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1955 vs 1956 4 bbl intake manifold


Bill Newland
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I have a 1955 Buick Roadmaster that I recently acquired with a 1956 322 engine in it.  The 56 engine has the 1955 intake manifold and 1955 Rochester carburetor.  I have heard that this is incompatible in some way.  Can someone shed some light on this?  The engine is running ok, except for very poor gas mileage and running rich. 

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17 hours ago, Bill Newland said:

I have a 1955 Buick Roadmaster that I recently acquired with a 1956 322 engine in it.  The 56 engine has the 1955 intake manifold and 1955 Rochester carburetor.  I have heard that this is incompatible in some way.  Can someone shed some light on this?  The engine is running ok, except for very poor gas mileage and running rich. 

 

Carburetors get blamed for a lot, some of which they actually cause.

 

Yes, a carburetor flowing too much fuel CAN cause the engine to run rich (about a 4 percent probability). A defective ignition system can also cause an engine to run rich (about a 93 percent probability). Underinflated tires, dragging brakes, your golf clubs in the trunk ;)  among other items can cause poor fuel economy.

 

Jon.

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I certainly agree that "95% of carburetor problems are electrical" (to quote an old saying), however I don't detect any misfiring of the engine.  Still a possibility I guess.  I do see a hint of black smoke coming from the tailpipe after the engine is warm, choke is wide open, and exhaust by-pass valve is open.  This car has an electric fuel pump (with no pressure regulator) with the mechanical pump bypassed.  The thought occurred to me that the electric pump may be supplying to much fuel pressure and overwhelming the float valve system in the carburetor. My next thing to do is to put a pressure gauge on the fuel line and see if the pressure is within the 4 - 5 psi spec. 

 

The P.O. told me he thinks the 1956 carburetor with the 1956 intake manifold on the 1955 engine is incompatible somehow.  If the exhaust by-pass routing is the only difference between the two intake manifolds, I don't see how this could make a difference - especially after the exhaust manifold heat riser valve opens.  I noticed that the 1955 Rochester carburetor has a different part number than the 1956.  Can anyone tell me what the difference in the two carburetors is? 

 

Thanks to all for their comments and suggestions.

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Both use Rochester mounting gasket 1170643, thus there is no incompatibility to the intake manifold.

 

Without pulling prints (on microfilm) the major difference MIGHT be in the secondary. Packard carbs have no auxiliary air valve on the 1955, and have it on the 1956. Buick may be the same; I don't have any to look at, and too lazy to pull the prints. The 1956 Packard 4GC is superior to the 1955 because of the auxiliary air valve.

 

Jon

Edited by carbking (see edit history)
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As far as the ignition is concerned: with the proliferation of aftermarket stuff on the internet, there is always the possibility of acquiring wires, coil, and condenser that do not match each other in impedance. If not matched, firing voltages can be low, resulting in an incomplete burn of the available fuel, and a rich condition. Worse yet, are the electronic conversions that should ALWAYS (one of the words I virtually never use) be installed ONLY after upgrading to an alternator. A generator does not supply sufficiently stable voltage at low RPM for the electronics.

 

Those of you that are old enough, and grew up in rural areas with REA's; do you remember watching TV and the refrigerator kicked on? And the TV picture shrunk about 2 inches at both top and bottom, and then went back to normal?

 

Jon

Edited by carbking (see edit history)
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