77 f150.

300 straight six

Three on the tree

Fresh distributor

Rebuilt carb

double checked floats

No traceale vacuum leaks. Disconnectex vacuum lines, sprayed ether, etc. Definitely not the booster.

Damn thing wont stay running. It will fire up and run like a top while cold. Will continue to run fine hot. Unless you try to drive it. Stalls every time you come to a stop sign. Will be just fine rolling along in neutral, but hit the brakes and it dies. I replaced the distributor because the vacuum advance was dead. Ran much better, same problem. Rebuilt the carb. Same problem. Adjusted the clutch. Made sure fuel lines were clear and filters clean. Changed fuel pump. Had ignition module and coul tested.

If you block part of the carb throat, it picks up hundreds of rpm. Like a vacuum leak. But no leaks found.

Ive adjusted the idle to 2500 rpm, and it does it less, but still does it.

Im just trying to get this thing running right so it can be sold, as my neice (whis truck it is) got a different truck to take to west point for school in the fall.

What am i missing??????

 

It is possible there may be a vacuum leak in the carb itself. The throttle plate shaft can wear out and bushings and/or a new shaft may be in order.

 

Good call, and definitely a possibility. This isn't something that's fixed with a regular rebuild.


Has the engine ever run right in your ownership? Meaning, did you acquire the vehicle like this? Or is this problem that arose after a particular event or sitting for a really long time or something?

So you can drive it normally while the engine is cold? But once the engine warms up, it'll stall and die at stops, etc?

When you tested the ignition module, did they test it 5 or 6 times so that it got good and hot?

 

Yep that's what I am thinking bad carb.

I had one did the same thing, rebuild did not fix it either so I replaced it, motor runs great now.
Dave - - - -

 

I am betting on a bad brake booster.

 

It cannot be the booster. Or any other external vacuum accessories. I have, as a diagnostics, pulled and capped everything that can be. Leaving only tbe carb, and intake manifold under vacuum.

it may be a carb, but the problem started suddenly according to my neice. Which would lead me somewhere else at this point, unless the throttle shaft hit a certain point of wear causing the issue overnight.

 

it ran great for 20k after she aquifed it. Issue started suddenly. Truck hadn't been sitting, and there was no specific event according to her.

we did not test tje module more than once.

i havent moved ut any more than driveway while on the choke, but warmed up and off the choke, it will stall in the driveway.

 

Have you checked the intake manifold for cracks?

Otherwise, the module needs to be repeatedly tested (about 4 - 5 times) so that it heats up. Most of them won't have issues until they're HOT. But a control module that's going out can cause very strange drivability issues.

 

I am not sure about your 1977, but on the later trucks [1984-1986], there are multiple vacuum ports on the intake manifold and fire wall. One vacuum "tree" has about 6 or 7 ports, and most are covered with a rubber cap. There is one particular port on that tree which normally is positioned toward the cab [sometimes pointed down, slightly] and nearest the engine [just under the carburetor and on outside of the intake manifold]. It is difficult to see. On two trucks, with eratic idle and running, I found that the rubber cap had deteriorated and was letting air suck in. It could not be heard [near the exhaust] and was difficult to find with carb cleaner [ether] since the port points away from the spray. Sometimes when I sprayed along the intake manifold, I thought I could see a difference, but could not get it to repeat. The spray must have bounced off the engine and was sucked into that port.

Good luck.