Engine idles & runs perfectly when cold, responsive to the gas pedal, plenty of power, but as it warms up after 15 minutes of driving it starts to sputter and misfire. First symptom, pressing on the gas pedal to accelerate from 25 mph, very noticeable hesitation. A few minutes after that, balks when attempting to go over 40 mph. 10-15 minutes after that the performance gets much worse, hard to keep running, and stalls at idle, even in neutral.
Engine was due for new points anyway, so I did that first. No change, engine still performs perfectly when cold, sputters and eventually stalls when fully warm.
Considered the possibility of a clogged-carb, thinking it was going way too lean as the engine warmed up and the auto-choke plate went to full open. So starting w/a cold engine I removed the air cleaner and watched the choke plate as the engine warmed up. No difference comparing a cold engine, choke plate nearly closed to warm engine, choke plate fully open. Idled and responded to gas pedal normally in both cases. After another 10 minutes of idling, engine warmed further, choke plate still fully open, starts to sputter and shake at idle, poor response to gas pedal. Exhaust pulse feels like engine is missing.
Immediately after it stalled in neutral I popped the top off the carb. Float horizontal,
fuel bowl level is correct. Measured fuel pump delivery rate, in spec.
I’m guessing the results above indicate a problem due to something in the engine compartment getting hot. I’m thinking coil or condenser. Any other ideas?
Fuel pump diaphram might be worth looking at, the other might be an exhaust leak hitting the metal gas line I would guess overheating the fuel being delivered to the carb. Good luck.
Thanks for the ideas. It seems like on the fuel level test I did immediately after a stall, the fuel level would have been low if the problem was the fuel pump. It was at the proper level. The exhaust gas heating up a fuel line seems plausible. Easy enough to check for. Wouldn’t that have resulted in a low fuel level? I’m also planning to remove the coil and the ignition’s condenser and measure their electrical properties as I heat them up w/a hairdryer.
George, doesn’t that engine have some temperature activated vacuum switches? Could be a vacuum leak that doesn’t show up until the valve opens. Also, does this engine use that oddball variable venturi carburetor?
The carb design is pretty simple, 2B Autolite 2100. That’s the predecessor to the very common MotorCraft 2150, the main difference is the 2100 doesn’t use metering rods. The heat actuated vacuum switch idea is very good, something I’ll definitely look into if the coil and condenser check out. There’s several of them and I can bypass them, one by one, as a test.
Edit: I posted a diagram of the vacuum system configuration for this truck a few years ago. The thread remains but the diagram seems to have disappeared into the bit bucket.
There was something else to look at on the old 70s stuff. Condensers could go bad, especially if they were external. Some cars had them attached to the outside of the distributor. I think they were on cars with electronic ignition and my roadside repair was to cut the wire to them and the car would run great afterward. You probably don’t have one on your car but if you didn’t change the usual one when you changed the points, maybe you should change it. A bad one just grounds out the ignition. Usually when hot too.
Even though it runs good when cold, there could still be an ignition problem. I would want to check for a swollen coil wire end or hardened plug wire boots. The black rubber ones are almost always bad. You’re right to check the coil. Cap too. I never had any bad carbs on my fords except for accelerator pump leaks which would cause long cranking for starts because the float bowl would be empty.
My recollection is a heat sensitive spring that controls the choke, and it can be adjusted. Don’t know them all but do not recall metering rods. I do not recall a temp activated vacuum switch. S since we are throwing spaghetti at the wall might as well consider a sticking mechanical advance in the distributor.
Yes it does, most engines only have one of them, but there are several different ones.
There are 2 port and 3 port ones, Also there are several different temperatures, you need to match the color of the plastic part. I think there are Black, Blue, Yellow, Green and Brown. I don’t remember what color is what temp.
When cold check the timing at idle and as you rev the engine and recheck when warmed up. I gave up on the thermo-vacuum switching valves long ago and that is a likely cause of the problem. You can connect the vacuum advance directly to ported vacuum and then set base time to spec to correct the problem…
Hi George:
While this may not be your problem, do look at your intake manifold exhaust crossover passage. I’ve cleaned out many blocked passages years ago.
They block from from built-up exhaust carbon. When they do, acceleration hesitation is a common symptom, even with a warm engine. Stalling at idle was not a symptom. Fords didn’t seem to suffer from this as much as Chrysler’s V8 engines did.
I used to be able to detect blocked passages by touching the intake manifold below the carburetor. If it was hot enough so that you couldn’t keep your finger on it, then the passage was not blocked.
Keep this in the back of your mind as you triage this problem.
The most common cause for your symptoms are a stuck choke. Choke pull off’s are very common but they usually cause the engine to run rough right after the first time you step on the gas and clear up as the engine reaches full operating temperature. In your case, I suspect the bi-metalic spring has gotten weak and is not opening the choke fully. With the engine warmed up, the choke plate must be fully open.
Another cause often overlooked is the float level in the carburetor. As the needle and seat wear down, the fuel level rises in the carb and starts to spill over into the intake. The tab on the float can also bend a little as well as the float itself which is usually made from a plastic can absorb gas over time and get heavier. All causing the fuel level to rise. The rubber tip on the needle could also deteriorate allowing extra fuel into the bowl and overflow as well.
If you have the two barrel carburetor, get a rebuild kit and a can of carburetor spray cleaner. Maybe two cans if you want to clean the outside as well. These are super easy to rebuild as carburetors go. Just do it on a clean table top or bench top because it may have a small spring and ball that you don’t want to lose. One of these medium size drip pans used under vehicle engines to catch oil drips would be a good idea, the shallow lip would keep anything from rolling off.
The rebuild kits usually have good directions. One thing that I always do a little different is to lower the float about an 1/8" below the specs. I find they last longer doing this.
One last thing is that you will also get a new accelerator pump with the kit. I don’t think yours is bad as if it were, you would have the stumble all the time, warm or cold.
Dont overlook the ignition coil… and if you are brave… you can remove your coil and put it in your oven at 100-120 or so degrees and then install it again… see if you start out with your stumble. If so, you found the issue.
Or you can just buy a new one for $30 bucks or so? They don’t fail often, but they do fail at times.
Update: I’ve eliminated both the coil and condenser. Both check out ok when I heated them with the hair dryer on the bench. I re-installed them , same symptom continues: Truck cold-starts fine, idles as it always has, and after 15-20 seconds to warm up engine runs perfectly for 10-15 minutes. Then it starts to stumble, so I put it in neutral, engine barely idles, have to manipulate the gas pedal to keep it running, then stalls completely. One interesting point, after it stalls it immediately starts and idles fine for a few minutes, after which the same symptoms occur and it stalls again.
I’ve eliminated the the vacuum switches also. Bypassing them had no effect.
A couple more clues, when it fails it fails really fast, like turning on a switch from running perfectly to barely running. Also I’ve noticed an unusual engine compartment noise that seems to occur along with the poor engine performance. I’ve been able to isolate that sound to the front part of the engine. It sounds like it’s coming from the water pump. Sort of a scraping noise. I’m now thinking the water pump may be corroded & locking up as the coolant heats up, which is lugging down & stalling the engine.
Also thinking the carb’s power valve might be involved as a back-up plan; however the symptoms are not very consistent with that theory. But it does have the merit that a leaking power valve would make the engine run rich and that might show up only when the engine was really fully warmed up. The problem is that an engine running that rich would probably pour smoke out the tailpipe and the exhaust would smell like gasoline. Also a leaking power valve would make it hard to start when warm. None of these are the case .
As the next back-up, I’m thinking timing chain problem. Are these symptoms consistent with any timing chain problem you’ve ever experienced?
Removing the gas cap and checking the ignition timing and how it advances w/rpm, esp for a stuck dist advance are relatively easy to do. Good ideas. I’ll give those a try.
While checking timing advance, make sure the vacuum advance works too. I find that spraying a little oil on the weights takes care of any mechanical advance issues. Then take a piece of vacuum hose and hook it up to the vacuum advance and suck on the other end. If you can’t pull a vacuum or if you do but the points plate doesn’t move, then either you need more oil for the plate or a new vacuum advance diaphragm.
I’ve already verified the distributor plates move with increasing vacuum for the vacuum advance. It could be sticking I suppose. But it has never stuck while I was testing it with a hand-held vacuum pump. I checked the ignition timing with a timing light when I was doing some ignition points repair a couple months ago. But I didn’t repeat that when I installed new points recently. I just verified the dwell was the same.
Turn the engine over until it is at the spec for timing advance. I.e., if the spec for timing is 4 degrees BTDC, then turn the engine until the timing pointer lines up at this mark and the rotor is pointing at the #1 cylinder. The points should just be starting to open. I loosen the distributor, turn the key to run and them move the distributor back and forth and stop just as I see the spark at the points. Then lock it down. Be sure to check the point gap before starting this or check the dwell if you still have one of those meters.
Possibly a vacuum leak which is masked by the carb running rich when cold or may be an issue with the coil ror condenser. A flaky condenser can mimic an engine sputtering like it’s running out of gas.