440 runs perfect for 5 seconds, then starts missing. Every time. Help.

[TASummers]

High compression & low octane gas?

10:1. It runs well in premium.

 

[Geoff 2]

Sounds like flooding. Check needle/seats & replace met block gaskets.

 

[diesel_lv]

You initially replaced a wire n plugs. Have you verified since then which cylinders are dropping? A spay bottle on the header at each exhaust port or a digital temp gun. Verify if cylinders are running cooler. Is it the same ones each time?

 

[PRHeads]

Here’s what I’d try to help determine if it’s a fuel control issue……

Have a flash light handy and remove the air filter assy.

Start the engine and let it settle in to it’s running rough condition.
While it’s misbehaving during idle(throttle closed), shine the flashlight down the carb and see if there is any fuel dribbling out of the boosters. Look at all 4.

If you see any fuel coming from the boosters there is a float control issue, or they’re set too high.

In carbs that routinely sit for long periods of time it’s not unusual for the needle & seat o-rings to get hard or shrink, and then allow fuel to pass thru.

 

[Nxcoupe]

Here’s what I’d try to help determine if it’s a fuel control issue……

Have a flash light handy and remove the air filter assy.

Start the engine and let it settle in to it’s running rough condition.
While it’s misbehaving during idle(throttle closed), shine the flashlight down the carb and see if there is any fuel dribbling out of the boosters. Look at all 4.

If you see any fuel coming from the boosters there is a float control issue, or they’re set too high.

In carbs that routinely sit for long periods of time it’s not unusual for the needle & seat o-rings to get hard or shrink, and then allow fuel to pass thru.

I like this, along with crud building up in the metering block orifices.

 

[B-Body 70]

I had this problem once on my daughters 85 Audi 5 cylinder took me awhile to find the intermittent misfire. It was a broken valve spring rotating just right.