Hi all, just jumping onto this topic as it's kind of related to the problem I have with a 1962 BSA C15 that I recently acquired.
A previous owner fitted PVL ignition (Rex Caunt) and changed the engine sprocket from the standard 23T to 18T.
She starts very easily and pulls well in low gears, but my problem happens when in 4th as she revs very highly but never achieves more than around 30mph and this is the main subject of this post. Sounds similar to problem that @jon v8
refers to above?
I was thinking that to address the high revs/low speed issue in top gear, but retain the low speed performance in low gears, I should leave the engine sprocket at 18T, and fit the C15 Trials 4th gear, wide ratio pinions on the lay-shaft and main-shaft. Although there would be a big jump between 3rd and 4th, at least I’d be able to cruise on the open road at a more respectable speed without it revving so highly.
Is my thinking sound? Has anyone else had the same issue and resolved it in a different way?
On a related point, when I stripped down the gearbox, I discovered that the 1st to 3rd gear sprockets are from a standard road gearbox, but the 4th gear uses the cogs from the Scrambler gearbox (main-shaft: 26T / lay-shaft: 17T) . Not sure if this could be a contributing factor to my top speed issue?
If the approach I outline above would work, is it just a matter of swapping the 4th gear pinions on the layshaft & mainshaft over? Or will I need to change anything else - eg the gear selector?
For reference, I have listed all the sprocket sizes on my bike in the table (not sure how to embed it into this text - so added as an attachment).
For others that might be interested in the future, I've also listed the standard sprocket sizes for C15 variants - road, scramber, trials.
Thanks all - hope I've explained this simply and it all makes sense - and thanks in advance for anyone that help me out.