'78 F150 with a 302 and 3 speed manual, no A/C.

When driving through town at a speed just off idle, the engine will cut in/out real bad and jerk the truck around. It needs a good amount of throttle when starting from a stop, or it will die. It does it even if I ease into the throttle real slow... then it hits a certain spot and it just takes off like crazy and runs great.

The choke doesn't work, so I plan to replace it with a manual choke. Do I still need to use the vacuum thing near the choke?

There is a vacuum line from a port on the back side of the carb to a tee fitting, and then to the choke thing and the vacuum advance. Is this the proper routing? http://i44.photobucket.com/albums/f2...2-15-06005.jpg

 

That is the choke pull off, it keeps the choke from staying completely closed after starting so that the engine can get some air.

Sounds like either the timing is too slow, your vacuum advance isn't working or you need to back the mixture screws on the front of the carb out a couple of turns.

 

Thanks, I will check it out.

The "search" is apparently broken for me because it won't allow me to search, so...

Do I still need the vacuum operated choke pull off if I convert to a manual choke?

Are the vacuum lines in my picture connected correctly?

 

It is just a safegaurd against closing the choke too far, but no you don't have to have it.

 

460- I think you need to change your vacuum lines. The choke pull off should be hooked to manifold vacuum(high at idle) and the distributor vacuum advance should be hooked to ported vacuum(none at idle, increases with light throttle).

The present hook up will advance the timing a lot at idle and the it will reduce timing under acceleration until the mechanical advance starts. It could be causing the hesitation and poor off idle performance.

 

not related...but do you not run a pcv valve? it looks to be missing in the picture

 

Nope, not yet. I recently bought this truck and it had no air cleaner at all... I guess the previous owner wanted to keep it? I bought a chaep chrome job at Napa for 20 bucks and stuck it on there. Do I just install the pcv valve in the valve cover and then run a hose to the fitting on the bottom of the air cleaner?







I can find only one vacuum port on this carb, and it is located on the back side near the mounting bolt under the pull-off. Right now it has a line connected to it that splits and goes to the pull off and vacuum advance. So, I can connect the pull-off to manifold vacuum (or eliminate it because I am switching to a manual choke), and then connect the vacuum advance to this port on the back of the carb? On my 351M, the vacuum advance connects to a port on the side of the carb, but this port is missing on my 302 carb.


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The PCV valve needs to be installed in one valve cover and then plumbed into manifold vacuum, make sure you have a breather type oil fill on the other valve cover.

You can eliminate the choke pull off with a manual choke, your finger pushing the cable open performs that function.

Does that port have vacuum at idle??

 

It looks like I may already have a pcv valve on the driver side valve cover. There is a hose from it that connects to a spacer plate between the carb and manifold. So what would I connect the passenger side valve cover to then? Thanks for all you help, I'm an idiot when it comes to engines. Here is a pic of what I have http://i44.photobucket.com/albums/f2...2-15-06001.jpg


I will have to check that port on the back of the carb to see if it has vacuum at idle...



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I would also check for proper operation of accelerator pumps which gives a extra squirt of fuel when you move/open the throttle plates at lower throttle settings.

 

Yes, it seemed to have quite a bit of vacuum at idle.





Checked, and it is working properly.



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Is there a vacuum port under the front of the carb somewhere that may be ported vacuum?

 

I haven't seen any yet. I will get my little mirror under there and check it out.

In the pic linked below, there is an arrow drawn to a hole in the side of the carb, this is where the vacuum advance connects to the carb on my 351M... but there is nothing to connect to on this one. Maybe the "nipple" is missing? http://i44.photobucket.com/albums/f2...2-15-06004.jpg



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It seems likely the nipple is gone. Try holding your finger over the hole and opening the throttle, is there vacuum when the throttle is moved??

The passenger valve cover should have a breather type cap or a hose running to the air cleaner INSIDE the air filter element.

 

Finally - all seems well. I pushed a small piece of plastic tubing (like the stuff used for an aftermarket oil pressure gauge) into the empty port and connected the vacuum advance to it as it is on my 351M and my 390. I bought a 90* plastic elbow from Napa that fit the gromet in the valve cover, and used a piece of heater hose to connect it to the fitting on the air cleaner base. I also installed a manual choke and replaced the plugs, plug wires, cap, rotor, and fuel filter and adjusted the idle speed and idle mixture. It runs fairly good now.

Thanks to everyone who helped