Carb tuning/building tricks from old timers?

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GriffinGuru
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Carb tuning/building tricks from old timers?

Unread post by GriffinGuru » Fri May 19, 2017 8:07 am

Hi everyone. I want to start a thread where any of those more experienced in the carb systems may share some old school tricks to pass along for ways of setting up and tuning carbs.

For example, one trick I was taught was after setting the float, you can double check where the needle will actually stop in case there might be some discrepancies between how the needle seats while upside down, vs. how it may seat in normal orientation. After setting the float height, you can double check this holding the float in normal orientation by blowing (with your mouth) into the fuel inlet and move the float until the air stops. Then double check if this is still the same height you need it set at. My experience with this technique though, is that you need to have the fuel filter out or it is hard to blow through, thus this is not something I have tried a second time!! :td: Wetting the needle with your tongue for this test also apparently helps.

This is one example I was taught, but I want to know if anyone has any other techniques they would like to share. How do I fine tune the idle mixture screw?, is there a way to test accelerator pump function before running it on the car and looking for the squirt?

anyone got any other tricks of the trade they use?
It keeps me humble:
64 Corvair Monza convertible called Lucy (work in progress)

66vairguy
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Re: Carb tuning/building tricks from old timers?

Unread post by 66vairguy » Fri May 19, 2017 9:10 am

No tricks needed. Just follow time proven procedures

Fuel pressure DOES make a difference on how high the fuel level will go in the bowl before the float develops enough pressure to close the valve. There have been a number of issues reported with the new mechanical fuel pumps have a 9 PSI fuel pressure - spec. is 4-5 PSI. Those that use electric fuel pumps sometimes use the wrong one with excess pressure.

Bob Helt's book "How to identify and Rebuild Corvair Rochester Carburetors" does a good job of explaining things and covering the many changes the carburetors went through. His book recommends lowering the fuel bowl level (increasing the distance from the gasket to the float) in the bowl 1/16" to prevent "dribble" out the venturi cluster at idle, not common, but I've seen the issue a few carburetors - dropping the fuel bowl level per Bob's suggestion fixed the issue.

For a while the myth circulated that tipped float needles have an issue with today's blended gasoline. Maybe years ago a few cheap kits had an issue, but I've never had a new tipped needle cause an issue except once when some debris got into the needle assembly. A ball bearing float is sold through Clark's, but over the years folks reported problems with them - recently a post revealed the check ball assembly had burrs (from machining) causing intermittent binding and carburetor overflow.

Check to make sure you have the correct float - there are two different types (in Bob's book) and the floats are fragile so make sure they are level and aligned so they don't hit the bowl walls.

When replacing the throttle shafts makes sure the throttle plate is centered (it can shift when tightening the screws) by using a bright light to check for gaps.

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GriffinGuru
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Re: Carb tuning/building tricks from old timers?

Unread post by GriffinGuru » Sat May 20, 2017 6:20 am

I just went through an issue where I eventually determined I had a bad accelerator pump plunger. I have a stumble on throttle acceleration, but was otherwise OK if you were gentle on giving throttle. When looking at the carbs running on the car, I was only getting a squirt out of one side.

I took the carb off and apart on the side with no squirt multiple times. Everything looked Ok, the accel plunger had no defects, and even when I worked the carb on the bench with fuel in the bowl it would squirt. Only when I put the same carb back on the car and ran it, was there was no squirt- only a least pitiful dribble.

I checked all passages, float bowl level, brass accelerator needle etc, and there were no problems or clogs. :dontknow: Only when I swapped accel plungers between left and right sides and leaving everything else the same in the carbs, did my no squirt issue then move to the other side- only when I bolted the carbs back on and watched with the car running. That proved there is an issue with the one accelerator plunger, but I could not see any flaws. I ordered two more accel plungers (so I have a spare) from Clarks and I should get them any day and fix this thing for good now.

That is the gist of my post: Not about setting up the carb per other specs than is called for, but if anyone has any techniques and ways to double check that everything is alright on the bench to avoid going through some trial and error.
It keeps me humble:
64 Corvair Monza convertible called Lucy (work in progress)

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bbodie52
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Re: Carb tuning/building tricks from old timers?

Unread post by bbodie52 » Sat May 20, 2017 7:23 am

Always check the floats to make sure they have not developed a small leak and have gasoline inside the floats.

Clark's Corvair Parts offers a replacement for the needle and seat that makes use of a ball in an all-metal assembly. It has many advantages over the standard needle and seat found in carburetor rebuild kits, as shown below...

:link: http://www.corvair.com/user-cgi/catalog ... ow_page=58
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Part number C3799: METAL FUEL INLET VALVE-60-69 FITS CARS & TURBOS

Weight: 0 lbs 2 oz
Catalog Pages(s): 48(10),55(12),58
Price:
1 - 3 $ 9.00
4+ $ 8.10

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Don't try to remove the throttle butterfly valve or the choke butterfly valve if you don't have to. I've seen instances where the carburetor rebuilder busily tries to remove every components that is held in place with a clip or a screw, in preparation for dipping the metal carburetor body and metal components in carburetor parts solution to clean them. The throttle and choke butterfly valves do not need to be removed to clean the carburetor bodies. The screws that hold the choke and throttle butterfly valves in place are soft brass screws that are staked at the factory to prevent the screws from vibrating loose. The special screws are easily stripped or damaged if you try to remove them without first grinding the staked (peened) area of the screw threads to release the screws so they can be removed. Once removed, it is usually desirable to install new screws upon reassembly, and new screws must be staked (peened) in-place to prevent the screws from vibrating loose and being sucked into the engine intake.

In one instance a first-time carburetor rebuilder stripped the screw head of the brass screw while attempting to remove it. He used a drill to drill the screw out since he could not remove it with a screwdriver. As a result he destroyed the carburetor throttle shaft, since it was no longer threaded to allow it to be reassembled using new screws to secure the throttle butterfly valve!

Part number C1835: CHOKE PLATE SCREW-EA -ALSO FOR THROTTLE* *PLATE

Weight: 0 lbs 0.5 oz
Catalog Pages(s): 55(23),56(36),183
Price:
1 - 9 $ 0.70
10 - 19 $ 0.55
20+ $ 0.45

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:link: http://www.corvair.com/user-cgi/catalog ... ow_page=56
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Carburetor rebuild kits often contain a thin gasket to seal the underside of the carburetor where it mounts on the intake manifold. The Corvair carburetors need a plastic insulator to isolate the base of the carburetor from the intake manifold, which prevents the gasoline inside the carburetor from boiling or vaporizing if excessive heat is allowed to transfer from the hot aluminum intake manifold on the cylinder head to the carburetor body. These plastic insulators can be easily damaged when removed. Clark's Corvair Parts bundles gaskets and insulators together. They are listed near the top of page 56 in the catalog (see the page above).
Carb Insulator Note.jpg
Carburetor Base Insulator and Gaskets.jpg
Carburetor Base Insulator and Gaskets.jpg (56.62 KiB) Viewed 272 times
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bbodie52 wrote:
flat6_musik wrote:...I bought a Uni-Syn and only used it once. It seemed to me that at idle, there was barely enough air flow to raise the ball. It pissed me off and I put it away...
The Uni-Syn Carburetor Balancing Instrument (listed on the Clark's Corvair Parts Catalog, page 58), is a tool that I remember as far back as the 1950s and 1960s. It was designed for use in multi-carburetor automobiles, motorcycles, etc. that were initially common in European vehicles, but were later found in American vehicles like the Corvair.
Image :confused:
:idea: The Uni-Syn is an airflow gauge that must be calibrated to match the specific engine airflow moving through the carburetor throat. The flow control in the center of the Uni-Syn is mounted on a threaded pin. Rotating the flow control raises or lowers it, which increases/decreases the air gap, which impacts the air velocity passing the orifice that allows some airflow to pass through the gauge. With the Uni-Syn held firmly on the carburetor intake with the engine idling, the flow control is adjusted so that the plastic float is approximately centered in the glass sight tube. Once calibrated, the Uni-Syn can be moved back and forth between the two primary carburetors to compare airflow at idle. The goal is to play with the idle speed screws to get an even airflow level that matches between both carburetors, and at the same time produces the desired idle speed. Before fine-tuning this idle balance with a Uni-Syn, the ignition system, idle speed and idle mixture should have been adjusted following the standard procedures as described in the factory shop manual. The balancing step with the Uni-Syn gauge is added at the end of the tuning procedure to measure actual airflow though both carburetors at idle, and balancing that airflow using the airflow meter as a measuring tool, rather than simply relying on a physical balanced "calibration" using a strip of paper to detect initial contact between the idle speed adjustment screws and the carburetor linkage, followed by counting the screw turns needed to obtain the desired idle speed (and ensuring that the same number of screw turns are applied to each screw). That procedure achieves a fairly close initial setup. The use of the Uni-Syn airflow gauge as a final step ensures balance between the two carburetors by measuring actual airflow.

:wrench: Once the balanced idle airflow and desired idle speed have been achieved, a similar procedure can be used to synchronize the airflow when the carburetors are held open at a faster engine speed by the throttle linkage. Again, the shop manual procedure can be used to set the initial mechanical balance between the two sides of the throttle linkage. The linkage segment connecting the accelerator pedal to the cross-linkage between the two carburetors is temporarily disconnected, and a turnbuckle is temporarily attached to hold the throttle opened against the pull of the throttle return spring.
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The turnbuckle can be adjusted to hold the throttle open at approximately 1500 RPM. The Uni-Syn center flow control is readjusted to re-calibrate the float to a position somewhere in the center travel of the sight tube, based on the increased airflow through the carburetors at the higher steady RPM maintained by the turnbuckle arrangement. As with the idle synchronization procedure, the re-calibrated Uni-Syn is used to measure the airflow moving through the two carburetors. The goal is to fine-tune the threaded portion of the carburetor actuation linkage (left primary carburetor) so that the same airflow reading is attained on each carburetor, but this time with the throttles held open by a pull on the throttle linkage, instead of by the setting of the idle speed screws. When the airflow has been balanced using the throttle linkage adjustment, the balancing procedure has been completed and the normal accelerator pedal throttle linkage can be reconnected.

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TIPS & CLARIFICATION...

With the engine at idle, you want to open the Uni-Syn flow control as much as possible, but still keep the float in the sight glass at about mid-level. Then check the other carburetor, which you want to read the same flow rate. You may have to adjust the flow control a few times as you adjust the carburetor settings. Just remember to check each carburetor with the flow control set at the same point, and to keep the sight glass in the vertical to prevent the float from hanging up in the tube.

The wheel in the Uni-Syn venturi controls the flow through the tube, or in other words how high the bead is in the cylinder for a given engine speed. You need enough flow to not strangle the engine, and the bead works just as well in the lower third (which still lets good air flow through). Make sure the idle speed doesn't drop when you place the tool on the carb: if it does, open it up.

Before setting the carbs, make sure that the ignition is right: points and timing set, good wires and plugs. Rich running is often blamed on carbs when in fact it's a weak, retarded spark. Check the throttle shafts: loose ones let in air and lean the mixture, raising idle speed, as well as throw off the linkage action. Check the linkage that connects the carburetors. if it's loose, one will open before the other. Make sure there are no vacuum leaks.

With a twin carburetor set up, disconnect the linkage that connects one carburetor to the other. Set the idle speed with the linkage screws first. Use the Uni-Syn to check that each carburetor is drawing equally at idle. This may take a few tries until you get both drawing equally at the speed you want. Blip the throttle to see if they come back to those settings (worn throttle shafts can fool you). Set the idle mixture screws in accordance with the shop manual instructions. Check the balance again. Hook the linkage back up. If one carburetor now draws more, adjust the linkage until it's back to roughly equal.

Once the carburetors are drawing equally at idle, hold an engine speed: at about 1500 RPM. This checks that the mechanical linkage is pulling equally. You'll need to open up the Uni-Syn center wheel to draw more air and bring the bead down in the tube. If both carburetors are within a bead's thickness of each other, that's good. If one is definitely off from the other, the higher flow carburetor's linkage is being pulled more than the other. You'll need to figure out why that's happening mechanically to rectify it. :chevy:
UNI-SYN Carburetor Balancing Instrument Instructions.pdf
UNI-SYN Carburetor Balancing Instrument Instructions
(56.72 KiB) Downloaded 9 times
TECHNICAL REFERENCES

In addition to the factory shop manual and supplements, these are very good...
DELCO ROCHESTER - Models H, HV Carburetor Service Manual.pdf
DELCO ROCHESTER - Models H, HV Carburetor Service Manual
(1.79 MiB) Downloaded 13 times
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:link: http://www.corvair.com/user-cgi/catalog ... w_page=251
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Part number C3939: BOOK-CARBS BY BOB HELT

Weight: 1 lbs 0 oz
Catalog Pages(s): 251
Price: $ 24.95



:link: http://www.amazon.com/Identify-Rebuild- ... 355&sr=1-2
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Brad Bodie
Lake Chatuge, North Carolina
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bbodie52
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Re: Carb tuning/building tricks from old timers?

Unread post by bbodie52 » Sat May 20, 2017 1:30 pm

:chevy: STOCK MECHANICAL FUEL PUMP
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The stock Corvair mechanical fuel pump has proven itself in performance and reliability for decades through millions of Corvairs. There is a tendency to quickly abandon it and go running to an electric fuel pump replacement at the first sign of apparent trouble. Old age can certainly be a drawback, or weakness with this pump, but many continue to survive. There was certainly a period of poor quality aftermarket replacement pumps, and rebuild kits are no longer available. But if you begin having what appears to be carburetor problems, don't be too quick to condemn the pump.

There are two relatively simple tests outlined in the Corvair shop manual. These tests measure output pressure and fuel delivery volume. The output pressure test utilizes a common vacuum/pressure gauge and is relatively inexpensive. The output pressure is regulated by an internal spring that is contained in the upper pump housing. Unless your pump springs a leak in one of the diaphragms or seals, it is likely that the output pressure will not change with age. However, new pumps may be fitted with a spring that produces an output pressure well in excess of the 4-5 psi standard. High-pressure can cause carburetor flooding, and is particularly a problem with the Carter YH found on turbocharged Corvairs.

A new fuel pump should always be tested for output pressure. If the pressure is found to be excessive it may be possible to cut the spring or to exchange the spring from the old pump and fit it into the new pump. This may correct the output pressure. It is also possible to purchase an aftermarket fuel pressure regulator such as the one shown below.

Here is a link for ordering the recommended Holley fuel pressure regulator...

:link: http://www.jegs.com/i/Holley/510/12-804 ... 1797507184

Holley 12-804 - Holley Fuel Pressure Regulators — $30.41
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Holley#510-12-804

Standard Pressure Regulator
Chrome Finish
For use with gasoline
Port Size: 3/8'' NPT Inlet/Outlet
1 to 4 PSI
Includes Mounting Bracket

The second test measures fuel flow at the fuel pump outlet. The standard in the shop manual states that the pump should be able to deliver 1 pint of fuel over a period of 40 seconds or less at engine cranking speed. If your pump does not appear to have any leaks and can pass the output pressure test, but fails to deliver the needed fuel volume, the problem may not be with the pump itself. There is a long fuel line that runs the length of the car from the fuel tank to the fuel pump. In order to pressurize and deliver fuel to the carburetors, the pump must be able to create a continuous vacuum in that fuel line to draw the fuel from the tank to the pump. Most of the fuel line is made up of steel tubing and is unlikely to develop a leak. However, there are two short lengths of rubber fuel hose in the fuel path. One section of hose is found at the fuel tank outlet, while the other is found adjacent to the starter motor — just before the line enters the engine compartment. The purpose of the second hose is to absorb vibration from the engine and prevent it from reaching the rigid steel fuel line. If either one of these two hoses develops a leak, the leak itself may not be apparent because the line is not under pressure so fuel will not be forced out. Instead, the leak amounts to a vacuum leak, which can allow air to enter the fuel line. This can prevent fuel from being drawn from the tank to the fuel pump, much like you might experience with a drinking straw if the straw was to split and developed an air leak in the side of the straw. What appears to be a faulty pump that is causing fuel starvation problems in the carburetors often turns out to be a leak in the fuel line at some point between the gas tank and the fuel pump. So if fuel starvation becomes a problem with your carburetors, there is a tendency to question the condition of the needle and seat valve inside the carburetor, or to blame the fuel filter at the carburetor inlet (thinking is clogged), or to blame the fuel pump itself. Before you blame the pump and toss it, or abandon it and replace it with electric fuel pump, be sure to check the condition of the rubber fuel hoses at each end of the long fuel line between the fuel tank and the fuel pump. You may find that a couple of hose clamps and a few inches of replacement fuel hose is all that necessary to get you back on the road! :doh:
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There is also a fuel tank strainer inside the gas tank on the outlet tube. It was listed as a part used in 1960-1965 Corvairs, but it could be installed on any model year. It is conceivable that this strainer could become plugged and could restrict fuel flow to the fuel pump.

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Also, if you have any reason to remove and reinstall the fuel pump, be sure that you have installed it properly. I would confirm that the fuel pump is correctly seated and installed. There is a hole in the side of the pump shaft that the tapered bolt tip must seat into. If the pump is sitting too high and the bolt is simply pressing against the side of the pump housing, rather than seating inside the tapered hole, the pump push rod will not be doing its job. So first confirm proper pump installation, and then check the fuel pump output pressure and volume, as shown in the shop manual pages above. Fig. 57 in the shop manual page shows the tapered hole that the tip of the bolt fits into. This ensures proper installation and seating of the pump.

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:chevy:
Brad Bodie
Lake Chatuge, North Carolina
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