Losing power when in high gears

[BILLINGTON]

I hope you can help me, my 1932 BSA W32-6 (500cc side valve) is losing power when I change up to third or fourth gear, but only when riding on the flat.

The bike is not easy to start but when it does start it pulls away just fine in first gear then pulling strong in second. However when I change to third or four gear the engine revs drop and the bike would stop if it did not change down to second again.

I find this strange but what I find even stranger is that the bike works fine when either going up or down hill in third gear.

What is causing this and how can I solve it?

Thank you for any helpful advice.

[BEVANC]

A contact of mine with  29 Sloper had similar issues. He solved it by easing the primary chain tension, which was tightening up as the engine warmed up, so it couldn't pull in high gear.
Worth checking I guess.
Cheers, Bevan

[BILLINGTON]

Thank you so much Bevanc for your response. I've recently replaced the rear tyre, which required me to aline and tension the rear wheel and chain. I've set the bike up with about 30mm of free play on the chain at it's middle point.

Do you think this would have the same result as your friends primary chain tension issue?

Also this is the first time I've attempted to run the bike in 29C heat (It's hot currently in Britain). Could this be a contributing factor.

[BEVANC]

I can't see that the final drive chain could be the cause, so if you primary tension is ok, I'm not sure what the problem could be.  If it was a magneto failing, that could make it hard to start and also falter when hot, but I would not expect it to recover when you changed down.  Is your advance\retard mechanism intact and working correctly ?  The only other things that occur to me are a valve or the piston starting to nip up, but that's not really consistent with your described symptoms.

I hope someone with greater expertise than me can offer you a useful suggestion.

Good luck !!
Cheers,
Bevan

ps - just a thought - you haven't got an intermittent fuel blockage due to some crud in the tank have you ?  Might explain the going ok up or down hill if the petrol is slopping around more than on the flat.

[BILLINGTON]

Bevanc thanks for your post. I've only done about ten miles since the dynomag was professionally rebuilt. Since the rebuild I've removed the mechanical regulator coil and replaced it with a modern semiconductor solid state encapsulated regulator for motorcycles using the Lucas E3 type dynamo.

I set the timing up as described in the period BSA manual; however the bike will only start on full retard and will then run best at full retard. I know this is counter intuitive and may result in the exhaust port becoming to hot (burning out). I'm not sure that this is related to my 3rd and 4th gear issue though.

I've also cleaned and rebuilt the carburettor. However I would not rule out the possibility that I have introduced a problem due things like float height or air screw being wrongly adjusted.

I've full restored the inside of the tank, this was one of my first problems that I solved.

[BEVANC]

Only starting and running on full retard surely must dictate there is a timing issue. Presumably it should be set with points just opening at tdc on full retard ?  Since its an sv, is it easy enough to whip the head off and check the timing and advance \retard accurately with a degree disc on the crank ?

[BEVANC]

This is a quote from one of the experts on the Ariel motorcycle forum, where i posted your issue this morning:

"When in doubt start with the magneto. Perhaps the timing between the points and the magnet is wrong due to mixing parts sometime in the past or the "weak" key is damaged on the taper of the points assembly where it fits onto the end of the armature. IF this is wrong you lose all spark at some place in the advance retard movement. Check also the High tension slip ring isn't worn hollow where the spark occurs or brush sticking and the spark is generally adequate. You should get about 1/4 inch spark onto the plug when the motor is at a fast idle . Don't use an "R" plug or resistor "suppressor" leads. Check point gap. It may also be geared too tall. and see if lowering the airslide helps at higher throttle opening when it dies. Nev"

Cheers,
Bevan

[BILLINGTON]

Thank you so much Bevanc. It will take me a while to process this but they are defiantly lines of investigation I need to pursue

[BEVANC]

Would be really useful to hear eventually what the cause was
Hope it is a simple one !
Cheers,
Bevan

[BILLINGTON]

THANK YOU!

Thank you so much for the kind help offered by the members of this (and two other) forum. Today I rode my bike for 10 miles in the glorious sun.

With your advice I've been able to identify and correct the problem of losing power when in high gears. It was a small head gasket leak which resulted in insufficient compression when dropping down the rev range in higher gears. A new head gasket and some advice on torque setting and I have a bike that runs sweet.

This type of support is invaluable for inexperienced vintage bike mechanics like me.