I have a Motorcraft 2150 on my '78 351M. Rebuilt it and got it tuned last week and it ran decent. This week it wants to stall everytime it's in neutral and I stop or am pointed downhill slightly. I did notice while tuning it by ear as I adjusted the air fuel, I was unable to notice much change as I backed them in and out.

I also noticed that if both screws were completely closed the engine would continue to run, but once i stopped the engine it would not crank back up until i moved them out.

Is this normal or is my carb just worn out?

 

It's not set up correctly.

If it stalls when you're pointed downhill, the float height is set incorrectly and the fuel level is too low in the bowl. With the carburetor already put together, you'll need to correct this by doing a wet float height measurement with the air horn removed and engine running.

When you say "air fuel" I assume you mean both idle mixture screws on the front of the carburetor? Are you saying the engine runs with them turned in completely? That's almost impossible, so please clarify.

You can usually get by adjusting the curb idle speed by ear, but you really need to use a vacuum gauge to set the idle mixture. Otherwise you're shooting in the dark and can't effectively troubleshoot. With the truck shut off, turn both mixture screws all the way in gently, then back out 2 turns each. Connect the gauge to a manifold vacuum source, start the truck, then adjust the screws almost simultaneously (they may need to separate some) to get the maximum vacuum reading (you want 19 to 21 inches of mercury). After that's pegged, bring the curb idle back down.

I'm also assuming you're only having these issues with the engine warm - if any of this happens before the engine reaches operating temperature, there's another group of things to assess.

Conclusion: no, you do not need to replace your carburetor; it just needs to be set up. And if you decide to forgo all the troubleshooting steps I have listed here and opt for a replacement anyway (which happens a lot on this website), a replacement carburetor will likely have the exact same issues you have now, because it will still need to be set up.

 

Ok. What should the float measurement be?

Also I am talking about the adjustment screws. If they are completely closed tight the engine will still run. I've heard if someone tights the screws to tight the seats can be ruined thus rendering the carb junk.

 

It depends on year, engine displacement, etc. It will be in your rebuild sheet.

That's possible. Remove the screws and inspect the tapered ends to see if this may have happened. See if you can peer into the seats with a flashlight and a mirror.

 

Sounds like a vacuum leak, check all vacuum lines and get yourself a vacuum gauge for tuning and diag. I'm going to assume you used correct new gaskets to reinstall carb and choke is working properly . While running spray some carb cleaner(use the extention tube that came with the can)around the base gasket an throttle shaft, listen for change in idle speed which would indicate a vacuum leak at these locations. Post back what you find.

Edit: Pay attention to what fmc400 is telling you, the results will be worth it! The 2150 is a fine carb.

 

Already used a vacuum gauge. It held steady. No leaks

 

Choke might be an issue though. It doesn't seem to be opening all the way once warmed up