Walbro carb problem

[spudulike]

8 hours ago, bmp01 said:

"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....

I distinctly remember one MS460 and the saw had no idle but would rev OK but the idle was non existent. I suspected the check valve was shot and a new secondhand carb resolved the issue.

 

All you say about the check valve is correct and perhaps my saying the idle was very rich may have been an overstatement but the check valve breaks the carb in to two distinctive circuits to supply fuel at high and low revs. If the check valve is stuck open, the carb will be pulling fuel through the low speed circuit and high speed circuit so by my reckoning, it will be rich and adjusting the L screw won't resolve too much because the carb wasn't designed that way.

 

The OPs carb may have a blocked high speed jet, blocked channel or a check valve that is stuck CLOSED or perhaps a diaphragm issue. The lack of high speed revs points me to a blocked high speed circuit.

 

[bmp01]

2 hours ago, openspaceman said:

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.

I've seen both ie some carbs where the hi speed screw controls all the flow and in others there is a fixed jet to control some percentage of flow and the hi screw controls an additional flow. I suspect earlier carbs fall into the "all the flow" category but dont know that for sure.

The boring reality is you need to investigate each carb to fully understand it .... unless you already have experience with the specifc type of carb (I don't).

 

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.

Yes, metering valve must be opening at full open throttle, t'is the only way fuel can get from tank to metering camber. The metering chamber and flexible metering diaphragm should behave like a (small) reservoir of fuel able to cope with transitional behaviour - clearly not in this case.

 

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.

I dont think it will go through the check valve, it goes through the jets or drillings into metering chamber. The check valve is on the other leg of t b e tee junction. To test further I'd suggest opening the metering chamber, blank off these hi speed drillings (or jets) with bluetack and try again, blow and suck through the hi speed screw hole and see if you can feel a difference in flow ie check valve working.

 

Tell me... do both saws have purge fittings ? Does the purge pipe fit snuggly on the bad carb? Any air leaks here with allow the carb to suck air into the metering chamber... Similarly if the purge bulb is split.

 

So here's the thing.... you dont need the carb pump, the check valve or the metering function to be operating 100% for the saw to run on choke or at fully open throttle - the magnitude of the vacuum in the carb inlet port will suck fuel through. At idle and low throttle, the depression in the carb inlet port is small, you need all the above to work well. Any air leak into the metering chamber eg, check valve leaking, purge bulb circuit Ieaking, acceleration circuit Ieaking, diaphragm or gasket leaking etc then air will be drawn into meter chamber in prefernce to creating a depression here which you need to open the metering valve.

 

 

 

 

 

 

 

[bmp01]

2 hours ago, spudulike said:

the check valve breaks the carb in to two distinctive circuits to supply fuel at high and low revs. If the check valve is stuck open, the carb will be pulling fuel through the low speed circuit and high speed circuit so by my reckoning, it will be rich and adjusting the L screw won't resolve too much because the carb wasn't designed that way.

Yes I agree it does divide the carb. I'd add this:

At low speed (low throttle openings) the check valve closes off the high speed circuit completely, engine is fed from low speed circuit only.

At high speed both the high speed and low speed circuits feed fuel to engine. I suppose a test for this might be to some how get the engine runnIng at a decent lick and close the low spoed screw. Look for a high no load speed....

 

Im gonna have to disagree with you 😨  with what happens if the check valve gets stuck open.

While the throttle is closed there is ambient pressure at the high speed opening in the carb inlet port so no means of pulling fuel into the carb inlet. I suppose there might be some vacuum in the inlet port if the air filter was monsterously blocked.... just like the choke does.

 

Edit:

Actually rather than worrying about what we think is giong on with presures and all that, just have a look at the check valve in the flesh as it were. It only checks the flow from reveresing. It does not stop the flow in the forward direction, that is to say fuel flow in forward direction is the same if the check valve is operating correctly or if it is stuck open.

 

Edited December 9, 2023 by bmp01
Added text.

[openspaceman]

2 hours ago, bmp01 said:

do both saws have purge fittings ? Does the purge pipe fit snuggly on the bad carb?

Yes and the bad saw had a split hose from the carb to the purge bulb which I have replaced with a tight fitting one.

[spudulike]

My thinking is that with the engine running, there is air flow through the carb so if a venturi in the bore of the carb is open (an open faulty check valve), fuel will be pulled up through it.

I know the check valve seals in the other direction but it still requires a reasonable amount of airflow for it to open as it would when the throttle valve opens from idle and enough air flow opens the valve.

This is probably like discussing religion and isn't helping the fella fix his carb......like I said.....he is lacking high speed fuel....blocked high speed jet, blocked channel or a check valve that is stuck CLOSED or perhaps a diaphragm issue.

[bmp01]

Point taken. My last input on check valves then, its gonna change your thinking though !

Picture below is a check valve in bits, its from a chinese carb, I was messing about with it at the time. But they are all the same.

The point is .... there is no spring (or equiv) holding anything closed. The little flapper disc moves open or closed with the breeze. Like a reed valve but with zero bias.

 

 

[openspaceman]

So crud could be either side of the flap to foul the working?

[adw]

20 minutes ago, openspaceman said:

So crud could be either side of the flap to foul the working?

Yep that's how they are, but take the bugger away and see what happens, you see a lot of it where people blast 120 psi through it and wonder why it doesn't run properly. 

[spudulike]

No change of thought here as I have had them in bits before so know how they are constructed. All they do is to stop any high speed circuit action at idle so the low speed circuit can do its thing by itself.

In regards to the "So crud could be either side of the flap to foul the working?" ....Yes, if the valve is stuck shut, the high circuit is shut down making idle fine but revving out near impossible.

If you give a blast of carb cleaner down the H screw hole, you should get a good blast out of the check valve in to the carb bore.

[Ollie_M]

my suggestion would be fill with aspen as can help clear gunk out and leave on as high of an idle as it will run for a good 30 mins or longer, just try and get some fuel flowing through it, done this with a freinds ms181 i had borrowed that had been sat for around 2 years, wouldnt rev up, hard to start etc, left to idle once started and over the course of probably 45 mins i managed to get it to pick up rpm when throttle blipped, h needle right out it was very lean and clearly gunked up but once again cleared through with time. should've had a carb rebuild but mate didnt want to spend the tenner and worked out fine in the end and runs lovely now