Advanced carb questions, calling elites

[Brent Nowell]

I have found some great great posts about carbs here, but there are still some questions I have. This all falls into place for my, what seems to be, never ending quest to diagnose poor performance.

Here is a pic of an old zama carb from a poulan 3314.



1. What is the safest way to test the main nozzle/check without blowing it?

2. What is the function of “unkown”. It appears to have some kind of plastic behind it that looks like a check, but it is two way when I use vacuum or pressure. What does it do?

3. This carb has an accelerator pump, by putting some 2 stroke oil behind the piston and pumping it, the oil came out of this oriface. This seems to be tied directly to the H jet and to the main nozzle.
If there are so many problems with it leaking air, and people purposely blocking it, why have it? I understand the function, throttle is hit fast and the piston pumps a shot of fuel through the main nozzle to make the transition from the L jet to the H jet seamless. But if people are blocking it off like with the stihl 200, and the engine runs the same, why have it???? What’s the trade off for blocking it?
I have looked and looked through diagrams for the carbs on my 550xp and the 395 and I cannot see any accelerator pump. I have to assume without taking them apart that they do not have it. Why is it used on smaller displacement? If u want to see a diagram of the 550xp carb it is attached below

4.primer oriface. If you really wanted to block this, to eliminate the possibility of air being introduced to the system, would this be a good idea? Older saws did not have it, and it just seems like a waste of time and a reliability issue. What do you think?


If you could take the time to read through the questions and answer them with an explanation, I think this would help a lot of people understand small 2 stroke carbs better.

 

[LegDeLimber]

Is "unknown" where the metering diaphragm gets its impulse through?
IF so, perhaps the floater could be a regulator?

 

[Old2stroke]

The "unknown" is another check valve just like the main nozzle one except this one is in the passage that supplies fuel to the low speed circuit. It can be removed (difficult to do without destroying it) and will have no effect on engine performance. It's only purpose is to allow the primer bulb to work, without it the bulb will just suck air instead of fuel up through the fuel line and carb. It can be tested by just holding the end of the right size fuel line against it and sucking and blowing on it. The main nozzle check valve can be tested the same way except it has to be punched out of the carb to do it. I have found these things can pass all the tests you can think of and still be intermittent in use. Also they are never included in a repair kit and if you have to buy both, it's cheaper to buy a whole carb from China. The low speed circuit has a port at the throttle plate to supply fuel when the throttle is closed and usually 2 more ports that supply extra fuel as the throttle is open to allow the engine to transfer from idle to full throttle, if the low speed needle is adjusted right an accelerator pump should not be necessary.

 

[Huskvarna hotellgäst]

Is "unknown" where the metering diaphragm gets its impulse through?

The metering side doesn't get crankcase impulses.

 

[Huskitoter]

The "unknown" is another check valve just like the main nozzle one except this one is in the passage that supplies fuel to the low speed circuit. It can be removed (difficult to do without destroying it) and will have no effect on engine performance. It's only purpose is to allow the primer bulb to work, without it the bulb will just suck air instead of fuel up through the fuel line and carb. It can be tested by just holding the end of the right size fuel line against it and sucking and blowing on it. The main nozzle check valve can be tested the same way except it has to be punched out of the carb to do it. I have found these things can pass all the tests you can think of and still be intermittent in use. Also they are never included in a repair kit and if you have to buy both, it's cheaper to buy a whole carb from China. The low speed circuit has a port at the throttle plate to supply fuel when the throttle is closed and usually 2 more ports that supply extra fuel as the throttle is open to allow the engine to transfer from idle to full throttle, if the low speed needle is adjusted right an accelerator pump should not be necessary.

Thanks for posting that. I mistakenly assumed the check valve was in the bulbs.

 

[Brent Nowell]

The "unknown" is another check valve just like the main nozzle one except this one is in the passage that supplies fuel to the low speed circuit. It can be removed (difficult to do without destroying it) and will have no effect on engine performance. It's only purpose is to allow the primer bulb to work, without it the bulb will just suck air instead of fuel up through the fuel line and carb. It can be tested by just holding the end of the right size fuel line against it and sucking and blowing on it. The main nozzle check valve can be tested the same way except it has to be punched out of the carb to do it. I have found these things can pass all the tests you can think of and still be intermittent in use. Also they are never included in a repair kit and if you have to buy both, it's cheaper to buy a whole carb from China. The low speed circuit has a port at the throttle plate to supply fuel when the throttle is closed and usually 2 more ports that supply extra fuel as the throttle is open to allow the engine to transfer from idle to full throttle, if the low speed needle is adjusted right an accelerator pump should not be necessary.

Thank you!!!!

Awesome explanation it is making sense now

I will say that a 550xp carb is about 150-200$ from dealer and there is no China carb to choose that is cheaper. I did a test on the main nozzle and it passes air, I plugged the fuel inlet and drew vacuum through the primer nipple. It passes slowly, my other carb with a similar config does not.

Problem is bogging on quick throttle snaps and the idle seems to be super rich which tells me that at least the main nozzle is passing at idle which the test shows.

 

[Old2stroke]

The usual problem with the main nozzle check valve is it likes to stick open and when that happens the saw will stall when you let off the throttle after a cut. Air bleeds back through the main nozzle into the low speed circuit and leans out the idle too much. If the 3 ports in the idle/transfer part of the carb are flowing fuel properly and the low speed screw is set on the rich side, the saw should not bog on a snap throttle opening. Perhaps on that engine/carb combo, the low speed system just doesn't supply enough fuel and that's why a carb with an accelerator pump was used. Just as a note, at idle there should be nothing coming out of the main nozzle and the check valve in it should be sealing it off completely.