How managing and stabilising fuels protect motorcycle engines and carburettors


Most fuels for pre 1990s engines incorporated a lead additive since GM invented it in 1921 to reduce the internal combustion engines’ arch enemy – knock (or ping) – and to lubricate the valve seats and guides. Lead was banned for use in fuels in 2001 due to environmental health reasons. For better or worse, most of the petrol you buy at stations around the country since that ban on lead has been “oxygenated” with any number of differing additives since a series of fuel amendments and eventual ban on lead were made by the Department of the Environment and Energy in the late 1990s – 2000s. The idea is to help the gasoline burn more cleanly and completely and thus deliver better economy and cut down on harmful emissions with the “pinging”, previously controlled by lead, now governed by knock sensors feeding back to the EFI ECU. So what about the poor old carburettor … TBSS?

One of these common (and cheap) additives in petrol formulation is ethanol, which – without getting into the political and environmental debates about its efficacy – is fine for use in fuel-injected vehicles that are run regularly and designed to use up to 10% ethanol (much more for some specifically designed engines) with mandatory knock sensors. These fuels contain little or no valve seat and guide lubricant as the engine manufacturers changed the technology of these engine components at around that time of lead abandonment to suit the fuel composition changes.

On the down side, ethanol enhanced fuel is not so great for any vehicles that sit between uses, and / or carbureted engines, like the one in your dirt bike or classic red plate motorcycle. Background – Ethanol is a form of alcohol formed in the fermentation process of yeasts and sugars, and can be corrosive to certain parts in older fuel systems. Methanol / Alcohol is also “hygroscopic” and likes water, so when water gets into fuel during a fill – up or from condensation (while sitting undisturbed), it can mix with the ethanol, creating a chemical combination that causes rust, corrosion, acids and, if left long enough for the rest to evaporate, sticky varnish that wreaks permanent havoc in fuel systems, especially carburettors. Ethanol fuels can even cause rubber parts and fuel lines to dry out, harden and deteriorate prematurely.

Fortunately (at this point in time) readily available alternatives to Methanol fuels for your street ride are 92, 95 and 98 RON pump fuels – or if you’re more into high performance and you are OK with paying $5 ish per litre for special fuel with RON’s much higher in drums from a dealer like VP racing fuels, Power Plus racing fuels to name a couple, most of which are oxygenated without little or no ethanol then you can comfortably steer well clear of those undesirable and destructive fuels.

Just to confirm, your modern fuel-injected vehicle (daily drive) that you store in a dry place and run at least twice a month is unlikely to suffer any ill effects, but what should someone do with their older carbureted bike (or boat, lawnmower, wiper snipper, generator, etc.)?The best advice is to not let them sit whichever fuel you use.

The best advice is to not let your vehicle sit.

The shelf life of un-stabilised gasoline containing ethanol is about one month. Running your vehicles weekly or at least fortnightly, until fully warm is the best way to prevent fuel delivery problems. When you can’t run them regularly, here’s what to do to minimise, but not necessarily eliminate, problems with your collection.On carbureted bikes with steel gas tanks, the fuel system (not the tank) should be completely drained, the tank itself should be left as close to full as practical. 

Carburettors and their tiny air passages and jets can become plugged with aged fuel and crud that deteriorates into that sticky varnish over time if left in the bowl. 

Since carburettor internals are made of non-ferrous, zinc, aluminium, brass, plastic and rubber that won’t rust (but can still corrode) if it’s practical to do so – drain them.

Shut off the fuel tap manually first (or look for a vacuum-operated-type petcock) this is your best bet for trouble-free operation when the bowls are again refilled. Remember the fuel tap is part of the system that is susceptible to corrosion and crud collection.

O-rings and seals have been known to dry out and leak when carburettors are left dry for a very long time (years) but this is less likely than the possibility of “varnish” left plugging jets or worse if they’re left wet. 

Some carburettors have a drain bolt in the bottom of their float bowls, others have a drain screw. Don’t over tighten either one, and only drain carburettors (into something please, not just onto the bike and your floor) when the bike is off and cold. 

Steel tanks on carbureted or fuel-injected bikes can rust inside, so it’s best to leave them as close to full of fuel as well to which you have added stabiliser (more on this later). Some newer models have plastic lined steel, aluminium or plastic tanks, in which case it’s up to you, but make sure you stabilise it if you leave fuel in the tank. In really humid environments I would still keep an aluminium or plastic tank full.

  1. On carbureted bikes with steel gas tanks, the fuel system (not the tank) should be completely drained, the tank itself should be left as close to full as practical. 
  2. Carburettors and their tiny air passages and jets can become plugged with aged fuel and crud that deteriorates into that sticky varnish over time if left in the bowl. 
  3. Since carburettor internals are made of non-ferrous, zinc, aluminium, brass, plastic and rubber that won’t rust (but can still corrode) if it’s practical to do so – drain them.
  4. Shut off the fuel tap manually first (or look for a vacuum-operated-type petcock) this is your best bet for trouble-free operation when the bowls are again refilled. Remember the fuel tap is part of the system that is susceptible to corrosion and crud collection.
  5. O-rings and seals have been known to dry out and leak when carburettors are left dry for a very long time (years) but this is less likely than the possibility of “varnish” left plugging jets or worse if they’re left wet. 
  6. Some carburettors have a drain bolt in the bottom of their float bowls, others have a drain screw. Don’t over tighten either one, and only drain carburettors (into something please, not just onto the bike and your floor) when the bike is off and cold. 
  7. Steel tanks on carbureted or fuel-injected bikes can rust inside, so it’s best to leave them as close to full of fuel as well to which you have added stabiliser (more on this later). Some newer models have plastic lined steel, aluminium or plastic tanks, in which case it’s up to you, but make sure you stabilise it if you leave fuel in the tank. In really humid environments I would still keep an aluminium or plastic tank full.

Fuel injection systems seem much less susceptible to the ravages of stale fuel as they are far better “sealed” to the environment, and once full of stabilised fuel are almost carefree. In fact, some manufacturers warn against running their EFI bikes entirely out of fuel.

If you can’t drain your carburettors, after adding stabiliser to the fuel in the tank run the bike long enough to ensure stabilised fuel has filled the bowls, then shut off the bike and petcock. Carry a small bottle of stabiliser with you when taking out one of the less frequently ridden bikes, and add it at the gas station before riding home. Err on the side of adding more stabiliser; you can’t overdose (within reason) with these products. Stabilised fuel in the carburettors does not guarantee that they won’t suffer from plugged passages or jets, however, and you should still run bikes kept this way at least every month or so. Running them more often is simple insurance that you won’t need an expensive service – compare the cost of Ethanol free fuel and/or stabiliser additive to that of a carburettor rebuild and the former starts to make a lot of economic sense. A full clean out and carburettor rebuild on a 4 cylinder HonKawSuzYam can be over $1,000! Good for me – not so good for you. Just make sure you run the engine until it’s fully warm (to burn off water and contaminants in the oil and exhaust). While you’re at it, pump the fork and shocks and work the brakes, clutch and shifter to keep those oil seals flexible and lubricated.

As already mentioned above, in the fight against bad gas and fuel delivery issues is a fuel stabiliser additive. They’re not foolproof, but they can provide consistent results with motorcycles. Nulon Fuel Stabiliser and Algae Killer treatment and the Penrite version Petrol Fuel Stabiliser are a couple of names. There are others of course but do your homework to ensure they are good for motorcycles. All make lots of claims about their effectiveness that you have no way of proving or disproving, so just buy some and use it, or spend hours online researching them before you just buy some and use it. All of them offer smaller bottles and/or containers with measuring devices built-in to make carrying and using it while out on the bike easier.

The instructions for each will tell you how much to use, how long the fuel is usable when treated, etc. There are some consistent rules of thumb. Generally you only need to stabilize fuel if you won’t be using the bike for two months or more (but carbureted bikes should still be run every couple of weeks as described above). 

Adding a little new fuel or stabiliser to old fuel won’t renew it, nor will adding more stabilisers to old stabilised petrol extend its usable life. Overdosing is not an issue, and none of these additives will cure an already plugged-up carburettor no matter how much you add to the fuel. Your best bet is to avoid plugging it in the first place!

Simply put, don’t use ethanol fuels, if your bike is older (pre 90’s) then use some additives in storage, start it and warm it up every couple of weeks – please?

Also take into account you only need to use the Ron grade of fuel the bike requires. The higher your engine’s compression ratio (and therefore likelihood of Knocking) the higher the RON numbered fuel to use. 

Good luck, and please email me with any questions, comments or similar experiences at [email protected]