Xplorer Ultraflight
Performance Paramotors
for powered paragliding

Articles of Interest

Adjustable carb pop-off pressure by Keith Pickersgill - March 2010

Most paramotors use a Diaphragm-type carb, and not a float-type carb.

These can operate for extended periods at any attitude (even upside-down) and is not affected by G-forces, centrifugal force, etc, so they are perfectly suited to PPG flight.

They operate somewhat like a gas pressure regulator, with a diaphragm to sense atmospheric pressure, a spring to regulate the overall fuel-flow and a Low and High speed jet, or adjustable fuel-air mixture screws.

The High Jet (or adjuster) is much like the jet in a gas burner, controlling the amount of fuel introduced into the air-stream (same as on standard float carbs), when at wide-open throttle. It is important that the paramotor delivers maximum power for takeoff and when climbing hard through turbulence.

On the Low end of the RPM range, is usually always a Low-adjuster, controlling the fuel-air ratio when the engine is idling. It is important that the engine does not die at idle, hence this aduster so that the pilot can ensure the engine idles OK, and can pick-up from idle when he whacks the power. All carbs also have an idle-speed adjuster, which is a mechanical adjustment of how far the throttle closes when you release the accelerator.

The mid-range, however is another matter entirely... and is the subject of this article.

    We have two main carb models on modern paramotors:
  1. The Walbro WB series (WB32, WB37, etc) which has a Low and Hi Adjuster.
  2. The Walbro WG8 which has a Low Adjuster, but a fixed Hi-Jet inside the carb.
However, the overall performance of the carb is set by adjusting the pop-off pressure. This is set by changing the spring on the needle-valve that the diaphragm presses on (via a lever).

So the fuel-pump tries to pump fuel into the carb, however the main needle-valve opens and closes to control the pressure of the fuel in a small reservoir, from where the fuel flows to the various jets (fixed and/or adjustable).

The pop-off pressure is so named because we measure it by fitting a small thumb-operated air-pump to the carb fuel-inlet nipple, which has a pressure-gauge integrated into it.

We first pressurise the supply side to approx half a bar (approx 7psi), then stop to ensure the needle-valve does not leak and is capable of holding that pressure.  Then we gradually increase the pressure, until the spring holding the needle-valve is over-powered, the valve opens, and the pressure drops suddenly with a loud "pop" sound. The pressure just before the valve popped, is the pop-off pressure.

Increasing the pop-off pressure, by stretching the spring, or fitting a stronger spring, makes the entire carb run leaner...
whereas reducing the pop-off pressure, by cutting a turn off the spring or fitting a weaker spring, makes the carb run richer throughout its entire range.

Once the pop-off pressure is set, e.g. 15psi on my engine, then we correct the two ends of the range with the mixture-adjuster screws, or changing jets.

So if the carb is leaned out by increasing the pop-off pressure, then the bottom end and the top end must be richened to compensate.

So essentially, the mid-range is set by adjusting the pop-off pressure, then the idle and top end is fine-tuned with different sized fixed-jets or mixture-adjuster screws.

Here are the the main problems with this system:

Firstly, even the manufacturers of these engines are not always sure what the correct, or ideal pop-off pressure should be. We set according to their recommendations purely because we have no better indication of what setting is better. 

If the mid-range is slightly too rich or too lean (or in many cases, much too rich or lean), the pilot would never know... All he cares about is that the engine does not die when it is idling, and that at full power, the engine is peaking.

If the engine is under-delivering in the mid-range (either too lean or too rich), the pilot simply adds more power by squeezing the accelerator a bit more... Both cases (too lean or too rich) are in-efficient.

Most pilots spend most of their flying in the mid-range while cruising. Full power for takeoff and hard climbs, idle for gliding and descending, but most level flight cruising is done slap-bang in the mid-range, where the carb is seldom set correctly.

If the mid-range is too rich, the fuel-consumption is higher than it should be, and the engine gradually fouls up with partially-burnt fuel clogging the spark-plug, the cylinder head, the exhaust port, the exhaust bypass valve, the decompression valve and also the tuned-pipe whose internal dimensions are quite critical.

If the mid-range is too lean, then the engine suffers very gradual, not noticable, cumulative damage that builds up insidiously until the day the pilot pushes the engine hard at full power, resulting in catastrophic failure of some critical part (rings, piston, bearings, etc)

I would guess that 95% of pilots are running far too rich in their mid-range, as evidenced by their very high fuel consumption on long cruising flights. I fly a smaller propeller diameter than others on the same engine, which should increase my fuel-burn very noticeably, yet my fuel-consumption appears lower than anyone else I fly with, even those with the same engine and wing and all-up weight. I can only explain this by my taking great pains to tweak my pop-off pressure to the ideal setting.

Now here is the single biggest problem with this entire system:

Adjusting the pop-off pressure is a painful, hit-and-miss affair that few pilots ever get right... or they don't appreciate how important this is.

First, you need a pop-off gauge, which is a delicate piece of equipment that get broken very easily. Then, you need to open the carb's metering side, which is not easy while its on the engine, so the carb should be removed from the engine.

Then, to increase the pop-off pressure, you need a set of different springs.
These are expensive and rather difficult to obtain.

Then, you need to fiddle around with the different springs, re-assemble the carb each time, measure, strip down, fiddle again, assemble. measure, and repeat until you get it right.

Most pilots do not have the set of different springs, so they start messing up their only spring, the one that was inside the carb when they bought the motor. Stretching the spring is considered an emergency field-repair and not a long-term solution. Cutting coils off the spring is permanent... and if you later try to re-stretch a shortened spring, it simply never works.

Even when the pilot gets the pop-off pressure readout on his pop-off gauge "according to the manual", its probably not correct, as the manufacturer of the engine has probably not taken the effort to measure and check the fuel/air ratio across the entire RPM band... as long as it seems to work, everyone seems to be happy... not an ideal situation.

Many pilots fit various sensors and instruments to their paramotor in an attempt to avoid costly repairs...

    All of these instuments have their problems:
  • Tachometer (RPM) - could indicate almost any problem without any indication of where to start looking.
  • Cylinder Head Temperature (CHT) - the thermal inertia is far too high... by the time you see the temperature go high, its already too late!
  • Exhaust Gas Temperature (EGT) - too easy to misinterpret... some seizures occur with a low EGT...

Strangely enough, the most promising of all instruments, the Exhaust Gas Oxygen sensor (EGO), has not yet found its way into mainstream paramotoring, however that is about to change...

All the other instruments measure just symptons of the real problem -- and not the problem itself, i.e. the fuel/air ratio.

An Exhaust Gas Oxygen (EGO) readout will indicate the actual ratio of fuel/air being burnt, right across the entire range, without any lag and with no possibility of misinterpretation.

So, let's say a pilot gets his EGO set up... and then discovers his mid-range is running rich... he knows he should increase the pop-off pressure, but by how much?  Trial and error, stripping the carb, fiddling with springs, re-assemble, go for another test flight watching the EGO indicator?

There must be a better way!

Here is where my new idea comes into play...

A screw-adjuster to tweak the pop-off pressure.

Without opening the carb, without removing the carb from the engine. As easy as adjusting one of the mixture-adjusting screws!

I built one some years ago and have been hunting for it, but will probably just build another one now to test with an EGO. We used my original one when developing paramotors, to ensure we found the optimum pop-off pressure, before EGO's were readily available.

Imagine being able to use the same screw-driver used to turn the mixture-adjusting screws, to adjust the pop-off pressure, without removing the carb from the engine, without opening the carb.

Though the main idea is for ground-adjustment, it could easily be extended to an in-flight adjuster using a twist-cable if desired, just as some pilots have added an in-flight high-mixture-adjuster.

Many LPG gas regulators already have this adjustment built in... if you see a plastic cap covering the protrusion above the diaphragm, simply pop it off (or screw it out by hand), where beneath it you find a slotted disk in a threaded housing, pressing on a spring on the outside of the diaphragm. By screwing the disk in, you apply more pressure to the outside of the diaphragm, increasing the gas pressure and hence its flow rate.

Now, how do we modify our carbs to achieve the same adjustability?

Actually its surprisingly easy... The current carb design allows for a simple modification.

Over the diaphragm, exists a hole in the carb-cover, to expose the diaphragm to ambient pressure. This is in the center of the diaphragm, directly over a small metal place in the diaphragm center.

If we attach to this hole, a tube on the outside of the cover, threaded internally, whose ID is suitable to hold a typical carb pop-off spring, then we can thread a grub-screw into the tube, to hold the spring in tension. Turn the grub-screw in deeper to add more pressure to the spring, to reduce the pop-off pressure, increasing the fuel flow-rate. So all we need to do now, is replace the original pop-off spring, with a stronger one, then set the new external spring until the pop-off pressure is identical to the original setting. (In fact, we move the original internal spring, to the new external mod).

Now we can test-run the motor in the mid-range, and adjust the pop-off pressure as required, for the smoothest possible mid-range. With a Tacho-meter, we could also find the highest RPM for a given throttle-setting in the mid-range. Then we can tweak the top-end and bottom end as before to compensate for the changed pop-off pressure.

However, with a EGO indicator, we can get the mid-range absolutely perfect!

Then we can also get the top-end perfect, as well as the bottom end, using the EGO.

So an EGO meter and a replacement carb-cover with the mod built onto it, makes a really great combo!

If pilots do not wish to purchase or make a modified carb cover, then they could borrow one to find the perfect mid-range setting, measure what pop-off pressure this delivers, then adjust their pop-off pressure in the old way until they get to this ideal setting. 

Voila, a perfectly running engine, with minimal fouling, and maximum fuel economy!

I believe an EGO meter will become a very popular instrument, to keep an eye on the engine, measuring the actual fuel/air ratio and not some sympton of problems relating to this.

Some links:

Building a DIY pop-off tester:

A commercial pop-off tester (this is the one I use)

From Footflyer (Jeff Goin, author of the PPG Bible)

alex Varv's very informative pages:
www.aerocorsair.com/id27.htm - (how these carbs work)
www.aerocorsair.com/id28.htm - (pop-off adjustment)

Any comment or suggestions would be most welcome.

Write to keith@xplorer.co.za