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.

[john.k]

Seeing as its 10 hp ,you dont need to lose much.............assuming the ignition timing and carburation are right,you may be overgeared.............peak power will be at around 4000 rpm,and sidevalves loose power quickly at higher revs,work on that for your gearing.......55mph would be max in favorable conditions anyway.........another factor is engine heating,power reduces as the engine gets hotter,which it does running flat out.

[mini-me]

another echo from the WD M20 forum?

[murdo]

Have you given the carb a good clean and everything tight? Have heard of a bike that went well until the vibration started to get bad and would cough and fart until engine slowed down again. Turned out the wiring was shorting on the frame.

[TGR90B]

I would agree with the gearing theory. In the 60s I had a ZB32 Gold Star, which was a bit of a bitsa. A very peppy engine at high revs and would just hit 70mph in second gear. The trouble was it wouldn't reach 75 in either third or top. I wasn't experienced enough at 17 to work out what the problem was.

[Billington]

Thank you for kind responses. I really do appreciate your help. I’m new to the world of old British motorcycles and forums like this are gold.

The bike is only claimed to have 4.99 bhp from new and the gearing box is a four gear box, which was and optional extra in 1932. The standard was only three gears.

This bike was originally marketed for sidecar use. I know that it had a sidecar fitted in the 1950s from the old tax discs.

The bike works fine a low and high revs in 1st and 2nd gear however when the it is shifted into either third or four gear the revs drop off and the engine will die.

I’ve not taken the gear box apart, however I think that first and second gears must be the optional lower ratio gear designed for sidecar use. I believe this to be the case because it really does pull well for such a light bike when in 1st or 2nd gear.

Murdo thank you for asking if every thing is tight, I’m going to check for this again in case there are any air leaks around the carburettor.

This is a new problem that has only just started to happen; previously the bike ran fine in 3rd and 4th gear.

[TGR90B]

So it's a new problem and was previously fine. Possibly better to say that in the first place. It's obviously not a gearing problem then.
Goes up hills fine in third, but dies on the flat. Have you got enough petrol in the tank? If you have we could be here for a very long time.

[Billington]

Thank TGR90B, yes the tank is half full with fresh regular petrol.

I’ve just replaced the rear tyre, could I have set the chain tension so tight that it can’t pull in 3rd and 4th. Could this result in it being okay in 3rd going up and down hill but not on the flat?

The problem has happened when running the bike in 29C heat. Could this just be a coincidence? I’ll try riding the bike when the weather gets cooler, which in Britain will mean waiting two days.

Also TGR90B is that a picture of a big Triumph. My father had two of these they were beautiful to ride in and fast.

[TGR90B]

1967 Triumph 2000 mk1 Auto with about 60k on it.

[Rex]

[The bike is only claimed to have 4.99 bhp from new

Not quite. What you appear to be quoting is the old RAC horsepower (HP) figure.


This is a new problem that has only just started to happen; previously the bike ran fine in 3rd and 4th gear.

So it can't be a gearing or gearbox problem then, but "ran fine" is a bit tenous. I would strip and clean the carb and check over the mag. You'll need to ensure the basics are OK before moving on.

[Billington]

Rex 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.

[petrolderek]

This is interesting it dose sound like a gearing problem. You say it was fine and its only just started with this problem. You also say you have just fitted a new rear tyre. Did the problem only start after the new tyre was fitted. If so have you checked the size of the new tyre, is the circumference  the same some tyres have a taller side wall this could have the effect of raising the gearing. Think back to when the problem started what did you do to the bike just before the problem started and work from there. The most difficult faults to sort are the ones that someone has put on ie changing a component for an incorrect one or not fitting it correctly.

[murdo]

As above but if the wheel has been out what about the wheel bearings? Does the wheel turn easily by hand when on the stand?

[john.k]

Try increasing the exhaust valve clearance........open it to 10 thou if necessary...........if the mag has been off recently,as you say.....then I suspect the timing is out........but with a manual advance you should be able to get around a fair error in timing.A simple check is the motor should sound flat on full retard,then speed up as you advance the ignition,gradually.On full retard,if you blip the throttle,the motor should stumble,and maybe stall,on full advance,blip the throttle,and instant revs,without hesitation.

[Rex]

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’ve also cleaned and rebuilt the carburettor.

That would indicate to me that the timing is way out, and that would affect (because of the reduced power output) it's ability to pull a higher gear.
If the rear hub was tight or the tyre a different diameter it would be noticeable in the lower gears too.