SCARY TYRES

Lightweight number two was a very smart mid 1970s soft-top, ex-british Army of the Rhine, built left-hand drive, converted to right-hand drive, more recently fitted with a new galvanised chassis, and now in for a service and MOT. All pretty routine stuff: it is one of those vehicles that only gets used for high days and holidays, doesn’t do many miles between MOT tests and should in theory not need any attention beyond the odd oil change.

However, it has had a problem with rough idling ever since the owner fitted a twin choke Weber carburettor and manifold of the type found on early petrol Ninetys and One Tens, so I was asked to take a look.

The Weber 32/34 DMTL is not a bad little carburettor in my opinion. It is a progressive twin choke design, which means that the throttle disc for the second choke does not start to open until the first one is fully open. This makes for reasonable fuel economy.

The jets are externally accessible and can be removed and blown through without dismantling the whole carburettor.

This particular Weber carburettor is festooned with pipe stubs, solenoids and other bits and pieces, all of which have a purpose and need to be connected correctly.

There is a solenoid valve mounted close to the fuel inlet pipe and powered via the ignition, and even I have absolutely no idea what that one does. But what I spotted straight away was that the overflow pipe from the float chamber (intended to route petrol away from the hot exhaust manifold if the float sticks) had been connected to the breather pipe which fed into the air intake. This was sucking petrol from the float chamber back into the intake which was not good for the mixture.

With the carburettor re-plumbed and normal idling restored it was time for a quick road test before the MOT. I still had some servicing tasks to finish (including a check of tyre pressures), and wanted to warm the engine through prior to changing the oil.

Off I went on the road test, and quickly found that the vehicle was very wayward indeed, to the point where above 30 mph on a single track country lane, just keeping it on the black bit between the grass verges was a full time job. Something was badly wrong.

Back at the workshop I pondered tyres. I had already noticed that the vehicle was shod all round with Firestone Super All Traction 7.50-16 crossplies, which will prompt fond recollections from older Land Rover enthusiasts. The tyres looked in good order with almost no tread wear, but the lack of any load or speed markings made me suspicious: I can’t remember when such markings became a legal requirement on new tyres, but it was an awfully long time ago.

While I was pondering I decided to check the tyre pressures: none of the tyres looked low on pressure, but it would do no harm to check. (I had, of course, overlooked one of the characteristics of these old crossplies, which is that the sidewalls are so stiff that the tyres will remain inflated and round-looking at very low pressures.)

The results were enlightening. The handbook for a civilian Series III 88in (which weighs about the same as a Lightweight) on 7.50-16 tyres suggests 25 psi front, 30 psi rear. On this particular vehicle the pressures varied from 10 psi (nearside front) to 20 psi (offside rear). With this corrected the vehicle was a whole lot better to drive, but still not entirely stable at speed: it reminded me of the 109 V8 I bought a few years ago which was on nearly-new crossplies and almost undriveable above 50 mph as it wandered around and followed every ridge, furrow and bump in the road surface.

Perhaps it is just that these particular tyres are old and hard: but I have for many years been a big fan of radial tyres on road going Series Land Rovers, and after my trip back to the good old days of SATS (as everyone used to refer to these Vee-treaded Firestones) I think I’ll stick to radials, thanks.