BSA B31 timing

[Steve.S]

I once had an engine with the same problem. It eventually turned out to be that the valves were only opening 1/8" due to cam wear. Also, a blocked exhaust can cause the same symptoms. Just a thought.
However, there seems to be something strange here. To start at the beginning, if your timing was set to 56 degrees fully advanced, this would be about 26 degrees fully retarded. I doubt if you could start the engine at that. I think it may kick back violently, if it fired at all. Well, you say it did start and ran at 50 to 55 mph. So you must have only been running with only a very small amount of advance on the lever. Fully advancing it would have caused extreme pinking and maybe stopped the engine. So it doesn't really make much sense.
You then set it to 38 degrees, and say it was hard to start. (Later, without making any further changes, you say it starts OK).
You say it splutters and pops when you retard the ignition. Is this at tickover, or on the road? I think the popping may be relevant. In my experience, this usually indicates an ignition problem.
Being new to all this, are you using a cigarette paper to check points opening? Also, (sorry to state the obvious) remember that when viewed from the right hand side of the engine, it turns clockwise, and when viewed from the left hand side it turns anticlockwise.
I wonder if you could paint marks on the cush drive and crankcase at TDC and a few degrees BTDC and see what's going on with a stroboscope?
Having said all this, if the motorcycle gets up to 55 mph without pinking, the timing can't be far out.
I wonder if you might check the fit of the points carrier on the brass cutout on the armature. These can wear badly. Is your armature coming apart due to loose endcaps?
One of the ways to check if it is advancing correctly, is to ride along with a small throttle opening at full retard. If all is well, the bike should surge forward when you advance the ignition.
It might be worth investing in a Spark Gap tester. A 5/16" gap will show a voltage of 15,000 volts. But don't go more than 5/16".
Finally, I once had an engine that ran fine, but faded going up hills unless I closed the choke. Turned out I had a bad air leak at the carburettor flange.
Hope all this may be of some help to you.

[briancairns]

Thanks for the advice, currently rechecking everything and taking carb off to clean in ultrasonic bath. I'm probably doing something daft !

Will let you know.

Thanks

Brian

[briancairns]

Hi Brian

You could have one of a variety of Amal carbs  ,  pre-monobloc, monobloc or concentric. Do you know which you have? They all work OK but your tuning strategigy might be a bit different for each.

this link is a good start for tuning info.   https://amalcarb.co.uk/downloads/

Concentrics are cheap and easy while pre-monobloc or monobloc are more "authentic"  Enter your bike details here in "vehicle search" to get info on which carb would be "correct" for your bike           https://amalcarb.co.uk/

Hi Pete, carb is a pre-monobloc 276DR/AT. I got the tuning guide but link I have an air leak at flange. Taking carb off to give a good clean and fix air-leak.
Thanks Brian

[briancairns]

I once had an engine with the same problem. It eventually turned out to be that the valves were only opening 1/8" due to cam wear. Also, a blocked exhaust can cause the same symptoms. Just a thought.
However, there seems to be something strange here. To start at the beginning, if your timing was set to 56 degrees fully advanced, this would be about 26 degrees fully retarded. I doubt if you could start the engine at that. I think it may kick back violently, if it fired at all. Well, you say it did start and ran at 50 to 55 mph. So you must have only been running with only a very small amount of advance on the lever. Fully advancing it would have caused extreme pinking and maybe stopped the engine. So it doesn't really make much sense.
You then set it to 38 degrees, and say it was hard to start. (Later, without making any further changes, you say it starts OK).
You say it splutters and pops when you retard the ignition. Is this at tickover, or on the road? I think the popping may be relevant. In my experience, this usually indicates an ignition problem.
Being new to all this, are you using a cigarette paper to check points opening? Also, (sorry to state the obvious) remember that when viewed from the right hand side of the engine, it turns clockwise, and when viewed from the left hand side it turns anticlockwise.
I wonder if you could paint marks on the cush drive and crankcase at TDC and a few degrees BTDC and see what's going on with a stroboscope?
Having said all this, if the motorcycle gets up to 55 mph without pinking, the timing can't be far out.
I wonder if you might check the fit of the points carrier on the brass cutout on the armature. These can wear badly. Is your armature coming apart due to loose endcaps?
One of the ways to check if it is advancing correctly, is to ride along with a small throttle opening at full retard. If all is well, the bike should surge forward when you advance the ignition.
It might be worth investing in a Spark Gap tester. A 5/16" gap will show a voltage of 15,000 volts. But don't go more than 5/16".
Finally, I once had an engine that ran fine, but faded going up hills unless I closed the choke. Turned out I had a bad air leak at the carburettor flange.
Hope all this may be of some help to you.

Hi Steve, I checked ignition timing and it was out ( don't ask me how) but have reset. I think I also have an air-leak at carb flange . Taking carb off for a good clean and fix air leak.

Many thanks

Brian

[Steve.S]

Brian, I wonder if your drive pinion might be slipping on it's taper? If you were to paint a mark on the points housing adjacent to the points when they are just opening, would this be a quick way of checking whether or not the pinion had moved?
I use wet or dry on plate glass to resurface the flange. You'll soon see if it's warped. If you still have a problem, it might be worth dribbling a little oil over the flange with the engine running. If it slows down, that indicated an air leak. You could also try the same on the inlet valve guide, or perhaps use spray grease?
Steve.

[briancairns]

Thanks Steve, I was going to do what you said regarding flange. Hopefully will get it sorted soon.

Thanks again all for your help

Brian

[idie]

Make sure the timing pinion hasn't got a hair line crack on it. It will slip if it has.

[briancairns]

Thanks , will check it out.

Brian

[Jim S]

Hi Brian

Have you determined the cause of your apparent power loss? If yes, please let us know. I am interested and others are probably also.

Jim

[briancairns]

Sorry Jim,
No updates yet, I'm very busy at work and haven't much time at the minute. I rechecked timing again and it was spot on. Currently I have the carb off and giving everything a good clean in sonic bath. Will probably skim the flange as I think it's slightly warped as think previous owner seemed to have over tightened it. There seemed to be an air leak from flange.
Hopefully I will have it ready for a test ride by end of next week and will give an update then.

Thanks for your help

Brian

[briancairns]

Hi All,

Delighted to report bike is working fine, took carb off and cleaned in sonic bath, although it looked absolutely fine.
I think issue was a mixture of things, timing, contact breaker gap, spark plug and dirt in carb.
Thanks so much all for your invaluable advice!!

Thanks again

Brian

[Derek996]

Glad to hear that it's all sorted. Having various item not right can be a bit baffling but working on one at a time usually gets you there. Well done   

[briancairns]

Thanks.
No doubt I will be back for more advice!

Cheers

Brian