Small engines and green fraud

Recent weeks have seen some catch up work on maintenance of small engine yard equipment. There are 22 engines in all, 6 4-stroke and 16 2-stroke.

For some years, ‘green’ measures implemented by government meant that ordinary unleaded petrol (ULP) was not available retail, one had to use E10 (ULP with 10% Ethanol).

Greens claimed that at such low Ethanol, that the fuel was compatible with all existing and new engine equipment.

Diaphragm carburettors

Most of my 2-stroke small engines use so called diaphragm carburettors. These ‘all position’ carburettors are common on yard equipment like brushcutters, leaf blowers, chainsaws etc.

Above is a Chinese after market clone of a Zama ‘butterfly’ type (note the throttle butterfly) diaphragm carburettor that suits a Stihl BG85 and similar leaf blower. (This cost $15 inc shipping on Aliexpress.)

Diaphragm carburettors contain seven components that degrade even in E10 fuel:

  1. metering diaphragm;
  2. fuel pump membrane;
  3. metering needle;
  4. primer / purge bulb;
  5. primer inlet check valve;
  6. primer outlet check valve;
  7. man jet nozzle check valve;
  8. fuel hoses.

Items 5, 6 and 7 are unservicable on many carburettors.

Item 4 is easily serviced. The primer bulb becomes hard and cracks, there are two common sizes, are inexpensive and easy to replace.

Some equipment contains a separate primer pump, often acommon pattern, and easily replaced.

Some equipment does not have a primer pump, so no problem.

Item 7 is commonly a small disc of rubber like material, 4.8mm diameter and 0.3mm thick for instance. It is subject to damage by some solvents. It is located in the main jet nozzle (the brass projection into the venturi in the pic above. It is very hard / expensive to service on some / most carburettors.

Item 8 are generally easy enough to replace, simply with Tygon hose or OEM parts or after market parts to suit. My three Stihl tools all had their tank fuel hose assemblies rot after less than two years. Chinese replacement parts are always a risk (the Chinese soft plastic / rubber like products often seem to have short life)… then the original Stihl parts (black rubber in their day) were a problem and Stihl refused warranty replacement which is testament to Stihl’s commitment to product quality. I have not bought any Stihl product since those hoses rotted.

Air leaks anywhere is 2-stroke systems can be a serious problem, be it crankcase shaft seals and cylinder gasket, carburettor isolator and gaskets, fuel hoses.

The good news is that most common problems are with items 1, 2 and/or 3 (but don’t overlook 8.

Above are items 1, 2 and 3. The most common cause of carburettor faults is one or more of these and / or dirt / debris / corrosion from unclean / stale / water beraring (due to Ethanol) fuel.

Item 1 is a soft rubber like supple membrane that may become stiff and even crack due to Ethanol. It no longer smoothly regulates the supply of constant fuel pressure to the jets and engine behavior becomes erratic.

Item 2 is the fuel pump diaphragm with flapper check valves, commonly rubber, acetate or woven PTFE fabric, though a newer PTFE film with spiral stainless steel spring reinforcement has been developed by Walbro. I believe this one is an acetate membrane. The common acetate membrane often becomes inflexible and even brittle with Ethanol exposure and incapable of supplying sufficient fuel to the engine. This one can be seen distorted in the area subjected to crankcase impulse. The PTFE membranes fare better, but they are uncommon in production carburettors (at this time).

Item 3 is a resilent tip needle, the resilent tip may be degraded by Ethanol and disrupt smooth regulation of fuel pressure to the jets.

The discussion above has been about butterfly carburettors, the other common kind is the barrel or rotary carburettor, and it has much the same components vulnerable to Ethanol.

Algal growth

Water bearing fuel (aka Ethanol bearing) promotes the grown of algae which blocks fuel filters, screens, jets and other small orrifices in carburettors and fuel systems.


Ethanol absorbs water, so Petrol with Ethanol absorbs water. (A simple test for presence of Ethanol in fuel is to put a small quantity of water in a jar of fuel and shake it up, if the water is absorbed by the fuel it probably contains Ethanol.)

So, what happens when the fuel dries up / evaporates in storage? Yes, the water is left behind to react with carburettors, fuel tanks and sintered bronze fuel filters. Rust and corrosion are the results. Some experts advise draining fuel from equipment to be stored, but my experience is that you cannot totally dry the systems (unless you disassemble them) and so you cannot eliminate residual water. If the equipment is full of fuel, it takes longer before water separates out, it reduces vapor pumping, and then dump the stale fuel when the equipment is to be used again.

Fuel filters

In my experience, sintered bronze in-tank filters (as used to be used by Honda) corrode, and Ethanol increases the risk. Modern engines seem to use a plastic gauze in-tank filter.

A lot of portable 2-stroke engines use a combination felt and screen in-tank fuel filter. The felt absorbs separated water (aggravated by Ethanol fuel) and will restrict flow.

Float bowl carburettors

The 4-stroke engines here are a mix of Honda, Chonda, and one Briggs and Stratton. From an Ethanol point of view, they are similar in that they do not have the diaphragms of the ‘all position’ diaphragm carburettors. They still have a weakness in fuel hoses and carburettor needles, fuel filters, and the general corrosion problems of carburettors and metal fuel tanks.

Now one of the small Chonda engines with a Huayi carburettor is on a water blaster which refuses to run for the first 5 minutes without choke… and of course it is so easy to leave the choke on that it runs way to long on choke, literally choking the engine up with carbon deposits and polluting the atmosphere with unburnt fuel. Of greater concern is that with choke off, it is probably too lean and that results in expensive valve damage from excessive temperature. After checking there were no loose parts, faulty gaskets, obstructed exhaust etc that might cause lean running, I bit the bullet and drilled the main jet out from 0.66mm to 0.8mm. It made the world of difference, choke can be reduced to zero when the engine starts (though it is still needed for an immediate cold start), no smell of unburnt fuel, exhaust and spark plug look good.


Well, you might say just put $80 + fitting ($60) on the counter at your local Stihl shop and get a new carburettor fitted from time to time.

The job of repairing these machines was informative… it gives me an informed lack of trust in green arguments and legislated green measures… if the measures made good sense, why would you need to embellish the case with lies?