5hp 2 stroke misfiring at higher RPM

[bulldiv]

Hi,

I have a Tohatsu 5HP 2 stroke engine which runs fine at low RPM but when I open the throttle, it starts misfiring badly. Reducing the throttle again makes it run smoothly.

I have had the carb apart and cleaned the jets but still doing it.

Any ideas please?

Dave

 

[Chiara’s slave]

Obstruction in the air intake? Choke stuck on?

 

[BobnLesley]

Obstruction in the air intake? Choke stuck on?

Those two; plus try fitting to a new spark plug and check the fuel flow from the tank to the carb (is the tank clean/water-free or maybe you have a collapsing fuel line?) Also maybe re-check that carb.

 

[bulldiv]

Thanks both will do some careful investigating!

 

[Chiara’s slave]

The other thing might be the fuel pump. Not completely familiar with this motor, it might well have a vacuum operated diaphragm pump. A tiny air leak there could result in fuel starvation, but I think it’s going rich on WOT. Fuel starvation would probably make it cut when you close the throttle.

 

[Stemar]

I had a similar issue on a 3.5. Obviously a fuel issue. Well, no, it turned out to be corrosion on the coil's earthing wire connection to the block.

 

[oldharry]

Depends whats happening: is it just fading and losing power, or is it a staccato popping on and off? Fading is nearly always fuel, but popping is plug/electrics with a spark not strong enough to sustain high speed running. That can be a range of things. Faulty plug, weak ht coil, bad connections etc

But it is a two stroke, and electrics can fade while fuellling faults can cause popping. Considering a 2T has only a few moving parts its astonishing how much can go wrong

 

[Chiara’s slave]

Depends whats happening: is it just fading and losing power, or is it a staccato popping on and off? Fading is nearly always fuel, but popping is plug/electrics with a spark not strong enough to sustain high speed running. That can be a range of things. Faulty plug, weak ht coil, bad connections etc

But it is a two stroke, and electrics can fade while fuellling faults can cause popping. Considering a 2T has only a few moving parts its astonishing how much can go wrong

Ain’t that the truth Speaking as the owner of an Aprilia RS250.

 

[bignick]

Have you changed the fuel? I’d also run it on super unleaded mix - standard unleaded how has 10% ethanol which some older engines don’t like. Super unleaded is still e5.

 

[BobnLesley]

Have you changed the fuel? I’d also run it on super unleaded mix - standard unleaded how has 10% ethanol which some older engines don’t like. Super unleaded is still e5.

That's a good shout, our 3.5s. hated E10, to the extent we bought 20 litres of expensive Bahamian ethanol-free and took it into the USA to carry us through until we returned to civilisation