Two stroke reeds

[BurnitBlue]

Hi, I presently have collected over a few years two mopeds, a chainsaw, and an outboard that now sit in the garage. They all have the same problem.

They start OK and idle but the slightest increase in throttle and the engine dies. I worked on one of the mopeds for days. Removed and cleaned the petrol tank. Removed and cleaned the carburettor. I felt absolutely sure it was a fuel problem. To be sure it wasn't something else I changed the spark plugs etc.

I would like to know what is causing these problems. I am running out of space in my garage. Maybe it's the two-stroke oil.

However, I decided to have another go at them so scoured the web for information. I came across a few references to something called a reed in the crankcase and it has something to do with fuel delivery. I cannot find any more information about them. Where they are, is it a pro job to look at them etc?

Can anyone help with info or a link etc.

Many thanks. I am off to the boat today right now so will be on line again this evening.

 

[rosssavage]

On the compression stroke the rising piston creates negative pressure underneath itself as it rises in the bore. This sucks the mixture from the carb into the crankcase. On the power stroke (piston moving down..) the mixture in the crankcase is pressurised by the down moving piston. As the piston uncovers the inlet port port towards the bottom of the stroke this pressurised mixture is forced into the cylinder. The reed valve sits between the carb and the crankcase and stops the mixture being blown back through the carb during this pressure / inlet phase. Not all engines use reeds though, some use a rotary valve (basically a spinning disc with a slot in it...) but all two strokes need some method to prevent this blow back through the carb.

USUALLY fairly easy to replace.... (remove carb, unscrew reed assembly, replace with new...)

Sounds more like a dirty carb issue though if the engine runs at all.

 

[rosssavage]

http://www.southernskies.net/page_info/runningtwostrokeengine.html

Clicky click....

 

[Lakesailor]

The two tongue-like strips of metal in the bottom (as seen) of the crankcase are the reed valves in a Tohatsu motor.
Very unlikely that reed valve issues are causing your problems on all three motors.
It is very likely it is gummed up carbs, the main jet and emulsion tube and if there is some form of accelerator pump perhaps that, but not that likely on small motors.

Try getting a motor running and squirting a few ccs of fuel into the carb throat whilst opening the throttle. If it revs up it's because you have supplied what it needs, fuel and air. If the reed valve was broken there wouldn't be the crankcase pressure to feed the combustion chamber and it still wouldn't rev up (very well).
Carry on cleaning.

 

[rosssavage]

I say easy to replace - obviously on lake sailors tohatsu they're not easy!! I agree that if they run at all it's very unlikely to be the reeds...

 

[BurnitBlue]

Thanks for the description and use of a reed in a 2-stroke engine. Exactly what I was hoping for especially the technique to determine whether the reed is the culprit.

Launching day soon and I will give the carbs another work-over on my Yamaha 8hp. I will also examine the reeds. I am not sure about the carb because it happened suddenly from one day of no problems to the next day when I started the engine easily as usual. Cast off the mooring lines, opened the throttle and the engine died. I was swept down river before I got my act together. Engine has behaved the same way for two years now despite attention and cleaning etc.

Bought a new Tohatsu last year as a replacement and after heading down the Baltic I entered the European Canal System. With the mast on deck I got as far as Berlin when the same darn thing happened to the Tohatsu. Had to use an old Johnson 3 hp to get back home. Very dangerous as there was not sufficient power to combat the suction from passing German barges. Never again.

Bought a replacement boat in Greece which has a diesel engine but I would still love to fix those 2-strokes at home mainly by studying them and working from strength rather than ignorance. I am now a two boat man.

Thanks again.

 

[SAMYL]

Sound suspiciously like blocked air vents. Have you checked them? Sometimes just a few turns is not enough and you may have to loosen the fuel cap fully.