What's on your bench today?

[spudulike]

I am not 100% sure how the primer works on this saw but on the majority of Stihl stuff and others, pressing the purge empties the bulb of fuel in to the tank and as the bulb expands it draws more fuel in to it from the metering section of the carb, on doing this, the diaphragm pushes on the metering arm, actuating the needle valve and more fuel is pulled through.

If you are getting air in, your fuel filter is not fully in the fuel or the line is split. 

The bulb shouldn't pressurize the fuel tank as you are just pumping fuel through from it and back in to it. 

Are you 100% sure the primer bulb lines are on the correct way?

Other issues are the one way check valve in the pump under the purge bubble can seal up and not work properly. With a MityVac, you can connect it to the carb fuel inlet and the purge should cause a vacuum. Connect it to the fuel outlet from the primer and it should produce a pressure. 

That is my  normal fault finding on purge bulbs!

[openspaceman]

5 hours ago, spudulike said:

The bulb shouldn't pressurize the fuel tank as you are just pumping fuel through from it and back in to it. 

Are you 100% sure the primer bulb lines are on the correct way?

The prinmer bulb is sucking fuel from the carb and dumping it to tank. Because it is sucking air from somewhere other than the tank this air goes to the tank and pressurises it. This is why I asked the original question; can the air be being sucked back from the jet and into the metering chamber? I have replaced the gaskets so will give it a try tomorrow as the gasket to the metering chamber was split.

[bmp01]

Opensaceman you are quite right in your thinking - there is nothing to stop air being drawn in via the low speed fuel circuit. But it should be a small proportion of what the purge bulb pulls through. It's easier to draw fuel (assuming the fuel filter,  carb micro filter, metering valve are flowing freely ) compared to drawing are through the tiny jets in low speed circuit.

Worth noting:

- the one way valve in the high speed jet stops air being drawn in via the high speed jet, (one way valves fail occasionally but pretty rare).

- there is possibility of drawing air from the dreaded accelerator pump circuit (accelerator pump o ring will wear out dependant on mileage).

- leaking metering diaphragm is strong candidate (you say gasket was split so I'd say that was your problem).

 

HTH with diagnosis.

Edited January 10, 2022 by bmp01

[openspaceman]

2 hours ago, bmp01 said:

- leaking metering diaphragm is strong candidate (you say gasket was split so I'd say that was your problem).

Thanks, I had replaced all those. It turns out it was a split fuel hose from the top of the tank to the carb.

[Stubby]

1 hour ago, openspaceman said:

Thanks, I had replaced all those. It turns out it was a split fuel hose from the top of the tank to the carb.

As spud predicted ! 👍

[openspaceman]

19 minutes ago, Stubby said:

As spud predicted ! 👍

Yes

[spudulike]

It was the most likely although BMP was pretty accurate in what he said.

It sounds like it is fixed now.

[bmp01]

Well, I try to be (accurate). Thought this was going to be something juicy, a little bit out of the norm. Could have gone the replacement carb, chinese vs genuine discussion ....again 😁

 

But anyway it's fixed so that's a good outcome, well done openspaceman 👍

 

[openspaceman]

49 minutes ago, bmp01 said:

Well, I try to be (accurate). Thought this was going to be something juicy, a little bit out of the norm. Could have gone the replacement carb, chinese vs genuine discussion ....again 😁

 

But anyway it's fixed so that's a good outcome, well done openspaceman 👍

 

Not fixed as I expect the piece in the post from L&S tomorrow as I ordered it just before middday. I couldn't see it was damaged when it was in its little clip and strangely it wasn't leaking much petrol, just sucking it in when the bulb was pressed. Fingers crossed tomorrow.

[openspaceman]

1 hour ago, spudulike said:

It was the most likely although BMP was pretty accurate in what he said.

It sounds like it is fixed now.

Funny thing is I have never come across this problem before. I could have fitted a plain plastic fuel pipe  instead of waiting for the new piece to arrive but as I only keep standard stihl small tube and had nothing bigger bore to hand I lashed out £6.75 on the moulded piece.

 

It was one of three stihls I was given to fix last Monday and now I suspect the bulb  was only ruptured because it was pumped so often to get the thing to run at all. The others were to replace a snapped throttle trigger, easy, and a spark plug blown out of its hole, to which I fitted a helicoil.

 

 

 

Edited January 10, 2022 by openspaceman
added bigger bore