classic motorcycle forum

Any questions?
 

Brilliant! I have questions!

Do you have a spreadsheet? I love a good spreadsheet.

So take 1931 for example. Here's p1 and p2/3 of the spare parts list for 1931-on Midget gearboxes. They call the non-ks model "B.J.", and the model with ks "C.J.". The part numbers are like BJ1/BJ2 and CJ1/CJ2 for the cases.

Was the first number for 1931 BJ001, or BA001? I assume BJ001?

In which case, were there 9000 "B model" gearboxes built before 1931 - numbered BA001 to BI001? (From my BTH magneto spreadsheet I found that BTH skipped the letter "I" in their dating codes, to avoid confusion with the number "1". Maybe Albion skipped I, too.)

Cheers

Leon

Gosh that gearbox is tiny !

Not just tiny... officially "midget"!!

I'd not realised the Excelsior version used foot change, something usually seen only on Isle of Man racers at the time! When hand change was used, there was a simple lever pivoting between the main shaft and the clutch cable boss - no external gate as used during the 1920s. Cheap and cheerful.

Edit: Here's a less shiny one https://www.andybuysbikes.com/archivehtml/6625exc.html# - foot change below

Leon

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