Issues with too high pop off pressure

[cookies]

I would start from the beginning using the factory procedure to set up the carb properly since it sounds like you have been all over that thing adjusting it. Typically you set the jet pins out a turn then adjust idle up then set it back down using the low speed jet and it sounds like you have insufficient fueling on the low speed jet when the saw needs to recover from heavy loading at high rpm in a hot engine that needs more fuel to idle properly.

 

[FunkyYota]

I would start from the beginning using the factory procedure to set up the carb properly since it sounds like you have been all over that thing adjusting it. Typically you set the jet pins out a turn then adjust idle up then set it back down using the low speed jet and it sounds like you have insufficient fueling on the low speed jet when the saw needs to recover from heavy loading at high rpm in a hot engine that needs more fuel to idle properly.

Thing idles and comes off WOT fine now.

Maybe I should take a video of this thing running. Still runs kinda weird. I’m starting to think maybe it’s not air/fuel related.

 

[cookies]

pressure/vac test it and its fuel hose/filter/vent to remove them from the equation

 

[Old2stroke]

Thats a good point - I got the thing running pretty well. The thing will idle through a tank of fuel I imagine. I've let it idle for five minutes no problem. So thats good.

When its pinned and not in a cut it sounds pretty good - seems to me just like the rev limiter.

What's got me kinda confused is it'll 4 stroke in the cut. I tried richening the H up to get it to 4 stroke out of the cut, then clean up in the cut, but can't make the 4 stroke in the cut disappear.

If it still 4-strokes well into a serious cut, you are still just a bit too rich on the H screw. If you are making a number of short cuts, it's normal for the engine to load up and 4-stroke for awhile the first time you make a serious cut and it might not clean out until you are well into the cut. Try cutting a couple of cookies in succession with a log that will put serious load on the saw and see if it will clean out. When you get it right the engine should run crisp and clean as soon as it starts the cut and if you back off on the pressure part way through the cut it should go back to 4-stroking instantaneously.

 

[DanAyo]

Yes I have had everything from about 12 psi pop off pressure up to around 20 ish, all work just fine. I don’t suppose this is your issue. I honestly don’t give pop off pressure much thought, I never see specs in any manuals and if it was so essential I believe they would.

Another Pop Off question
I am in the process of rebuilding a couple of WT Series carbs by the book. While researching YouTube the subject of Pop Up pressure surfaced.
1 - Should I completely disregard this test? I read there is a danger of raising the pressure so high that the diaphragm could be damaged.
2 - Should I stick to 7-10 pound pressure test using the input fuel fitting.

Last question -
If there are large discrepancies in the pop off pressure, how does that not affect the operation of the carb?

Thanks,
Danny

 

[J D]

Make sure it holds the 10psi. If you can give the dimple on top of the diaphragm a light tap & it should release.
Think of it kinda like a tap, doesn't matter how much pressure it will hold, as long as it holds enough not to leak under normal working conditions, & when it's opened it opens & flows as it should

 

[SteveSr]

2 - Should I stick to 7-10 pound pressure test using the input fuel fitting.

Yes, All the needle and seat has to do is to hold off fuel pump pressure which should be under 7 psi.

 

[DanAyo]

Yes, All the needle and seat has to do is to hold off fuel pump pressure which should be under 7 psi.

Please let me know if my understanding is correct.
1- The vacuum and pressure from the Pulse line only affects the fuel pump.
2- The metering diaphragm is lifted as the air flowing through the Venturi increases. More air requires more fuel. As the diaphragm is lifted upwards from the increased vacuum, the metering lever is depressed (according to the vacuum pressure). This causes the metering needle to lower, allowing fuel into the reservoir feeding the high/low needles.

Up until now, I assumed the engine pulse controlled the movement of the metering diaphragm. I’m learning.

If my understanding is correct and since you say that the engine develops ~7 pounds of vacuum, it seems to me that measuring the spring tension from the fuel intake is not the best method. Isn’t the spring tension directly affected by the flow of air through the venturi and acting upon the metering diaphragm? If possible, it seems best to measure the spring tension from the vacuum on the diaphragm instead.

I am thinking out loud and trying to learn…

Thanks for the help.

 

[DanAyo]

In the illustration above, I had thought the funny looking face in the dark circle, at the top of the high speed path was Walbro’s way of making a comical gesture. Not Kidding!

I now believe it is the check valve. Is this correct?

I’m a little slow but catching on. LOL

If that is a check valve, how does it work?

 

[huskihl]

Yes, All the needle and seat has to do is to hold off fuel pump pressure which should be under 7 psi.

I think you mean fuel tank pressure. With tank vents the way they are nowadays not allowing pressure to bleed out of the tank, I would be shooting for more like 15 psi minimum. Many of the newer saws are above 30 PSI pop off pressure

 

[huskihl]

In the illustration above, I had thought the funny looking face in the dark circle, at the top of the high speed path was Walbro’s way of making a comical gesture. Not Kidding!

I now believe it is the check valve. Is this correct?

I’m a little slow but catching on. LOL

If that is a check valve, how does it work?

Check valve, main jet nozzle. Same thing

 

[Vintage Engine Repairs]

That is the main nozzel, inside the main nozzel is the nozzel check valve. That serves a few functions, mainly stopping air going back into the metering chamber and thus causing a lean condition. This is only applicable at idle and part throttle. After that that nozzel check valve is open as it allows mains fuel supply.

 

[Old2stroke]

The type of check valve in the wt carb is just a small round rubbery disc mounted on a fuel orifice in the main jet and can cause major running issues if it becomes distorted and sticks in either open (more common) or closed position. Alcohol in the fuel is the usual fault or sometimes the disc can be blown out by cleaning the carb with high pressure air. They are never included in a standard carb rebuild kit, they can be tested but may still be intermittent and often become the reason for trying an AM replacement carb. If you really like repairing things and time isn't an issue, do a search on "repair nozzle check valve". Good info to be had.

 

[DanAyo]

The type of check valve in the wt carb is just a small round rubbery disc mounted on a fuel orifice in the main jet and can cause major running issues if it becomes distorted and sticks in either open (more common) or closed position.

See Illustration -
If this is the check valve, it seems like vacuum through the venturi would suck the check valve upwards - stopping the flow of fuel through the main jet.
Please explain how the check valve operates.

 

[Vintage Engine Repairs]

See Illustration -
If this is the check valve, it seems like vacuum through the venturi would suck the check valve upwards - stopping the flow of fuel through the main jet.
Please explain how the check valve operates.

At idle there is no venturi effect occuring over the H nozzel. However there is vacuum acting on it from the metering chamber.

 

[J D]

The lip that retains the valve disk isn't a full circle so it only seals with pressure from the venturi side

 

[DanAyo]

The lip that retains the valve disk isn't a full circle so it only seals with pressure from the venturi sideView attachment 940164

I just took a close look at my check valve and realized my assumption was wrong. According to the Walbro illustration it appeared to me that the check valve seated on the top. After inspection I see that the check valve seats on the bottom. Things make sense now.

If a piece of fuel hose is sealed against the bottom of the check valve (in the venturi) and sucking is possible while blowing is not, can the valve be considered good with no need to remove and inspect it?

 

[heimannm]

Yes

 

[Old2stroke]

I just took a close look at my check valve and realized my assumption was wrong. According to the Walbro illustration it appeared to me that the check valve seated on the top. After inspection I see that the check valve seats on the bottom. Things make sense now.

If a piece of fuel hose is sealed against the bottom of the check valve (in the venturi) and sucking is possible while blowing is not, can the valve be considered good with no need to remove and inspect it?

You will have difficulty sealing a tube to the valve while it is in the carb. They are easy to remove by just punching them through into the carb throat and can be reinstalled by punching them back in from the fuel chamber side, to the same depth as original. Just be aware that even though they test good, they can become intermittently sticky and drive you nuts.