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A Shaky Buick

Q. I have an 1985 Buick 5th Avenue. I have installed a new carburetor (rebuilt), new spark plugs and wires but the engine is shaking.

Any help on this would be highly appreciated.

Thank you,

A. You didn't mention if the engine had a rough idle before you put the carburetor on. In any case, there are two possibilities. First is that when you replace the carburetor, it has to be adjusted. This is done best with a four gas emission analyzer. Lacking that, you can come close by turning the adjusting screws until the engine smooths out. Be sure you turn both screws evenly.

The second possibility is that there is a vacuum leak. There is a fairly large vacuum line going from the back of the carb base to a fitting on the top of the engine. This is the vacuum supply for the Positive Crankcase Ventilation (PCV) system and it is very common for this hose to split open. Many a carb has been replaced needlessly due to this hose. On the other end of this system is the PCV valve. This should be checked and replaced whenever you get a tune up as well.

If it did have the rough idle before the new carb, then it would more than likely be the vacuum leak. I would carefully inspect all the vacuum lines for cracks or broken hoses.

If you take a can of carb cleaner and squirt a quick spray into the carb while the engine is running and the rpm's pick up, then you know you have a lean fuel mixture. Ether from a misadjusted carb or a vacuum leak.

In a high milage vehicle we can not discount the possibility of a weak or dead cylinder, burnt or sticky valve or a bad head gasket. These could cause the rough idle as well.

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