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How To Set Pop-Off Pressure

How To Set Pop-Off Pressure

By Len Emanuelson

First, let me start by stating that I’m no 2-stroke carburetor expert. However, I’ve been forced to learn certain things about how to setup and service my carbs as part of my race prep. One of those procedures is checking and setting pop-off pressure — the pressure required to unseat the inlet needle and seat, or in some cases, the carburetor’s gross jet. What this affects is the carburetor’s calibration (rich or lean) especially in the lower rpm ranges. There are as many target pop-off pressure settings as there are opinions and engine combinations, but the generally accepted settings are 8.5-10.5psi for gas and 5-7.5psi for methanol. You can’t go wrong picking a mid-point in those ranges as a starting point.

After you remove the pumper stack, what you see is the “brain” of the carburetor, the fulcrum arm/inlet seat assembly. It’s hard to believe, but this crude little lever spring arrangement operates at engine rpm, or in my case 15,000.
After you remove the pumper stack, what you see is the “brain” of the carburetor, the fulcrum arm/inlet seat assembly. It’s hard to believe, but this crude little lever spring arrangement operates at engine rpm, or in my case 15,000.

There are a couple of other factors that affect low-rpm mixture control. The most important is the fulcrum lever height. A good starting point is with the top of the arm adjusted (bent) so that it is flush with the adjacent “flats” on the carburetor. If you bend the arm up, you richen the mixture. Typical settings for raised fulcrum arms (at least on my carbs), is .030” for methanol carbs and .060” for my gas carbs. Another consideration is the type of inlet valve, needle and seat or gross jet. My Buller methanol carbs run dual-ball gross jets that flow more fuel (even at smaller openings) than needles and seats. My gas carbs use needles and seats that run cleaner on the bottom end and seem to give snappier response.

The way you check pop-off pressure on a typical Mikuni or Tillotson pumper carburetor is to remove the pump “stack” breaking the carb down to the carb body with it’s inlet valve, fulcrum arm and spring. Next, you pour some type of liquid in the inlet valve well. Kermit Buller recommends and sets his carbs up with Marvel Mystery oil, but gas, solvent, WD40 or most lubricants will work. Then you take your pop-off gauge and insert it in the passage that leads to the chamber below the inlet valve (see photo), and slowly pump pressure into this chamber. Keep watching the fluid around the inlet valve for bubbles if the gauge doesn’t seem to be holding pressure as you pump it up. Dirt or a bad gasket or surface under the inlet valve often leads to leaking here (you will see bubbles around the inlet valve).

Most inlet valves, needle and seats or gross jets require a liquid to provide a seal. I use Marvel Mystery Oil because it’s what Buller recommends for checking gross jets, but lighter oils like WD40 or your actual premix fuel work well too.
Most inlet valves, needle and seats or gross jets require a liquid to provide a seal. I use Marvel Mystery Oil because it’s what Buller recommends for checking gross jets, but lighter oils like WD40 or your actual premix fuel work well too.

When you get to the set pop-off pressure the inlet valve will open releasing pressure (usually spraying oil all over). If you are using a needle and seat, it should be an audible “pop”, initial pressure release and then the gauge stabilizing at a “holding” pressure. If you are using a gross jet, the pressure will simply release at the pop-off number and then stabilize at a lower number. As Buller’s info sheets describe it, the gross jet acts more like a fuel regulator.

To change pop-off pressure you need to adjust that very tiny coil spring under the fulcrum arm. It’s good to have a large supply of pop-off springs as they have a nasty habit of launching, never to be found. For a given spring wire diameter etc., the length determines the pop-off pressure. I purchase my springs from Comet Kart Sales, but they are available from others such as E.C. Birt. As they come from Comet they are approximately .750”-long and produce a needle and seat pressure of 9.5psi in a Mikuni carb. That is nearly ideal for many gas foreign motors. By compressing the spring (squeezing it several times to coil bind between my fingers) to .570”, the result was 6.5psi in my gross jet methanol carb. It’s strictly a time-consuming trial and error operation. However, after you establish the correct length spring for your application, you can usually hit the pop-off number in just a couple of tries.

Get a good pressure check gauge. If you run 2-strokes, you’ll need it often. I found this one on the internet for about $35.
Get a good pressure check gauge. If you run 2-strokes, you’ll need it often. I found this one on the internet for about $35.
To check the pop-off pressure, slowly pump up the gauge until the inlet valve releases the pressure. This carb popped-off at around 6psi and held at 3.5psi, which was perfect for methanol fuel.
To check the pop-off pressure, slowly pump up the gauge until the inlet valve releases the pressure. This carb popped-off at around 6psi and held at 3.5psi, which was perfect for methanol fuel.

The photos here (iPhone7 camera) show me checking the pop-off pressure on an eBay carburetor I purchased (atomized methanol Mikuni) and also setting the pop-off pressure on an existing Buller atomized Mikuni I have been racing with that was too lean causing engine damage. It’s not a bad idea to check your carburetor’s pop-off pressure whenever you are installing a fresh diaphragm kit.

If the pop-off wasn’t correct, you remove the fulcrum lever and axle by simply loosing the Philips-head screw seen here to the right of the gross jet ball. Just loosen so that the fulcrum axle tilts up and slips out. If you remove the screw every time you need to grow a third hand to put it all back together.
If the pop-off wasn’t correct, you remove the fulcrum lever and axle by simply loosing the Philips-head screw seen here to the right of the gross jet ball. Just loosen so that the fulcrum axle tilts up and slips out. If you remove the screw every time you need to grow a third hand to put it all back together.
The stamped steel fulcrum lever is pretty fragile, so if you need to bend it to adjust fulcrum lever height (described further on), be gentle. The bottom side has a dome that helps retain the spring. Make sure the fulcrum lever rotates freely on the axle.
The stamped steel fulcrum lever is pretty fragile, so if you need to bend it to adjust fulcrum lever height (described further on), be gentle. The bottom side has a dome that helps retain the spring. Make sure the fulcrum lever rotates freely on the axle.
Spring length determines pop-off pressure. The longer the spring, the higher the pop-off. Once you determine your correct pop-off spring length through trial and error, use that as your baseline.
Spring length determines pop-off pressure. The longer the spring, the higher the pop-off. Once you determine your correct pop-off spring length through trial and error, use that as your baseline.
If you noticed leaking around your inlet valve during pop-off testing, or the inlet valve just wouldn’t build to a pop-off pressure, there’s a good chance it is leaking under its seat. As you can see the debris in my second carb here that was allowing the gross jet assemble to leak by it completely. I cleaned it with a small screwdriver and added a fiber washer.
If you noticed leaking around your inlet valve during pop-off testing, or the inlet valve just wouldn’t build to a pop-off pressure, there’s a good chance it is leaking under its seat. As you can see the debris in my second carb here that was allowing the gross jet assemble to leak by it completely. I cleaned it with a small screwdriver and added a fiber washer.
There is a love-hate relationship with both gross jets and conventional needles and seats. I’m an equal opportunity hater – I use gross jets for methanol and needles and seats on my gas carbs. The gross jet on the left is from Buller, the needle and seat in the center is from E.C. Birt and the need and seat on the right is from Speed Parts.
There is a love-hate relationship with both gross jets and conventional needles and seats. I’m an equal opportunity hater – I use gross jets for methanol and needles and seats on my gas carbs. The gross jet on the left is from Buller, the needle and seat in the center is from E.C. Birt and the need and seat on the right is from Speed Parts.
Fulcrum lever height plays a role in determining a carburetor’s inherent richness or leaness. Level with the surrounding “flats” is kind of the neutral position. This lever is actually raised .040” above the carb body for a richer mixture.
Fulcrum lever height plays a role in determining a carburetor’s inherent richness or leaness. Level with the surrounding “flats” is kind of the neutral position. This lever is actually raised .040” above the carb body for a richer mixture.
The best way to check fulcrum lever height is with the depth end of a caliper. My methanol atomizers are .030”-.040” higher, and my gas Mikunis are setup with the levers .060” higher.
The best way to check fulcrum lever height is with the depth end of a caliper. My methanol atomizers are .030”-.040” higher, and my gas Mikunis are setup with the levers .060” higher.
Here are some brutalized fulcrum arms in my collection. The one on the bottom with the sharp kink came off a carb with a gross jet. It is a good example of how to “dog-leg” a lever so that when you raise it, it is still parallel with the top of the carb. Likewise for the top lever.
Here are some brutalized fulcrum arms in my collection. The one on the bottom with the sharp kink came off a carb with a gross jet. It is a good example of how to “dog-leg” a lever so that when you raise it, it is still parallel with the top of the carb. Likewise for the top lever.