I have a 1979 Ford F150 with a 302 and a 2150 2 barrel off a 78 LTD.


My truck starts fine and idles great but after a few minutes it dies if you don't give it gas. Once it gets warm it is harder to start and you have to hold the gas down to get it started. As long as it is running (going down the road), it runs really smooth. No misses or backfires, plenty of power, etc.


It just started doing this. I set the timing and set the carb using a vacuum gauge a few weeks ago.


I can set the idle a little higher at the carb and it will idle a little longer but eventually it starts to die.

My guess is that it is flooding and that it could be caused by the float sticking, getting too much gas somehow (but it ran fine up until a few days ago) or by plugs fouling out (but it doesn't backfire and doesn't sound like it is missing or anything).


I am going to pull the plugs tonight and look at them and maybe change them out. Also going to clean the rotor and distributor contacts and the pickup coil if it's not too much of a pain.


Just wanted to know if I am on the right track and any other quick checks I can run.


Thanks as always!

 

Sounds to me like your fuel is getting hot in idle and fine when driving. Make sure your fuel lines are not near your headers or exhaust. If you haven't already place a clear fuel filter just short of your carb to monitor the fuel supply.

 

I don't think it would be a heat issue since nothing has changed with the lines or exhaust, but the clear fuel filter is a great idea. I will do that for sure.
Thanks.

 

Am I understanding it wrong or does it idle fine until the engine is fully warm? If that's the case, I have to wonder about the choke as well as the idle screw/idle speed adjustments that you made. The adjustments should be done with a vacuum gauge to obtain maximum vacuum once the engine is fully warmed. Many do this far too soon to properly set the idle screws. The engine must be run for at least 30 minutes prior to adjustment or the engine is not properly warmed enough for those adjustments to be accurate. I also suspect that the idle speed is starting out at what the choke idle speed is, which should be higher than the warm idle speed I believe (I have a Holley, so hard for me to say for sure how the Motorcraft choke system works). If so, the engine is supposed to slow down from a high idle speed on choke to a slower idle speed off choke, which may be exactly what it is doing, but you may have set the idle to be normal while still on the high idle setting. Once the engine is warm, I think that any blip of the throttle will drop the idle to the low idle setting, but if you don't do that, you stay on the high idle setting. Hope that makes sense!

I would definitely double check the float level. That takes only a couple minutes and eliminates a basic operation of the carb from being the culprit. Otherwise, if you're mixture screws are turned in a tad bit too far, the engine will want to die and will do so eventually.

If it were me, I'd start the truck cold and look for vacuum leaks while it is still idling fine. Blip the throttle once in a while during this time. Once it gets to the point of dropping rpms and starting to die, I'd adjust the idle speed screw to get it to stay idling at the proper RPM. At this point, you should have a vacuum gauge and a tachometer attached (either an engine analyzer/dwell meter style tach or something like the Harbor Freight "automotive multimeter" which has a tach function). Once you have the idle in the neighborhood of 800 rpms, start adjusting the idle mixture screws to obtain maximum vacuum. If they were off, you may have to adjust the idle speed again from time to time to get it back to 800 if it climbs. Keep fine tuning the mixture screws and idle speed from time to time until it is completely warm at 30 minutes of running. Once you get past 30 minutes of running, you then want to make final adjustments to the timing, then final adjustments to the idle speed, followed by final adjustments to the idle mixture.

For the idle mixture, turn the screw out until it no longer increases vacuum, then turn it out maybe half a turn more. Do the same for both, then turn each screw in until you see the vacuum just barely start to drop, and turn the screw back out 1/4 of a turn. I usually do this half a dozen times to be sure that I like where both screws are at. Once this is done, observe the motion of the needle on the vacuum gauge. It usually has a slight vibration to it at this point, and I do mean almost unmeasureably slight as if vibrating from sitting on the engine, adjust one of the idle mixture screws very, very slowly in or out to see if you can steady the vibration of the needle. When you get the steadiest reading, do the same with the other mixture screw and try to get a dead still needle. If the engine is in decent shape, you should be able to get the mixture screws adjusted so that the vacuum gauge needle literally does not vibrate at all. There may be slow meandering motion in the total vacuum level of perhaps 1/4" hg up or down which I think is due to variation in fuel pressure if you don't have a pressure regulator (I still need to buy one), but the needle should look otherwise dead still or as close to that as you can get it.

The difference from getting it to the first point where you have maximum vacuum for both mixture screws and getting it to where the needle is dead steady is very noticeable in how the engine runs when driving. I'd be curious to know for sure how it measures out, but I would not be surprised if it makes a 10% difference in total power output. I used to go by the rule of obtaining maximum vacuum, then backing out 1/4 of a turn, but that's where I would stop tuning. Once I learned about trying to fine tune it further to eliminate any needle vibration, I was stunned at the difference you could feel just in taking that one additional step. It has a pretty significant affect on both on and off idle engine performance from what I can feel.

Once you get the warm running idle sorted out, then you can adjust the choke settings. I don't really know much about those for your carburetor, so you'll have to look up the instructions for that. I am pretty sure that the choke settings need to be done after you get the warm engine setting correct first.

 

Wow, thanks for the great info! I did set the vacuum cold. Today once it started to stall, I was able to adjust the idle to keep it running. Wish I had seen this post sooner!
I'll try to reset it tomorrow.
I'll bet your right about needing to set the choke too. I noticed it wasn't fully open after the engine was warm. I found a good YouTube video for that.
Ill post back once I get to mess with it some more.

 

I pulled the fuel filter off at the carb and put the gas line in a bottle and it pumped out fine. I ran it right into the carb without the fuel filter and same result. It will start, run and idle fine for a few minutes and then it will slowly bog down and die. To get it started back you have to hold the pedal to the floor and let it crank (like it is flooded).
Then to keep it running you have to keep tapping the pedal. It doesn't backfire, stumble or anything as long as you keep it running. I am hoping to be able to try and readjust the idle tomorrow but it is supposed to thunderstorm of course. It's driving me crazy not having time to work on it.

 

I had the same problem, with my 73, 360, 4bbl. Started doing it, and wouldn't restart until you primed it. I just kept driving it, and riding the throttle at stop lights (manual, neutral, foot on brake and gas). I drove it around a hundred miles that way. My issue was gone after the next tank of gas.

 

Bad gas, huh? I have had issues with bad gasoline before, I guess we will see. I will reset the idle mixture screws on the carb and go from there. It needs a fill-up now.

 

Well something is definitely off with the carb. One of the idle adjustment screws has no effect at all on the carb, even screwed all the way in. Also, it floods every time you shut it off. So to start it, you have to turn it over for four or five seconds then push the gas down a little for it to start.
Nothing a rebuild kit and a few beers won't solve I bet.