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Discussion Starter · #1 ·
I put a rebuilt 304 into my CJ in December and its been running great since. In the last week its been backfiring when I hit the gas. It idles fine. I assume this is either timing or the carb. I played with the timing a bit but it didn't help. Any suggestions before I mess up? lol

1997 TJ w/ 33's & 1984 CJ7 w/35's

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6,784 Posts
1. Check the vacuum advance on your distributor.
Flip the distributor cap so you can see the vacuum advance arm move the trigger plate.
Put a clean hose on the vacuum advance and pull some vacuum.
If the trigger plate moves, and the canister holds vacuum for one minute, and the trigger plate returns when the vacuum is let off, it's not the vacuum advance.
2.Check your cap and rotor. I don't know why, but winter time is the worst for showing bad caps & rotors. Probably the drier air causes more ozone, and that shows up as cross fire and ground fires (dead miss).
3. Advance weights stuck. While your vacuum advance line is disconnected and plugged, use a timing light to see if the timing goes up and back down when the RPM is increased and allowed to go back to idle.
4. Cold has affected your ignition module. It is unlikely, but if it is the module, it will be very hard to diagnose, because intermittent modules are notorious for working correctly when they get warm, and doing strange stuff when the temperature changes.
5. Check your accelerator pump. (With the engine off)
Take your air cleaner off, and look straight down the carb throat with a light.
Now, pull your throttle linkage all the way open with one smart stroke.
Did you see a good, 'thick' stream of fuel get shot into the Venturi bores?
No bubbles, sputtering, or weak 'gurgling' pump shot?
If it was anything but a good, long, strong pump shot, have someone good with carbs look at it.
NOTE: Cold air is more dense, and needs more fuel for proper fuel ratios. A lean idle mixture or weak accelerator pump shot, or combination of both will really show up on cold days.
5. Fuel pressure and float level. Self explanatory.
6. Make sure your choke is pulling all the way off.
7. Check each end of every vacuum line for leaks. A little leak can cause a lean problem, and this sound like what is going on.

If that don't kill the problem, let us know and we'll throw blind guesses at it until we get it, or you get sick of tinkering with it in the cold and take it somewhere!!

Good luck, Aaron.

So many cats.... So few recipes...
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