Morris Minor Owners Club

I've been trying to find the cause of an intermittent lumpy tickover on my 1971 traveller, and the end result may be a useful tip if you have the same problem.

After trying all the usual suspects, timing, plugs, ignition harness, coil etc., and servicing and adjusting the carburettor, the "lump" was still there. The engine would tickover smoothly, then every now and then it would drop revs and pick up again.

At one point, to relieve my aching back after leaning into the depths one time too many, I lent on the rocker cover filler cap, and low and behold, the "lump" went away.

The extra pressure from me leaning on the cap was enough to provide an air tight seal between the cap and the cover, and even though this is the "breather" type cap that allows air in through the mesh filter, the leak between the tired seal and the cover was enough to cause the lumpy running.

I had been too mean to replace the cap when I serviced the engine, because it was still "breathing" cleanly after I'd washed out the mesh filter.

It looks as though the replacement recommendation is because the gasket and bayonet moulding on the cap get tired, not because the filter gets blocked.

A new cap is now on order.

Moral, if it says "replace the filler cap" in the service schedule, then replace it.

More likely your hand blocked off the cap - and stopped the ingress of unwanted idle air!

No, it was definitely pressing down on the cap that cured it. (As evidenced by the fact that fitting a new cap with a good seal cured it, sans hand).

The vents are at the side, under the cap. Hand comes nowhere near them.

I once had a similar problem with rough running on an MG Metro, where one of the lugs had broken off the alloy rocker cover, so the filler cap appeared to fit and lock into place, but not tight, because of the missing lug. It had the same breathing arrangement.

The cure in that case was a new alloy rocker cover since the lugs are part of the casting. I still have the old one, if anyone wants it.