vacuum.

Technically, the highest vacuum would occur when the throttle is closed at speed, but that is a transient value.

On most carbs won't allow an engine to idle with the butterfly totally closed because there isn't enough air flow to keep the engine running. A few designs have been built with complete idle bypass circuits.

At WOT there is virtually no vacuum.

With the W-1's, I would not set for highest vacuum.

I wouldn't use the Rochesters, period, so no suggestion if you do.

Jon.

Some of us use Rochesters for authenticity. We appreciate any helpful suggestions and information about them.

Jim,

That is what I thought regarding measuring vacuum with a closed throttle (on a standard 216/235/261 carburetor/engine).

Thanks,

TC - a well broken-in 216 or 235 will idle with the throttle plate closed with either a properly restored Rochester or the W-1.

As I have never personally adjusted a 261, I cannot say.

It is rare today to find a workable Rochester, although they do exist. Issues include the well-known warpage, but just as bad is the mixing of parts of different carburetors by the commercial rebuilders. If I were going to use one, I would not set for the highest vacuum.

Most believe that a hesitation off idle is always a faulty accelerator pump; when in fact, the off-idle or idle transition circuit (call it what you wish) on a carburetor with a properly adjusted idle actually provides fuel to the engine for a fraction of a second PRIOR to the fuel provided by the accelerator pump. As the lower edge of the off-idle slot would be covered by a closed throttle valve, no fuel would flow through this circuit, and the entire idle fuel/air mix would be forced through the lower idle port. As mentioned above, this causes "puddling".

As engine displayment grew, and camshaft duration became larger, less of a chance that the engine would idle with the plate completely closed. In fact, a longer camshaft duration would cause the throttle to be opened so wide that the vacuum signal at the idle ports would diminish (low air velocity past the port) to the point where some carburetors had factory drilled holes in the throttle plate(s). As this would add air to the idle, the throttle plate(s) could be adjusted closer to the throttle bore, thus increased air velocity resulting in a better vacuum signal in the idle ports.

There is really no rule that works in all cases.

GENERALLY, to set the idle on a relatively stock engine, I use the following procedure:

(1) look up the factory recommended "range" for idle mixture screw adjustment
(2) divide that range in thirds.

Where I set the idle mixture screw then depends on the condition of the engine.

(1) freshly rebuilt up to maybe 500 miles - 2/3 of the range
(2) engine with 500+ miles up to ??? but still with good compression and not burning oil - 1/3 of the range
(3) worn-out engine (low compression, and burning oil) - the upper range limit

Examples:

Consulting the manufacturers specifications for an original carb shows an adjustment range of 3/4 turn to 1 1/2 turn. Divide the difference by three gives 1/4 turn.

So 1/3 of the range distance would be 1 turn (3/4 initial + 1/4)

2/3 of the range distance would be 1 1/4 turn (3/4 initial + (2) x 1/4)

The full range adjustment would be 1 1/2 turn (factory spec).

So in my examples

Freshly rebuilt - 1 1/4 turns
Good engine broken in - 1 turn
Worn-out engine - 1 1/2 turns

Once this setting is complete (it takes a lot more time to explain than to do), I would run the engine at a high idle until normal temperature is obtained, and then set the curb idle RPM with the throttle positioner screw.

Finally, I would now use the vacuum gauge to verify that the idle vacuum is within the expected range for the engine tune (read camshaft duration) and the engine condition. If the vacuum falls within this range, road test the vehicle.

Note that adjusting the idle mixture screw BEYOND the manufacturers maximum generally will not change the A/F ratio.

Not saying this is the absolute best method; but I have been using this method successfully for over 50 years.

Jon.

Thanks very much, Jon

Since we are discussing the idle/off idle circuit, perhaps now is the time to discuss what some might call semantics:

Generally, changing the adjustment of the idle mixture screw does NOT change the idle mixture!!!

The idle circuit in many (virtually all of those with which I am familiar) carburetors is FIXED. The idle circuit generally consists of the following:

(1) the low speed jet (a.k.a. idle jet, idle tube, etc.) FUEL
(2) the idle bypass AIR
(3) the idle economiser (a.k.a. idle restrictor)
(4) the idle air bleed AIR
(5) the idle mixture screw (really should be called a volume screw)
(6) the ports (lower port and transition slot)

Without getting into atmospheric pressure (in the bowl) and negative pressure (vacuum) in the ports:

Fuel passes from the bowl through the low speed jet into a passage. Further along this passage, air is drawn into the fuel through the idle bypass, and mixing begins. The air fuel mixture is now directed through the idle economiser (restrictor). As the name implies, the idle restrictor is a necked-down diameter in the passage. As the dynamic volume is reduced, the velocity of the air fuel mixture increases, adding to the mixing of the air and fuel. Past the idle economiser, the passage diameter is again increased, and additional air is bled into the fuel via the idle air bleed. The idle mixture is now complete and it travels into the final passage to the ports. The first port encountered is the transition slot. IF the throttle plate is not completely closed, air passing by the throttle plate creates a negative pressure in this port, causing some mixture to be delivered into the throttle area. If the plate is closed, no mixture will pass though this port. The remaining mixture continues to the the idle mixture screw, where the position of the screw controls how much of the mixture is passed through the lower idle port.

I like to use the analogy of an old-fashioned shower with three controls (hot water faucet, cold water faucet, and volume). Think of the hot water faucet (think fuel) and the cold water faucet (think air) being controlled by the carburetor design engineer, and the volume or pressure control (idle mixture screw) as being controlled by the person taking the shower.

The user CANNOT change the hot water or cold water setting in the shower!

However, IF NECESSARY, the mixture in the carburetor MAY be changed by replacing either the low speed jet with a larger or smaller one or changing the value of the two air jets.

Jon.

Actually, the idle mixture screw controls a highly aerated emulsified mixture of air and fuel that is then mixed with the air going past the throttle plate; hence it does in fact control the air/fuel mixture going into the engine at idle. In tech terms, the idle screw port is a variable discharge restriction that is adjusted by the engine tuner to achieve the desired A/F ratio at engine idle.

If I read all that right, the air to fuel ratio is set by the carburetor (engineers who designed it), and the idle mixture screw varies how much of that mixture is let into the engine.

I always thought it was adding fuel only. Now I can understand how it can create a hesitation, since backing mine off (closing them) the truck doesn't smell like fuel when shutting it off in the garage.

Thanks, Jon

Joe

One thing I might point out, that I neglected to mention earlier:

Many UPDRAFT carburetors and a very few downdraft carburetors have an idle mixture screw that is actually the final idle air bleed (the mixture screw actually meters air). In this case the mixture screw truly does alter the mixture.

Since this type of idle circuit predates the more modern circuit described previously, this is probably why the screw is labeled a mixture screw rather than a volume screw.

Jon.

It is a mixture screw because it controls the amount of air/fuel mixture going into the throttle bore; simple as that.

Many other carbs use an "idle air screw" also, particularly in two stroke engines.

So back to the question, how is the truck running know?