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I am helping a friend restore a 1963 Corvair convertible. The good news is we are so close to being finished, the bad news I am stuck and not sure how to proceed.
Specifics:
We just installed a new engine, and the carburetors were the only items carried over from the old one. Basically, everything else has been replaced with new or rebuilt items.
Problem:
I cannot get the engine to run smooth or idle at low RPM. The strange thing is the engine will start up and run great for a minute or two, but once it warms up it will not say running.
When I restrict the airflow to both carbs at the same time the engine starts running smoother. Also, I was checking the temperature readings on the outside of the engine and the driver’s side head, was about 40 to 60 degrees warmer than the passenger. The former tells me that I am getting too little fuel to both carbs, and the latter is telling me that I am getting too little to the driver’s side carb.
I would appreciate any help or advice that anyone has.
Since you are new to the Forum and I don't know anything about your experience, technical background, etc., I have included a great deal of information and material below to try to provide the information you may need...
If the carburetors have not been cleaned and rebuilt, it sounds like they both need an overhaul. Both are likely partially clogged within the fuel and low-speed/idle circuits inside the carburetor bodies. One may be worse than the other, or otherwise mismatched which may account for the differences in apparent head temperature you mentioned. When the engine is cold the automatic chokes are set, which enriches the fuel mixture with a closed butterfly valve in the intake of each carburetor. The throttles are also held open on a fast-idle cam to increase the idle speed until the engine warms to operating temperature. But in your case either the idle mixture screws are too lean, the idle speed is set too low, or both, and/or the idle passages are partially clogged.
With a freshly overhauled engine (if "new" = "rebuilt" or "overhauled") there is no reason to ignore the needs of the intake fuel system. An overhauled engine deserves a pair of overhauled carburetors. Spray carburetor cleaner is not enough. It would perform a superficial cleaning without really cleaning the internal passages. The correct method involves dismantling each carburetor completely and then soaking all of the metal components in a chemical carburetor cleaner (parts dip). The carburetor components are then rinsed, blow dried and carefully reassembled using a carburetor rebuild kit for each carburetor. These kits replace all of the gaskets, needle valves, seals, and other components that are usually subject to wear.
TIPS: When dismantling the carburetors, it is not usually advisable to remove the throttle mechanism and butterfly valve, or the choke butterfly valve in the top of the carburetor body. The screws that hold the round choke and throttle plates to the shafts are staked at the factory and the exposed threads must be ground down to permit removal. Then the special screws must be replaced and re-staked. These should only be removed if some seal or throttle repair kits are to be fitted to correct throttle shaft wear. Otherwise the carburetor bodies can be cleaned in parts dip with the throttle and choke mechanisms left in-place.
Two gaskets with a plastic insulator/spacer sandwiched in-between are needed at the base of each carburetor. This provides a proper seal to prevent vacuum leaks, and isolates the aluminum carburetor bodies from the hot aluminum cylinder head intake manifolds. Without the insulators the fuel in the carburetor bodies can overheat, boil or vaporize during operation, which can cause engine operating problems.
The carburetors should be carefully synchronized (idle mixture screws, idle speed screws, throttle linkage adjustments) using a vacuum gauge or Unisyn gauge. If you think one carburetor would benefit from an overhaul, then both carburetors would benefit. You want a balanced pair of carburetors that are both clean and equal, and balanced for airflow at idle and at part throttle. A Unisyn air flow gauge is an inexpensive tool that helps to accomplish this. If the base of the carburetor had an air leak, you might want to replace the insulators and carefully tighten the carburetors when they are reinstalled. Also check the condition of the short vacuum hoses on the balance tube at the intake manifolds. Also, consider ordering new choke vacuum break plastic choke controls. The old ones are often leaking, cracked or broken.
The bottom of the above catalog page from Clark's Corvair Parts also lists professionally rebuilt carburetors. There is a core charge which is refunded upon the return of old, serviceable carburetors. Rebuilt Corvair carburetors can also be sourced on eBay and from other Corvair parts suppliers — some of which are listed below... Part number C2755EP: PREMIUM CARB REBUILD-62-63 *INCL NEW THROTTLE SHAFT, C506H GSKTS & C506
NOTICE: Price includes core value: $85.00. You will be issued a refund of $85.00 when you return the used part.
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The two videos below will walk you through a normal series of tune-up tests and adjustments. The videos explain the reasoning behind each check as well as illustrating the adjustment procedures. I have also attached several printed references that I have found to be helpful. These references include numerous photographs and descriptions that include operating checks, adjustments, and carburetor synchronization procedures. The Rochester HV manual starts with a very good theory description of each carburetor subsystem that should provide you with a better understanding of how the carburetors function, and should help you with terminology as well.
Tuning the Corvair Engine — Part 1
Tuning the Corvair Engine — Part 2
As I mentioned before, if you think one carburetor would benefit from an overhaul, then both carburetors would benefit. You want a balanced pair of carburetors that are both clean and equal, and balanced for airflow at idle and at part throttle. A Unisyn air flow gauge is an inexpensive tool that helps to accomplish this. The following Corvair Forum link will provide you with a description of the function and use of a Unisyn gauge: viewtopic.php?f=55&t=5922&p=40585&hilit=air+flow#p40585
I have attached a number of technical references which should prove to be useful if you choose to overhaul the carburetors yourself. The DELCO ROCHESTER - Models H, HV Carburetor Service Manual is particularly useful in explaining the theory of the various Rochester carburetor subsystems, overhaul, and adjustment procedures.
Welcome to the Corvair Forum!
If the carburetors have not been cleaned and rebuilt, it sounds like they both need an overhaul. Both are likely partially clogged within the fuel and low-speed/idle circuits inside the carburetor bodies. One may be worse than the other, or otherwise mismatched which may account for the differences in apparent head temperature you mentioned. When the engine is cold the automatic chokes are set, which enriches the fuel mixture with a closed butterfly valve in the intake of each carburetor. The throttles are also held open on a fast-idle cam to increase the idle speed until the engine warms to operating temperature. But in your case either the idle mixture screws are too lean, the idle speed is set too low, or both, and/or the idle passages are partially clogged.
1966 Corvair Corsa Convertible
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