Series IIA Tune Up
One of my local customers has a rather sweet little Series IIA (88in petrol), which visited the workshop for the first time a few months ago to be fitted with a new set of parabolic springs. The customer rang me up not long ago to ask if I could take a look at the vehicle as it had started cutting out when pulling up at junctions. I asked if he knew what kind of carburettor it had, and he replied Weber, so I thought I already knew what the problem would be. Series IIAS left the factory with one of two types of carburettor: Solex PA40 up to 1967, and Zenith 36IV (the same as a Series III) from 1967 to 1971. Both types of carburettor have their foibles, especially in old age, but they are simple devices and give very little trouble if in good order. However, when they wear out they are very commonly replaced with a Weber 34ICH. This is an aftermarket conversion, marketed originally as providing better fuel economy, which it achieves through being rather smaller in bore than either of the factory fit carburettors. This tends to strangle top-end performance a little, but most owners will happily live with that. What is much harder to overlook is the tendency of some (but not all) 34ICH installations to suffer from icing in cold and damp weather conditions.
Carburettor icing is a not uncommon phenomenon with fixed-jet carburettors, and back in the days before fuel injection became common, most cars had some form of air intake ducting to draw warm air from around the exhaust manifold in cold weather. I learned the importance of this many years ago when I owned a Fiat Uno. This had a short piece of corrugated tin foil ducting between the airbox and the manifold. It fell apart when I was changing the air filter and I did not bother replacing it. A couple of weeks later I was driving along a busy dual carriageway at night when the engine started to lose power. The vehicle gradually got slower and slower until I finally had to pull off onto the verge, at which point the engine stopped and would not restart. I sat there for a couple of minutes with the hazard lights on, then turned the key again and it fired up and ran perfectly. I set off, and a few miles later the same thing happened. The carburettor was icing up due to the missing ducting: when the car stopped, heat from the engine would melt the ice within a short time. The next day I bought a new piece of ducting, fitted it and had no further problems.
Series vehicles have no provision for air intake heating, and in my experience the Solex and Zenith carburettors do not give any problems as a result. However, I have had severe icing problems on some Weber-fitted engines. The worst one I have seen would ice up and cut out if left idling outside the workshop. Strangely, other vehicles with the same engine and carburettor combination have given no trouble whatsoever. I have been unable to establish any reason for this variation in behaviour. In this case, given that my customer’s vehicle had only started giving trouble since the onset of cold weather, and that the problem was intermittent in nature, it seemed likely to me that carburettor icing would be the cause. So I ordered a Chinese-made reproduction Zenith carburettor (I have now fitted several of these and they seem fine) and booked the vehicle in.
I have quite a few Land Rovers come through the workshop each year and do not always remember the peculiarities of each one, so it was only when I lifted the bonnet that I was reminded that this particular Series IIA had lost its original engine in favour of a 2.5-litre lump from a Ninety or One Ten. It did indeed have a Weber carburettor – a factory-fitted 32/34DMTL twin-choke, which in my experience does not suffer from icing problems when installed in a Land Rover. So, my theory about the cause of the problem went out of the window.
On starting the vehicle I noticed that the idle speed was rather higher than it should have been, so I thought that before going any further I would set up the carburettor properly. These old fixed-jet carburettors can usually be tuned pretty well by ear: the first step is to set the idle speed, then tweak the mixture screw to obtain a nice smooth idle, readjusting the idle screw as necessary. I dropped the idle speed, which gave a rather rough and lumpy idle, started to turn the mixture screw and found that it had been wound right in against the stop. I had to unscrew it two full turns in the end, with a correspondingly large adjustment to the idle speed screw to get the speed back down to the acceptable 850 rpm or so.
I have seen this kind of tuning a few times before. A long time ago one of my customers admitted that he had done it on purpose, because he had been told that winding the mixture screw all the way in would give better fuel economy. I doubt that is the case, what it certainly does is give a very inconsistent and erratic idle. I took the little Series IIA for a five-mile drive around the local country lanes, with plenty of junctions along the way: it behaved perfectly with not so much as a hiccup. No doubt I will find a use for the new Zenith carburettor at some point.