Walbro carb problem

[adw]

If the check valve is not closing off then air from the Venturi will enter the main jet causing it to run lean.

[openspaceman]

6 minutes ago, adw said:

If the check valve is not closing off then air from the Venturi will enter the main jet causing it to run lean.

Thanks but is there a graphic to explain how this happens? I know there is a fixed jet  plus the jet the Hi screw adjust and the idle jet but I cannot see how this weakening happens, though it does sound very much like the problem.

 

I have a few busy days so not looked at it again yet.

[bmp01]

This is from the Zama tech guide, you'll have to find the original source if this is not readable.

 

[bmp01]

Couple of things, Zama isnt the same as Walbro but the principles are the same.

In the pictures above the air flow is from right to left ..... chinese innit.

The check valve is required to work for idle conditions. It stops air from going the wrong way through the high speed nozzle / jet / check valve what ever you want to call it. With out the check valve, air from the intake in the carb would be drawn the wrong way through the check valve and high speed circuit, into the metering chamber due to the suction fom the idle circuit. Engine would run lean at idle if it ran at all.

 

Check valve should be open for wide open throttle, its possible for it to be stuck closed, would send the engine lean as soon as throttle was opened.

 

In your opening post, did the original saw idle ok or was that only on choke too (you described the good saw with duff carb only running on choke ... I think...). If it only ever ran on choke that would point to the pump circuit not able to supply fuel to the metering side of the carb (pump circuit isnt needed on choke, the intake depression from the engine sucks the fuel through). 

 

 

[openspaceman]

14 minutes ago, bmp01 said:

did the original saw idle ok or was that only on choke too

On both saws they would only run on choke and died as soon as the choke flew off when throttle depressed.

 

Thanks for the graphic, I'll look at it properly tomorrow evening.

[bmp01]

Ah-huh, consistant then. Well, definitely gettting fuel to the metering side of the carb while its on choke so its moving the diaphragm, operating the metering lever and opening the valve. Coming off choke you'd expect it to run for a few sconds while it uses up the fuel left in the metering chamber... doesnt sound like it does.

On the good carb, how many turns out on the mixture screws?

Here's the running on choke page, same guide....

 

 

Edited December 7, 2023 by bmp01
Typo

[openspaceman]

23 hours ago, bmp01 said:

Ah-huh, consistant then. Well, definitely gettting fuel to the metering side of the carb while its on choke so its moving the diaphragm, operating the metering lever and opening the valve. Coming off choke you'd expect it to run for a few sconds while it uses up the fuel left in the metering chamber... doesnt sound like it does.

On the good carb, how many turns out on the mixture screws?

 

Okay I have got my head around why the check valve is on the main jet, not taken it all on board but got the gist.

 

Anyway I got back today with a bit of daylight left and started the saw in for repair with the carb from the good saw. It starts and runs as normal.

 

Did the sane with the good saw and the bad carb and it would start on choke and run  if the throttle stop screw was wound in, clutch engaged and chain turning. Again if fluffed to a stop when the throttle was operated. Then using @spudulike's suggestion I wound out the Hi screw a few turns. Saw then ran flat out with the throttle open but was weak and running too fast, when I released the throttle a bit to slow it it fluffed and would not pick up when the throttle was opened again and died like it had no fuel.

 

Strangely the good carb is not a walbro, I know it is original as I was given the saw several years ago when I was in a Stihl dealers and the customer handed it in. He had broken the original chain and wanted rid of it, I took it when the dealer would not. There was and is nothing wrong with it once I replaced the chain which is now ready for replacement.

 

I'll have the bad carb off and test the check valve, I will see the owner tomorrow and have to make excuses for not fixing it in the month.

[spudulike]

sometimes you can push a bit of soft fuel line against the H screw hole (with the H screw removed) and if you blow and suck on the pipe, you should be able to feel the check valve popping shut and open. 

Most faulty check valves I have seen are stuck open making the saw run very rich at low revs so it kills the idle and pickup but the saw WILL run flat out.

You must have a lack of high speed fuel and that is most likely the high speed circuit from the diaphragm chamber, through the jets and in to the check valve. 

Have you closely inspected the pump diaphragm? Sometimes small holes in the pump or holed sealing flaps can cause issues.

[bmp01]

16 hours ago, spudulike said:

Most faulty check valves I have seen are stuck open making the saw run very rich at low revs so it kills the idle and pickup but the saw WILL run flat out.

"Most faulty check valves I have seen are stuck open...." airline pressure into the high speed screw hole has that effect on the one way valve. Carb cleaner ok, airline not so much.

 

"....run very rich at low revs..... " I dont think thats right, definitely not right at idle.

Check valve exists to stop air going through the the high speed circuit and into the metering chamber.

Logic: at closed throttle and low revs, there's (nearly) ambient pressure where the high speed circuit enters the carb - its before the closed throttle. The metering chamber is at slight depression due to low speed circuit sucking fuel. Air wants to flow from high go low pressure, so metering chamber would fill with air, not useful. Further bad consequence is metering chamber cant get sucked into depression to move metering diaphragm / open the metering valve.

 

".....but the saw WILL run flat out" Oh yes !  Gloriously, given the shite part throttle behaviour.

 

But im not convinced the problem is the check valve either. Do the pipe over the hi speed screw hole test like adw and spud described.

 

Sounds like issue is getting fuel into metering chamber - could be check valve screwing thing up - but also the usual suspects: the wee fuel pump or the metering side ie lever height or diaphragm....

Edited December 9, 2023 by bmp01
Typos

[openspaceman]

6 hours ago, bmp01 said:

Sounds like issue is getting fuel into metering chamber - could be check valve screwing thing up - but also the usual suspects: the wee fuel pump or the metering side ie lever height or diaphragm....

I was thinking that initially but after winding out the Hi screw and it running over fast I got confused. Tried it today and exactly the same happened. The thing is I didn't think the screw controlled all the flow through the mainjet/check valve I thought it just added to the flow through a parallel drilling; i.e. does screwing in the Hi screw cut all the flow to the main jet? Other than what goes through the idle jets.

 

Because it fluffs and will not recover makes me think that the diaphragm  is not opening the metering valve yet it must to some extent for it to continue running when the throttle id held full open.

 

I removed the Hi jet screw today and pushed a plastic tube in, when I blew I could not get a good seal but I saw bubbles moving into the purge bulb so the air must have passed through the check valve and into the metering reservoir.

 

BTW I met the owner at the repair cafe today and he said he had opened up the carburetor and cleaned it before he asked me, so he may have blown compressed air into the check valve. I will take the carb off again and test the check valve from the reservoir side.