Small Engine Secrets (Part 1)
Figure 1 John Deere Mower used for testing.
Lawn mowers, go-karts, mini-bikes, generators, pumps, scooters, and many other things are powered with single and 2-cylinder small engines. They can range from 3.5 to 22 HP, horizontal or vertical shaft. A small push mower now costs close to $200 for the el-cheapo model. That is a lot of money for a “disposable” product! With any sizable yard, it may cost $100 to mow the back 40 with today’s gas prices! Recreational toys like go-karts & mini-bikes cost money for fuel as well — you must pay to play. Here are a few things that will improve combustion efficiency on small engines. These tricks improve both performance and economy, while reducing harmful emissions. In fact, they should keep your small engine running much longer than the manufacturer claims (in conjunction with regular oil changes). Part 1 covers conventional tuning.
Baseline
The mower is a John Deere Hydra 165 with a Kawasaki FB460v 12.5 HP single cylinder flat head engine (see Figure 1). I got it used (and abused) about 7 years ago, so it’s old. I changed the oil with NAPA 100% full synthetic at the beginning of the season (3 months ago or so). Just before the baseline emissions testing, I cleaned the air filter; the foam surround was washed with soapy water, and the paper element blown out. The spark plug was a Champion 843 EZ “Easy Start” (looks like a well-worn platinum plug).
Figure 2 EMS 5003 5-Gas Emissions Analyzer used for testing.
Emissions testing is performed with an EMS 5003 5-gas emissions analyzer with Lab View software (see Figure 2). The test runs for a period of 2 minutes, where an average reading is taken. I ran the mower for about 15 minutes prior to testing (mowed some grass) to reach operating temperature. First test is at idle, second test is at normal run RPM (at the “rabbit” on the throttle control), and the third test is with the PTO engaged for a load.
Idle: HC = 686 ppm; CO = 8.43%; NOX = 13 ppm; CE = 71.3%
Run: HC = 2225 ppm; CO = 7.13%; NOX = 13 ppm; CE = 59.4%
Loaded: HC = 481 ppm; CO = 10.3%; NOX = 11 ppm; CE = 65.6%
Quite frankly, these numbers are horrid — even for a small engine!
New Spark Plug
You may be able to look up the spark plug part number and cross reference it to a non-resistor version. Resistor type plugs usually have an “R” in the part number, whereas non-resistor types do not. If not, you can modify some resistor-type plugs to make them non-resistor. For this project, the owner’s manual recommends NGK-BMR4A spark plugs (which I was not able to modify). The NGK web site’s Compare function allows a comparison between the BMR4A resistor type and BM4A non-resistor type plugs (the “R” in the part number denotes Resistor). Curiously, even the non-resistor plug is rated at 1k ohms, while the resistor version is rated at 5k - 10k ohms. This means that even if we opt for a non-resistor type off-the-shelf spark plug, we still get a resistor type plug. Oh, and the non-resistor BM4A was listed as “Not Available”.
I replaced the old plug with the NGK BMR4A. Measured resistance was right around 10k ohms. It had 0.025” spark gap out of the box, which is how I installed it. (We’ll play with non-resistor plugs in Part 2). Retesting emissions, it was obvious the old plug needed replaced:
Idle: HC = 586 ppm; CO = 8.15%; NOX = 16 ppm; CE = 73.0%
Run: HC = 1734 ppm; CO = 6.94%; NOX = 11 ppm; CE = 60.6%
Loaded: HC = 449 ppm; CO = 10.0%; NOX = 11 ppm; CE = 65.8%
The HC emissions dropped 100 ppm at idle, 491 at high speed, and 32 under load. The rest of the numbers didn’t change appreciably.
Solid-Core Magneto Wire
Figure 3 Stock Magneto
Figure 4 Disassembled magneto with replacement solid core wire.
Figure 5 Rebuilt magneto; note RFI ring at magneto end of HV wire.
The stock magneto was equipped with a resistor-type HV wire, which was getting old and somewhat brittle. I replaced it with Jacob’s 8 mm solid core wire. See Figures 3 through 5. If anything, emissions may have slightly increased when not under load, but they went down under load:
Idle: HC = 635 ppm; CO = 8.27%; NOX = 15 ppm; CE = 73.1%
Run: HC = 1950 ppm; CO = 7.32%; NOX = 13 ppm; CE = 60.5%
Loaded: HC = 425 ppm; CO = 9.51%; NOX = 9 ppm; CE = 65.9%
Magic Spark Plugs
Figure 6 Mangled "Magic Spark Plug"
As outlined in the Ignition Insights section, drilling a cup in the center electrode, and a hole in the outer electrode will turn your spark plug into a mini-blow torch. I bought 3 new NGK plugs,. One of them I converted to a Magic Spark Plug — sort of. I couldn’t find the drill chuck for the lathe. So I decided to try making a Magic Spark Plug on the drill press. What I found out is that a poorly made version causes more harm than good (see Figure 6). After this test, I went back to the standard plug.
Idle: HC = 580 ppm; CO = 8.13%; NOX = 16 ppm; CE = 72.8%
Run: HC = 2426 ppm; CO = 7.27%; NOX = 15 ppm; CE = 59.0%
Loaded: HC = 539 ppm; CO = 9.97%; NOX = 9 ppm; CE = (?)
Coiled Coil Wire
Figure 7 New magneto wire with 26 AWG inductor wire wound around it.
When an electric current passes through a wire it creates an electro-magnetic field. This field can be inductively coupled to another wire. By wrapping the coil wire with an inductive winding, you can capture this normally-wasted electromagnetism and divert it back into the spark. The results should be a more intense spark. This is another “Gadgetman” trick.
Figure 8 Aluminum tape covers inductive windings.
Whereas Ron used 18 AWG wire, I have some 26 AWG enameled solid core wire typically used for custom winding inductors and transformers. I wanted to try it as I can wind more turns on the same length of ignition coil wire, yielding more step-up voltage (see Figure 7). Plus, thinner wire is more flexible. Ron gave the wire an aluminum tape wrap over the coiled wire, so I did too (see Figure 8). Somehow the aluminum tape wrap shorted out the spark — no start condition. Once I removed the outer foil tape, I had spark again. The numbers weren’t bad, but they weren’t really improved either:
Idle: HC = 528 ppm; CO = 8.30%; NOX = 13 ppm; CE = 73.0%
Run: HC = 2112 ppm; CO = 8.15%; NOX = 12 ppm; CE = 60.8%
Loaded: HC = 498 ppm; CO = 10.1%; NOX = 9 ppm; CE = 65.6%
Carb Tuning
The emissions show the engine is running pig-rich all of the time. Both economy and emissions can benefit from leaning it out quite a bit. If you have an older engine, it may have a mixture tuning screw on the bottom of the float bowl. Adjusting AFR is a simple matter of turning the screw. Most small engines built since 1996 have a fixed orifice replacing the adjustable. This makes adjusting the AFR a bit more challenging. Perhaps the easiest way to adjust AFR on a fixed-orifice carburetor is by adjusting the float level, if you have a brass float. Bending the tab that presses against the fuel inlet plug allows fuel level adjustment.
Figure 9 A #72 hole drilled in venturi fuel feed tube.
If, however, you have a fixed orifice plug and a plastic float, AFR adjustment becomes quite a bit more challenging! I removed the carburetor and gave it a thorough cleaning. There was no main circuit AFR adjustment anywhere. There was a screw-in jet that I suppose could be changed, but I didn’t have any on hand. Instead, I drilled a #72 hole in the vertical fuel feed tube just above the venturi throat (see Figure 9). After reinstalling the carb, I used the gas analyzer to adjust the idle mixture screw.
Idle: HC = 231 ppm; CO = 2.95%; NOX = 21 ppm; CE = 88.1%
Run: HC = 1504 ppm; CO = 8.01%; NOX = 13 ppm; CE = 63.8%
Loaded: HC = 392 ppm; CO = 9.79%; NOX = 25 ppm; CE = 66.9%
The adjusted idle circuit shows the engine is capable of running much cleaner. Adding the #72 hole made a dramatic difference, so I added another just above the first one:
Idle: HC = 202 ppm; CO = 1.56%; NOX = 22 ppm; CE = 93.4%
Run: HC = 675 ppm; CO = 7.82%; NOX = 15 ppm; CE = (?)
Loaded: HC = 294 ppm; CO = 9.20%; NOX = 18 ppm; CE = 69.1%
Spark Gap Adjustment
I opened up the spark gap from 0.025” to 0.030” and the emissions went up slightly. I backed it off to 0.027” and got the best numbers under load. Since it spends the vast majority of time under load, this is where I focused:
Idle: HC = 235 ppm; CO = 2.89%; NOX = 24 ppm; CE = 88.1%
Run: HC = 721 ppm; CO = 8.42%; NOX = 15 ppm; CE = 67.7
Loaded: HC = 249 ppm; CO = 8.23%; NOX = 23 ppm; CE = 69.3%
And One Other Thing
Though it cannot be measured with a gas analyzer, sharpening the blade(s) allow you to plow through the yard faster while still maintaining a clean cut. I wanted to include it in the “conventional” section of Part 1. I have been amazed how narrow leaf plantain stalks simply bend over with a dull blade, but are cut clean with a freshly sharpened blade. With a dull blade, I have to back up and hit them again (and again…).
Summary
Spending some time with the mower proved there is probably room for much improvement on small engines. With the modifications listed, the emissions were reduced drastically. This inevitably equates to better fuel efficiency.
Stock HC; Final HC; Change; Stock CO; Final CO; Change:
Idle: 686 ppm; 249 ppm; -63.7%; 8.43%; 2.89%; -65.7%
Run: 2225 ppm; 721 ppm; -67.6%; 7.13%; 8.42%; +18.1%
Loaded: 481 ppm; 249 ppm; -48.2%; 10.3%; 8.23%; -20.1%
Testing fuel economy on a riding lawn tractor is not easy. How do you measure it? Acres per gallon? Rows per gallon? Hours of operation per gallon?? To complicate it even more, taller grass puts a higher load on the engine and may even require slower speeds to get a clean cut. It would be hard to detect a 5% improvement. However, anything above 20% will surely be noticeable.
Part 1 covered basic fundamentals, conventional tricks that have stood the test of time. In Part 2 we’ll delve into some cool stuff to see how add-ons can improve efficiency.
MPGenie Basics 051 Training - Small Engine Secrets
