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Posted by Vaughn [99.68.110.107] on Saturday, July 10, 2010 at 10:29:37 :
In Reply to: One more slant 6 question posted by David Sherman [72.47.9.228] on Friday, July 09, 2010 at 23:32:27 :
Back firing through the carb is more typical of electrical shorts and rich/lean fuel mixture.
1. Star the engine at night in the dark and look for arcing around plug wires and coil. Pay close attention when the engine sputters.
2. Pull the dist. Cap and look for arcing across cap terminals
3. With the engine running, turn the choke butterfly slowly towards the closed position and see if the engine speeds up and the sputtering stops. If it does, there is a vacuum leak, or incorrect float level, poor fuel pump pressure, carb adjustments or jet size may be the problem. If the engine slows down, it could be running rich, examine the spark plugs.
4. The difference in timing is more likely from the engine slowing down and speeding up. It could be due to weak counter weight springs or cracks in the vacuum advance diaphragm if one is used, but probably is the result of changes in vacuum pressure.
5. Timing chain may not be a factor. The cam always has tension applied to it from the oil pump, dist., and valve springs, so there isn’t much of an oscillating effect. The rest side of the chain is swung out from centrifugal force.
6. The harmonic balance may not be a factor at all. This is it’s function:
Power impulses tend to set up torsional vibration in the crankshaft. When a piston moves up and down on its power stroke, it thrusts, through the connecting rod, against a crankpin with a force that may exceed 3 tons. This force tends to twist the crank ahead of the rest of the crankshaft. Then, a moment later, the force against the crankshaft is relieved. The crank then tends to untwist, or move back into its original relationship with the rest of the crankshaft. This twist-untwists, repeated with every power impulse, and tends to set up an oscillating motion in the crankshaft. This is torsional vibration; if were not controlled, it would cause the oscillations to build up so much that the crankshaft might actually break at certain speeds. To control this, a harmonic balancer is used. Today’s Auto Shop 101.
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