Please Help. Rear Engine Is Stumbling.

[5-7HEAVEN]

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 top speed AND 8900RPM. 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

 

[grouchyolfart]

Ooops. Never mind.

 

[jg767]

Check for an intake leak. And the fuel line and filter. Good luck.

 

[Mountainman]

5-7
just wondering if your lean mixture of 50:1 has caught up with you ?

but then again a simple thing such as the new plug you have planned may be a fix

ride that thing sideways

 

[5-7HEAVEN]

Well Mountainman, 50:1 is the recommended gas/oil mix. I'm pulling the plug soon to replace it. The old plug will show if it's lean.

I just got a WYK-58 Walbro carb. I'm gonna try that soon.

jg767, I'll look for intake leak. Come to think of it, I removed the carb a few weeks ago to check measurements. And lately the engine HAS been idling erratically. In fact sometimes it'll die while idling.

 

[Happy Valley]

Sounds like the engine is fuel starved at high demand.....or an air leak which is the same thing. Since you had the carb off I'd start there but check fuel lines and filter too. How many hours are on this engine? If it's fairly new I wouldn't think the problem to be electrical.

As an aside, while I doubt fuel mix ratio is causing a problem if cylinder compression is still good, I'm convinced the rec on 50:1 is to pass emissions. I broke mine in at 40:1 and have kept it there, produces a tan plug and good looking piston wash.

 

[5-7HEAVEN]

Happy I believe you're right about fuel delivery causing the stumbling.

BTW, both engines have less than 1,000 miles on them.

In the morning the tanks are partially full. After 1.5 miles of travelling at 25mph, I hit the highway for two miles of full speed. Engines are fine, lots of fuel. Two more miles of 25mph riding.

The bike sits for nine hours in covered parking. The engines' fuel levels are low. Although the Mitsubishi engines are identical, each has a different tank design. They also have a different configuration of fuel lines to the reserve tank. The front and rear engine also sit at different heights as compared to the reserve tank, which is slightly higher than both tanks. This might explain why sometimes the rear engine runs outs fuel. On other occasions the front engine runs out of gas.

On the way home it's about two miles of riding 25mph or less, then two miles of sustained high speed. On the third mile the rear engine starts to stumble. Flooring it makes it worse. If I drop speed down to 30mph the engines runs fine.

Well I replaced both sparkplugs and they look lean. I also removed the carb to look for broken gaskets or loose bolts or fuel lines. Carb looks fine.

I have a new WYK-58 high-performance carb, so I tried to bolt it onto the dual-port(air/gas) manifold. No luck, the carb's larger port does not match the Mitsu's intake manifold. The carb bolts directly on, but ports don't match.

Need to find a matching intake manifold.

I'm willing to bet that if I fill both engines' tanks via siphon hose then my problem will disappear. If the rear engine does NOT stumble at sustained high speeds, then I need to come up with a more efficient fuel delivery system that feeds from reserve tank to BOTH engine tanks.

I think the final solution will be a low-pressure electric fuel pump and a small Whizzer-type battery.

Then of course I must also solve the engines' lean conditions, probably with HP carbs and filters.

 

[kerf]

Check the fuel screens inside the carb diaphragm fuel pump.

 

[5-7HEAVEN]

Well today was a blast. The roads were wet, the tires slipped constantly and it took a bit longer to reach work. Both engines ran well with no stumbling.

This afternoon's ride home went very well. Traffic was extremely congested, but I managed to get home in 19 minutes, a great time.

Both engines' tanks were full and the rear engine never missed a beat, even after two miles of sustained high speed.!!!

I checked fuel level when I reached home. Both tanks were quite full, so some fuel WAS being delivered from the reserve tank.

SOLUTION: I need to keep spending 30 minutes daily manually priming the fuel lines or siphon fuel directly from reserve tank to each engine tank...

or install electric fuel pump with small battery.

 

[5-7HEAVEN]

Electric Fuel Pump Being Installed!

A comedy of errors dictated that today was the day to begin installing a low-amp/low volume/low pressure fuel pump.

Yesterday the fuel line from reserve tank to rear engine got punctured. Today I went home during my lunch hour to repair it. After doing so, I filled the reserve tank and motored to work. A mile away the rear engine started stumbling at 30mph. This problem had gotten worse over the months. Once either engine starts stumbling, it's because of low fuel level at the engine tank. The carb's primer bulb cannot pull fuel from the reserve tank. Tanks are only slowly filled by siphoning effect that takes hours.

So I ride home to siphon fuel from reserve tank to rear engine. Forgetting myself, I take a deep suck to prime the siphon line...and get a mouthful of gas!

Then I spill gas on my workpants.

That's it! Workday is over. I call my secretary to let her know I had car trouble. She laughs because she knows I ride my "putt putt" to work.

Fuel pump mounts on the studs of the Happy Time tank. Brass fittings and cutoff valves are installed. Tomorrow I'll run 1/4" fuel hose. The Whizzer 12volt/1.2amp/hour battery will not be mounted on the bike, but carried in my backpack.

This should PERMANENTLY resolve my fuel-delivery issues.

It is the same fuel pump I used on my modified VW Bug engine 25 years ago.