classic motorcycle forum

Take a photo / measure the combustion chamber while you are at it. ?

That does look like a copy casting, its not clean enough to be a Villiers product. ?
Your combustion chamber possibly hasn't had the final machining to give the correct dimensions ?

The advent of ashless oils was a huge leap forward in oil technology, especially for 2 strokes.
I'm likewise unaware of these 'inertia rings', and thought I'd kept up with most such developments ...

Hi Keith,

No I don't think the "inertia ring" (see collected guff below) has anything to do with it. Sounds like the compression is unnecessarily high? Maybe the head has been shaved, or if it's new perhaps it has a smaller combustion chamber than the original. I bet it would run better and last longer with lower compression... try a thick copper head gasket?

Fitting a decompressor would work too, but I suspect the compression needs to come down a bit too. The Brooklands Villiers in the photo higher up had pretty high compression, but was not hard to kick over. Ditto various Bultacos in my younger days.

Cheers

Leon

"We may mention the provision of a single thin ring, called the inertia ring, above the top of the first pressure ring on the piston. This ring is free to rotate and to move up and down slightly, and it has been found to be very effective in keeping the rings free from gumming deposits."

"The Villiers 346cc long-stroke engine was fitted with a patented inertia ring, which was one of the most important improvements made in two-stroke engine design for many years. The object of the inertia ring was to prevent the gumming up of the ordinary piston rings, and it achieved this object in a very simple manner. In practice, it was found that a film of oil formed above the piston rings gradually became burnt and carboned, eventually fixing the rings solid in their grooves. The inertia ring, which was fitted above the top piston ring, was designed so that it could not touch the cylinder walls. It was permitted to have a slight up and down movement and to rotate freely. This movement prevented any film of oil forming above the piston ring and so kept the rings quite free."

"It is interesting to note that the latest 148 cc. Villiers engine has an inertia ring in the same groove as the upper piston ring. Because of its “floating” the inertia ring scours the top ring and groove and prevents it sticking up for long periods."

"The piston is of cast iron and carries two compression rings; in the upper groove is a special device patented by the makers of this engine, known as an inertia ring. The object of this ring is to prevent the piston rings from sticking in their grooves. This inertia ring is clear of both cylinder and piston walls, and is free to move within its own clearance limits, thus tending to keep the ring and groove clar of carbon deposit."

Correction first: Above I said that KZ was the prefix for the 196cc Brooklands engine; I meant Super Sport. (I don't think there was a 196cc Brooklands?)

Anyway, thanks for the photo of the engine. Two thinks are very clear, both of which you mention in your first post: the boss on the head is not machined for the decompressor (in fact, the head looks a bit like a recent casting?), and the boss between the exhaust ports is not machined for the oil pipe that comes with the Villiers automatic oiling system. Reading Browning, it seems that this connection, when present, is for the oil to go in via internal drillings; the pressure is provided by the other connection lower down on the crankcase.

Browning lists the 196 Super Sport prefixes as KZ and KZS, without saying what the different prefixes refer to. The VMCC machine register lists 3 KZ prefixes, 7 KZS and one lonely KZB (KZB 398) all fitted to FBs 1929-1930-ish. I guess the prefixes refer to combos of oiling system and lighting coils, with KZB being petroil and no lighting? Let us know when you figure it out!

I assume most/all KZ engines had the decompressor originally (see photo), but I doubt it's really necessary. Most useful for stopping the engine?

Cheers

Leon

Some clarity needed here
He is talking heads and you are talking cylinders 

Might be a slip of the pen.
But that photo would help ...

That's interesting. I think we might need a photo.

Most Francis Barnetts used the 172cc Super Sport engine, which has prefix BZ. The hot Brooklands version has prefix Y.

KZ is the prefix for for a 196cc Super Sport [edit: not Brooklands as I first said] engine, but I'm not familiar with KZB... Maybe it's some variant on the Brooklands/Super Sports.

The drilling in the cylinder between the exhaust ports is for an oil feed. The Brooklands head has a boss for a decompressor, but it is usually not fitted. Perhaps super-sporty riders/racers made do without a decompressor. If it's a huge problem, maybe you could use a thin aluminium gasket under the cylinder (not too thick or it will change the port timing) or a thicker head gasket to lower the compression a little.

Leon