Chainsaw Repair

Hi all,

This is my first post so be gentle 

I've owned a used 254 for the last 3 years and to be honest its been a bit of a let down. After about 20 mins work (firewood prep) it begins to bog down and eventually no amount of throttle feathering will keep it running. Once dead it won't start again until cold.
My worst fear is that the crank case bearings have wear but looking at it and checking for play there doesn't appear to be any.
It's had a new air and fuel filter but to no avail. I have supplied a new (non OE) pot and piston to my hobby mechanic father but i'd also like to supply him with a carb service kit.
I am struggling to find a suitable kit because the information he gave me from the carb is not recognised. I wondered if anyone could help?
SN from the saw : 254 9380977
Carb PN: Walbro 21-2457-4

I read somewhere that a certain carb (140?) could replace mine but having searched ebay they are about £90 delivered from the states.
I'd really like to get the saw running properly either to keep or sell but at the moment its totally unreliable.
Thanks in advance

Yes, it should be a HDA-35 - and the saw was made 1989 week 38.

It may be a good idea to check the fuel lines and the tank vent as well, to rule the really simple things out.

If opening (and closing) the fuel cap helps (temporarily) when the saw starts bogging, the tank vent is involved.

Coil might be going dead too after it warms up.  Check for fire after it does this.  If its got fire, open the gas cap very slowly while listening for a suction sound.  If it has one, than this is the problem as Sawtroll said, which is the tank vent.

A coil that is acting up when warm is of course a possible reason - but I would want to rule out the more simple (and cheaper) ones before going there.

It usually is best to start with eliminating what is easiest and/or cheapest to fix, before going to the more "advanced" issues.