Slightly off engine but it's old and loud My 650 vintage Triumph race bike was on the dyno,2 hours of abuse.This plug was new for the last pull,the engine wide open throttle and held to 7200 rpm by the dyno for about 15 seconds, ignition killed,clutch pulled in. It made excellent power for being built by a hillbilly like me. VP 107 octane leaded race fuel, timing at 37 degrees which is sightly less than most guys use.
Looks good to me. If you went one or two steps up the heat range for that plug the nose might change from black to a dusty brown - which is supposed to be the best. Just curious - what kinda power did she make?
The engine needed bigger than usual jets for this kind of build ,but you gotta run it the way it wants to be run.We couldn't get the A/F meter hooked up because no pipe bung and the end of the wand blocked off to much of the 1-1/2 inch exhaust pipes. You guys are looking at the ground electrode color and the carbon on the steel shell ?
If its not running hot, and makin good power, I say leave it.... to me it seems ever so slightly lean, but, you may never get it perfect..... If its runnin good and makin power leave it.... If your in a humid climate, all the more reason to leave it...
Looks a little lean to me as we'll.The thing you want to watch is the temp, especially on an air cooled engine. You could just richen it up a tad to be on the safe side, but then again if it's running cool and making good power let her rip and keep an eye on it. Ken
A pro told me to look at the porcelain down inside with a magnifying glass. There will be a place where it turns from one color to another. Is that place up, down, or half way? Perfect is half way. I think that plug needs to be run more to make a better decision, but it does look like it wants to be a bit fatter. If it wants to load up at low RPM's, then go back to this setting.
There was a few random misfires as the engine got near the HP peak at 7000,but the engine continued to build rpm to around 7200.The dyno operator thought it might a touch of lean out caused by lack of fuel flow.That may be why the engine wants big main jets? I had the plug gap at .030 (full electronic ignition),then .028 cleared up most of sporadic misfire.I believe .025 will be correct. The gravity fuel flow is 8.5 ounces per minute for each carb as measured at the bowl drain.That should support 35-40 HP per cylinder. This 40 cubic inch lump made 52 HP @7000 rpm at the rear wheel,that would be about 62-64 HP at the crank....I have higher flow float needle jets to put in....
Well the first thing that came to my mind was. A high rpm rumble may seem like a stumble or misfire, when infact its an exhaust leak. Ever so slight, but when the hot gas hits cold oxygenated air, it causes and rumble, back fire, thunder, ect.... Runnin to lean, you risk burnin a valve.... SO here is the deal.... If it stumbles over 5000 rpms, but not under, ask your self how often are you going there, and how bad is the stumble.... You don't want to over saturate the fuel mixture. Raise the main jet and try it......
raced m/c for many years. that thing looks way lean. one question are you running cam2. they will show a plug looking white
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