The Stovebolt Site Forums The Shop Area Engine Shop Rochester type B adjustment question

New to the type B. Finishing up a rebuild and need to get it adjusted correctly. Simple enough procedure on the web. I was going to put a vacuum gauge on it and adjust the idle speed screw for maximum vacuum. But in reading past posts on this carb, “Carbking” stated that was not correct as it lead to hard hot starts and poor fuel mileage. Question is: if Carbking is correct, how or what do you do to adjust this carb? Thanks.

That is how I have been doing it on my 1954 standard/original style equipment truck for a few decades.

I have never had a starting/running/accelerating problem.

Idle speed screw or idle mixture adjustment screw? They're two different animals.

Before I did anything I'd screw the mixture screw in and then back it out about 2 turns. Then I'd let the engine get to normal operating temp and make certain choke is fully open. Set idle speed screw between 500 and 550 rpm and then adjust idle mixture screw until the engine is running smoothly. I've tried this with a vacuum gauge and have never really felt it told me much one way or another.

Here is the technique that I have used for many years.
A real mechanic might cringe.

One minor change I now make on doing "vacuum timing" is to rotate the distributor about 4 degrees clockwise after getting the maximum vacuum reading.

The Book says to adjust idle screw to within specs, then adjust mixture screw to highest RPM. Back off the idle screw if the idle is above specs, then repeat the "highest RPM mixture screw" procedure.
Repeat this until your mixture screw adjustment results in the correct RPM spec.
Note: I have paraphrased the wording of The Book for ease of understanding (for me anyway).
Carl

tcleaderman has it just about right. Setting the timing first is the best way to go. His back set from "Max Vac" is also a good idea. Then set the mixture for max vac and you should be OK.The only issue I have had, hot starts or other wise. Is mine wants to stumble just off idle. After the max vacuum is set with the mix screw, I have to back it out(rich) about 1/5 to 3/4 turn. I think the power piston is a bit sticky with the crappy gas we have here in Utah. The local altitude is about 5000 feet so when it works it works anywhere I have gone. Hot starts can be an issue. 2 pumps on the pedal is all I have ever needed. We do have hot days(over 100F) so I cheated. Put an electric fan on with a switch. In town there is not quite enough airflow for short hops to cool down properly. 5 min with fan and it has. Then the fan is turned off and all is good. The fan is behind the grill and is run as a pusher. The last test I do is simple and kinda fun. We have a hill, about 1.5 miles long right in the neighborhood. I come up that hill at wide open. It will tell me right now if it is gonna ping. No ping? Stop at the top and add a little advance. Repeat and wait for some random pings. Turn it back just a bit and forget it. If it pings Turn it back until it stops. If I drive it daily(most of the time) no problems. Starts cold and works all day. If I let it sit(3 days or more) the check valve will bleed off and it takes a bit to start.Mine is a 235(2) all stock. in a 52 1/2 ton. So the stock fan is too low and I think that is why it won't quite cool the swamp heat on short trips. There is a Tech Tip that shows how to relocate the water pump. Using a 21 pump and a mount. That also lets the radiator set in the proper place. I can't remember, but others will who makes a "kit" with all you need. If you still have the 216 then no problems. The 235 was in the pile of rust I got as a project. So it got used.

There is a difference in setting for the highest vacuum versus setting for the highest RPM.

Historically, there were some low compression engines (not that the stovebolt is a high compression engine, but it better be better than 4 1/2 to 1 ) in the 'teens and 'twenties that DID have the idle set to the highest vacuum. Once a "rule of thumb" is established, one almost has to break the thumb to change the rule.

If one examines the throttle body on most modern (1935 or so and newer) carburetors, one will find two idle ports: (1) the round hole through which the idle mixture screw can often protrude called the lower idle port, and (2) the idle transfer or transition slot which is just above and just covered by the throttle plate.

What condition of the throttle body will always give the highest idle vacuum? The highest vacuum will be acquired when the throttle plate is completely closed. Opening the plate lowers the vacuum.

With the throttle plate completely closed, ALL of the idle mixture will flow through the lower idle port. Why is this important?

One of the tenets of Carburetion 101 is that there are three ways to increase fuel atomization: (1) add heat, (2) increase the air velocity, and (3) add additional fuel. Since the throttle plate is completely closed, there is virtually zero air velocity in the intake manifold; so ADDITIONAL FUEL MUST BE ADDED. Again, with the absence of air velocity, some of this additional fuel will "puddle" in the intake manifold. Now when the throttle is opened (to pull away from a stop sign), the increased air velocity in the manifold sweeps all of the puddles into the cylinders, and there is an instantaneous overrich condition which causes a hesitation.

But the fuel necessary to keep the engine running BEFORE fuel flows from the accelerator pump comes from the idle transfer slot. But there was no fuel flowing through the transfer slot because the throttle plate was closed. So right after the instantaneous overrich condition, there is now an instantaneous overlean condition before fuel flows from the accelerator pump, and the hesitation continues. We are talking milliseconds, but the hesitation is noticeable. And the owner will curse the "defective accelerator pump"

Note that this hesitation will occur ONLY when the engine comes off idle, and will occur even with a new accelerator pump. In fact, literally millions of good accelerator pumps have been replaced (at the owners expense) because of this simple adjustment.

Just to take this thought a bit further, look at the two and four barrels GM used beginning in the mid-1950's (compression was increasing). The idle was set with an "air screw", that basically was an adjustable vacuum leak!

A few thousandths opening of the throttle plate will cause the vacuum to be a bit lower than maximum, and will minimize or eliminate the off-idle hesitation.

Jon.

I hope I did not confuse/confound (or, cause concern to) anyone.

I was referring to "standard/original" stovebolt engines - 216/235/261 truck engines and carburetors (like the original poster referred-to).