A few days after my last post I pulled the spark plugs for a baseline reading and was a little surprised by what I saw. The carb had only been partially tuned, and so sooty plugs weren't totally unexpected. What was unexpected, though, were the locations of the richest running plugs.
Model radials, similar in design to the Hodgson with its centrally-fed rear carburetor, typically suffer from an uneven mixture distribution between the upper and lower cylinders. In order to improve the H-9's distribution, an impeller was added to the original design and then revised a few years later. The result was a performance improvement, but builders still experience rich-running lower cylinders when the carb is tuned for the upper cylinders. This mirrors my own experience with my H-9. The consensus is that fuel tends to puddle in the lower portion of the plenum and enriches the mixture drawn by the nearby cylinders. The H-9 design even includes drain holes in the lower plenum to reduce this problem.
The plugs on my T-18, however, showed a completely opposite result. Its rear row cylinders have intake tubes similar to those on the H-9. The rear row plugs, though, clearly showed the lower cylinders had been running leaner than the upper cylinders. The plugs in the lower cylinders had a slightly rich to almost ideal color while the plugs in the top cylinders showed those cylinders had been running rather rich with obvious soot on the plugs. The front row cylinders with their much longer intakes showed a similar distribution but, overall, they looked as though had been running very rich. The top plugs in the front row were all excessively sooty while the lower plugs were comparable to those in the top cylinders in the rear row.
These first baseline results were obtained using the 1401 Perry carb (.312" Venturi) with its high speed needle set at 8 o'clock (just over a half turn open from fully closed) and the idle disk still set at stock neutral. For fuel I'm using a product called Four Cycle Tru Fuel which is available from our local Lowe's store. This is an ethanol-free 92 octane gasoline designed for lawn equipment, and it contains an added stabilizer for long shelf life. What caught my eye about this fuel for my application was a claim that it also contains friction modifiers which I thought might be beneficial to the distributors' lower bearings and gears. Of course, this fuel could also be mostly advertising hype in order to justify its premium price.
A majority of the 5 or 6 minutes runtime that produced these plug results was between 2000 and 2500 rpm, but it also included periods of idling as well as a pretty ragged restart. The restart was attempted during a hot soak period with the throttle insufficiently open. The engine eventually started, but it required the drill starter to spin the engine for several seconds. The sooty plugs indicated that either the carb needed a lot more leaning, or the unfortunate re-start had badly fouled the plugs; and the engine hadn't been run long or hard enough afterward to burn off the excess accumulated carbon.
To prepare for the next set of runs the plugs were cleaned, and the high speed needle was leaned another 1/8 turn by turning it CW from 8 o'clock to 10 o'clock. (Since the needle on the Perry carb actually rotates with the throttle, my needle positions are referred at w.o.t.) This setting turned out to be too lean, and the engine consistently died several seconds after being choked and started. The engine needed at least another 1/8 CCW turn in order to sustain smooth running. As a result, I returned the high speed setting to its previous 8 o'clock position.
I then broke the set on the idle disk and turned it about 1/8 turn in each direction while the engine was running but could see little or no effect on the idle. It's possible that I was fooled by the grip of the o-rings as this adjustment, semi-unique to the Perry, isn't all that user-friendly. I then decided to rev the engine up to 3500 rpm and hold it there while burning the remainder of the fuel in the tank. The reason I did this was to color the plugs for the next reading with a substantial high speed run with no restart nor idling time. I was still curious about the previous upper/lower plug differences and now really bothered by the earlier overly rich plug readings. My plan was to revisit the idle adjustment after the second plug readings. I then would be in a position to connect any further changes in plug conditions to changes in the idle setting.
At this point the engine has plenty of throttle left to rev past my self-imposed maximum of 3500 rpm, and so I'm confident the carb size is adequate. When starting the engine with the drill starter there is a neat sounding intake hiss that's louder than the fuel pump. Also at this point the engine likes timing advance up to about 20 degrees.
After the engine cooled, I once again pulled each spark plug and recorded the results. Fortunately the heavy soot was gone indicating what I previously saw was more of an operator problem than an engine or carb problem. The upside-down mixture distribution differences remained though. The four photos show typical plugs from the tops and bottoms of the front and rear row cylinders. The important part of the plug is the white insulator. The photos continue to show the lower plugs in both rows are running leaner than the upper plugs. This may be a result of my impeller/plenum design, but it's not at all obvious to me how I was able to invert the result compared with the H-9.
A model builder used to running alcohol may find these 'filthy' plugs appalling; but, for a gasoline model, they really aren't that bad. A proper heat range plug in a modern full-size fuel-injected engine typically won't show any insulator coloring when running on unleaded gas. In a model engine, though, carburetor tuning is always a compromise; and plugs with heat range options aren't available. Carb tuning and, to some extent, the reach of the plug into the combustion chamber are the only tools available to control carbon deposits on the insulators. If the carb is leaned as much as practicable, but the insulators still accumulate deposits that eventually affect the spark then a shift to an alternate fuel may be the only option. Personally, I've come to prefer gas over alcohol because of gasoline's easier starting due to its higher volatility as well as its lower production of water in the crankcase due to combustion.
The cold rainy weather has settled back into our area. When it clears out I plan to work on the idle mixture.
While tying up some loose ends I also added a crude oil filter to the engine's oil line just after the drip feeder. It was made by drilling an array of .020" diameter holes through an inline bronze plug. It's purpose is to prevent chips that might have been inadvertently left behind from being recirculated. The most difficult part of its construction was clearing the chips left behind during its own construction. - Terry
