What's on your bench today?

[spudulike]

If you have one turn out on the carb and it wont pull around 14krpm then something is tight, the engine is making very poor compression or the carb is over fuelling. The things that stop a two stroke revving are load, wear or fuel. It could be the check valve in the carb is stuck open.

Have you checked the compression yet? I have had some very worn engines do the same as yours, not make top revs and lack any go.

One other thought, someone hasn't swapped the original black coil for a limited one at 13Krpm...a 390XP at a guess?

[billpierce]

If you have one turn out on the carb and it wont pull around 14krpm then something is tight, the engine is making very poor compression or the carb is over fuelling. The things that stop a two stroke revving are load, wear or fuel. It could be the check valve in the carb is stuck open.
Have you checked the compression yet? I have had some very worn engines do the same as yours, not make top revs and lack any go.
One other thought, someone hasn't swapped the original black coil for a limited one at 13Krpm...a 390XP at a guess?

Thanks spud. Dont know what the check valve in the carb is? Compression feels good so havent checked with a gauge but will have look at piston. Will get numbers off coil and look them up check its the right one. Also will run it with no bar on see what its doing

[spudulike]

The check valve is where the high speed fuel mix enters the carb throat. It is a valve that lets the venturi effect of the air ripping through the carb pull fuel through from the metering section but then seals in the opposite direction allowing the low speed circuit to control the mix at low revs when the throttle is closed. 

If this is stuck closed, you will fail to get high revs or power, if it is stuck open, idle will be non existent and the high speed will be very rich.

[billpierce]

The check valve is where the high speed fuel mix enters the carb throat. It is a valve that lets the venturi effect of the air ripping through the carb pull fuel through from the metering section but then seals in the opposite direction allowing the low speed circuit to control the mix at low revs when the throttle is closed. 
If this is stuck closed, you will fail to get high revs or power, if it is stuck open, idle will be non existent and the high speed will be very rich.

Thanks spud, idle is fine so possibley not this. Failing to achieve more than 13k with no bar. Piston looks fine, slightly carbony but not scored. Ignition module is 537 16 21 01 which a quick google says is for 345, 346, 350 etc so probably correct. I guess a vac test is next but seems unlikely and air leak from symptoms

[spudulike]

That's a strange one then....fuel line and fuel filter? I have had saws not make top revs and it is either too much fuel, too little fuel or engine wear although a faulty coil/spark can do it if you are loosing spark when flat out. My suggestion is to make sure all the fuel system parts are checked and then whip the cylinder off. Has anybody rebuilt the bottom end recently and not de-stressed the bearings? Is the saw easy to pull over with the plug out?

How is the saw cutting, lifeless or full of go? Any history on the saw and what others may have done to it? Is the carb being held fully open by the throttle? These throttles do wear a fair bit on the plastic bit! I usually just pull it back a bit so it is a bit longer.

[billpierce]

That's a strange one then....fuel line and fuel filter? I have had saws not make top revs and it is either too much fuel, too little fuel or engine wear although a faulty coil/spark can do it if you are loosing spark when flat out. My suggestion is to make sure all the fuel system parts are checked and then whip the cylinder off. Has anybody rebuilt the bottom end recently and not de-stressed the bearings? Is the saw easy to pull over with the plug out?
How is the saw cutting, lifeless or full of go? Any history on the saw and what others may have done to it? Is the carb being held fully open by the throttle? These throttles do wear a fair bit on the plastic bit! I usually just pull it back a bit so it is a bit longer.

No history on the saw, bought second hand, a few different shades of orange plastics so probably atleast has a few bits replaced if not rebuilt. How to destress bearings?

The saw runs ok, cuts strongly, just not very zippy, as fast to pick up as id like, and sounds like it has a bit more to go but wont. Ive had a new fuel filter on but nor checked the lines, suppose it could be a kink or something, but my gut feeling is its something inside the engine, as no real difference in rpm with the bar off.

How on earth could it still rev fine with the h needle turned right in?

Yes saw seems ok to pull over with the plug out. Very slight turning a 50pence feel to it but i dont think any more than i have felt on other saws that have worked fine.

[billpierce]

Yes have checked throttle opening fully, linkage is doing its job and appears fine

[spudulike]

You only de-stress the main bearings if the cases have been split. You just tap both sides of the crank with a copper mallet until the crank is smooooth and doesn't feel tight.

Very strange!

Edited February 3, 2020 by spudulike

[StihlGreen]

Been lurking for a while, finally signed up today. Just finished reading this thread, all very interesting....learnt a lot from it.

 

I've just put a new p+c on my ms261, she runs well. I'm now thinking before I use it too much that i should find the original cause of the damage.

 

Any advice on where to start would be greatly appreciated.

 

Cheers

Harvey

[spudulike]

Old fuel oil mix, straight petrol, incorrect adjusted carb, air leak, carb issue etc... That's about it. Just tach it so it is 500 -1000rpm below maximum and all should be fine.