I have a '65 F100 with a 240 CI, 1bbl carb, 3 speed manual tranny. My truck has begin to lurch and chug (not sure if these are the proper descriptive words or not). It was running perfectly fine one day. The next day I started it and drove it down the road about five miles and it began to chug and lurch. I turned arounds and struggled to get it home.

Pertinent information that may assist you in helping me out:
I completely restored this vehicle over the past two years. I have put about 10K worth of new parts in it (yes, probably too much), to include the following: Alternator, battery, voltage regulator, distributor, distributor, cap, rotor, points, plugs, plug wires, coil, water pump, thermostat, radiator, fuel pump, fuel filter, and carburetor (adjusted to manufacturer specs for 8,000 ft elevation), just to name a few relevant parts. I recently had my harmonic balancer rebuilt. I then reset my timing, accounting for my high elevation.

The truck starts perfectly and idles very smooth at 800 RPMs. It accelerates very smoothly up to 4-5K RPMs, while parked. When I get the truck down the road about 5 miles it begins to "act up"--chugging and sputtering and wanting to stall. I struggle to get it home in first and second gear, chugging all the way.

I recently drained and cleaned the entire fuel system, thinking I might have gotten bad fuel somewhere. This did not help!

One additional note: The motor has always appeared to lack power. I struggle to get it over 65 MPH, unless going down hill. When any load is put on the motor (e.g., going up hill or into a strong WY wind) I loose power quickly. I tested the cylinder pressures, and they are all from 90-100 lbs. Which seems within specs for this age of a vehicle and motor. I have no idea what condition the valves are in.

I am not a professional mechanic, but I've been working on cars and small engines for thirty-five years or better. I feel that I am fairly competent under the hood, to a certain degree. This one has stumped me!! I'm hoping that someone out there has seen a similar issue and can provide some advise.

Please request additional details if you think that might help. Any assistance would be greatly appreciated.

Thank you for looking,
Brad

 

Starting with the simplest and most basic thing that can cause at least some of the symptoms you describe would be an incorrect gas cap. If it's not able to draw in air, then the tank creates an internal vacuum and won't let gas go down the fuel line to the fuel pump very easily. Five miles is roughly the about of time it takes to draw enough fuel from the tank to create the vacuum and then the problem occurs.

Likewise, if the cap can't release air and fumes, then on hot days as the gas expands it can create a pressure build-up and in extreme cases can actually cause the tank to permanently bulge in a worst-case scenario. If that happens it's best to replace the tank.

The correct gas cap has a check ball in it, perhaps the check ball is stuck.

 

X2 on this. Sound like it could be starving out, drive it for a mile or 3, pull over and open cap and listen for vacuum or sucking sound, I had a vented cap, brand new, no bueno, took me a day to go oh, duh, unscrewed cap to where it was loose, drop to NAPA, got a new one and haven't looked back.

 

Wondering the same thing if it might not be gas cap/check related. Had something similar happen back in mid-eighties with a 70 F100, I replaced the old cap for a bright shiny new one that wasn't vented and had a similar problem. Driving the few miles to work in the morning it started cutting out and stalling and eventually quit on me.

 

Thank you all for the suggestion. I will order a new gas cap. Just seems strange that it was running perfectly fine one day and then it wasn't.
I'll check back in after I replace the cap in case that did not help.

Brad

 

You can test a gas cap using nothing more than oneself. I hate to have you sucking and blowing through your gas cap but it’s an easy test. Ask if you have never done it and need directions.

 

One thing I'm thinking of is the 240 had the load-o-matic distributor is some cases and it needed/needs the carburetor that is designed for that distributor which only has vacuum advance. Some people do swap to a dual (vacuum and centrifugal) advance distributor to get away from that and be able to use any carburetor. But, if it is the load-o-matic distributor and you aren't using the correct carburetor design like original then performance overall can suffer like you describe as an overall symptom of lack of power.

If you search just the 1961-1966 forum for either loadomatic or more common load-o-matic there is lots of good reading material. The shop manual also goes into some detail of the combination of the distributor and carburetor. Tough to tell from looking if that's what you have, but it was very common.

 

Heck, take the cap off, stuff a rag in the filler next and head down the road and see if the problem occurs. If not then buy a new gas cap.

 

Also tell us if you can - after the chugging and sputtering and wanting to stall all the way back home what happens to clear it back up? A wait overnight? Just a few minutes? Just getting it back home and then it starts to run correctly once in your driveway? Just curious if it might be some sort of heat soak or fuel vaporization issue. How hot is the outside temperatures when this happens? Does it always happen day or night? The further you go the deeper things get and trying to troubleshoot without being there riding along with you makes it especially difficult. We don't get to hear or feel or smell or any of the senses that can help. But we can try...noticing you don't have a phenolic or aluminum (some people even use wood, but I wouldn't recommend it) spacer between the carburetor and the intake manifold. That might cause some issues.

 

Thank you very much for the information. I will look into the carb/distributor compatibility issue.

 

I notice that after i finally get the truck back home it will start and idle fairly well but still sputters and struggles. I normally just shut it down and don't mess with it again until the next morning. Then it starts perfectly well and then five minutes down the road we repeat the process. I understand the difficulty diagnosing the problem if not riding along or being able to look at the problem in person, but I appreciate any guesses.

The gasket/spacer under the carb is the one that came with the new carb.

Thank you for your comments.

 

I am in WY at elevation so it rarely get above 80-85 here. And yes, it happened day or night. What strikes me as strange is that it was running perfectly fine with the carb and distributor combo (minus the lack of power) and then one day I started it up and tried to drive to town (35 miles) and this problem began.

 

Thank you. I just ordered a new cap, just in case that is the problem.

 

Thanks for the suggestion. I already purchased a new cap. I've syphoned (and drank) a lot of gas in my time.

 

Looking back at your old posts. You were having about the same problem last year.
The fuel cap suggestions are good to check.
What I'm seeing in that picture may be the wrong carburetor for a load-o-matic distributor if that is what you have. A Holley 1908 would have a spark control valve, but I don't see one on that carb. The shop manual lists three different carbs for a 65, 240. Two are for load-o-matic distributors and one for dual advance distributors. None are Holley's. But if you have a dual advance distributor that Holley might work if it doesn't have a spark control valve in it.
Unless your engine has a dual advance distributor, it should have one of these. Tomco Ford 1BBL Carburetor 192 | O'Reilly Auto Parts (oreillyauto.com)
Whatever you have you need to lose that lawn mower air filter at your altitude you need something big enough to let some air in.