As I mentioned in earlier posts, I had some trouble with a loss of power when jumping quickly onto the gas - the engine would just stall. The only way round it was to come right off the gas and onto the clutch before the engine completely cut out. Slowly pressing the pedal was fine - it was only a problem when I pressed too fast.

In the Zenith carb that I have, there is a small piston which squirts a shot of fuel into the throat of the carb when the throttle is operated. Normally, fuel is “sucked” into the engine by the vacuum in the inlet manifold - the higher the revs, the lower the pressure, the more fuel is sucked into the engine (regulated by the throttle setting). When you jump on the gas the engine is not revving fast, so there isn’t enough vacuum to suck in enough fuel to get it started. Thus the extra little squirt mechanism, which gives a quick boost.

Sure enough, I found that I wasn’t getting any little squirt when I pressed the throttle. So the carb had to be opened up. Not as complicated a job as I’d thought, but a very stinky one!

Here’s the carb with the lid off. There’s a bit of rusty gunk in the petrol, but nothing to worry about.

Here’s the top part of the carb, float mechanism (which makes sure the reservoir in the carb is always at the right level) detached.

The squirty piston. Looks very much like you’d expect his larger cousins in the engine to look!

And here’s the problem. At the bottom of the piston is a ball bearing valve which ensures that petrol is pumped into the engine rather than just squirting back into the carb reservoir. You can see that the ball has jumped out of the hole and got stuck.

So I fixed the problem and got the engine up and running again. Things are slightly better… BUT… I can still stall the engine by jumping on the gas. Very odd. More investigation needed.