Pop-off Pressure

by Had Robinson
updated May 31, 2022

Do not waste time measuring the pop-off and rest pressures if it has been a while since the carburetor was rebuilt.  A malfunctioning inlet needle will have virtually the same appearance as a new one.  As the needle ages, the microscopic shape of the rubber part (which is part of the valve) changes shape so that more pressure is required to open the valve.  This is why carburetors should be rebuilt often or when they are suspected of not functioning correctly.  Remember that the cue for a carburetor rebuild is an erratic idle or it becomes impossible to adjust the idle.

The pop-off pressure affects the fuel/air mixture across the entire throttle range and is why it must be set correctly.  Do not waste time rebuilding the carburetor without also checking the pop-pressure.  Pilots might get lucky with just a rebuild but if there are problems, the gauge must be used.

Note: While the pop-off and rest pressures are close for the WG and the WB carburetors for all engines, the pop-off springs are NOT interchangeable between these models.  You must use the pop-off spring that is specified for that specific carburetor model (WG, WB, or Tillotson).  Using the incorrect pop-spring for your carburetor can burn up the engine.

1. Obtain a gauge  A genuine Walbro gauge costs around $190.  The Mikuni MK-BN PMP pop-off gauge costs less than $60 on eBay and works exactly the same.  Note: the stock connecting tube on the Mikuni is too big for the Walbro WG so pilots will have to purchase a 1/4" ID x 1/8" ID barb reducing coupler.  You will also need a foot of 1/8" ID windshield washer hose which can be found at any auto parts stores.  BEWARE of the Chinese knockoffs of the Walbro gauges because they do not have the correct pressure range and who knows how accurate they are.

2. Watch the video  Learn how to test the pop-off pressure by watching the Walbro carburetor service video.  Be extremely careful with the pop-off gauge pump.  Hard or fast pumping could damage the delicate valves inside the pump chamber.  This is also why an compressed air hose should never be used to "clean" an assembled carburetor.

3. General test  Walbro recommends a complete pressure test of the system (see below).  You can find leaks, soft spots, sticking valves, and a plugged inlet filter screen, as well as measure the proper pop-off and re-seat pressures.  The nice thing is that the carburetor does not have to be disassembled to do this test.  If the carburetor is already disconnected from the fuel system, the pop-off gauge can be connected directly to the fuel inlet pipe on the carburetor.

Note: If you can raise the pressure above 24 psi, whether it pops-off or not, STOP and disassemble the carburetor.  It is likely that water has gotten into the fuel system including the carburetor.  This is especially a problem in humid climates where water in the gasoline will give a high pop-off pressure if, indeed, the valve even opens.  This is because water will not go through the inlet filter screen easily.  Other reasons for a high pop-off pressure include a deformed inlet needle, a clogged inlet filter screen, and a rusted/brittle metering lever springs.  Rarely, the metering lever assembly can be mechanically damaged through mishandling.

WG pop-off pressure: 1.2-1.3 Bar (17.5-19.0 psi)    Rest pressure must be above 0.68 Bar (10 psi.)

WB pop-off pressure: 1.3-1.4 Bar (19-21 psi)    Rest pressure must be above 0.68 Bar (10 psi.)

If the pop-pressure is too high, rebuild the carburetor AND replace the metering lever spring.  An excessively high pop-off will lean out the engine.

If the rest pressure is too low, the valve is leaking and a rebuild is probably all that is needed.  Theoretically, the rest pressure can equal the pop-off pressure.  In real life, the rest pressure will always be lower than the pop-off because the valve will open, even a tiny bit, at rest until the spring pressure closes it completely.  If the rest pressure is less than the fuel pump pressure of 0.34-0.48 Bar (5-7 psi), the valve will open and richen the air/fuel mixture, especially over the lower throttle range.  This will cause the engine to idle poorly, if at all.  If the pop-off and/or rest pressures are *NOT* correct, continue with step #4, otherwise go to step #5.

4. Inlet valve seat condition  Determine if the inlet valve seat is worn out/damaged.  If it is, the carburetor must be replaced.  To perform this test, the carburetor must be removed.  a.) Replace the inlet needle valve with a new one (from a kit) but do not install the metering lever diaphragm and cover.  b.) Put some penetrating oil in the inlet needle valve hole and the surrounding area around the metering lever.  c.) Pressurize the fuel system with the pop-off gauge to 0.14-0.34 bar (2-5 psi).  If there are any bubbles around the needle valve, the valve seat is bad.

The leak is due to a corroded valve seat or goo e.g. from old fuel, water contamination, and/or ethanol fuels contaminated with water.  The technically gifted can try this: Use a modified cotton swab with the cotton end cut off.  Shape the end into a tip, like a pencil.  Put some carburetor cleaner or rubbing compound on the tip.  Push the tip of the swab into the inlet valve seat and rotate the the tip for (10) seconds.  This would work only for light corrosion or the presence of goo.  Retest the seat.  If the valve still leaks, the carburetor must be replaced.  If this test passes, rebuild the carburetor and perform test #5.

5. Measure the pop-off and rest pressures  (If the General test #3 above passes, you do not need to perform this test.)  The Walbro video does not mention it but it is necessary to wet the inlet needle valve and seat with penetrating oil or some fuel/oil mix in order to get a more accurate pressure reading.

Note that you should fully open the ML with your finger a few times under pressure from the gauge/pump to clear any microscopic debris that might be between the valve and the seat.

If the pop-off rest pressure is less than the fuel pump pressure of 0.34-0.48 (5-7 psi), the valve will leak and richen the mixture over the entire throttle range.  This will cause the engine to idle poorly, if at all.  As Gerry Farell notes, the rest pressure is more important than the pop-off pressure.

6. Analyze the results  The pop-off fuel pressure is determined by the strength of the metering lever spring.  The spring forces the inlet needle valve closed except when acted upon by the metering lever diaphragm which, in turn, is activated by the amount of air flowing through the carburetor.  For detailed discussion of this, consult the Walbro service manual.  As parts age, the pop-off pressure increases because the rubber tip of the inlet needle deforms.  The rest pressure may also decrease.

View of the WG with the metering diaphragm cover and diaphragm removed.

Walbro WG-8 metering lever details

If the pressure is too high, the engine will run lean and may overheat.  If too low, it will run rich and full throttle power output will suffer.  The engine will run roughly.

If the valve leaks, the idle will be difficult to adjust and/or the engine will stall.  The engine may also run roughly off idle due to a richening of the air/fuel mixture.  A badly leaking inlet needle valve will make the engine difficult to start, if at all.

A weaker spring will cause a low pop-off pressure.  A stronger spring increases the pressure.  A high pop-off pressure is usually caused by a corroded spring and it should be replaced.  Before replacing the spring, rebuild the carburetor, and check the pop-off pressure again.  Generally, it is good to replace the spring every time you rebuild the carburetor.  The springs are inexpensive.

Over time, the pop-spring will harden, become brittle, and/or weaken and affect the fuel pressure, usually lowering it (and richening the fuel/air mixture).  This is why the pressure must be checked every time the carburetor is rebuilt or when there are complaints about engine performance.  Pilots can increase the pressure by adding special brass shims under the spring (available from Southwest Airsports) but this is not necessary in most situations and is not recommended for increasing the pressure due to a weak spring.  It may be done if the engine is used at high altitudes.  In the photo below, the spring on the left (marked red) was unable to maintain the correct pressure and caused a rich running condition.  The spring on the right is new.  Note the difference in length.  New springs should be 15mm long.  However, old springs can also be the same length but, because of age (hardening), will give a higher pop-off pressure.

the pop-off spring on the left is old and brittle

The combined functions of the main jet AND the pop-off spring determine the air/fuel ratio delivered to the engine at around 3/4 throttle and above.  It is better to change the jet size than change the pop-off pressure.  We have the high altitude jets, if needed.  Contact us to purchase.

Do NOT stretch or cut the pop-off spring as this will negatively affect the spring at high engine speeds when it is being vigorously exercised.

Walbro pop-off springs (L-R)  WB; WG; WG(special)  Note that they are all different lengths and each gives a different pop-off pressure.  These springs help determine the idle, mid-range, high speed fuel/air mixture going into the engine.

Walbro pop-off springs

Pilots who fly at high altitudes (1,200m MSL/4,000' MSL and above) may want to adjust the pop-off pressure by adding shims to the existing spring or replacing the spring with one of greater compression strength.  We can supply these special shims and springs in different sizes.  Contact us for details.  However, it is usually simpler to change out the main jet for the high altitude jet.