John Deere 425 dirty carb, old gas, additives, cleaning and results...

Decided to do some forensic analysis of old fuel and bad carbs today.

We fired up a JD 425 that had been sitting for a year a couple days ago. It ran poorly on the old fuel and sort of OK with a little choke [more on this in a second].

So I added some acetone and some Marvel Mystery Oil and let it idle a few minutes, still ran poor.

I pulled the air cleaner, brackets, etc and the top half of the carb and cleaned it thoroughly. There is a jet in the lower half [that I didn't pull because of additional brackets, cables, etc] and just unscrewed, inspected, cleaned the upper jet and replaced.

Put it all together and it still ran bad. The symptoms were it idled fine but required a little choke to get to run smooth [sort of like leaning out an aircraft engine] and if it wasn't leaned right there was a pronounced knock.

I showed my SO how to keep the engine mixture set so as to avoid detonation and let her run it awhile hoping that whatever was restricting the fuel flow would clear.

Didn't clear.

I could increase the throttle quickly, the engine would run OK and then after about 3 seconds slow and begine to knock. Increasing a bit of choke smoothed it right out. So I knew there was fuel to the carb. The fuel pump is in the tank and pressure checked OK earlier.

Got out the shop manual to see wazup. [There was almost zero info on the carb internals but some clues]

Turns out the jet I pulled was a secondary jet and there was a primary jet [that I saw but thought it might be part of the12vdc shutoff solenoid valve assembly]. It was at the bottom of the lower housing and I decided I'd have to pull the whole thing.

I pulled it tonight and got the lower half out and decided to inspect VERY carefully to see what was going on and if there was any deposits or gum, etc.

The lower carb housing was very clean with just traces of a brown dust [that I've seen on many other tractor and blower carbs that we used when mulching leaves and branches, and we used to spend almost as much time mulching as mowing in some areas].

I pulled the fuel solenoid and carefully removed the primary jet and took it to my lab to inspect optically.

Sure enough, there was a small amount of the brown dust visible under magnification, restricting the orifice just a little, maybe 5% of the flow. The dust was sort of caked on and spraying with Electrocleen <trichlor] and carb cleaner didn't faze it.

So I took a plastic toothpick with a tiny 'brush' on one end [Walmart] and inserted a piece of UltraFloss into the jet and ran it back and forth to clean it.

The deposit was pretty resistant to cleaning. It didn't seem to be loosened by strong solvents. Anyhow, I cleaned it.

Put the carb back together and the engine ran smooth as a baby's behind.

Now, looking at the facts, year old gas ran fine, sitting for a year there were NO deposits in the lower half of the carb, just the restriction of the fine leaf debris accumulating in the jet like cholesterol in an artery.

In the past I remember taking the rear body of another tractor off to access the fuel pump because the motor was stuck. There was a some leaf dust on the pump filter and in the tank. Unavoidable.

There is a small clear plastic filter inline right before the carb and it flowed well.

I think the 'dust' started attaching itself to the jet bore over time [last year IIRC, the engine was running a little funny but not bad enough to be concerned] and just built up and unbalanced the critical operation of the carb. I'm surprised that it took such a small amount of restriction to do this.

This year I changed the jet on a Honda EU2000 for high altitude operation.

The jet's part number is associated with a drill size. There was just enough difference to be easily visible between the two jets.

The amount of reduced diameter in this tractor jet looks to be close to the % of reduction of the genny hi altitude jet.

Just remembered, we used to run a JD 318 tractor and I often had to pull the jet on it [easier than the 425] and clean it after dust started reducing the jet diameter.

I got a really good deal on a Honda mower recently that had issues.

It would run but at idle the engine would hunt up and down in RPM.  Turned out that the foam air cleaner was pinched and unfiltered air was getting in causing the motor to run lean.  Fixed the wrinkle and it runs like a champ.

My 335 runs like crap if I leave fuel in it over the winter, even stabilized fuel.  Since I started running it dry in the fall it has been OK.