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425 dual carb setup question


64rivguy

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I need some technical advise on getting my dual carb operating smoothly..very difficult cold starting and after it has warmed up the engine still continues to shake as if it has high performance cam (but it does not) I've read articles on proper gaskets etc but can't recall where it was that I saw it.... the engine was rebuilt .030 over and only has approx 5000 miles it burns very rich, has a strong gas smell even after it is warmed up and gases my wife right out of the car..any help would be greatly appreciated...

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I would start with the bad idle first - I would look for a vac. leak. - bad seal - I had the same thing happen with my 61 - plug on the bottom of the intake went bad. In that case I found the vac leak by useing a hose to my ear and searched all around the eng. . You may have some bad plug wires also. The hard start when cold could be another issue. You may need to get the carb rebuilt by a REAL carb. rebuilder.

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It could be a vacuum leak, resulting the no real manifold vacuum to keep the power enrichment part of the carb from working ALL of the time.

It could be a wrong gasket inside the carb, too. OR it could be a plugged air bleed somewhere in or close to the venturi cluster. All of those little holes on the top of the venturi cluster are calibrated air bleeds which fine tune the basic fuel supply (fuel curve) for the carburetor. If one is plugged, that circuit goes "full rich", no matter what. If it's only partially plugged, the same thing to a slightly lesser degree.

On many of the earlier 4bbl carbs, when you pull the venturi cluster out of the throttle bores, there will be some tubes extending down into "wells" in the carb body. One is a larger tube with "emulsion holes" on it for the "main system" and the other (smaller) one is for the idle system. There should be corresponding air bleeds on top of the venturi cluster for each of these tubes. Sometimes, the holes might not be conspicuous, but they are there . . . somewhere. Making sure they are open and of the correct size is important.

One thing to make sure of is that the carbs are correct for that application. IF the engine has been rebuilt and a "stock" cam was put back into the engine, some of the "stock" spec cams from aftermarket suppliers are NOT the exact same as what the OEM manufacturer used. I know of one Corvette, years ago, that had a "stock" replacement Mellings camshaft that sounded nothing like the factory cam did. Upon checking, some timing event specs were a few degrees different, but still had the same total duration and such.

Also, what kind of manifold vacuum is being generated at idle? Just curious. Perhaps the cam was installed one-tooth-off or similar?

Although it could well be a carb failure, it could also be that some finer-tuning is all that might be needed. Those older carbs were generally pretty easy to rebuild and were pretty trouble-free.

It might be easy to point some fingers in particular locations in this sort of tuning situation, but until you check and verify everything BEFORE focusing on the carb issues, you might end up doing some unnecessary repairs. Those would include (as mentioned), vacuum leaks, correct carbs for application (by the numbers rather than just cosmetics), correct carb base gaskets (OEM style!) for the manifold being used and the respective carbs being used, making sure each carb is in the place it belongs(!!), and that the basic idle adjustments are balanced between the two carbs.

As bad as it might seem, I suspect it could be a reasonably easy "fix". But it might take a while to get to that point (in the diagnostics of things).

Just some thoughts,

NTX5467

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