Small 2-stroke engines and popoff pressure

This article explores the physics of fuel metering in a typical small diaphragm carburettor (carburetor to some) as used on small 2-stroke chainsaws, leaf blowers, brushcutters etc.

The discussion following is in terms of absolute pressure, and it is assumed that atmospheric pressure is 100kPa. ALL pressures are absolute unless stated otherwise, to find gauge pressure, subtract 100kPa.

Above from Zama is a cross section view of the metering chamber of a typical butterfly carburettor. The metering lever pivots on an axle (1), the distance from the axle to the needle (2) is 3.5mm, to the spring (3) is 3.5mm, and to the contact to the metering diaphragm (4) is 8mm. The needle seat is 0.55mm diameter.

The main jet is fed by the needle screw at left, and the low speed progression holes are fed by the needle screw at right.

So, time for application of a bit of high school physics.

If crankcase impulse reach 150kPa (appears a commonly quoted figure), so the fuel pump supplies fuel at about 150kPa.

The metering lever has the following forces acting upon it:

  • fuel pressure under the needle tip;
  • spring force;
  • atmospheric pressure above the metering diaphragm; and
  • fuel pressure  under the metering diaphragm.

Each of these give rise to components on moment in the metering lever.

Let’s work through them, results are rounded for presentation but greater precision is carried forward in calculations.

Moments, forces and pressures


We can calculate the force on the needle due to the fuel supply to be \(F_n=P_iA_s=P_i {\frac{0.00055^2 \pi}{4}}=0.0356 \text{ N}\). This acts about a radius of 3.5mm, so the moment on the lever due to the needle is \(M_n=F_n*r_n=F_n 0.0035=125 \text{e-6 Nm}\).


The spring compresses to the closed needle position at about 60mN force, so it exerts a moment of \(M_n=-F_s*r_s=-0.06 \cdot 0.0035=-210 \text{e-6 Nm}\).

Atmospheric pressure

Let’s take atmospheric pressure P to be 100kPa. Atmospheric pressure acts on the metering diaphragm and produces a force on the metering lever \(F_a=P A_m=\frac{0.023^2 \pi}{4}=41.5 \text{ N}\).

The moment on the metering lever due to atmospheric pressure is \(M_a=F_a \cdot r_a=41.5 \cdot 0.008=0.332381 \text{ Nm}\).

Net moment, force and pressure

The net moment \(M=M_a+M_s+M_n=0.332295 \text{ Nm}\), marginally less that that due to atmospheric pressure alone.

The net force acting on the metering diaphragm then is \(F=\frac{M}{r_a}=41.5 \text{ N}\).

The net pressure acting on the fuel below the diaphragm is \(P=\frac{F}{\frac{0.023^2 \pi}{4}}=99.974 \text{ kPa}\)


So, this system maintains the fuel chamber at a pressure of almost atmospheric pressure, admitting fuel through the needle as needed to maintain that pressure. The fuel chamber below the diaphragm supplies fuel at regulated pressure to the jets / orifices / nozzles in the carburettor body where fuel is drawn into the lower pressure area on the intake side of the venturi (and choke).

We might expect that at WOT (wide open throttle) that the pressure on the intake side of the venturi during induction is less than 70kPa (or -30kPa gauge pressure), so there is a pressure difference of around 30kPa between the fuel chamber and the main nozzle tip. The effects of the metering lever spring and the force exerted by the fuel pump on the needle tip at around 26Pa are near insignificant, they have very little influence… as one would want.

Note that being 2-stroke, there may be instants in a cycle when pressure at the main nozzle tip exceeds atmospheric, hence the nozzle check valve (which can be seen in the diagram above).

Popoff pressure

Popoff pressure is the fuel pressure that unseats the needle.

It needs to be greater than the fuel pump pressure, otherwise the carburettor will flood or at least run excessively rich. For that reason, it is wise to pressurise the fuel input to say 170kPa (70kPa gauge remember) to be sure that the system does not leak down. A wet carburettor should hold that pressure for 1min with very little leak down.

Some practitioners use a Sphygmamometer scaled to 300mmHg (140kPa absolute), not sufficient for popoff testing, but will reveal a leaking needle, gasket or membrane.

Popoff testing means raising the pressure until the needle unseats due to the supply pressure. The pressure may be from 200 to 500kPa, though around 200-350kPa seems ‘normal’. There are two questions that should arise:

  1. is it safe to pressurise the system to that degree, particularly the fuel pump membrane; and
  2. is the value obtained a critical performance parameter.

In respect of 1, Zama advise against popoff testing, suggesting that it may damage the carburettor (presumably the fuel pump membrane).

In respect of 2, popoff pressure is dependent on the spring force amongst other things, and low popoff pressure might indicate a weak spring… but if it withstands a leakdown test at 170kPa, is that not adequate spring tension? If the popoff pressure is extremely high, it might indicate a faulty needle tip or blocked orifice… but if the risk of membrane damage exists, isn’t it preferable to just dismantle the carburettor to inspect?

In any event, fuel pressure and spring pressure normally make only a very small contribution to the regulated fuel pressure, so small differences in either should not make much difference to system operation. It is mostly about ensuring that the needle does close off properly as needed so that fuel pressure is effectively regulated.

That being the case, online expert’s advice to ‘tune’ popoff pressure or spring pressure seems quite misguided for these small diaphragm carburettors.

Joe Pace on pressure

Joe Pace (ex Husqvarna tech school?) has produced a number of training videos that are very professional.

His training video on carburettor pressure testing recommends not more than 7psig and 10psig, the metric equivalent gauge pressures are 48kPa (0.48b) and 69kPa (0.69b) respectively.

My own practice

A pressure test of the wet carburettor fuel input at a little less than 200kPa  (~170kPa or 70kPa gauge) is meaningful in identifying leaking gaskets, needle, membranes.

Above is a cheap Chinese vacuum / pressure pump, pumped up to 0.5b gauge pressure (50kPa gauge, 150kPa absolute).

Response to primer pump operation under pressure provides hints of improper primer components (like poor check valves).

Repeating this test after repair and reassembly is good quality assurance, more than good, and can save having to uninstall and reinstall a leaking carburettor, or an incipient problem that will get worse in time.

I do observe that the metering lever operating force seems reasonable, if I have suspicions it is easy to put the carburettor on a gram scale and measure the force on the lever to unseat the needle… it is usually less than 100mN (10gf) in small carburettors.

I do not perform popoff testing.