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Chevrolet Old Cars Motor Misses at Speed Troubleshoot Guide

Motor Misses at High Speed Only

There is insufficient gasoline flowing to carburetor due to obstruction in gasoline line or filter screen.

A valve may be sticking slightly and does not come to its seat properly.

There may be a loose electrical connection.

The spark plug points do not have the proper gap.  The points should be separated about A of an inch.

Motor Misses at All Speeds

Porcelain in the spark plug may be broken, allowing the spark to jump from the electrode in the center of the porcelain to the shell of the plug before entering the combustion chamber.

One or more spark plugs may be fouled.  Thoroughly clean the sparking points and porcelain with cloth dipped in gasoline.

A valve may be sticking. Remove and thoroughly polish the stem.

Compression may be poor due to pitted or warped valves.

A valve spring may be broken.

Push rods may be adjusted too tight.

Adjustment for the push rods may have become loosened and valve is not opening.

Filter screen in carburetor clogged and gasoline not flowing to carburetor properly.

One of the ignition wires may be loose and due to vibration makes and breaks the contact.

Contact points in distributor are not opening and closing proper

Contact points in distributor may need  cleaning  or filing.

The spark plug gaps are not adjusted properly.   (The gaps should be about -31,- of an inch.)

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The carburetor may be flooding, causing the mixture to be too rich.  This is usually caused by the needle valve not seating properly.

 

 

Motor Misses at Low Speed Only

Compression is weak due to leaky piston rings or valves not seating.

There may be a leaky gasket where the carburetor is attached to the intake manifold or where the manifold attaches to the cylinder head, permitting air to enter, weakening the mixture.

(To detect the leak, take a squirt can filled with gasoline and squirt around where the connections are made.   If any gasoline enters the opening, the speed of the motor will immediately increase thereby indicating a leak )

The regulator screw which regulates the flow of gasoline at low speed only, may not be adjusted properly. Set the throttle for low speed running and turn the screw in and out to obtain the best low speed running adjustment.

The spark lever may be advanced too far.   When running at low motor speeds the spark lever should be retarded.

When running at low motor speed the generator does not deliver electric current to the storage battery as the circuit breaker makes an “open” circuit in the line and ignition current is then supplied from the storage battery.

If the battery is in a badly discharged condition it ofttimes happens that insufficient current is being sup-
plied for ignition purposes.

There may be one or more weak exhaust springs and with the throttle practically closed the vacuum created in the cylinders by the piston on the suction stroke will open the exhaust valve, drawing in burned gases and weakening the mixture so it will not burn.


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Chevrolet Old Cars Motor Misses at Speed Troubleshoot Guide Article Source –

This article courtesy should goes to : Chevrolet Repair Guide of 1923 by Chevrolet Motor Company.

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