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British Bikes / Re: Tiger cub valve timing.
« Last post by Rex on Today at 09:59:01 AM »
I think I'd rather look into skimming the piston crown before altering the valve timing.
Personally I can't see how altering the valve timing would or can alter the CR, given that it's a ratio of swept volume to compressed volume of the cylinder.
The CR would be the same even if there were no valves in place.
Altering the amount of time that the inlet valve is open may alter the amount of mixture entering the engine, but that's not the CR.
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British Bikes / Re: Tiger cub valve timing.
« Last post by Vreagh on Today at 09:19:54 AM »
Thanks Leon, although that doesn't sound much, I think it's worth a go. The effect would be greater with 7:1 piston so might be just enough.
  Cheers, Keith
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British Bikes / Re: Tiger cub valve timing.
« Last post by cardan on Today at 06:51:44 AM »
I've calculated the change in compression ratio, guessing that a Cub con rod is about 4 1/2" long.

Delaying the close of the inlet valve from 55 degrees after BDC to 60 degrees after BDC changes the (static) compression ratio by about 3% - if it was 9:1, it would drop to around 8.7:1. Not sure how that feels in real life.

Leon
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British Bikes / Re: Tiger cub valve timing.
« Last post by cardan on Today at 03:00:01 AM »
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
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British Bikes / Re: Tiger cub valve timing.
« Last post by Vreagh on April 21, 2026, 02:53:54 PM »
What started me thinking about altering the valve timing was Cub guru Paul Henshaw. He mentioned that a combination of the 9:1 piston and standard cams caused difficult starting due to early inlet closing making higher pressures. He said the 'R' cams closed the inlet later thus lowering the pressure at TDC. I compared standard timing with 'R" timing  and they 'looked' similar if the standard was retarded one tooth. I couldn't see any problem with valve / piston entanglements. It was just a thought to get me back in the saddle. I might have to try leg exercises as a last resort.
Cheers,
 Keith
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British Bikes / Re: Tiger cub valve timing.
« Last post by Rex on April 21, 2026, 11:20:39 AM »
My Venom was a tooth out on the valve timing and that bike could've been kicked until the next Millennium without starting.
I have two 350 singles, both with decompressors, and using the Big Single starting technique (easing over compression, etc) neither tax my knee, hip or heart too much to fire up. Not like twins, where you're always kicking through compression.

When I inevitably get to the "I can't start them any longer" stage I'll treat myself to a new Bantam at 350cc, or even one of those little AJS 125s, both cheap as chips and leccy start.

That's if I'd still trust myself to last on busy roads!
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British Bikes / Re: Tiger cub valve timing.
« Last post by cardan on April 21, 2026, 12:17:29 AM »
Yes getting older presents a few challenges. I'm lucky that (heading into my eighth decade in a couple of years - wow) I can still start and ride and maneuver bikes around, and get a lot of my enjoyment from researching and fiddling in the shed. I have to admit that I do lots more riding on my ebike than I do on old bikes these days, and I do the riding off road to minimise the chances of being cleaned up by increasingly dangerous traffic.

I was going to suggest starter rollers as a solution to your starting problem, but getting the bike on and off the rollers has so many opportunities for disaster that it might be best avoided.

With new knees, have you considered an ebike? If you have low-traffic places to ride near home it's great fun, and - if what I've read is correct - good for knees and general health. Mobility is such an important part of our lives.

Must be frustrating, though, looking at the that little Fanny B... dream machine!

Leon
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British Bikes / Re: Tiger cub valve timing.
« Last post by R on April 20, 2026, 11:23:32 PM »
Practice your run-and-bump start ?
Its generally easier to kick over a bike once its warm.
Well tuned, they practically start themselves.

Hmmm, so thats why LE Velos are becoming popular again..
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British Bikes / Re: Tiger cub valve timing.
« Last post by Vreagh on April 20, 2026, 04:22:12 PM »
I was toying with the idea of trying to fit an Enfield decompressor but there's not much head to play with. Someone suggested a larger engine sprocket and that might be a way forward. It's demoralizing to get old, I even tried my  FB with the 196 super sport, but same thing, can't get enough momentum to get over compression. Might be some  exercises that'll built up the leg.
Cheers,
Keith
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British Bikes / Re: Tiger cub valve timing.
« Last post by cardan on April 20, 2026, 10:12:52 AM »
Pity the blipping thing doesn't have a valve lifter!
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