Please Help. Rear Engine Is Stumbling.

5-7HEAVEN · #1 04-18-2009, 01:34 AM

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

bobdirt · #2 04-18-2009, 12:39 PM

Did you replace the fuel filters? A fuel filter can begin to clog, still pull fuel at low to mid RPM's even reach high RPM for a breif time before the engine begins to starve.

5-7HEAVEN · #3 04-18-2009, 03:04 PM

Thanks. I'll check that.

Michigan Mike · #4 04-19-2009, 04:33 PM

Sounds like the rear engine is running out of gas at high speed.

I'd start by replacing the fuel filter and making sure the tank vents properly.

Always good to start with fresh gas and new spark plug too.

.

5-7HEAVEN · #5 04-20-2009, 04:17 AM

Bob. Mike, both engines share the same large automobile fuel filter. It is transparent; when I prime the fuel lines I can see the air bubbles flow through the filter and into the Happy Time reserve tank. If it was defective both engines would have high-speed stumbling.

I just replaced both sparkplugs. Since I use the bike every day there is always fresh gas in the tanks.

ZnsaneRyder · #6 04-20-2009, 07:10 AM

Have you tried removing the carburetor and cleaning and reassembling it? That usually works for me when I have an engine issue.

Michigan Mike · #7 04-20-2009, 11:14 AM

Quote:

Originally Posted by 5-7HEAVEN

Bob. Mike, both engines share the same large automobile fuel filter. It is transparent; when I prime the fuel lines I can see the air bubbles flow through the filter and into the Happy Time reserve tank. If it was defective both engines would have high-speed stumbling.

I just replaced both sparkplugs. Since I use the bike every day there is always fresh gas in the tanks.

Heaven,
If the fuel filter has some age on it I'd still replace it just to be sure ... shouldn't cost much. Seems like it might be possible for a partially clogged filter not to let enough fuel through to run both motors at high speed.

But, as someone else said, it's starting to sound like the carb may need cleaning. Here's a way to check ... when the back engine starts to falter, try choking it a little bit. If it picks up speed with this richer mixture it may indicate that the carb jets need cleaning and/or the air mixture needs adjusting. This would at least confirm that it's probably a gas problem.

Hope that helps ...

.

HoughMade · #8 04-20-2009, 11:28 AM

Another vote for fuel starvation. You have the classic signs.

I don't know where the blockage is, but I feel pretty confident that you have a fuel restriction. I had this exact issue on my bike and it required a thorough dissassembly and cleaning of the carb. I have also seen this exact behavior on numerous small engines (I used to work at an equipment rental, maintaining and repairing small engines) and it was pretty much always a lack of fuel.

Bikeguy Joe · #9 04-20-2009, 02:34 PM

Maybe it's just not-quite-keeping-up, unloads and breaks into a four cycle?

5-7HEAVEN · #10 04-20-2009, 05:19 PM

Methinks it's fuel starvation too. The engine isn't sucking fuel from the reserve tank.

This morning my engines' tanks were almost full. This afternoon I'll do a full throttle run on the highway and maintain that for two miles, traffic lights notwithstanding.

If both engines perform well, then I'm gonna install an electric fuel pump in the line. That should solve fuel delivery problems.

Usually if I have time it takes me about 30 minutes using the priming bulb to fill the engines' tanks.