Hey guys:

I've got my '68 Valiant registered now, so I've been driving it around my small town accruing miles close to home in case something weird happened, and it did.I was out this evening for a whirl around town, and the Plymouth had an "episode".

To preface, this is a stock 225 automatic with a "reman'd many years ago but barely used" 1920 carb that came on an engine I was gifted. It *looked* nice, so I thought why not even though I know all about reman carb quality.

This carb actually works well for a 1920, but it started displaying some erratic behavior recently. I occasionally got a huge tip-in hesitation(just one) when the engine was nice and hot. The next thing that started happening is the engine stalled about 4 different times, all when the engine was warming up. Three times when I was slowing down for a turn, once when it was running on fast idle. That issue was not predictable. After I restarted it ran fine. Also, it has this weird hang-throttle thing going on where the idle speed hangs about 300 rpms higher for a couple seconds when you rev the engine, then drops off to it's normal level. It is not the mechanical advance sticking(I checked) and it is not the throttle hanging open.

Here's where it gets interesting...

On my circuit of town I decided to pull in and top off the tank. Right at the moment that I stopped, the engine idle dropped and it started running unevenly. I aborted the gas stop and headed for home. I had to keep my foot on the gas to keep it running...it acted like it wouldn't restart easily if I let it quit running. It was very unhappy at light throttle the 2 miles home, and when I turned into the driveway and gave it gas, it backfired through the carburetor loudly. Then it ran fine!

For full disclosure, I'm not a fan of these carburetors. I think they have numerous poor design elements and I'll build a BBS to replace it. As far as what happened, I have my suspicions and I think the effect of the backfire is a significant clue.

What do you think?

PS, please, no boilerplate text about remans are bad, tank is rusty(it's not, it's new), fuel pump is bad(it's not it's also new), fuel filter clogged(it's not, it's new as well), etc etc. Let's put some thought into it. Thanks!

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1966 Chrysler 300 coupe
1968 Plymouth Valiant 100
1969 Dart Custom 273 V8

Wild guess, normal flow in idle circuit is to manifold low pressure vacuum. Backfire no vacuum high pressure flow in reverse direction, which may have blown some dirt out of idle circuit threw idle air bleed hole. Throttle shaft wear and bore holes worn egg shaped, Throttle does not close to same spot all the time. A stiffer spring may help.