Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 22 Times in 17 Posts
Re: 258 Low speed miss
Your miss is almost certainly due to carburetor problems; most likely blockage in the mid-range circuit. Your best bet would be to get a rebuild kit and dive in. The kits are cheap, and the rebuild easy. You've nothing to lose but a sawbuck and a few hours.
Be sure to note the setting on the needle valve whan you're disassembling it. When you have everything apart, blow out all of the passages with compressed air. Be sure to check the weight of the float with a postage scale. The foam ones soak up gasoline over the years, and the brass ones sometime get a pinhole leak.
Put it back together according to the specs in the rebuild kit; don't try to make any 'improvements' in the process. And put in new fuel and air filters while you're at it. Almost certainly the problem will be gone when you get it back together.
My carb goes wonky about once a year. I never see anything obviously wrong with it when I open it up, but it's always better afterwards. One of life's little mysteries I've given up trying to understand.
I might have been born at night, but it wasn't LAST night.