Sometimes a large gap will keep an engine from missing at WOT...I have experienced this on Sachs 5.8 singles...More than once....
Suzuki spark plug gap is .028-.031......I guess .031 is "just about" the same as .020.....
Bottom line DA also says use Lawn boy ashless oil in their engines...We know how well that works...

that sounds more to be a timing issue if you need to enlarge the gap. by increasing the gap you will retard the timing slightly because the coil will need that extra time to soak the secondary more to increase the initial spark energy. if you look at an oscilloscope graph you could actually see that happening. i can tell if a cyl is rich or lean based off of that reading. spark plug gap really plays a big part when you start adding superchargers and turbos on an engine. you need good coils and smaller gaps. like in the .035" range. but that is usually do to the higher compression that the spark energy has to go through. and .031 is still less than .040".

http://www.bikerenews.com/Stories/Sp...park_Plugs.htm
http://www.spyke1.com/manuals/sparkplug.htm
here is some good reading on proper spark plug gaps.

Originally Posted by rcign

Sometimes a large gap will keep an engine from missing at WOT...I have experienced this on Sachs 5.8 singles...More than once....

i was thinking could it be that you are running a very rich oil ratio and the hotter spark is needed to keep the oil from fouling the plug. a freind of mine has if i remember right a zenoah 26 in a race boat that has been modified to run 17k rpms and makes like 5 hp. he runs it at 16:1 because i guess thats recommended. i will ask him about gap. just a thought i had about the missing at wot. maybe even your elevation has something to do with it.didn't you say that you were at like 6k asl. thinner air,lesser compression ,the energy needed to jump the gap will be lower because of the lesser resistance. and what was the first gap you used and the end result gap?
i was wondering on what you did to the ignitions on your race engines also.im rather curious on how you changed timing and if you ran modified coils or anything.

The race engines in the stock class can't be modified in any way...
In the open classes we run glow..many years ago we ran ignition with glow fuel in our 289cc Herbrandson twin..It ran just as well timed at 22 BTDC as it did at 28, so we ran 22 to keep any heat down...We have found that the glow plugs last a long time so no one runs ignition in the race engines....
Timing could be changed by removing the rotor key...a key is not necessary on any engine with a tapered crank...Haven't used a key in any engine for about 18 years...
First gap was about .025, opened to about .040...
The stock Zenoah mag coils are about as good as you can get..They work very will with electronic ignition, been using them for 21 years for conversions...

in the open class it is a gas motor that you use glow fuel with? also i tried googling that henderson engine, you have any links. peaked my curiousity

GT80, 19x18 carbon prop, 11,500+ rpm, 190 mph, on a GR7 F1 plane, 100" wingspan, about 25 lbs..
289cc Herbrandson twin drone engine, 22x31 carbon prop, 8200 rpm, 235+ mph..Tsunami airplane, about 40 lbs
Methanol with about 10% Klotz and 22% nitro....
The stock F1GT gasser class uses GT80s, gas with Horizon oil, 9000 rpm, 140 mph, 25 lb 100" planes...
usrainfo.org

[quote=rcign;951649
289cc Herbrandson twin drone engine, 22x31 carbon prop, 8200 rpm, 235+ mph..Tsunami airplane, about 40 lbs

usrainfo.org[/quote]

holy crap that is fast. your into the speeds that the redbull full scale racers are doing.