LPG propane conversion of petrol generators explained

What is LPG?

Liquefied Petroleum Gas, also known as Propane, is widely used for BBQ’s, caravan cookers, patio heaters, catering wagons etc etc . It comes in red bottles and is notably supplied by Calor although there are many others. It is distinct from the other common bottled gas, Butane which comes in blue bottles. Most gas conversions in the UK use Propane. It is possible to use Butane, but due to its higher boiling point it is more useful in warm climates and for summer use.

How does an LPG generator conversion work?

LPG conversions for portable generators are a little different to the conversions used in motor vehicles. LPG converted cars feed liquefied gas to an evaporator that then feeds to a regulator/gas controller. This evaporator is not necessary for small stationary engines as they can draw the vaporized gas straight from the top of the Propane bottle without the risk of the liquid splashing around and entering the regulator. Gas is fed through a standard 37mbar Propane regulator such as you might have for your caravan or BBQ and then into a gas controller which controls the rate at which gas is fed into the engine. Inside the gas controller is a large diaphragm that responds to the pressure pulse generated by the induction stroke of the engine. This pulse travels up the same pipe that feeds gas to the carburetor, acting upon a large diaphragm in the gas controller which in turn opens a valve allowing the regulated gas supply to escape through the controller to the engine. The mixture of gas to air is determined by the size of a jet that is usually located at, or close to, the carburetor.

Introducing the gas to the carb

Feeding the gas to the carburetor can be accomplished in two ways

Collar method:(dual fuel)

Using an aluminium collar situated between the air filter and the carburetor allows the carburetor to remain unmodified and still capable of running on petrol if desired. The collar has a pipe connection in the side that usually contains the gas jet. The collar is designed with a slight constriction to maximize transmission of the pulse created by the induction stroke of the engine without acting like a choke and effecting the mixture when the machine is running on petrol. It is of such a thickness that it usually requires longer carburetor mounting studs or the use of specially adapted nuts to secure the whole arrangement. On some machines, particularly those in the EM range and the EU26i and EU30is, the inclusion of the collar results in the air filter either hitting against the frame of the machine or making the air filter very difficult to access.

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Spud method. (LPG only)

The spud method requires the replacement of the main jet and emulsion tube with a combined pipe connection and jet called a spud. The main jet and emulsion tube are removed altogether and the spud screwed in their place. This renders the carburetor unsuitable for petrol operation but it is much simpler to fit than the collar method and requires no modifications of the frame and studs and no awkwardness when accessing the air filter. Spud is probably an acronym for something but who knows what? so if you want to make one up, I’ll go with your version as mine is just daft. Screw Pipe Under Dere.

Dual fuel or LPG only conversions?

On the face of it, being able to operate on either gas or petrol appears to be a desirable thing. However, an examination the logic underlying this assumption reveals it to be flawed for most applications. Let us assume that you are operating primarily on LPG and one day you run out of gas. You have petrol in the fuel tank so you just switch over to petrol operation until you get more gas. What’s wrong with that? Well for a start, how old is the petrol in the tank? If it’s more than a few months old then the chances are that it has stopped being petrol and become something akin to French polish.

Petrol degrades in storage as the more volatile elements evaporate away leaving a soup of less volatile components mixed with condensed additives. This will have a lower calorific value than fresh petrol and the engine will not be able to produce full power. This degradation is accelerated by the fact that it is stored in a vented petrol tank above a hot engine. If the engine cannot produce full power then it will slow down under load. If the engine slows down under load then so does the fan that cools the alternator. See where this is heading?

OK you think! How about if I use fresh petrol from a gerry can.? Well, Yep, that would be just fine, but why would you be carrying around a can of fresh petrol rather than a spare bottle of gas? In most instances you will have a spare bottle of gas as a matter of course. If you had only one gas bottle then if at the end of one days use you had less gas left in the bottle than required to get through the following day you would be forced to have the bottle refilled before it was empty, thus wasting gas.

Dual fuel just does not add up. In most cases the compromises in performance and the extra hassle in terms of fitting are not justified. The same logic that underlies dual fuel gas conversions would have you carrying a horse around in the back of your car, just in case you run out of petrol. It’s what I do but it hasn’t proved popular with passengers

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Manufacturers of LPG gas conversions

Those of you enamored of the pastime of stick shaking will find ample targets in the variety of gas controller manufacturers available. However, unless you relish a life of torment and unreliability, the only manufacturers worth considering are Garretson or their close imitator Gemini/Century. They make two basic types of controller that are of interest to us. The standard Garretson controller requires a feed from a regulated supply such as a 37 mb regulator . The Beam controller has an integral regulator and is more compact than the standard controller. Generally speaking I prefer the standard controller as its larger diaphragm makes it more responsive to the induction pulse created by small engines. However, the standard controller does usually require priming manually by the press of a button prior to starting. This makes them less suitable for machines to be operated with remote starting. The Beam controllers require no priming and allow a controlled leak through of gas for starting. They are usually fitted in association with a solenoid operated shut off valve and a safety switch that detects if the engine has stopped. This can require modification of the generators battery charging system to cope with the extra current draw of the solenoid.

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Advantages of LPG conversions


Running on LPG is cheaper than using petrol. How much cheaper depends on what size bottle you buy. The biggest 47kg bottles offer the greatest economy and usually cost from £50 to £60 to refill. Running from a 47kg bottle will roughly halve your fuel costs over petrol. The economies diminish with bottle size and once you get to the 13kg bottle they pretty much disappear. Economy is not the only reason to run on LPG


If a machine is likely to remain idle for long periods, as back up generators do, then the chances are that any petrol contained within them will degrade and block the carburetor making it unusable when required. LPG does not have this problem. An LPG machine can remain idle for years and still start perfectly when called upon. LPG has no decay products to block a carb or hinder its operation. The bottle containing the LPG is sealed so there is no danger of degradation by evaporation


LPG is seen as being less of a fire hazard than petrol or diesel as there is less danger of a spill. This is particularly true in boating applications and marinas where a spill of diesel or petrol can spread rapidly across water and endanger many vessels. Propane gas is heavier than air so it is important to ensure adequate ventilation in the floor where lpg is used or stored.


Many event organizers will no longer allow the operation of petrol and diesel generators at their events and insist upon LPG due to its decreased fire hazard and less noxious fumes. This is a trend that is on the increase and is likely to become standard practice in time.

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Operation of LPG converted generators

Some types of gas conversion require starting on petrol and then a switch over to LPG. The best way to operate conversions that require starting on petrol is not at all. Such conversions are a failure from start to finish and should be abandoned at the earliest opportunity. A proper gas conversion will start easily from cold.

Gas conversions require no choke. For machines with auto choke, this must be disabled (via a switch if it’s a dual fuel system). For conversions with a primer button, a quick press of the primer introduces a shot of gas to the carburetor and the machine will start as easily as if it were operating on petrol. To stop the machine the supply is shut off, either by turning off the gas bottle or in the case of Beam controllers by the associated shut off solenoid valve. The usual ignition switch or stop switch method of stopping the machine is to be avoided where possible as it leaves unburned gas in the engine which will smell in storage and transit and may also cause a backfire when the machine is restarted

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Fuel consumption of LPG converted generators

Every machine will have a slightly different characteristic but, as a rule of thumb, an LPG converted machine will use 0.4kg of LPG for every kw/hour of electricity generated.

DIY gas conversions

DIY kits are a real hit with suppliers. They are expensive kit with a healthy mark up. Fitting by the customer absolves the supplier of responsibility for the end result. Unless fitted to a brand new machine then a DIY conversion is a gamble that’s probably not worth taking. Every machine, even of the same model develops its own characteristics over time reflecting how it has been used. Unless a machine has been returned to 100% original operating order then there is no reason, other than hope, to suppose that fitting an off the shelf conversion will do more than add to problems that were previously undetected.. Getting it wrong can waste fuel, risk damage to the valves in the cylinder head and overheat the alternator due to reduced engine power. To set up a gas conversion properly requires as a minimum, access to a load bank with a readout of frequency. In addition a big healthy dollop of experience goes a long way to achieving the proper result.

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LPG conversion and the law

Anyone can legally fit an LPG conversion to a portable generator but when the generator is then used in close association with a home, dwelling or place of business it may come under the remit of the GAS Safe regulations and will require inspection and approval of the installation by a GAS Safe engineer. The GAS Safe regulations will be concerned with various aspects of safety including the way bottles are stored and the security of supply pipes leading to the generator. Full details of the regulations can be found here:http://www.legislation.gov.uk/uksi/1998/2451/contents/made

The effect of LPG conversion on noise

There is a popular rumour that LPG converted machines are quieter than petrol machines. It’s not something I have noticed.

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LPG conversion of petrol generators and the effect on the Honda 5 year warranty

Fitting a gas conversion to a new Honda generator voids the Honda worldwide warranty. Seems harsh but look at it from the point of view of Honda. They regard it as modification of their machine with another manufacturers equipment. Most dealers still offer the same warranty as an unconverted machine but the warranty is with the dealer not with Honda. Take an LPG converted machine to another dealer and the worldwide warranty would not apply. If a generator developed a problem in France, when it was purchased in Bristol, there may be a problem getting it fixed there and then.

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Effect of LPG conversion on longevity of the machine

When a petrol engine ejects burned gases from the cylinder, it opens the exhaust valve and the gases are forced out of the exhaust port by the rising piston. With the exhaust valve open, a small part of the valve stem is exposed to the hot gases and any oil on it tends to be evaporated away. Petrol contains additives that help to improve the lubrication of this exposed portion, LPG does not. For this reason, LPG machines tend to wear the exhaust valve stem and it’s guide more quickly than petrol machines. In Honda engines the lubrication delivered to the valve gear is very generous and the wear negligible. In other makes such as Robin and Chinese machines, the effect can be more pronounced. It may be worth adding an oil improver to aid the lubrication of the valves. In any case the cost of replacing an exhaust valve and valve guide is insignificant compared to the economies of running on LPG.

Additive systems are available designed to protect the valves from this extra wear and you will find something about this on my Blog but in my opinion the extra wear is so minimal that in most cases it can be safely discounted.

Spark plugs and ignition coils

The air/ LPG mix in gas converted machines has a higher electrical resistance than the air/petrol mix in petrol generators. This results in higher voltages being generated in the ignition circuit and possible overheating of the spark plug or ignition coil. To counteract this effect the spark plug gap is reduced as much as possible (to about 0.4mm). Iridium type spark plugs that have conical electrodes are designed to further reduce the ignition circuit voltages. As Iridium spark plugs are very expensive it is probably more economical to reduce the gap on an ordinary spark plug and accept that they will need replacing more often than the plug on a petrol machine. In any case, whenever an LPG machine starts to misbehave the first thing to do is to change the spark plug and check that the gap is reduced to about 0.4mm.

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Machines that are especially tricky for LPG conversions

The Honda EU30is can be tricky to convert to LPG. I have seen plenty of EU30i LPG conversions where they do not work properly on ECO throttle even from new. Definitely not a good DIY conversion. Similarly the Honda GX620 V twin engine has a reputation for being a temperamental machine to gas convert.

In fact all models in the EU range are trickier to convert than a conventional generator. The complicating factor is the ECO throttle feature which varies the engine speed to suit the load. This requires the gas controller and associated jetting to function over a wider range of conditions than in a fixed speed generator. For EU models I have found that most off the shelf dual fuel conversions fail to produce 100% perfect running in all scenarios. Often there is a forced compromise between running too lean at full power and running too rich at idle.

I have found that the best way to achieve perfect running in all scenarios is to keep the pipe between gas controller and carburetor as short and unobstructed as possible and to introduce the gas straight into the carb using a spud or direct connection to the carburettor down tube. Dual fuel gas collars are not made or designed by Honda and as such they can never attain the same smooth and effective venturi effects as a proper Honda carburettor. Dual fuel collars are good enough for simple single speed conversions and “good enough” for the EU range of machines but they are rarely perfect.

The Honda EU65is and EM65is. The EM65is is sort of doable for the gifted amateur but the EU65is is a minefield and if you want to use either machine in conjunction with a remote start then you are really getting into deep water. I have seen some horrendously complicated attempts at converting these, even by major players. If you want it done properly then I am possibly, the only game in town.

The Honda EX650 has long been regarded as a no go for gas conversion since Calor discontinued the specialist gas collar required. However I have developed a bespoke conversion for this machine that is convenient to use and works beautifully. The cost of this conversion is £229 fitted.

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Gas converting Chinese generators

Many Chinese brands use copies of the Honda engines in their generators. Other brands will use genuine Honda engines in their generators but cut costs by using Chinese parts for the peripheral components like engine mounts, exhausts ,recoils, filler caps and most importantly ignition coils. If you are attempting to convert one of these machines then you are probably going to experience problems at some point as the ignition coils are not as good as the genuine Honda ones. LPG converted machines are very fussy about the integrity of the ignition system. Your best bet is to buy a fully fledged Honda machine but failing that you might consider changing the ignition coil and plug cap for a genuine Honda one.

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by Peter Noble