You’ve got questions. We’ve got answers—the Summit Racing tech department tackles your automotive-related conundrums. This week in our Mailbag, we’re tracking down the cause of misfiring.

Q: I have a 1991 Chevy half-ton truck. It has a 305 TBI motor with underdrive pulleys, ACCEL distributor cap, rotor, and 8.8mm wires, platinum spark plugs, and a JET Stage 1 computer chip.

Here’s the problem: when the engine has been warmed up and is sitting at idle in very warm or rainy weather, it has a slight miss like there is a bad spark plug or loose plug wire. The problem has gotten worse as I put more miles on the truck, and it’s starting to miss when in gear. I had two sets of Taylor Spiro-Pro wires on the engine, with no improvement. Some other people with the same kind of truck as mine have the same problem.

A: One of the biggest problems with this truck’s ignition is the inadequate pickup coil in the distributor. After a while, the coil gets weak and can’t produce enough voltage, causing a misfire.

Another problem could be leaking throttle body-to-intake manifold gaskets. A vacuum leak is virtually guaranteed to cause a misfire.I would check these two items first. If they are OK, look for mis-adjusted timing or valves, low compression in one of more cylinders, low fuel pressure, or dirty fuel injectors.

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