Hydraulic lock a chainsaw ?

[peatff]

Can someone explain how you could hydraulic lock a saw without pouring fuel straight down the plug hole and fitting the plug back in then pulling it over ? Someone has told me their saw hydraulics and breaks the pull cord but I have never seen it happen and can't see how enough fuel would get into the squish area when it would have to travel through the crank case and in through the inlet port as a liquid and not as a vapour which the carb puts out. Once the plug is wet you can smell the fuel so you would have to be pretty dumb to keep pulling long enough to fill it with fuel.

[spudulike]

A leaking needle valve would be the most obvious way. I have seen it on a blower in recently!

[peatff]

Not something you could ignore and break the cord 3 times really then ? I just couldn't see how enough neat fuel got above the piston through the carb on a saw when the tank is lower down than the carb. Cheers for the answer Spud.

[spudulike]

Not something you could ignore and break the cord 3 times really then ? I just couldn't see how enough neat fuel got above the piston through the carb on a saw when the tank is lower down than the carb. Cheers for the answer Spud.

 

The blower came in and the owner said it was tight to pull over, got lots of fuel out of it but guess if the combustion chamber is at least 1/3 full of fuel, then the compression would be excessive. The owner thought it had seized but in reality, the needle valve was stuck open and the tank pressure just pushed fuel in causing a fluid lock.

[cornish wood burner]

The blower came in and the owner said it was tight to pull over, got lots of fuel out of it but guess if the combustion chamber is at least 1/3 full of fuel, then the compression would be excessive. The owner thought it had seized but in reality, the needle valve was stuck open and the tank pressure just pushed fuel in causing a fluid lock.

Our 365 has had similar symptoms from new. Poor starting after being left for a while and also a bit hard to pull. I have now discovered fuel is leaking into the chamber.

[spudulike]

Our 365 has had similar symptoms from new. Poor starting after being left for a while and also a bit hard to pull. I have now discovered fuel is leaking into the chamber.

 

I would personally pressure check the carb and make sure the needle holds at least 15psi pressure. It may just be a worn needle valve, a badly adjusted metering arm, baggy diaphragm or dirt in the needle valve.

 

With 365s it is best not to use the decomp valve, they don't make massive compression and using the decomp doesn't help especially when cold.

[cornish wood burner]

I would personally pressure check the carb and make sure the needle holds at least 15psi pressure. It may just be a worn needle valve, a badly adjusted metering arm, baggy diaphragm or dirt in the needle valve.

 

With 365s it is best not to use the decomp valve, they don't make massive compression and using the decomp doesn't help especially when cold.

 

Thanks for the suggestions. I must have a look at it before we need it again.

[peatff]

Our 365 has had similar symptoms from new. Poor starting after being left for a while and also a bit hard to pull. I have now discovered fuel is leaking into the chamber.

 

That's the bit I can't understand. The carb is not connected to the cylinder direct as the fuel vapour goes in, round the crank and then in through the port and the saws I have all have the tank lower than the carb and fuel has to pump in via the metering diaphragm unless the saw is hung up in some way.

[treequip]

That's the bit I can't understand. The carb is not connected to the cylinder direct as the fuel vapour goes in, round the crank and then in through the port and the saws I have all have the tank lower than the carb and fuel has to pump in via the metering diaphragm unless the saw is hung up in some way.

 

But as you pull the string the engine draws whatever is in the crankcase into the combustion chamber, so any excessive amount of fuel gets dragged up from a relatively large space into a small one.

[peatff]

But as you pull the string the engine draws whatever is in the crankcase into the combustion chamber, so any excessive amount of fuel gets dragged up from a relatively large space into a small one.

 

But how did it get into the crankcase via the carb, is the fuel under pressure and forced through the carb because the tank vent is blocked ? I have been tinkering with engines for 50ish years from Lambretta and BSA Bantam days and when they leaked fuel it was under gravity and any fuel in the crank evaporated leaving the oil so when you got a start it was very smoky till it burned off or it oiled the plug and wouldn't go