Modern fuel and older engines.

Modern fuel and older engines.

June 26, 2018 0 By Ray Bohacz

A love for older equipment that still has life in it are the hallmark of farmers. I have never been to even the most modern farm operation that did not have an old gas engine that was still in use. It may be in a grain truck, welder, irrigation pump, or utility tractor.

The gasoline that goes into the tank of that machine shares little with that produced when it rolled from the assembly line. It is not better. It is not worse. It is just different; and it is imperative that you understand how.

Modern gasoline

Over the years gasoline has changed in chemical composition due to its main use in automobiles. It can be considered “redesigned” for lower emissions, the removal of lead, and to meet the needs of the fuel delivery method; the transition from carburetors to fuel injection.

None of these alterations were made for agricultural engines, but were for the most part designed to be backward compatible to older engines with some caveats.

Today, there are “boutique” fuels that are made to meet the emission control standards for a given geographic area that may or may not include the blending of 10% by volume of ethanol.

There are three concerns:

  1. The weight (specific gravity) of the fuel is different from what the carburetor was designed to.
  1. The burn rate (flame speed) is different for each fuel.
  1. Modern gasoline is designed for a fuel injection system that is closed, not a carburetor with a bowl vent and vented gas tank.
  1. Ethanol has the possibility to attack certain rubber parts over time.

The difference in specific gravity means that the carburetor will need to be adjusted (both the idle and power circuit) differently than with the original design fuel or even gas from a few years back.

Most likely the fuel today will be heavier (do not confuse this with energy density) and thus, may need more of the transfer port in the carburetor exposed (slightly higher idle speed), the mixture and power screw at a different setting, the float slightly higher, and in a very rare instance, a larger main jet. Start with the adjustments and float setting and all should be fine. Today’s fuel in most areas is oxygenated so the mixture will usually need to be set to compensate for that.

The ignition timing and rate of advance is based on the flame speed in the bore. The slower the burn speed the more advance (lead) the engine needs. It will be impossible to calculate the flame speed, but the engine will let you know by running better. Do not be surprised if the timing varies as much as 6 degrees from the factory specification. Play with the settings. The engine will let you know what it likes.

The bowl vent of the carburetor will expose the gasoline to atmosphere and it will allow it to degrade over time. Modern fuel will also evaporate at a higher rate in the bowl.

If possible, the best approach is to shut the fuel off from the tank and run the carburetor dry, or if it has a bowl drain, use that too. This way it is not gumming up the inside of the carburetor. If you are using E-10, the gaskets/rubber is not exposed to the alcohol when not in use.

I also like to take a plastic bag and tightly rap it around the gas cap and then put it back on the tank. Remove the bag when you plan on running the engine. This will greatly diminish the amount of air influencing the fuel in the tank.

Contrary to what many believe, today’s gasoline is excellent. Making an older engine run fine on it requires only the simple steps outlined here in the Farm Machinery Digest.