Tune her up! Getting your tractor to run as intended.
June 3, 2020With the distributor and carburetor already covered in previous installments, it is now time to make your tractor run as the factory anticipated. The impetus for this series being there are too many beautiful antique tractors that run poorly.
The root causes
The engineering community uses the term degree(s) of freedom (DOF). It identifies the areas that can be changed or, in this instance, adjusted.
To finely tune your tractor, you need to employ the DOF it has to evoke the delicate balance between mixture strength (air/fuel ratio) and ignition timing. It must be recognized that these areas are often not linear and need to be tuned for the engine’s various operating states.
Before this can be done, you need to confirm that all is well. The spark plugs, wires, rotor, and distributor cap need to be in proper order, the air filter clean along with the engine oil.
If the tractor is running poorly, experiences many cold starts, or is only running for a short time, it will, over time, put raw gasoline into the crankcase. This will dilute the oil and destroy its lubricity causing excessive wear. This condition will also artificially richen the mixture as the hydrocarbons are drawn past the piston rings and into the combustion event.
A good check is to pull the dipstick and smell it. Also, if your engine tends to “make oil,” it is filling the pan with gasoline.
Another concern of short duty-cycle operation is the coating of the spark plugs. Unleaded fuel can create a clear coat on the plugs from the short running time that cannot be seen but will cause the engine to misfire.
I have fixed many engines with fresh plugs even though the ones I took out looked new.
The best way to avoid this is to use your tractor and do not keep “cold starting” it so your friends can hear it run.
A lack of use and short run times are the bane of the collector tractor and are responsible for ruining many engines or making them run poorly.
Specifications
A cook follows a recipe, but a chef follows their taste buds. To be a tuner, you need to let the engine tell you what it wants and use the specifications as a starting point. I like to take the following approach:
Point gap: If a range is given, such as 0.014 to 0.017 inch, I like to hit the middle of the specification. I want to confirm the feeler gauge setting with a dwell meter, if possible.
Spark plug gap: Start at specification. If the tractor is not used under load, then increase the gap by a few thousandths of an inch for a hotter spark and larger ionization window. The plugs will stay cleaner that way.
Timing: I start with the base specification and then alter it in two-degree increments each way (advance and retard) to see what the engine likes. I will go as far as six degrees from the specification.
I then confirm the rate of advance using a dial-back timing light and alter the centrifugal mechanism to achieve the best results at high rpm and under load.
This way, you have the optimal idle setting independent of the high-speed timing.
Mixture/idle speed: Your goal is to have the engine idle as slow and smooth as possible.
Use the base idle speed specification and then turn the mixture adjustment in (lean) until the rpm drops slightly. This is delicate —- do it in 1/8 turn increments waiting for the engine to respond.
Then in less than 1/8 turn increments, back out the mixture screw and wait for the engine to respond. Repeat until the highest engine speed via the mixture is achieved and the best exhaust note. This is the optimal tune.
If the rpm is now too high, turn the idle speed screw down and then reset the mixture using the same procedure. A tune-up style tach/dwell meter is best for this.
Power jet: With the engine at high speed, follow the same procedure as for the idle mixture until the highest speed, and the smoothest exhaust note is found. Test under load and alter the tune if necessary.
If you follow these simple steps, your tractor will run better than you ever knew possible!
