Greetings. Guitar truck here and I am finally back to working on the Old Blue as I am now calling her. Have finished up a few home projects and now it's back to the truck. To recap : 1982 F150 -XLT Lariat, 2 wheel drive, 302, C6, A/C all emissions equipment in place and passed AZ emissions in January Number 6 cylinder tested at 50psi when we got her back in the road. OE M/C carb rebuilt by a shop in Nebraska. Whole new fuel system from tank forward. The issue I am dealing with is when the truck gets warm, occasionally she will stall, especially with the A/C on. She starts right back up, however upon acceleration she stumbles with no power, catches then accelerates. The stumbling occurs even if the truck doesn't stall. Does not happen when the engine is first started in the am. Otherwise she starts and runs as well as a truck should on 7 1/2 cylinders (see compression issue above). Since I am new to this I don't want to start ripping into things without a plan of attack. Any advice on what that plan should look like is most appreciated.

 

Have you adjusted the carb after you had it rebuilt? I would back the mixture screws out some and see what happens. They are hard to see, they are way under the front of the carb. Always count the number of turns you move them so you can put them back if it doesn't help it.

 

No Franklin I haven't touched. Makes sense, since the shop who rebuilt the carb is located in Nebraska and probably set it up for colder conditions, read slightly richer mixture. Out here where I am it will never get below 50 and frequently like today is over 100.

 

Does the hesitation go away once warmed up? After I rebuilt my carb I had that issue and it was corrected by setting the choke slightly richer as the choke was coming off too soon on warm days and would cause a hesitation some times a actually drop and pick back up but once warmed up ran like a top.

I also had it doing it when hot but I was able to correct it by moving the accelerator pump rod up to the top most hole (mine only had 3 and was already set in the middle), this gave the pump more stroke which sprayed more fuel into the engine.

 

No Rusty it runs fine when cold. The hesitation starts when the truck is warm. The truck starts fine when cold. Two pumps no throttle kicks right over. Runs nice and strong till temp gets up to normal operating. Then the hesitation and occasional stalling starts. Could be choke not coming off enough??

 

How long does it take to "warm up" (from a dead-cold start) and for the problem to appear? Does it ever get any better as the engine runs longer/warms up more?

 

Since we are in the middle of the summer and I am using the a/c it takes maybe 3-4 miles of driving to get the middle of the temp gauge. Once it gets there is when the problems start. Doesn't get any better or worse the longer I drive.

 

I can't tell if you're fuel injected but I will assume not since you mentioned having a carb.

First think I'd look for is a vacuum leak. If you're too lean (because of a vacuum leak) you'll idle rough, stall and stumble. You can try spraying just a little starting fluid around your intake and base of carb. If the motor speeds up a bunch you may have found a leak.

Another thing to check is your ignition timing and vacuum advance. With the engine off, pop the distributor cap off, attach a long vacuum hose to the vacuum advance on the side of your distributor and suck on it. If you can draw air through it your vacuum advance diaphragm is toast and needs to be replaced. Otherwise you should see the pick up plate move as vacuum is applied.

Also while you're in there, give the rotor a little twist to check your mechanical advance. You should be able to turn it a few degrees and feel the resistance of the springs. Then when you let go it should spring back.

There are a variety of ways to set base timing but you can't always rely on the timing mark on the crank to be accurate because on certain models the damper can rotate if the rubber inside gets cracked and wore out. Sometimes advancing the timing until it starts to ping under a load, or doesn't want to turn over when hot, and then backing the timing off until things smooth out is effective.

 

Thanks guys. I will try things in this order. Vacuum leak check, vacuum and mechanical advance then accelerate pump then mixture. Let's see what happens.

 

Perhaps the choke is not opening fully when warm?

 

that is a vary good plan.
When you check vacuum & mechanical advance thru a timing light on it to see were base is at. If you have a dial back light you can check base, vacuum & mechanical timing and see what each adds to the base for a total.

Adding a little more timing can help cure a hesitation just watch for spark knock.
I am going to say choke is opening all the way other wise he would have poor MPG and black smoke out the tail pipe(s).


It sounds like it is running lean when it is up to temp, read choke off all the way. Now it may be jetted right so not lean in that way but pump squirt lean and when the choke is on it richens it up just enough to cover up the lean squirt.
Dave ----

 

EGR valve opening too soon, or sticking?

 

Running good cold, bad warm is the clue I am concentrating on. It could be many things, you just have to start fiddling with different things. Just make sure you when you move something you mark it or make a note of where it was when you started. It's not uncommon to come out of this with the same problem and a few others added that you have created by adjusting different things. That's why you need to keep track of how much you turn the mixture screws if you do adjust them. I would get the engine hot so it's not running good, and then get out and turn each screw counterclockwise 1/2 to 1 turn. These emissions carbs have very fine threads on the mixture screws, so sometimes you have to turn them alot to have some affect on the engine.