pop off pressure

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turtle561

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what would be a good pop off pressure be for a tilly hl 180B? it is on an old poulan 91 i have finally gotten back together after a lengthy search for parts.
i rebuilt the carb with a full kit. it doesn't want to pick up any fuel.even after putting it together wet it wouldn't move any fuel into the engine.it fires when i prime thru the plug or carb. the carb looked untouched when i pulled it out but the metering spring feels pretty stiff. i guess it ran this way,though it did have a cracked piston from overheating. what do you think?
where can you get spare metering springs? the kits don't have them.
 
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A lot of the experts today would recommend against puming them up until they pop off. Evidently the new diapragms are not as flexible as they use to be and could be damaged. Most recommend pumping them up to 10 PSI and verify that they will hold.

You may have to pull another HL apart and try switching the springs.

Are you absolutely sure you got the orientation of the parts right on the fuel pump side? The HL's are normally pretty reliable.

Check you carburetor gasket to where it attaches to the air box or manifold, make sure the impulse hole in the two are aligned with the hole in the gasket.

Mark
 
thanks for that.everything checks out fine assembly wise.the motor has new seals too,so there's plenty of crankcase pressure.found a tutorial on the web on testing hl carbs used on powered gliders.it shows them testing dissasembled so no harm to the diaphragms.
i'm rigging up a tester with the tubing from my blood pressure cuff and a guage i got at the sporting goods store.
i'll clip off a few coils and see if i can get it to work consistedtly at the 10-12 lbs. they reccommend. they said the stock springs can vary quite a bit.
ordered a few spare springs,just in case i go too far.
 
heimannm said:
A lot of the experts today would recommend against puming them up until they pop off. Evidently the new diapragms are not as flexible as they use to be and could be damaged. Most recommend pumping them up to 10 PSI and verify that they will hold.

[snip]
Mark
Click to expand...

I've read this too, Zama for one specifically recommend against doing a complete pop off test, they recommend 10psi held for a minimum of 15 seconds but...

The pop off pressure affects carby and hence engine tune !
It was something we always set pretty accurately on the HL Tillo's used on kart engines. (and sorry, no I can't remember, it was over twenty years ago :cry: )

This is for the ultra light guys, but IMO it applies equally well to us, neither want engine seizures
The pop-off pressure is the pressure at which the needle valve will open thus letting the fuel to enter the metering chamber. The value of this pressure is a function of the engine, power output and other factors and it will directly affect the air-fuel mixture that the carburetor will deliver to the engine. It is true that the mixture adjustments are done by the HI and LO screws but if the pop-off pressure is too high, it can lead to fuel starvation especially at high RPM. Such a situation will cause engine overheating and possibly lead to a seizure.If the pop-off pressure is too low, the carburetor will have a flooding tendency especially at low and mid RPM. It will also cause difficulties in restarting a warm engine.
After years of experimenting with the Walbro carburetor, I can tell that every carburetor that I tested had a different pop-off pressure setting. The big problem here is that these carburetors were bought from different dealers that sell paramotors. In other words, some carburetors mounted on these engines having a too high pop-off pressure may present an engine seizure hazard.
Most of us, PPG pilots, use Solo based paramotors. The great majority of these machines use the Walbro 32. After many experiments, I found that the best pop-off pressure for this type of carburetor is from 10psi to 12psi. The Cors-Air engine works best with the pressure not exceeding 11psi. All four off-shelf carburetors that I presented at the top of the previous page had different pop-off pressure settings. Three out of four had a too high value. (14psi, 15psi and 15psi) This pressure is dangerous for the Solo engine. As stated above, it can lead to overheating and seizure.
Click to expand...


From here http://www.aerocorsair.com/id28.htm
 
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bingo ! after clipping 3 or 4 coils from the metering spring & getting a 11psi pop off, i finally got it to run. a pull with the plug out got the fuel moving.plug in, one pull and it was running like a clock
guess this is a simple, often over looked thing that can effect a range of fuel related symptoms.
i have a partner R-16 that is a touchy/lean starter that i'll check next.
 
The pop-off pressure is the pressure at which the needle valve will open thus letting the fuel to enter the metering chamber...
Click to expand...

No, the pop-off pressure is when the needle valve will open when one forces it with more fuel pressure than the carburetor is designed for. In normal operation the needle valve opens when the pressure in the metering chamber drops below atmospheric pressure due to low pressure in the intake. Atmospheric pressure pushes on the diaphragm, which in turn pushes on the lever to lift the needle off the seat.

The force acting on the lever is the difference in pressure between atmosphere and engine vacuum times the area of the diaphragm. The spring is selected so that the needle valve opens at the desired pressure differential. The diaphragm area is one critical factor that determines at what pressure the needle valve opens. Yet the pop-off test is done with no diaphragm in the carburetor. It is not correct to say that there is an optimum design pop-off pressure (10-12psi) for all carburetors because this observed pressure does not determine when the needle valve opens on a running engine.

BTW, cutting coils off the spring shortens its free length but makes it stiffer. When installed in a carburetor the preload is lower so the initial force on the lever is less and the pop-off pressure is reduced. But the stiffer spring requires more force to lift the needle higher off the seat, which may restrict flow when higher rates are needed (as compared to the original spring).
 
drkptt said:
[snip] It is not correct to say that there is an optimum design pop-off pressure (10-12psi) for all carburetors because this observed pressure does not determine when the needle valve opens on a running engine.
Click to expand...

Absolutely correct. The original author was talking about a particular model carburettor on a particular engine. I don't think anyone has suggested 10-12psi is ideal for all impulse type carburettors.

Carbies with more efficient/powerful pumps need a higher (sometimes much higher than what is used in a saw) pop off pressure to prevent an overrich condition.

BTW, cutting coils off the spring shortens its free length but makes it stiffer. When installed in a carburetor the preload is lower so the initial force on the lever is less and the pop-off pressure is reduced. But the stiffer spring requires more force to lift the needle higher off the seat, which may restrict flow when higher rates are needed (as compared to the original spring).
Click to expand...

I could never work this recommendation out either.
A 'softer' (lower rate) spring has more winds for a given wire diameter, not less. This is basic spring theory.

For those that don't follow, a coil spring is merely a torsion bar wound into a coil.
The longer the bar, the softer rate. The smaller the wire diameter for a given length (number of turns) the softer the rate. Chopping coils off a spring makes a stiffer spring.

Everyone does it, so reducing the pre-load (the captive length) works. I'm guessing the rate doesn't increase enough to be a problem.
 
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