Single Cylinder Gas Engine Tuning

From the responses received from the IMAC list and from Brison and BME manuals, this is what I have come up with. Please feel free to disagree, debate is welcome.


The following information is not specific to any one engine and is intended as a general guideline. Use it at your own risk. No warranty is made as to its accuracy. I strongly suggest that you follow the manufacturers instructions that came with your engine.


Do not use the largest recommended prop. Use one that allows the engine to operate within its maximum power band.

Break the engine in with petroleum based oil (10 to 15 gallons of gas) then switch to synthetic oil. Use good quality regular (87 octane) gasoline. Follow engine manufacturer's guidelines for oil to gas mixing ratio.

Set the timing first, then low needle and finally high needle. Fly the plane and check the engine response in the air. Adjust as necessary. Remember to adjust the high needle after adjusting the low needle as the low needle setting affects the gas supply throughout the entire throttle range.

All readings should be taken when the engine is hot. The needles should be adjusted with the cowl on. Stop the engine when adjusting the needles and then start it again. Allow it to run for at least one minute before taking an RPM reading.

Above all be patient. Don't take any short cuts and don't try to adjust the engine while it is running.


Running rough : Popping, spitting, spluttering, burbling.
Goosing : Opening the throttle from idle to full very quickly.
Lean : The term lean means that there is less fuel being delivered to the engine from the carburetor for a given throttle setting. The engine will run faster, smoother and hotter as the mixture is leaned, provided that it is not leaned too much.
Rich : The term rich means that there is more fuel being delivered to the engine from the carburetor for a given throttle setting. The engine will run slower, cooler and less smoothly and will probably foul the spark plugs if it is too rich.

Process Should be preset by the factory. Exact settings depend on the engine type and model but are usually around 2-5 deg BTDC at idle and 28-32 deg BTDC at full throttle. Timing should be set as retarded as possible without max RPM loss.

For those without fancy testing equipment, run the engine until hot then measure max RPM. Retard the timing a little and re-measure. If max RPM drops, advance timing a little.

Symptoms Too Advanced:
  • Loss of Max RPM
  • Kick back and possibly throw prop when starting
  • Pings
  • Hard to start
  • Engine will miss
  •   Too Retarded:
  • May start backwards
  • Slow to accelerate
  • Low power
  • Runs hot

    Low Needle
    Process Keep in mind the low needle affects the high one but not the other way round.

    The low needle should be set as lean as possible without displaying any of the bad symptoms mentioned below. First get it to run smoothly at idle, then try goosing it to about 1/3 throttle. Adjust until the transition is smooth.

    Symptoms Too Rich:
  • Runs rough and shakes badly (may hop from wheel to wheel)
  • Burbles in midrange and smoothes out at full throttle
  • Engine will transition slowly
  • May die after idling for a long period
  •   Too Lean:
  • Hard to start
  • Won't decelerate (hangs at higher revs for a while)
  • Dies when 'goosed' to full throttle
  • Warnings Setting too lean may cause a dead stick when opening throttle after a long downline.

    High Needle
    Process Keep in mind the low needle affects the high one but not the other way round.

    Bring the throttle up to high and back the high needle out till it runs rough / rich. If you can not get the needle to produce an over rich condition then you have insufficient fuel flow (example: tubing may be too thin). Fix it and then restart the tuning process.

    The high needle should be set to gain maximum RPM without overheating. When running in it should be set slightly rich.

    Measure the RPM and slowly close the needle as long as RPMs continue to climb. If RPMs flatten out or fall then back off about 1/16 of a turn.

    Test the high needle setting in the air by pulling vertical and flying some loops. If it loses RPM it is probably set too lean. Open it about 1/16 of a turn and try again.

    Symptoms Too Rich:
  • Full throttle blubbers, cleans up only on uplines
  • Will not develop full power
  •   Too Lean:
  • Runs hot
  • Sags on uplines
  • May die in flight
  • Will not run at full throttle
  • Warnings Setting too rich may cause the engine to generate carbon deposits and foul the spark plugs.

    Conclusion / Confusion

    Mixed symptoms

    Runs hot : May be caused by high needle being too lean and/or by timing being too retarded.
    Hard to Start : Could be timing, needles or any of a host of other causes. Check low needle too lean and/or timing too advanced.

    If you are still having problems contact the manufacturer. They are always willing to help.