2 Stroke Design Query

[KOPON]

Hi all,

New to the forum but I can see some well informed members here regarding the workings of the 2 stroke engine.

In my case it's a chainsaw that only goes when it has sat for about 6 months and then only until I stop it and won't start again. The spark plug becomes soaked with fuel regardless of choke or throttle position. So I've stripped it down for a look and with the plug out, the carb off and the exhaust off I can see the piston in action. My (nonsensical) question now is this: Why is there no valve visible on the inlet port to prevent blow back on the down stroke of the piston?? Some diagrams online indicate that there IS a one-way valve here but no information on what it looks like. My limited mechanical knowledge tells me there OUGHT to be a non-return valve on the inlet port to encourage the fuel mixture to exit the crank case via the transfer port for final combustion. What am I missing?

Thanks for any assistance on this.

[Stubby]

19 minutes ago, KOPON said:

Hi all,

New to the forum but I can see some well informed members here regarding the workings of the 2 stroke engine.

In my case it's a chainsaw that only goes when it has sat for about 6 months and then only until I stop it and won't start again. The spark plug becomes soaked with fuel regardless of choke or throttle position. So I've stripped it down for a look and with the plug out, the carb off and the exhaust off I can see the piston in action. My (nonsensical) question now is this: Why is there no valve visible on the inlet port to prevent blow back on the down stroke of the piston?? Some diagrams online indicate that there IS a one-way valve here but no information on what it looks like. My limited mechanical knowledge tells me there OUGHT to be a non-return valve on the inlet port to encourage the fuel mixture to exit the crank case via the transfer port for final combustion. What am I missing?

Thanks for any assistance on this.

You need a carb kit and to re set the metering arm me thinks . 

[spudulike]

It is a two stroke, there are no valves, all the timing is down to port openings and durations or those openings, that is what makes two strokes what they are.

You may be thinking about the powervalve system as used in the Yamaha YPVS - this was merely a way of changing the exhaust port timing by raising and lowering the exhaust port top to change the power characteristics at different revs.

You need to ensure the engine has compression (was the piston clean looking when you removed the muffler?), other than that, if the saw is old, try a new fuel line, fuel filter and carb kit.

 

[KOPON]

Thanks for the replies.

Yes the piston and cylinder both look good. I've now got into the crank case and found a flood of oily fuel so I've washed it out with clean fresh petrol and left to dry overnight. Tomorrow I'll make up a fresh fill of 2 stroke at 40:1, fit a new plug and see how it behaves.

There are numerous diagrams on line apparently showing a typical 2 stroke engine operation, e.g. https://thundercoastmarine.com/blogs/blogs/understanding-2-stroke-engines-how-they-work

This is one of many similar and obviously misleading sites. Nevertheless, I'm still trying to understand how the fuel mixture is not partially forced back through the inlet port prior to being forced into the transfer port. 

[Stubby]

6 hours ago, KOPON said:

Thanks for the replies.

Yes the piston and cylinder both look good. I've now got into the crank case and found a flood of oily fuel so I've washed it out with clean fresh petrol and left to dry overnight. Tomorrow I'll make up a fresh fill of 2 stroke at 40:1, fit a new plug and see how it behaves.

There are numerous diagrams on line apparently showing a typical 2 stroke engine operation, e.g. https://thundercoastmarine.com/blogs/blogs/understanding-2-stroke-engines-how-they-work

This is one of many similar and obviously misleading sites. Nevertheless, I'm still trying to understand how the fuel mixture is not partially forced back through the inlet port prior to being forced into the transfer port. 

If you are going to use a modern synthetic 2t oil then mix it at 50:1 . 40:1 could be part of your oily residue problem .

[SJT]

Thy this for a diagram.

 

[SJT]

2-Stroke_Engine.exe

[KOPON]

Thanks for the reply. Sorry but I can't seem to open the file. On my laptop it says I'm trying to download an app. On my iPhone it says 'download completed' but then I can't find the file in my downloads. I've googled how to do it but again lots of instructions to follow that don't work for me. I've obviously got a bad iphone and a bad laptop!😊

[Sauen]

22 hours ago, KOPON said:

Hi all,

New to the forum but I can see some well informed members here regarding the workings of the 2 stroke engine.

In my case it's a chainsaw that only goes when it has sat for about 6 months and then only until I stop it and won't start again. The spark plug becomes soaked with fuel regardless of choke or throttle position. So I've stripped it down for a look and with the plug out, the carb off and the exhaust off I can see the piston in action. My (nonsensical) question now is this: Why is there no valve visible on the inlet port to prevent blow back on the down stroke of the piston?? Some diagrams online indicate that there IS a one-way valve here but no information on what it looks like. My limited mechanical knowledge tells me there OUGHT to be a non-return valve on the inlet port to encourage the fuel mixture to exit the crank case via the transfer port for final combustion. What am I missing?

Thanks for any assistance on this.

The diagrams you're seeing are from reed-valve two stroke engines, this indeed prevents the fuel/air mixture from backflowing. Such engines were used in early chainsaws, but nowadays most chainsaw engines do not use such a valve. They rely on the momentum of the air/fuel mixture moving through the case and cylinder to pull air and thus fuel into the case. Indeed, some of this can be blown back if the intake duration is too much.

[KOPON]

9 minutes ago, Sauen said:

The diagrams you're seeing are from reed-valve two stroke engines, this indeed prevents the fuel/air mixture from backflowing. Such engines were used in early chainsaws, but nowadays most chainsaw engines do not use such a valve. They rely on the momentum of the air/fuel mixture moving through the case and cylinder to pull air and thus fuel into the case. Indeed, some of this can be blown back if the intake duration is too much.

Thanks, Sauen. I suppose the speed of the transfer is what effectively prevents blowback i.e. proximity of inlet port to transfer port.