PUMP PROBLEMS
A FEW years ago I was approached by a distributor and offered the chance to buy reproduction Zenith 36 IV carburettors (as fitted to many Series Land Rovers) at an unfeasible low price. I tried one and it seemed fine, so I ordered a few more and carried on selling them for as long as it took for people to start returning them as faulty. Clearly they were cheap for a reason, and finding one with the float chamber full of wood chips was the last straw.
Since then I have always advised people to get their Zeniths rebuilt by a carburettor specialist. But when Dave the landlord ran into problems with the life-expired Zenith on his Series III I thought it would be worth ordering a reproduction Zenith to see whether things had improved.
Once it was on the vehicle it was obvious that all was not well. The engine started and idled just fine, but opening the throttle too rapidly caused the engine to die, and any attempt to drive the little Land Rover resulted in all kinds of pops and bangs. Once under way it was fine as long as you didn’t slow down and then try to accelerate again. So Dave asked me to take a look.
The Zenith 36 IV is a very simple, conventional fixed jet carburettor with an accelerator pump, which is supposed to squirt an extra dollop of fuel into the air intake when the throttle is opened, to fill in the short gap between the airflow increasing and the fuel delivery via the main jet catching up. The symptoms pointed to a non-functioning pump. With the engine off and the air intake elbow removed I peered down inside the carburettor and moved the throttle arm a few times. There should have been a squirt of petrol into the manifold every time the throttle was opened, but this one remained as dry as the Kalahari in midsummer.
Access to the pump is not difficult but requires the top of the carburettor to be removed (four screws and two split pins). The choke/jet block is attached to the underside of the carburettor top (two screws and the float jet holder). The pump operates inside a long cylinder in the choke/jet block with an opening at the bottom, which picks up fuel from the float chamber. As I was removing the choke/jet block I inadvertently pushed down on the pump plunger and got a squirt of petrol in the eye. Aha! There is a one-way valve at the bottom of the pump – a simple ball bearing held in place by a spring. On this carburettor the spring had been fitted without first locating the ball bearing in its seat. In a matter of minutes the carburettor was back on the vehicle and normal driveability restored. I will not be going back to selling cheap reproduction Zeniths any time soon.