Motorcraft 2150 Throttle Sticky With Engine Running, OK when off

[OldBlue76]

I have a stock 1976 with 302 auto and 2150 carb. It is a remanufactured unit (National, which I absoutely do not recommend). The carb has very few miles on it. I got the carb tuned up pretty good but the throttle sticks making it hard to press the accelerator pedal smoothly and the rig jumps forward. I thought I had a linkage issue, but after messing around I determined that it operates smoothly with the engine off, but as soon as the engine is started the resistance to rotate the throttle increases dramatically. Very odd.

I searched this and the only thing I found was some thought that it is caused by the vacuum moving the throttle plates against the bell. I checked it and this does not appear to be the case. No rub marks and moving the shaft latterally does not seem to cause the resistance to increase. It does not seem to be worn or loose too bad.

It's not the springs. I removed them and it behaves the same way. I have another rig with same setup and an original 2150 (that I rebuilt) and do not have this problem.

Help! Any ideas???

 

[Speedrdr]

I’m not sure if it’s possible that when the carb was rebuilt that the shaft the butterflies are on got a little bent. Doesn’t seem likely that there would be any difference between off and running, but we have to address Murphy’s Law in n this instance.

Randy

 

[Oldtimer]

The throttle plates may not be centered in their respective throttle bores.
Engine vacuum may be enough to causes plates to bind.
Mine were binding at front or back edge (can't remember which).
I loosened screws holding plates to shaft, shifted plates, and tightened screws.

 

[DirtDonk]

Seems like a common theme with modern rebuilt carburetors.
More than just a few times we’ve heard of the throttle blades not being installed perfectly on center. It’s not a hard thing to do, but maybe takes a few extra seconds during the installation process.
The “rebuilders“ are probably paid by the unit.

 

[73azbronco]

Disconnect throttle from carb, move butterfly using carb linkage, then try it engine on.. Change? problem with carb. No change?, then your issue is pedal to carb.

 

[OldBlue76]

Thanks for all the input, but none of the above is the issue I believe.

I did some further investigating and testing. I used an old diet scale (all I could find) to measure the resistance running and not. The resistance I'm experiencing is in fact a characteristic of these carburetors and is not butterfly binding. I put the old carb back in and it behaves the exact same way, but with less resistance than the new one.
I have another 76 and checked it (which I should of done from the get-go). Same result, there is more resistance to rotation when engine is running, but less than the “new” carb I was fussing with. My explanation is that the vacuum pulls down hard on the throttle plates which puts pressure on the shaft which increases the friction. It’s different on each carb probably due to wear difference on the shaft and housing. Because the scale did not go past 16oz, I didn’t confirm the max force reading to move the throttle when running, but the engine off force reading on the “new” carb was higher (9oz vs 6oz) and the user experience definitely tells me the “new” carb friction is much higher when running than the others.

So, I am left with an explanation but no solution. I will try spraying some silicone on it but not sure how long that lasts on a carb. After I put the old carb back in I was reminded why I replaced it with the “new” one. It does not set up well even after I rebuilt it (but it does rotate easier!!!). So I can either have a soft touch ruff idle carb, or a stiff response good idle carb…..

If you have access to Face Book, here is my post on it with video:

https://www.facebook.com/groups/110...8&notif_t=feedback_reaction_generic&ref=notif

 

[knack]

Are the engine mounts OK? Engine moving around making the linkage bind up?