My Plymouth runs great right after I start her up, but after about 20 minutes of freeway or surface street driving (after heating up to normal operating temp), she starts running rough: The idle drops, and when I take off from a stop light under normal load, she backfires a few times through the exhaust.

I'm running a 413, MSD-6AL, MSD wires (new), Speed Demon 750, recurved FBO distro, new FBO coil, Accel header plugs (I've been running them about a year now - 500 miles), and 101 octane pump gas (real pump gas, not octane boost). Timing is set to 38* (per the distro spec sheet).

Any thoughts?

is your fuel line a little too close to your exhaust or trapped against the block some place where there isn't circulation. Maybe you aren't getting enough fuel to your bowls due to vapor lock. Do you run steel braid/hose or brake/fuel line pipes?

Sorry to resurrect a two month old thread... but the problem is still there. I put in a phenolic gasket / billet spacer from Don at 4 Seconds Flat yesterday (in case I was boiling the fuel), and it still seems to have this backfire problem that seems to be tied to heat (by the way, for those who don't have the 4 hole spacer on a dual plane... get one. The throttle response is great!).

To answer the above poster, I am running 3/8 aluminum tubing all the way from the tank in the stock location. eight inch section of 3/8 rubber fuel line to the fuel pump (Carter mechanical, 120 GPM @ 6 - 8 PSI), three inch section 3/8 rubber from fuel pump to Edelbrock billet fuel filter, short 3/8 rubber connector to 3/8 aluminum tubing running up to the carb in the stock location (behind the alternator, underneath distro and over intake manifold). It's not running near the headers at all, and is about a 1/2 inch away from the block at the closest (below the distro). Other than that, fuel line is away from the block.

Would a braided line cut down on the heat soak (if there is any)? Also, I have a new (stock) vapor separator. Should I throw this in, and set a return line to the tank? Would this help at all (no return line now).

This problem has had me scratching my head for a long time now. Any help would be appreciated.

Possible lifter pump up but I cant xplain why only exhaust, do you have adjustable rockers a little to tight ?

Rockers are non-adjustable. Engine has about 500 - 750 miles on it, tops.

This is the same thing my 83 chev short box truck is doing, it's driving me off the deep end trying to fix it.

Get rid of those accel plugs and go with some NGK's. The screw on terminals on those accels NEVER stay tight or contact worth a darn. Sounds like youre loading them up, and whatever mixutre you have to run with the way they work isnt liking you once its warm. Put in a heat range hotter NGK plug, and lean out the idle circuit a bit. It will be a tad bit more tempermental when cold, but will be a world different warm. Also toss in a backup distributor of any kind to confirm that your pickup isnt having a problem, or its not having a phasing problem etc.

Just a thouught, sounds as if ign. up to snuff.
Maybe check intake for a small leak.
Once it gets to operating temp and everything has expanded, might develop a small intake leak.
Like I said, just a thought.

Sounds like a problem I had years ago, on mine there was a leak at the carb base.

Is the choke fully open when warm? Have had that happen a few times.

Thanks for the tip on the plugs. I was actually thinking I should switch to a hotter plug. Any recommendations on which part number NGK I should start with?

No choke on the carb, so it can't be that.

sounds like it needs to be tuned at operating temp. engines tend to reach operating temp in 15 min or so.

the most likely cause is the choke, or timming.