What can cause engine to missfire?

[2badgn]

1984 GMC 3500 2wd 350 engine

Last year, I had the timing chain changed along with cap, rotor, plugs, wires. truck was back firing after I turned off engine. A timing adjustment fixed that.

Truck is now at mechanic.
When driving, truck will misfire for like 2 seconds and then back to normal.

What things can cause misfiring?

Thanks

[old Rusty C10]

there are many things.. timing.. carbuerator off.. distributor.. crossed ignition wire...

[James McClure]

As Bob said, these things will cause a misfire or skip. There are other things to be aware of. The FIRST question you should ask yourself is "what was done to the eng (if anything) before this started. You covered the basics, BUT, GM made the screws that hold the coil down just a pinch to long to work well with aftermarket caps. The screw makes a small crack in the plastic and the spark will jump to that screw. Also, if you have vac advance, the leads on the pick-up coil tend to break insidee the insulation. When you mash the gas, the advance plate rotates to raise timing and the pick-up coil lead(s) loose connection. An easy way to check this is to remove the vac hose and drive it and see if it skips. Do this check a few times to make sure. If the skip goes away, the pick-up coil is toast. Next test would be a spray bottle filled with water. With the eng running, starting from the plugs, work you way up to the dist and see if it skips at all. Next, check under the coil cover and make sure the ground tab is installed from the coil frame to the connection block on the side of the cap. If all that is good, make sure of good connections to ign + (pink wire at cap) and 3 wire connector that goes back to the ign module and all the connections inside the dist. jim

[motornut]

check that the advance weights move freely/or worn