Getting a misfire at idle

MISTERMICHAELJ

MEMBER
1974 FORD F-350
100,000 MILES

Hello, I have a 1974 Ford F350. With 3 cylinders not-firing at idle. Replaced wires, plugs, did compression test, etc. (Old plugs looked normal It's possible that the cylinders 'kick in' when I apply 1/2 throttle or more, but, it putts' on mild acceleration, (between idle and appr. 1/2 throttle) The old plugs were clean, light tan, looked great. It almost appears (to me) that these 3 cylinders aren't getting fuel. Is it possible for, say, 3 intake valves to be hanging up?

3 Replies

CJ MEDEVAC

MECHANIC
11,004 POSTS

WHICH 3 CYLINDERS?

HAVE YOU DONE ANYTHING WITH THE CARB?

THE MEDIC

Was this

answer

helpful?

Yes

No

Friday, August 26th, 2011 AT 1:35 AM

MISTERMICHAELJ

MEMBER
2 POSTS

Cylinders # 3, 4 & 7. I installed a rebuild kit on the carburetor in an attempt to eliminate this problem. Initially, it seemed obvious that it was fuel related: checked fuel pump, replaced both fuel filters, etc. Compression check showed 120 psi in these cylinders. New plugs look very clean even though I ran it a few times for testing purposes, etc. No matter what I do/did, those 3 cylinders appear to not-fire at idle. I pulled every plug wire while idling; these 3 had NO effect while the other 5, when pulled, had obvious effect.

SPONSORED LINKS

Was this

answer

helpful?

Yes

No

Friday, August 26th, 2011 AT 12:50 PM

CJ MEDEVAC

MECHANIC
11,004 POSTS

Is this thing points?

You know your way around it well? Or are you sorta a beginner?

To me, if it runs well upon "more throttle". Sounds like it's getting enough fuel off of the "mains"

I don't know your "status". So I might be talking, as if you might be "Dummer than me!"

Did you rebuild your carb like a pro. Or did you fiddle with it for hours (like I do) wondering how it all goes back together?

At this point, to me it sounds like idle circuit problems.

Other things to check:

Pull dist cap, see if rotor shaft wobbles if you push it left, right, away from you, or towards you

Condition of dist cap and rotor

condition of wires

Just for "Giggles"---with a bottle of some sort. With the sorta "traditional similar" 'Elmers Glue Top'. Something you can increase or decrease a "Gas Drip" out of.

Dribble in each barrel, at different rates----while at idle. See if you can "Force an improvement" just by a little extra fuel dripping in.

If it works. Might need carb adjustments. Improved rebuilding. Or maybe a "Parts Store" rebuilt one

Let me know what you find.I'm old school, got 2 "Real Jeeps", w/ carbs. Kinda really what I grew up on!

Might be able to make suggestions, before you go Hog Wild, doin' stuff

Results?

The Medic

Images (Click to make bigger)