Technician’s Tech Tip


June/July 2012 Tech Tip

A bi- monthly heads-up is provided to assist customers with maintenance items that almost anyone can perform. Hopefully my Tech Tips segment will lead to big $$$$ savings for the reader.

Tech Tip for June and July, 2012, is about “Bad Gas”. While it sounds like a personal problem, stagnant, untreated, varnished fuel is the single most expensive, preventable, seasonal contributor to poor engine start / poor performance, or no start at all………

Left over fuel in a casual-use small engine fuel system, for months between start-ups, without some form of fuel stabilizer such as Lucas or Stabil, will turn into a varnish that becomes gummy & hardens over time. Preview pic #3 of hardened fuel / left-over fuel residues. The example shown on the cardboard backing is from an old McCulloch chain saw,  a 1960′s vintage, having been stored in a barn, with fuel left in the tank and fuel system. This is an extreme case, that required carburetor replacement and chipping of the fuel tank interior to remove the worst of the crust. Quite naturally, the same left-over fuel crust had hardened within the circuits and fuel passages of the carburetor, making it necessary to replace same and related components. Even the strongest of carburetor dipping basket-type cleaners would not have removed the hardened crust.

Other photos, are of a Lawn-Boy main nozzle, taken from the pictured black plastic carburetor, that has a “very distinctive throttle control modification”, (pic #4). The discoloured fuel residue shown, (pic #1), was also found inside the nozzle, but was soft enough for easy removal, allowing the bleed holes and main passage to be free of old fuel scum, (pic #2). 

The original design plastic body modular carburetor ( pic #7), was envisioned by Lawn-Boy engineers to be a labor-saving innovation. The dealer was to have a supply of these “replacement, plug-in carburetors” on the parts shelf available when the customer’s mower appeared for repairs. Simply remove the customer’s defective modular carb, install either a new or rebuilt likeness, keep the exchange and rebuild it for the next unit requiring carburetor service. The only design problem encountered with the initial run of these modular carbs, was the high placement of the nozzle in the main body mold, ( pic #5),  that would create a stall condition when mowing on a terrace. The nozzle was positioned too high in the carburetor body and could not draw fuel when mowing on a normal incline, creating the stall. This was a warranty issue, requiring complete replacement of the affected modular carburetors. If the reader has an older Lawn-Boy with a “plug-in style” modular carburetor, look into the air horn ( pic #6) and you should see just the tip of the nozzle, almost flush, in the venturi. If the nozzle tip ( pic #5) extends well into the venturi,  you are the proud owner of a modular carb. that missed the recall.  The “plug-in” design was achieved with a round male fitting on the outlet end of the carburetor ( pic #7) that was accepted into a female / “O” ring connection on the reed plate. Simply insert the carburetor into the lubricated reed plate connection and install the anchor screw / washer @ 12:00 position. This connection negated removal / replacement of the reed plate for carburetor service, and was later returned to the earlier design flange-style, two bolt mounting, ( pic #8).

ASIDE:  I included a reed plate photo, (pic #10), crankcase side, showing what are called “stuffing blocks”.  These create more efficient fuel transfer by reducing space inside the crankcase; downward pressure below the rings remained constant. A subsequent reed plate design removed these stuffing blocks, in an attempt by engineers to save material costs, that “added” more crankcase space for fuel transfer pressure, by virtue of their absense. It’s anybody’s guess if there was ever an appreciable loss in engine / mowing performance due to this design change.

These “bad fuel” conditions require expensive “professional” restoration of carburetion and fuel system components. With the advent of unleaded gasolines, fuel stored in quantities of less than five gallons has a useful shelf life just over thirty days. Oil mixed with fuel decreases that window. I remember my Dad saving outboard fuel from the previous year’s trip to northern Minnesota in the 50′s and 60′s. Never a problem with fuel longevity from year to year, until the advent of unleaded fuels in the seventies………………..

While Stabil and other fuel conditioners are a great resource, I have found in some 40 years of service experience, that simply running the engine completely out of fuel at the end of a mowing or boating season is the best solution for keeping carburetors and fuel system components free from fuel storage problems. Remember also, that today’s fuel residues within the carburetor’s idle/intermediate/hi-speed circuits will create problems if stored without a fuel stabilizer when the engine is run dry………………….

For those who have the old OMC pressurized  outboard fuel tanks, with a twin hose feeding the engine connection, (pic #11), be sure to allow fuel in the hose to run through while winterizing. Simply loosen the tank cap for pressure to escape, allowing fuel to run thru’ the hose as the engine runs dry. For subsequent single hose fuel pump-type applications, disconnect at the tank fitting while the engine is running and simply allow the hose to run dry.  You’ll observe the primer bulb collapse with this process. To re-inflate, gently insert a non-aggresive instrument, to slightly depress the spring-loaded tank end fuel fitting, which allows fuel to continue thru’ the hose. Remember, no smoking around fuel tanks!!……………

If folks did this one simple task of fuel removal / treatment with a stabilizer, they’d hardly ever see a shop, at least for fuel-related problems…………….. Go Forth and Enjoy……………Jim……..


2 responses to “Technician’s Tech Tip

  1. I’d like to give my son Matthew a big round of applause, for his terrific job of website design. Without his computer expertise’, this presentation would still be just an idea without form or function. Thanks so much, Matt, you’re the best!
    The photos were taken by this author, about 8:00p.m. on Three Island Lake, near Longville, MN about three years ago. Matt set the background using the same photo shown @ the top of the home page. Don’t ask me how he did it.

    Be well all………Jim………

    • It was the least I could for you. We’ll get this thing figured out for you so that readers and customers can navigate through this with ease. It’s looking really great, thus far!

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