Slight stumble, flat spot

[Vanspeed]

Hey guys, I got my 48 F-1 now to the point of where you can reach in the window, turn the key and hit the start button and she fires on the first crank, idles like a sewing machine at about 450 rpm. When you get in and hit the gas, it misses for a second, if you get the rpm's up it is smooth but when you let it come back down to idle, is has a flat spot at the very beginning. I have put a new accelerator leather pump and it didn't really do anything. Also replaced my power valve while I was in there. I still feel like it is a carb problem. What are your thoughts?

[Bassman/NZ]

If you dont have an accelerator pump shaft with a leather plunger that is likely the problem. The chinese ones with blue neoprene plunger are crap. I had same problem, changed to a USA leather one and put a very light coat of oil on it ...... accelerates smooth as.

[Binx]

Off-idle stumble is usually (USUALLY) a lean problem. Check to see there is no slack in the accel pump's mechanics.

Lonnie

[54vicky]

late timing can also cause off idle stumble

[Barry-ct]

Try it with the choke slightly closed up, maybe a 1/4 of the way closed. if it goes away, then perhaps a little lean. Have you set the 2 idle screws at 2-1/2 turns out and tried adjusting them in or out slightly to see if that would help?

[Vanspeed]

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Hi Barry, yes I have played with those a bit. Turned them in to make it stumble then backed them off a turn. Maybe I should go more. Brand new leather accelerator pump. I will try the choke and see if that helps. Maybe it is running to lean like you guys think.

[Vanspeed]

What slot should the little bar be in that connects the accelerator pump to the throttle shaft? There are three holes marked 1-3. I have it in the 1 spot right now.

[wga]

Van - keep changing the accel rod holes, and what works

[Mart]

Look down the carb and see if you can see 2 clear jets of fuel squirting into the venturis. You can do this with the engine not running, but probably best to have had it running first to make sure the carb has fuel in it. If both jets are squirting cleanly then play with the different holes to see what suits best. You should be able to have it idling and snap it wide open and it should rev up immediately.

Tinker with it a bit.

Mart.

[estout81]

I have mine in hole #2. Seems to work good.

[zzlegend]

Originally Posted by estout81

I have mine in hole #2. Seems to work good.

X2. Have three of them and all set like this.

[scicala]

Originally Posted by Binx

Off-idle stumble is usually (USUALLY) a lean problem. Check to see there is no slack in the accel pump's mechanics.

Lonnie

I would do what Lonnie said before you do anything else. If this is the problem, no matter what pump hole you use, or no matter whether you use a leather or rubber pump piston, you will still have the hesitation if there is slop in the pump linkage (throttle shaft moves significantly before you can actually see the pump rod go down).

Sal

[Vanspeed]

Checked it with choke partially closed and same thing. Took off the carb and took it apart. Nothing inside that is clogged or blocked. With top off carb with gas in the bowl is shoots gas very smoothly. Put it back on and adjusted the idle air screws open two full turns and put the accelerator pump linkage in the number 2 hole. This helped it a lot. I don't know if I have found the sweet spot yet but better.