New carb - can't get RPMs down from 2500

[emac]

I cant speak for running a 2F carb on an F motor. But, the idle solenoid has to be hooked up for the carb to run properly. It activates the idle circuit. Without it hooked up then it wont idle without turning the idle up real high. It should ONLY get gas through the idle circuit during idle. If running right, unplugging the wire will cause the engine to stop.

Make sure the butterfly is not hanging up. Hook up idle solenoid. Turn idle adjustment screw to get low idle, adjust idle mixture screw and idle screw until you get it proper (lean drop).

If you cant get idle low enough, then check linkage and throttle cable, timing and vac leaks. The 2F shop manual has a very good chapter on "tune up".

 

[emac]

Fast idle screw is adjusted based on the choke. As you pull the choke out you get fast idle. I have mine set pretty low as I dont like a lot of fast idle. Just enough to keep it idling in the winter when I need the choke to run the first mile or two.

 

[Vae Victus]

I cant speak for running a 2F carb on an F motor. But, the idle solenoid has to be hooked up for the carb to run properly. It activates the idle circuit. Without it hooked up then it wont idle without turning the idle up real high. It should ONLY get gas through the idle circuit during idle. If running right, unplugging the wire will cause the engine to stop.

Make sure the butterfly is not hanging up. Hook up idle solenoid. Turn idle adjustment screw to get low idle, adjust idle mixture screw and idle screw until you get it proper (lean drop).

If you cant get idle low enough, then check linkage and throttle cable, timing and vac leaks. The 2F shop manual has a very good chapter on "tune up".

Idle solenoid is hooked up. When I did so, idle went UP 1000 RPM.

 

[Solace in Solitude]

See @emac ‘s response above.

The last link I included contained the following:

“The second picture is a view of the side of the carb that is next to the fire wall. There are two adjustment screws on this side. The lower screw is the idle speed screw. The higher screw that is partially hidden by the U shaped linkage is the fast idle speed screw (used when the choke is engaged). Turning these screws clockwise increases the speed.”

I asked about fast idle because, I had a carb I couldn’t bring down to proper idle. I discovered the fast idle was way too high and the choke breaker was stuck.

Yor fast idle needs to works as emac described.

You need to be sure: you have no vacuum leaks; the fuel mixture is correct; the timing is correct and the idle is at 650-700.

Is this truck desmogged?

 

[Solace in Solitude]

I’ve had to

• Set idle as close to 650 as possible
• Lean drop
• Set idle as close to 650 as possible
• Lean drop
• Set idle as close to 650 as possible
• Lean drop
• Three times before I could reach 650.

 

[Vae Victus]

Turned the idle Speed screw until it’s about to fall out. No change. Something is hung is guess.

Taking off air filter to see inside.

 

[Vae Victus]

Biggest issue is that I can’t get close to idle. I’m at 2500-3000 rpm.

Until I figure that out I cannot
- set fast idle (choke isn’t being used)
- do a lean drop
- check for vacuum leaks (it’s closer to 15 in HG now that I’ve tightened a couple of hose clamps - but that’s not at idle so no idea what vacuum would be at 700 rpm)

Plus it’s hotter than hell today.

 

[Vae Victus]

I checked throttle cable - no difference when not connected.

I retarded and advanced the distributor as much as possible in both directions. Too rich and it won’t start and backfires. Just a little leaner and it jumps back up to hugh RPMs.

I turned the idle speed screw all the way loose and all the way tight. Neither reduces RPMs.

 

[Solace in Solitude]

Maybe something blocking the idle circuit... maybe @Racer65 or @ginmtb have seen it on Roger’s carbs... I know TrollHole had some issues with new carbs that came with metal shavings in them.

Otherwise, maybe one or more of the pros (@Pin_Head @FJ40Jim @65swb45 ) are online.

 

[Vae Victus]

Have removed the air filter and put cardboard on the carb to see if it showed a vacuum leak (if the rpms went up before it stopped) it did not indicate a leak.

What else makes the RPMs go up that high?

If I pull the carb, what am I looking for? The throttle portion of the carb seems to operate smoothly, but that's a guess since I don't KNOW what a properly functioning one looks like.

Since it's new I'm just assuming....

 

[Pin_Head]

Remove the carb and check the throttle plates to make sure they are closed. It won’t run at 2,500 with them closed, so something is holding them open. Fiddle with the linkage and speed screws until you find what is holding them open.

The vacuum reading is meaningless at 2,500 RPM. You need it to idle at 625 RPM first.

 

[Jdc1]

What happens when you throw another carb on?

 

[Vae Victus]

Hmm. Well I do have a new troll hole 2F carb. I could try that.

I'll pull mine and see if (why) it's sticking.

 

[Jdc1]

Hmm. Well I do have a new troll hole 2F carb. I could try that.

Cant hurt, is the idle solenoid grounded too?

 

[Marn0]

Have you put all settings back to their original position?

 

[Vae Victus]

Cant hurt, is the idle solenoid grounded too?

Not that I see right off. Will have to investigate.

 

[Vae Victus]

Have you put all settings back to their original position?

No but at 2500 RPM, and having tried all settings, I don't feel like that could be it. Could be wrong....

 

[Vae Victus]

Pulled carb. What am I looking for?

Butterfly valves on bottom seem to work fine.

The gasket probably needs to be cut out in the middle since the intake manifold is a single barrel but the gasket is a 2 and it's sagging. Even if not a problem now, it may become one.

Do I need these spacers on the bottom to remove the top spacer between air filter and carb?

 

[Vae Victus]

I noticed that the fast idle linkage seems to bind up on the choke cable screw so that could have been part of the issue but doesn't explain the idle speed.

Obviously I can adjust this so it's not rubbing but in its normal state it was pressed against the solid bar that goes to the choke circuit. Before I removed it and noticed that I could not move that bar up and down at all because it was being bound by the choke screw.

 

[Vae Victus]

Normal state of the throttle plates