Petrol for the Classic MG – But Which?
Few MG owners will be unfamiliar with the Leaded/Unleaded debate and nothing has changed since our last feature on the subject ‘LRP – RIP’ published in Enjoying MG in November 2002 as that was just prior to the start of the demise of LRP (Lead Replacement Petrol) since it was only a ‘fill in’ fuel for four years after the 1999 demise of true Leaded from general sale. The fact is that LRP has only really vanished from forecourts (in the UK) during the latter part of 2005 so many newer members may have missed this.
This time it is worth concentrating on other fuel issues that affect a considerable number of MGs, especially those with the B series engines and to a lesser degree the A series and others. Most relate to poor running of the engine, sometimes cutting out and refusing to restart until a cooling period has passed, or running on, or fuel leaks, and as these are such regular questions to the Club’s Technical Dept I thought I’d cover them here.
Over time the evaporation of modern fuels can gum up SU carburettors, stripping down the units and applying carburettor cleaner is the best cure
Part dismantled SU in the process of being cleaned
The root problem stems from the simple fact that as car engine design has changed the petrol formulations have also changed and today the characteristics of the modern pump fuel on the forecourt are simply nowhere near as well matched to classic power plants as that available decades ago. This is a problem that will continue and so encompass more and more cars not previously affected.
Carburettor Fuel Leaks…
Looking at the various common problems begins when you first switch on the ignition (electric fuel pump) or start the engine (mechanical fuel pump). Sometimes the problem is manifested by a very lumpy engine, which will often splutter to a halt often with a very strong petrol smell. The cause is fuel leaking from a carburettor, and in the case of the SU, usually from the float chamber overflow.
This sort of problem is nothing new, but with modern fuels it appears more widespread. A sharp tap on the float chamber or side of carb body with a wooden or plastic screwdriver handle or similar is the time honoured first air treatment and it may still work. However, one of the newer difficulties is the type of residues that are left when modern fuel is allowed to evaporate in the float chambers between periods of use. This is especially evident over winter periods when the MG is in hibernation. These ‘gummy’ deposits are so tacky they could be sold as an adhesive so they do no favours to the operation of the float valve and can also block the main jet flow path too.
Dried fuel deposits are slow to be dissolved by fresh fuel, but melt on sight when sprayed with ‘Carb’ or ‘Choke’ cleaners that can be bought for under £5 from virtually any motor shop and many other high street stores. Removing the float chamber lid on an HS series SU carb is simple, but not so with an HIF series that requires the removal of the carburettor and inversion to access the float chamber. However, if this residue problem is present it’s worth the effort as this is by far the most effective way to removing it. When spraying carburettor cleaner to remove any residues you should take off the dashpot, piston, etc to allow spraying down the main jet back to the float chamber and around the visible interior. The dirt dissolved will be quite obvious and perhaps surprising.
When working inside the carb float chamber it’s always worth the time to examine the float valve and seat not just for dirt and debris, but wear too. The clearest indicator for wear is on the brass conical seat of the needle valve in the form of a visible ring mark. If wear is detected then renew the valve and seat on all carbs, be it one, two or three. Logic also says to look at the float and make sure it is not punctured and that the float height when refitted is correct. Most carbs now have a plastic float with preset float height, having replaced many of the earlier floats, which had a metal connection to its pivot that could be bent for adjustment. Carburettor service parts are stocked by MGOC Spares.
Float chamber needle is particularly vulnerable to sticking
Effectiveness of carburettor cleaner is apparent on this SU piston and needle assembly, but do take great care not to bend the needle during the cleaning process
Sudden Cutting Out and Poor Running…
This often occurs when enjoying a drive on a warm day and you’re forced to stop or remain in a stop start situation for a period of time, and the problem grows in intensity. Indeed severe cases can lead to the engine cutting out and not restarting until a cool down period has elapsed. This usually points to a problem where engine bay heat has got into the fuel system and is vaporising the fuel and causing vapour locks restricting flow, or the expanding fuel is being forced into the engine and choking it.
The older MG fuel systems are simple affairs with fuel from the tank being pumped on demand to the carb(s) and then being fed to the engine in a measured way. When the cars were built fuel was less volatile and whilst problems were experienced in some hot climate areas, cold climate areas were unaffected. Now though it has almost become an expected aspect to see older cars affected by fuel problems on hot summer days or when stuck in traffic for more than around ten minutes.
The problem is that too much heat is able to transfer into the fuel system but whilst the car is being driven at anything more than around 30mph the airflow through the engine bay keeps those temperatures down and also the fuel flow is greater as the engine uses more. This combination means that fuel temperatures never get high enough to cause a problem. However, as soon as you slow down the airflow reduces, so under bonnet temperatures rise, often very quickly if slowed suddenly by something like a motorway queue, where the generated heat from higher speeds has yet to be dissipated. At the same time the engine needs less fuel so fuel flow slows. This results in more heat with more time to percolate into fuel that isn’t moving very quickly. The bottom line is that the critical temperature before the fuel vaporises is soon reached and the problems become obvious in the way the engine runs – or doesn’t run!
In standard form on most applications there is a heat shield between the exhaust and carbs, and usually there are specific pieces of heat resistant materials on the exhaust side of the heat shield. These often degrade and fall away. If this occurs then much more heat energy will penetrate the fuel system and create earlier problems. The solution here is obviously to replace the heat shield.
Exhaust manifolds have a bearing too, as the cast iron manifold is slower to radiate heat compared with the performance orientated tubular steel types. There is some advantage to consider wrapping the tubular manifolds with the specific exhaust wrap materials to reduce the radiated heat, although wrapping the cast iron manifold is not recommended as it may cause cracking. In the same way that exhaust wrap helps keep heat in the exhaust manifold, similar materials are available to wrap fuel lines (and wiring) to keep heat out. This will be helpful to a small degree, but the carb bodies are the biggest surface area for heat to be absorbed into the fuel system and reduction of this transfer here is the target. All of these parts are stocked by MGOC Spares.
Assuming the standard heat shield is in full working condition then the next biggest source of heat is that which is radiating from the exhaust and especially that from the lower part of the manifold and which can rise unobstructed straight into the carb area. Creation of a simple additional heat shield from polished alloy or stainless steel that connects to the bottom edge of the standard shield and goes under the carbs will reflect and deflect at least some of this heat getting into the carbs.
Engine bay air will take on heat and as this moves around the engine bay it will come into contact with the carbs and transfer more heat. It is possible to look at some reflective material, rather like cooking foil, to attach to some parts of the carbs to reflect heat, but always use purpose design products as ordinary insulation absorbent material would be unwise in case of fuel being soaked up and acting like the wick of a lamp. Another solution is worth considering.
Some export MGCs, 1.6 litre S series engined MG Maestro, all MG Maestro Turbo and MG Montego Turbo petrol models had a reasonably effective carburettor cooling fan, which was a thermostatically controlled fan, similar to a heater fan and which blew cooler ambient air at the carbs to keep problems at bay. Not a 100% cure but quite significantly effective. This fan was operated only after the engine was switched off, but with the current problems that can affect cars whilst still on the move in traffic, a manual override switch could offer some relief from the onset of poor running in traffic. It may be possible to fabricate a similar system using ducting and a switchable 12-volt computer fan.
Special exhaust wrap and heat insulation materials are also available from MGOC Spares
Running on…
This is another growing problem and is usually the result of series of small conditions that cumulatively have a larger impact. Generally the tried and trusted methods of reducing this condition are still perfectly applicable, correct ignition timing, mixture setting, lowest comfortable idle speed and if needed an anti run on valve, but modern fuels increase the tendency for running on, a point that needs to be taken on board as so many classics are affected you are not on your own if your classic suffers from these problems.
One often effective method of running on reduction is to use a higher-octane fuel or raise the octane by using a fuel additive. The Castrol Valvemaster Plus additive provides an effective lead replacement function and additionally has the ability to increase the octane rating by up to two points. There is a law of diminishing returns with the increase in octane when adding to a higher octane fuel, but it is reasonable to expect a two point increase when looking at 95 octane fuel, something less if adding it to a 98 octane fuel, but still achieving a noticeable gain. Remember that the MGB was initially recommended to run on 5 Star fuels with a 100 or 101 octane rating in decades past!
Improve octane levels with Valvemaster Plus
Wider Fuel Choices…
In the UK and certainly some other markets too there is a growing variation on what fuels are available. Over the last few decades we have experienced the rapid loss of lead from petrol and with it there has been an associated drop in the fuel’s octane rating. Not unexpected as lead in fuel had both lubrication and anti-knock functions. The lead was used to effectively enable higher octane ratings. Recalling the early days with my MGB I used to periodically fill the tank of my MGB with 5 Star (usually on pay day) and enjoy a boosted engine performance. In those days I was not observing fuel consumption with today’s interest. The rest of the month I had to ‘slum it’ on ‘ordinary 99 octane 4 Star and felt a less responsive engine.
By the time that leaded petrol vanished at the end of 1999 it had dropped to 97 octane, but as petrol is a mix of compounds that mix was different to that of 25 years before. That left two basic choices (in the UK) mainstream 95 octane unleaded and a higher octane, higher price, unleaded at 97, titled Super Unleaded. B series engines would often complain on 95, but run acceptably on 97. In the background small quantity supply of genuine leaded has remained, but at a significant price premium from a very limited number of outlets, usually privately owned petrol forecourts. On the plus side the fuel was rated at 99.7 octane and of the few cars I have heard running with this there were no problems.
Limited supply and around a 25% price premium doesn’t help the mainstream classic MG owner, so with the arrival of 98 octane (minimum rating) fuels from Shell (Optimax) and BP (Ultimate) on a nationally available basis and not too much more expensive than base unleaded, many took notice. Of the two, Shell Optimax has gathered more B series engined car owners support than the other, with general comment on their engines running noticeable smoother than on other fuels. This has been the case for several years now so is still currently valid for UK owners.
Recently we seem to have entered into a flurry of fuel related product releases, with Tesco releasing 99 octane fuels from some of its sites, and more recently followed by BP creating Ultimate 102, which as the title suggests is a 102 octane fuel. This release has been surrounded with quotes of independently verified gains in power on a number of different engines over normal pump fuels. The presentation looks very attractive until the price of £2.42 per litre is seen, (compared to an average price of around 95p per litre for 95 octane – at the time of writing) which corresponds to £11 per gallon. Adding Castrol Valvemaster Plus or other octane booster to sensibly priced higher octane fuel would go a little way to bridge the octane gap to the 102 octane and be an awful lot cheaper.
In addition to the clear memories of the old 101 5 Star I have also in the past been able to access 105 octane fuel in very small quantities. Using at a mix of 50/50 with then standard leaded pump fuel (97 octane) the result gained the title of ‘Rocket Fuel’ such was the effect it had on engine performance in a standard MG Metro. This was seat of the pants feel, not anyway scientifically measured before and after gains. These experiences do give the indication that the higher octane fuels will work well with the B series engine, but the advantages will probably be in a smoother and sharper engine feel and response rather than any significant power gains.
However, as some engines need to have their ignition retarded significantly to work even reasonably, then the use of a higher octane fuel allows the engine to be set to its optimum rather than the previous compromise, so here there is some scope for bigger gains. It also gives a greater window of opportunity for tuned engines to be taken further in respect of compression and ignition timing, but remember that there are other factors that limit how far each of these can go, so it is not an open ended book, just a couple of extra blank pages. This assumes that the extra cost can be justified, but a competition motor having to use normal road pump fuel stands to gain quite a bit.
The fuel issue is far from stable and the future will see further evolution of engines and fuels, hand in hand with each other to comply with legislative and other requirements. This will leave the older classic engines further behind and needing more innovative adaptations to continue running well. We will always be keen to hear from members who have suffered and then overcome these problems with specific modifications that they feel they can share with others.
Roger Parker