Vacuum Gauge Reading

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Yellow
Posts: 51
Joined: Thu Mar 15, 2018 2:30 am

Vacuum Gauge Reading

Post by Yellow »

Hello All. I have been fighting with my Motor for about a week. Wants to stall on acceleration from Idle.

I've put a vacuum gauge on the choke pull off ports and the needle has a steady fast vibration. When acceleration is added quickly drops to zero.

Recently replaced mech fuel pump prior to problem

Checked coil with another same problem.

replaced fuel filters

replaced vacuum lines at vacuum balance tube

replaced cap and rotor

New plugs and vacuum advance will go in today.

Running pertronix 1 on powerglide

At a loss if there is any thing anyone might suggest???

Thanks All!!
1962 Convertible Monza Powerglide w/ 64 differential and transverse spring and 65 carbs.
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bbodie52
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Re: Vacuum Gauge Reading

Post by bbodie52 »

You may have a problem with the accelerator pump in one or both carburetors. When the throttle is first opened there is a slight delay before the airflow through the carburetor throat begins to draw gasoline from the float bowl — so the engine only gets a burst of air with no fuel mixed in. The venturi effect develops a vacuum to pul the fuel from the float bowl, but it takes a moment to get the fuel flow moving. To get past this delay, the carburetor has an accelerator pump to mechanically squirt a jet of fuel into the throat of the carburetor to prevent the engine from stalling.

You can peer down the throat of each carburetor while holding the choke open. Open the throttle quickly. You should see a squirt of fuel injected into the carburetor throat from the accelerator pump. This confirms the presence of fuel in each float bowl.

With the air cleaner assembly removed, when holding the choke open and peering down the throat of each carburetor, do you see a squirt of fuel from the accelerator pump in each carburetor when you open the throttle rapidly? If the jet of fuel is not observed, the float bowls may be dry, possibly due to stuck needle and seat assemblies blocking the fuel inlet. There may also be a worn or damage accelerator pump boot or a blocked fuel passageway that is preventing the fuel from being injected into the carburetor throat.

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See page 6 of the attached DELCO ROCHESTER - Models H, HV Carburetor Service Guide for a description of the accelerator pump system in the carburetors.
DELCO ROCHESTER - Models H, HV Carburetor Service Guide.pdf
DELCO ROCHESTER - Models H, HV Carburetor Service Guide
(1.79 MiB) Downloaded 20 times

You also mentioned that you installed a new mechanical fuel pump. If you determine that fuel starvation is a problem and that the carburetor float bowls are empty or near-empty, you may need to check the fuel pump and the pump feed lines from the tank. Also check to ensure that the pump is properly installed at the correct insertion depth by the tapered bolt.

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 that 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 carburetor found on turbocharged Corvairs, which is sensitive to excessive fuel pressure.

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.

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 it 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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To remove and reinstall the fuel pump, be sure that you have installed it properly. It is important to ensure 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. 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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Brad Bodie
Lake Chatuge, North Carolina
Image 1966 Corvair Corsa Convertible
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Yellow
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Joined: Thu Mar 15, 2018 2:30 am

Re: Vacuum Gauge Reading

Post by Yellow »

Just to finish up here. Replaced a miss matched set of carbs. A 65 and a 66 both had all kinds of assorted and missing parts including wrong venturies. Two fresh rebuilt 65s and fresh sparkplugs and she is running smooth!

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1962 Convertible Monza Powerglide w/ 64 differential and transverse spring and 65 carbs.
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