I have a problem that started about a week ago. On my morning rides both TLE Mitsubishi 43cc engines on "The Dragon Lady" perform flawlessly. The temperature is in the mid-70's.
On the afternoon ride home both engines start out okay. Temps are in the low-80's. Everything's fine until I reach and maintain 8900rpm or higher. This is where maximum hp is attained. Then slowly the rear engine starts sputtering like it's running outa gas. If I floor the throttle the engine falls flat until I let up. If I give it part-throttle it'll struggle and catch itself. Then I can go full bore for a few seconds before it falters again.
Both engines have their 24oz tanks being fed by a single Happy Time 2-liter tank. Fuel/oil ratio is 50:1.
The loss in performance is only when top speed is above 34mph and 8500rpm. Any speed lower and both engines respond beautifully.
Both engines have ADA S1 expansion pipes. The front engine has 39" exhaust tubing connected to the bleed port and exits below the crankset. It has no muffler/silencer.
The rear engine has the standard silencer and four inches of connecting exhaust tubing.
I washed both air filter elements but that didn't help. Tomorrow I'll replace both sparkplugs. Then I'll replace the expansion pipe's silencer.
Can anyone figure out what this problem is? It doesn't seem to happen in the morning