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Hi, I have a 1981 250 and had some of the same problems you are experiencing.
If you can get a propane cylinder, you can use the gas to help you find vacuum leaks. I would check the base of the carburetor where it mounts to the manifold, and also the throttle shaft to make sure there isn't excessive wear in the bore. The EGR valve to make sure it isn't partially stuck open, or the gasket is leaking at the base of the valve. Sometimes you can wiggle the sub-assembly while the engine is running and that might cause an idle change. Check the brake booster line to verify that it hasn't gone hard and isn't sealing properly.
If you have a non-integrated head, you might check for vacuum leaks at the juncture between the head and the manifold.
I don't know if you have changed the fuel filters. I don't know about the 1 bbl units. I had a bad one in my 2bbl Varajet. Truck wouldn't go above 35 mph. Rough idle. Starting fine. Sluggish performance. Dieseling. Same as what you are describing
A lean condition will not allow complete combustion of the fuel, due to an erratic flame front. That will give you a gassy smell at the exhaust. Did the smell show up at the same time as these other symptoms?
You might also want to check operation of any heat stove to make sure it is operating correctly.
What kind of gas mileage were you getting before this problem showed up?
Last edited by Kazoocruiser; December 29th, 2018 at 2:07 PM.
Reason: additional cogitation
Kevinkpk and Kazoocruiser, you both gave me some things to check.
When we bought the truck 3 years ago it was getting around 16 mpg.
Thank you both for your input. As a new wrench turner, there are several terms and suggestions which will require some research on my part.
Update: So, according to the vacuum hose schematics, the engine is the 912F2 Fed. Light Duty. I have diagram, but it is difficult for me to decipher where what goes.
Easy: Canister, Carb, Air Cleaner, PCV, EGR valve, Distributor,
Difficult: VAC BRK TVS, TVS, Delay Valve, Manifold Vac Source, EFE (Early Fuel Evaporation) TVS.
The military and working in education did not train me for these acronyms.
One photo below is of the vacuum diagram. The line (I assume) should go from the canister to the EGR valve is plugged with a small bolt.
Nothing is attached to the EGR valve.
The next photo is of a small piece attached to the air cleaner housing. One of the nipples is bare.
I will pull the plugs tomorrow to see if any are fouled.
Manifold vacuum source is from the pcv (positive crankcase vent.) and manifold. These are the vacuum source for the entire system. You might be better off removing the emissions stuff if possible. The TVS thermal vacuum switch. On the spark plug issue, when you pull one, post what it is i.e. Delco R44T etc.
Spark plugs are AC R45TS, and they look pretty cooked. Plugs 1 and 2 had visible oil on them and turned pretty hard from what looks like a small leak in the valve cover gasket near the front of the engine.
The last (bottom) 3 look well worn, but normal. I'd replace them (check proper gap), then if you are still having rough idle (which I suspect you will have) chase a vacuum leak. The source is from the intake manifold, and PCV valve, I'd suggest remove all the end devices except the carb, and distributor, that will tell you that if it clears up, you have a vacuum leak on one of the end devices, same with electrical (or replace all hoses) hook vacuum lines up individually till it misses, that will be the culprit. Don't be surprised if it has more than one leak. The R45 (5 being the heat range) should be good with a six cylinder, but make sure the plug gap is correct. That is a very common plug for alot of engines. You might check the existing gap on one of the old plugs, verify it is correct to that specific engine.
The new spark plugs (same brand and type).
Replaced two vacuum lines.
Replaced the cracked PCV grommet on the valve cover.
She starts right up and purrs. The timing and/or the carb still needs some adjustment, but it no longer diesels after shutdown. A friend has a timing light and we will tackle it next week. I also found out my neighbor had a very similar truck long ago.
He has experience with this carburetor, and his age and lack of hair seem to back up the claim.
Check for the proper operation of the choke and related items. Also check the timing of the engine.
Search the distributor. New rotor, cap, and wires? How is the vacuum advance? The nylon cam spur gear at the base OK ? Clean the distributor cap thoroughly, make sure there is no carbon tracking on the inside or outside.
Look for water in the oil, and run a compression test, and check the head bolts with a torque wrench.
IIRC, these I6s were famous for blown head gaskets,
and head bolts which liked to back off their torque.
Not to rain on your parade, but at 40 years old, the valve train, timing chain(which could have the factory nylon timing gears, with worn or broken teeth) and cam could need work. Or the timing chain could have just stretched, and jumped a little.
Last edited by therewolf; February 27th, 2019 at 7:32 PM.
December 29th, 2018, 1:53 PM
