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Classic Chevy

I have a 1975 Chevy Caprice Convertible with a 454 cubic inch “big block” engine with a 4 barrel carb. The car is in mint, original condition with 24,000 miles. Usually the car runs flawlessly, however, lately I have noticed that the car doesn’t like going uphill. On level ground and downhill, everything is fine. However, when the engine and car are hot, and I come upon a hill, the car will start to buck and sputter as if the gas is being chocked off. A few extreme times, the car has stalled out completely. Once I role back down the hill onto level ground, the car starts up and everything is back to normal. The problem is not noticeable when the car is cold. Any insights would be appreciated.

I would have the catalytic converter checked. Sounds like a “classic” case of a clogged converter. The fuel pump is the next suspect in line so have the fuel pump pressure tested if the cat checks out.

Does a 1975 Chevy even have a catalytic converter? Possibly a carburetor problem or issue with the fuel pickup?

Nice car BTW.

Oblivion…1975 was the first year for catalytic converters. A few 1974 vehicles were equipped with catalytic converters as an experiment of some sort. I owned one of these vehicles (1974 Monte Carlo) but it had all sorts of problems. The HEI ignition system was on it as well but they used standard gap plugs (.035) and they did not use silicone under the ignition modules. This oversight caused a lot of burned out plugs and ignition modules. Finding unleaded gas was also a headache.

I think 1975 was the first year for widespread cat use.

I like the convertible best, then the sedan, and the coupe is a distant 3rd. I just don’t like that rear side window in the coupe.

You get these problems that make you check everything until you find the answer. On carbureted cars; if it seems like a fuel problem, it’s bound to be an ignition problem. Be sure your plug wires are good, replace the coil, make sure the center distributor contact is OK where the rotor contacts it. Check the rotor. Remove the distributor and change the pickup and module then see what you get. These parts are not expensive so I wouldn’t even bother checking coil, pickup and module, just change them. Those other suggestions are not bad to do either.

It would still be good to verify whether this car has a catalytic converter or not. I had a late 1974 GM car. It had no cat. Since the problem seems to happen on an incline, I’m inclined (haha) to think it could be a float problem in the carb.

If the fuel pump has 3 lines, block off the smaller output line and road test. The input from the tank is a 3/8" rubber line and the return line is a smaller 5/16" rubber line while the feed to the carburetor is 3/8" metal if all fittings remain OE. When used, the third line goes to a pressure regulator at the tank which dumps excess pressure. It often fails and greatly reduces the pressure and volume of fuel to the carburetor.

Many thanks to one and all for your input. The car has a catalytic converter. All of the mentioned items will be checked. This Community is fantastic. FYI…I thought I’d attach photo for you all to see Black Beauty.

Your converter is not the substrate type…It’s full of ceramic peanuts and they seldom clog up. If they do, there is a handy plug you can remove and using a shop-vac, remove the pellets. The fuel filter is located in the carburetor where the fuel line goes in… It’s small and may need changing. be very careful not to strip any threads here…Inside your HEI distributor is an ignition module with 4 wires attached, two at each end…Movement of the vacuum advance flexes these wires and can cause them to break inside the insulation…Pull on these wires gently and see if they pull away from the module…There are several places where rubber hose is used in the fuel lines. This hose is now 37 years old…It should ALL be replaced. There are some short sections located on top of the fuel tank. Have a real mechanic run a “volume test” on your fuel pump. When pulling a hill, your car requires large amounts of fuel. Make sure your fuel system can supply that fuel…

I guess I would also be looking at the carb float level adjustment. If it is set too low there may not be enough gas in the fuel bowl when it goes up hill so it shloshes to the back of the bowl.

I ran into a strange situation like this one time on a low mileage car. Seems there was a layer of sediment floating in the tank. If the car was on a certain grade the sediment would wick to the fuel pick-up sock slowly choking off the fuel. Was much worse on lower levels of fuel in the tank. When below a half tank the car would do what you are describing on flat ground. Have you noticed anything like this on you car?
I was able to drain the tank and see fresh gas flowing from the bottom and then this rusty mess come out after. 65 Mustang with a drain plug in the tank. You won’t be that lucky I guess.

Caddyman I think you mean an old school mechcanic somebody over 50years old who knows points hei and what a carbanator is.

I’m seeing two variables that when combined cause the symptoms, heat and “angle of attack”. If it were ignition related, as many heat-induced symptoms are, it’d be symptomatic at all angles. If it were just float related, it’d be symptomatic at least when under load whenever the engine were at operating temperature, regardless of attack angle.

Which leads me to suspect a possibility of low fuel pump pressure perhaps even manifesting itself as vapor lock when hot. Start by testing the fuel pump pressure. Post back with the results.

by the way, congratulations on a fantastic ride.