Issues with too high pop off pressure

[FunkyYota]

I’ve been messing with this echo cs590 for a while. It wasn’t idling well so I threw a carb kit in. I also put in a main jet/nozzle/check valve that didn’t have the bypass orifice so I can actually lean it out enough (I live at 8800’). That really cleaned up the idle.

I’ve been having a heck of a time getting this thing to run right. I’ve got the idle pretty well dialed and the WOT sounds ok, but when I let off fast it’ll die if I don’t blip the throttle on the way down.

I checked the needle pop off pressure at 25psi. Seems way too high. Before I messed with that wanted to check to see if I’m chasing a red herring.

 

[heimannm]

As far as I know, the only way to lower the pop off pressure would be to change the metering spring. You can go the other way by stretching the spring a bit but reducing the spring is more complicated.

Mark

 

[FunkyYota]

As far as I know, the only way to lower the pop off pressure would be to change the metering spring. You can go the other way by stretching the spring a bit but reducing the spring is more complicated.

Mark

Yea I think I can clamp it or cut some off to reduce the pressure, just curious what the symptoms of too high of a pop off pressure is

 

[Vintage Engine Repairs]

Sounds like too much fuel delivered at WOT. You may find that when you’re actually cutting that excess fuel gets used up and then when it returns to idle there won’t be excessive crank case fuel remaining to drop the idle so low.

 

[FunkyYota]

Nice I’ll take a stab at leaning out the high speed jet.

any concerns in your experience with the pop off pressure at 25psi?

 

[Patrick62]

I had one recently. It was a MS230, and it would run, idle was ok, but just would not wind up right, and real anemic. High screw was pretty much non functional.
I took it apart, and checked the carb. IT sure seemed like there was a LOT of pressure on that needle spring! I decided to switch it out for a lighter spring from another carb.
Fired it back up, and my gosh, night and day. Idle was supurb, and no bog at all. Adjusted to 4 stroking perfectly. Sent it out, and customer is happy.

The logic of this is that the carb and engine have to supply sufficient vacuum to pull that diaphram down to open the needle. if there isn't enough... then she runs lean, if at all.
On the opposite side, I suppose you could have it too light, and the pump side would over fill the diaphram and she would flood pretty quickly.

 

[FunkyYota]

good deal thanks for that. I’ll order up a lighter spring or maybe try to cut a few coils from this one.

 

[Vintage Engine Repairs]

Nice I’ll take a stab at leaning out the high speed jet.

any concerns in your experience with the pop off pressure at 25psi?

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.

 

[Mad Professor]

Is the spring from the kit? If so put the original one back in.

 

[FunkyYota]

Is the spring from the kit? If so put the original one back in.

Good call. See if I can’t find that

 

[Vintage Engine Repairs]

Good call. See if I can’t find that

Is the spring from the kit? If so put the original one back in.

Good call, let us know what the pop off was with that original spring. I must admit I only ever use OEM carb kits, AM carb kits may not have such levels of quality control and thus the springs may be an issue!

However you speak of rich symptoms, if pop off pressure is increased, you’ll actually get lean symptoms not rich. I still don’t think this is your issue but I’m very curious to see. Also let us know if new the spring is AM or OEM.

 

[J D]

In all the discussions I've been privy to I can only ever recall people being concerned with a pop off pressure of at least xxx...
That said, I can see where you're coming from considering the impulse has to move the diaphragm to overcome the spring. On that line of thought however I would think a leak on the impulse circuit, stiff diaphragm or incorrectly set metering lever would be as likely to cause issues as a particularly stiff spring

 

[rd35]

Very interesting thread! These discussions make me think! So let me see if I understand this correctly. Saw running WOT...release throttle trigger....engine coasts down with throttle butterfly shut (which creates high vacuum in the crank case). This vacuum reduces the impulse (no air = no impulse) which results in low fuel pressure against the metering valve. If this is true then I can see how a lighter spring would help. I would also consider adjusting the idle up a tad to reduce the low speed vacuum as this would likely increase the fuel pressure against the metering valve as the engine slows. Am I thinkng about this correctly?

 

[FunkyYota]

Very interesting thread! These discussions make me think! So let me see if I understand this correctly. Saw running WOT...release throttle trigger....engine coasts down with throttle butterfly shut (which creates high vacuum in the crank case). This vacuum reduces the impulse (no air = no impulse) which results in low fuel pressure against the metering valve. If this is true then I can see how a lighter spring would help. I would also consider adjusting the idle up a tad to reduce the low speed vacuum as this would likely increase the fuel pressure against the metering valve as the engine slows. Am I thinkng about this correctly?

Ya that’s kinda what I was thinking. The idle screw turned up a bit definitely helps it recover from dropping from WOT to idle but sometimes the chain will rotate a bit at idle. I don’t love that

 

[J D]

You could richen up the L a smidge to bring the idle down

 

[Old2stroke]

By fooling around with the spring you may be introducing variables that aren't meant to play any part in the tuning process. It should be possible to get the saw properly tuned with the regular adjustment screws, especially now that the rich high speed elevation problem has been solved.

 

[FunkyYota]

By fooling around with the spring you may be introducing variables that aren't meant to play any part in the tuning process. It should be possible to get the saw properly tuned with the regular adjustment screws, especially now that the rich high speed elevation problem has been solved.

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.

 

[J D]

Sounds like it may be a bit rich on the H & loading up when you come off WOT. A tach could be a good investment

 

[SteveSr]

In all the discussions I've been privy to I can only ever recall people being concerned with a pop off pressure of at least xxx...
That said, I can see where you're coming from considering the impulse has to move the diaphragm to overcome the spring.

No! There are two different and independent circuits. The impulse drives the fuel pump and nothing more. The fuel pump will put out less than about 7 psi.

The job of the inlet needle and metering diaphragm is to keep the metering chamber full with a certain amount of fuel. As the metering chamber empties, pulling down the metering diaphragm, the inlet needle opens allowing more fuel in to the chamber. This process keeps the metering chamber filled with a specified amount of fuel.

When you pressure test the fuel inlet on a carb you are actually making sure that the inlet needle can hold off the fuel pump pressure. This is usually tested at 7-10 psi which should hold indefinitely. Needle pop-off / re-seat pressure is basically irrelevant in this application.

 

[SteveSr]

In all the discussions I've been privy to I can only ever recall people being concerned with a pop off pressure of at least xxx...
That said, I can see where you're coming from considering the impulse has to move the diaphragm to overcome the spring.

No! There are two different and independent circuits. The impulse drives the fuel pump and nothing more. The fuel pump will put out less than about 7 psi.

The job of the inlet needle and metering diaphragm is to keep the metering chamber full with a certain amount of fuel. As the metering chamber empties, pulling down the metering diaphragm, the inlet needle opens allowing more fuel in to the chamber. This process keeps the metering chamber filled with a specified amount of fuel.

When you pressure test the fuel inlet on a carb you are actually making sure that the inlet needle can hold off the fuel pump pressure. This is usually tested at 7-10 psi which should hold indefinitely. Needle pop-off / re-seat pressure is basically irrelevant in this application.