classic motorcycle forum

Yes Rex you're absolutely right, but reducing the inlet charge by 3% is more-or-less like reducing the CR by 3% - there's less gas in there to squish. Of course when everything is spinning at 6000 rpm all the static calculations are out the window anyway!

Leon

Hi Keith,

You could certainly give it a go.

The inlet valve closes when it does because someone at the factory has determined that this is the point, when running "normally", that gets the most mixture into the cylinder. For a sporty little engine like the Cub the inlet closes quite late, because running at higher revs there's a lot of intertia of the gas flowing from the inlet port into the engine. So the cylinder keeps filling even though the piston is moving UP the cylinder on the compression stroke.

Cub timing figures attached, from www.triumphrat.net/attachments/cub-useful-data-page-with-timin-specs-1966-jpg.755483/

At starting, the engine is spinning very slowly. Let's imagine that we turn it really slowly, say by hand. Letting the inlet valve close later will mean that the piston is slightly higher up the bore before the inlet valve closes - the extra volume of gas (equal to the distance up the stroke times the area of the piston) has been pushed backwards out the inlet valve, so the amount of gas to be compressed is less. We could calculate it, but I really doubt it could make too much difference to the compression on the first rotation of the engine. Once spinning, the gas flow is a very dynamic thing and depends on all sorts of stuff I'm not qualified to talk about!

What it will almost certainly do is cause blow-back through the carb when the engine is running.

But give it a go and let us know how you get on. Good luck!

Leon

Pity the blipping thing doesn't have a valve lifter!

Chater Lea's own spring fork, introduced in 1910.

Leon

Here's the engine in a ROC.

"I fitted the 0.060 thrust washers on the timing side, and a 0.023 on the output side..."

I guess this is part of the mis-matched engine thing. It would be nice if there were no shims between the face of the crank and the bearing inner. Usually this is accomplished by shimming the bearing outer in the crankcase. As it stands, your shims are pretty large and heavy, and there could be quite a "hula hoop" effect as they spin around the main shafts (or the shafts spin inside them), which could cause damage to the shafts and/or the shims. In reality, if you're not going racing, it will probably be ok. But it would worry me. Maybe check with your JAP man?

You mention there is not much end thrust when the engine is vertical: true provided the piston is square and centred. You might be amazed to see the inside of an engine that has been running with a bent con rod!

Leon

Thanks - satisfying to read to the end.

Leon

Now I am NOT going down the "Villiers Inustrial Engines" rabbit hole, but I will admit there is one! As Reg says in his memoir, P&R Williams were a big part of the Australian end of the business, in the 1930s, with a gap for the war, then 1946 into the early 1950s.

I suppose the 98cc Villiers Midget in the Ransome lawnmower is akin to the engine in the 98cc Waratah of the time? On second thoughts, don't answer that!

Leon