Backfiring and flames from carb B31

[Pete C]

Have you checked the valve timing? BSA service sheet 305. Dash is inlet and dot exhaust.

[Janacek]

Have you checked the valve timing? BSA service sheet 305. Dash is inlet and dot exhaust.

No not yet.The reason being it was running perfectly fine (albeit at tickover and low revs) but when I gave it a bit of a rev,that's when it came to a fire spitting stop.
I do, like you,suspect valve timing,but I can't think of any way that it could alter,as it was running fine.

[scifi]

That sucking and blowing is quite normal for all cams, as they have 'Overlap' at TDC.   The more racey the cam, the more overlap.
You could check for a build up of carbon in the cylinder and plug, because if this is still glowing, it might cause a mis-fire.
As for it not starting, have you checked that it is still producing a spark..?   Also is the fuel getting through the carb to the engine..?

[limeyrob]

I would check for a blocked idle jet, common and it can cause spitting back.

[Pete C]

Have you checked the valve timing? BSA service sheet 305. Dash is inlet and dot exhaust.

No not yet.The reason being it was running perfectly fine (albeit at tickover and low revs) but when I gave it a bit of a rev,that's when it came to a fire spitting stop.
I do, like you,suspect valve timing,but I can't think of any way that it could alter,as it was running fine.

Sounds to me like the ignition timing has slipped. The taper on the pinion and magneto drive shaft need to have a good fit. You may need to retime and I would advise lapping in the pinion and shaft to ensure good contact and reduce the chabnce of slipping. I would strongly recommend following the information from the WM20 website here http://wdbsa.nl/ignition_timing.htm
Good luck and I hope you solve your problem soon.

[Janacek]

Thanks for all the suggestions folks.
I have checked everything,ignition,carb,and plug and all are ok.The sucking and blowing I know is usual,but it is really excessive.Kicking it over the blowback from the carb really wets my hand,and the suck from the exhaust is very powerful indeed.
Another clue is very poor compression.Previously compression would take my weight (12st) for about 3 seconds before sinking.Now,there is little compression.This is I think due to the air being expelled back through the inlet so there is very little air in the combustion chamber to compress.
It appears to me that the valves are opening and closing late,and the only thing I can think of is the pinion driving the cam wheels has slipped.As it is held on it's shaft with a woodruff key and nut,it should not be possible to move position,however,it may have been assembled without the woodruff key.I am hoping that it is something as simple as that.
I am about to don my overalls and go into the cold damp garage and attack the engine with my spanners to see what horrors await when I take the timing cover off.
Wish me luck.

[limeyrob]

The crank pinion slipping - key sheared is possible and it would explain a lot.

[idie]

Check the mag pinion they can develope a hairline crack across the taper so will slip under a load. Like a backfire. Happened to me.

[scifi]

I can't understand how you are getting a lot of suction at the silencer tailpipe, as the volume of the pipe and silencer is much more than the cc of the bike.

However, a quick way of seeing if your cam timing is somewhere near is to get the piston at tdc on the non compression stroke.  Each valve should be open a little bit, and equal.  The exhaust will be closing, and the inlet will be starting to open.  This is called the overlap, where the valves are 'Rocking'.

This method relies upon both In and Ex cams to have the same period and lift, if they are different (why would they be?) then the equal overlap may be a couple of degrees off tdc, but near enough.

It is cold in these garages these days, my thermal insulation keeps the cold in.  It is warmer outside..!

[Janacek]

Limey Rob,you are right.
Took the pinions off and found that although there is a woodruff key in the crankshaft pinion,it is too thin and too short.It is only about half the length of the slot in the shaft and is flush with the top of the groove so was not gripping the pinion.
I am surprised that it ran as long as it did,and only when I gave it a good rev it slipped.
Glad I found it,as my usual Sunday run is over the lonely moorland roads between Yorkshire and Lancashire,and being dumped up there with a dead engine is not recommended.