My 86 e350 runs smooth but there's times when I have to feather the gas and that's from a complete stop and it sputters and when I make a left turn it kinda sputters a little more but not bad at all but my main concern is that when I get in between 50-55mph it backfires not hard or anything but only at that speed. Once I'm past that speed it goes back to running smooth.

Would this be a timing issue ? I have new spark plugs in it...
would it be a fuel filter deal ?

thanks for reading and everything helps !!

 

Posting the size engine and the mileage helps answer your questions

Year and model is a must, but can't guest what engine you have.

But, when was the last time you checked the timing??

You replaced the plugs? Did you replace the wires too? Cap and rotor?

You could be having a problem with just bad plug wires.

 

It's a 1986 E350 with the 460. I'm gonna try the wires plus cap and rotor next. I got it from a friend and he said he just changed the plugs. The van is here in Oregon so the weather sucks and I'm guessing everything has some rust in it. The van is a work in progress. It's gutted except the front seats slowly gonna rebuild the entire thing. But that engine is strong and runs really smooth except for the couple little problems.

Thanks for the response !!

 

My 1985 E150 with 302 had a early computer system with a feedback carb. I believe your 1986 with a 460 still used the Dura-Spark ignition system with a standard Holly 4180 carb? Do you have an electirc fuel pump, mechanical pump, or both?

I don't know much about the 4bbls but with the 2bbls, a bad accelerator pump on the carb can cause it to spit and sputter when starting off..

Does it sputter when hot or cold or both?
Do you have to feather it when hot or cold?

 

backfires are typically a spark or lean condition
to check the wires drive it at night with the doghouse off and watch for sparks between the plug wires