Why won’t that old truck run properly?” I bet all of us on the farm have said this at one time or another, though perhaps we’ve used different names and adjectives. Old trucks are common on most farms and with the advent of long hauling for grain, our two-and three-ton trucks are not replaced as often because they get so few miles put on them. I’ve talked to dozens of owners who want to keep the old three-ton running, but don’t want to spend hordes of money because, “We only put 500 miles a year on the old girl.”
Here are a few tips for keeping the engine in shape. One of the keys to repairing any gas engine is to remember that they need three things to run: ignition, fuel and compression.
Let’s start things off with ignition. Some vehicles are prone to ignition switch problems. Make sure you’re getting good spark to the coil all the time. I once had a truck that would cut out when you were driving down the highway. All I had to do was wiggle the ignition switch and it ran fine. I replaced the switch and that was the end of the problem. Many of the trucks with dash-mounted switches had this problem.
Plugs and wires. These can cause lots of problems because even though they look fine, they can be faulty. Spark plugs that look perfectly fine may not fire. They can be defective, old and tired, or if the vehicle has flooded, the excess gas often ruins the spark plugs. We had a severely flooded truck in the shop once that would not run. We took out the plugs (which were fairly new) and found that only one of the eight plugs was firing. A Ford mechanic told me that in the ‘70s, if a new car or truck right from the factory didn’t run right, the first thing they did was put in new plugs.
Likewise with the wires. They may look fine, but if the plug wires are five or 10 years old, they may not be working well. I once spent countless dollars on our van because it wasn’t running right only to find that the plug wires were the culprits. If you’re getting new ones, get a new set that’s custom made for the vehicle you have. Make sure you’re careful not to mix up the firing order when you change the wires.
Distributor. A truck won’t run properly if the distributor is worn out. It will misfire, backfire and perhaps even catch on fire if the plugs aren’t getting the right spark at the right time. This is common especially on the Ford 330, 361 and 391 V8 engines. For some reason these distributors tend to wear the top bushings out. Then the point opening will vary significantly between cylinders and this will cause misfiring.
Points are typically set around 0.017 inch, which is about the thickness of a paper match. If you have 0.010 inch of wear in the top bushing, that means your point gap can vary from 0.012 to 0.022 of an inch. Your truck will not run properly. Replace the distributor but make sure if you’re installing a used one that it isn’t worn out, too. A rebuilt distributor only costs $80 and up, so used or rebuilt, it’s a cheap fix. One customer had two Ford grain trucks. He bought a rebuilt distributor for one and it ran so well he was back shortly for one for the second truck.
Numerous other things can go wrong with a distributor. To check for bushing wear, just take the cap and rotor off and see how much the shaft moves back and forth. Another problem on all distributors is with the wires inside the distributor. They are constantly moving with the advance system and the wire can break. Often you won’t see the break because the wire will break inside the plastic insulation, so it looks fine. You have to feel the wire to see if there are any breaks inside the insulation.
Before you get too worried about the carburetor, make sure all the fuel and air filters are good. Don’t always trust a fuel filter if you can blow through it. We have an old one ton, and even with a new fuel pump it wouldn’t draw fuel through the screen in the fuel tank. We could blow through the screen but the pump wouldn’t suck fuel through it. We took the screen off and it runs fine. I’m not sure if this is a sign of a worn lobe on the camshaft that drives the pump or if it’s just the world trying to persecute us. However, it runs OK with an inline fuel filter.
Make sure your air filter is clean. If you have an old oil bath air cleaner, check the mesh on it. If the mesh is kind of gooey looking wash the mesh out with some varsol.
Here’s another piece of knowledge that might help on some old equipment: If you have a remote air filter check all the pipes. These pipes can start to plug from a build up of dirt. Also check all your vacuum lines, fuel lines and any other rubber parts. These don’t last forever. A vacuum leak will cause any vehicle to run poorly.
Now you can move to the carburetor. Working on carburetors is soon to be a lost art. They don’t teach about them in most of the technical schools anymore. I’ve never been a great carburetor fixer, but I do know that if they don’t work well nothing else does either. The use of unleaded gas has been particularly hard on carburetors and often just parking them over winter will cause the carb float to stick. Sometimes you can get lucky and repair a carb just by replacing a part or two, but if it gets really troublesome it will need to be rebuilt.
If you’re doing it yourself, putting a kit in a carburetor is fairly cheap provided you know what you’re doing. Haynes makes some good books on carburetors and these will be a help if you need them. However if you’re having someone else rebuild it, the cost of the rebuild can be close to the cost of a factory rebuilt carburetor. I much prefer to sell a customer a good used carb or a factory rebuilt carb because I believe it’s better than putting a kit in a worn out carb.
One of the wear points in a carburetor is the throttle shaft. This wear is caused by the pressure of the return spring on the shaft. Eventually you end up with air leakage at the shaft, which disrupts the airflow causing poor carb operation. A good factory rebuilt carb will have the throttle shafts rebuilt to eliminate this problem. Costs for factory rebuilt carbs run from $100 to $450 depending if it’s a one barrel or a four barrel. Used carburetors are about half that price, but make sure if you’re buying a used one you get a guarantee that it will run well.
If your Holley four barrel on your grain truck is worn out, you might think that’s a lot of money, but let’s do some math. Say you only drive 500 miles a year and now it pops and putts and is maybe getting six miles per gallon. If you put on a rebuilt carburetor, let’s say you get eight mpg. At six mpg, that’s 83 gallons a year at $4,