Dolmar PS-52 wont run.

[lukey]

hi guys, i have recently had some running issues with the above saw which i diagnosed as the fuel pipe being split. pipe has been changed and new filter fitted but it still wont run.

it will fire and start with the choke on but as soon as you go near the throttle it cuts out. have stripped the carb and everything seems fine in there. to me it would appear that the fuel/air mix is wrong but cant see how it would have changed on its own as i havent touched it.

any help would be gratefully recieved.

 

many thanks

Lukey

[peatff]

A friend of mine has a MS390 which would only run flat out no matter what you did with the screws. We stripped and cleaned the carb and it looked fine but fitted a carb kit anyway and it sorted it. Only thing it could have been was hardened rubbers or a microscopic hole in the old one which we couldn't see.

[spudulike]

The fact the saw fires and runs a bit on choke means that the rest of the saw is sort of OK.

 

If the saw won't idle but will run if you give it a handful means you have issues with the low speed part of the carb, if the saw will idle but not run flat out means you have issues with the high speed part of the carb.

 

If it won't run at all, then it is most likely fuel starvation.

 

Has it got a separate impulse line - if so , check there isn't a hole in it. Is the gauze filter in the carb blocked?

 

Have you popped the H&L screws out and passed some cleaner down the holes?

 

It is always worth putting carbs that are misbehaving in an ultrasonic cleaner - it really does sort out many issues!

 

You could just try turning the L&H screw out by 1/2 turn and try that - it may be the original setting was a tad lean and hardened diaphragm is now making it even leaner!

[lukey]

sorry, forgot to say the saw only runs till the fuel is used up in the carb. have checked the new hose and filter and everything ok there. have also switched back to old filter "just-incase" but still no change. it almost as if the motor isnt getting any/enough fuel.

its a very frustrating problem coz prior to the fuel hose split the saw was faultless starting and running great everytime i picked it up.

many thanks for the replies guys, keep them coming.

[peatff]

sorry, forgot to say the saw only runs till the fuel is used up in the carb.

 

The carb doesn't really hold any fuel, it doesn't have a float bowl or anything and the diaphragm pumps the fuel. If some fuel is getting through and then it stops there is something wrong with the carb. When it's on choke the suction from the piston will pull fuel through but when the venturi opens the suction goes if the revs drop.

[lukey]

just ordered a carb kit (diaphrams, needle jet, spring etc) as it was prob 2yrs ago since it was all changed. will try that first before adjusting the h/l settings. cheers

[spudulike]

The carb doesn't really hold any fuel, it doesn't have a float bowl or anything and the diaphragm pumps the fuel. If some fuel is getting through and then it stops there is something wrong with the carb. When it's on choke the suction from the piston will pull fuel through but when the venturi opens the suction goes if the revs drop.

 

The diaphragm doesn't pump, it allows the carb to hold a little fuel that is pulled in to the carb throat by the vacuum formed in the carb throat by the engine pulling air across the low speed jets and high speed check valve (venturi effect)

 

The diaphragm also meters the correct amount of fuel to be held in this part of the carb by pushing on the metering arm and then having the fuel hold it off from letting more fuel in by breaking contact with the metering arm.

 

It is the pumping membrane in the top of the carb that creates the pumping action and this acts against a closed needle valve that opens when the petrol is used in the metering section and the diaphragm rivet contacts the metering arm!

 

Thought everyone knew this:001_tt2:

 

No offence bud - just like facts to be accurate:thumbup:

[GardenKit]

But to be competely fair, the pump 'membrane' is correctly referred to as the 'pump diaphragm'

As opposed to the other one on the other side of the carb which is the 'metering diaphragm'

[spudulike]

But to be competely fair, the pump 'membrane' is correctly referred to as the 'pump diaphragm'

As opposed to the other one on the other side of the carb which is the 'metering diaphragm'

 

Bet you vote Liberal as well don't you Barrie:001_rolleyes:

 

I guess if Peatff meant that then OK - will take it back, I would say people think of the metering diaphragm as "the diaphragm" but will find out later when he pops up again:lol:

 

Just wanted to make sure people didn't believe the metering diaphragm did any pumping - easy mistake to make!

 

The top part of the carb does the pumping acting like a mini fuel pump as the fuel is often lower than the carb, the bottom "Metering" section with the metering arm in it, meters out the fuel in to the carb choke/throat and in to the engine.

[GardenKit]

No, True Blue, me.