Running up my engine gave it a blip prior to shutting down and it started spitting quite scary flames out of the carb.
Sticking inlet valve or broken spring I thought.Kicking it over afterwards I noticed that turning the engine over slowly,there was compression,but kicking quickly,compression disappeared.Kicking it over with my hand over the carb,there was suction but it was also blowing back petrol over my hand.
Took the head off,fitted new valve springs,cleaned the valve stems and on re-assembly found it was just the same as before ie compression when turning the engine over slowly,but quickly,almost as if the valve lifter is being used.
Am at a loss as to what the cause is.
I think I will take the timing cover off to have a look,but I don't really know what I am looking for.
Any suggestions welcome
Was it running fine before?
Quote from: limeyrob on 31 Oct, 2025, 17:19 Was it running fine before?
[/quote
Yes it was.First or second kick always started and a lovely slow tickover
Could it be a sticking tappet? Never heard of that,but looking at the manual,I can't see what could cause this.It's almost as if the valve timing has slipped but I can't see how that could happen.
Think I have found the cause.All the collets look a bit bruised and they are all the same size
Exhaust collets are thinner as the exhaust valve stem is thicker than the inlet.Not sure if the collets fitted are for the inlet or exhaust as they are all the same,but I have ordered inlet collets but can't as yet find anyone with exhaust collets in stock.
It appears that a collet on the inlet valve let go,pushing up the valve cap and preventing the valve closing properly and turning my engine into a fire spitting scary monster!
Hopefully new collets will cure it.
Well that's a new one, I know I would never have worked that one out!
Lucky that the collet didn't let the valve go altogether. If the valve had dropped into the cylinder the consequences would have been far worse.
Quote from: Janacek on 07 Nov, 2025, 20:01 Think I have found the cause.All the collets look a bit bruised and they are all the same size
Exhaust collets are thinner as the exhaust valve stem is thicker than the inlet.Not sure if the collets fitted are for the inlet or exhaust as they are all the same,but I have ordered inlet collets but can't as yet find anyone with exhaust collets in stock.
It appears that a collet on the inlet valve let go,pushing up the valve cap and preventing the valve closing properly and turning my engine into a fire spitting scary monster!
Hopefully new collets will cure it.
Check your retainer for a polished looking top edge the rocker can get actually touch and push on the retainer first, mine did, I fitted some thicker lash caps, check your valve spring tension, weak spring will cause a lot of reversion out of carb, also check your cam timing. Cheers
Regarding the collets it would pay you to either make them yourself of commission an engineer to make ,as the chances of any new pattern collet matching the correct angles /taper of your collets would be very slim
Thanks for the replies.
Haven't got around to re-assembly yet,but put the valves,collars and collets together on the bench and all looks ok.The new collets look to have much sharper edges than the old ones,and have checked that the valve collar has no abrasion marks.My valves have valve caps so it is unlikely for the rocker arm to touch the valve collar.
Quote from: Roy on 12 Nov, 2025, 10:20 Regarding the collets it would pay you to either make them yourself of commission an engineer to make ,as the chances of any new pattern collet matching the correct angles /taper of your collets would be very slim
The collets I obtained were advertised for an A7 and B31 inlet valve,and I assembled the inlet valve,collar and collets on the bench,and found that with a bit of a pull the collar holds tight and looking at the underside of the collar,the bottom of the collets is just about flush with the collar aperture.So I think these are ok.
Quote from: Roy on 12 Nov, 2025, 10:20 Regarding the collets it would pay you to either make them yourself of commission an engineer to make ,as the chances of any new pattern collet matching the correct angles /taper of your collets would be very slim
8
You have given me something to think about.
Bought the collets and springs from different suppliers and re-assembling it I found the exhaust rocker shaft went in easily,the inlet was a pig!I noted that the shafts did not match,so it's likely that one or both are pattern parts.It's also likely that the valves are pattern parts,so buying pattern collets and valve springs hoping that they match pattern parts already fitted as you point out is unlikely.
I suppose the best course is to try to source new old stock original BSA parts.
Thanks for your input.
For my recent A10 rebuilt I had to buy valves, springs, collars and collets from the same supplier (in this case SRM) because they were able to confirm that all the parts would assemble correctly. The existing parts were an odd mix and I was not confident which were correct and which wrong.
Quote from: limeyrob on 14 Nov, 2025, 14:39 For my recent A10 rebuilt I had to buy valves, springs, collars and collets from the same supplier (in this case SRM) because they were able to confirm that all the parts would assemble correctly. The existing parts were an odd mix and I was not confident which were correct and which wrong.
I should have bought the parts from the same supplier,but I didn't think
I should have known,as some years ago I had an MZ and all the available parts were pattern.Bought a new piston and found that the skirt was hitting the flywheel at the bottom of it's stroke.Either the piston was innacurate or the flywheel
Shaved a couple of mil off the piston skirt and all was well.
Sadly the valves,springs,collars and collets are rather more crucial on a four stroke,so I will have to try again.
I feel for you , its so frustrating, but the days of genuine parts where they made 100,000 all identical seem to be over.
Assembled the engine after new valve springs and collets,but engine will not start. As in a previous post the engine stopped when I gave it some revs to check the charging rate,with flames shooting out of the carb.Having done the valves they seem to be ok and the pushrods are straight.
While I was standing around wondering what could be wrong,I noticed something rather peculiar.
Kicking it over with my hand over the carb,it sucks a bit and then blows petrol over my hand.Hand over the exhaust,it blows a little and then there is a quite powerful suck!
It appears that the carb is sucking then blowing and the exhaust is blowing then sucking.Most puzzling!The engine won't start,not even a cough so something is wrong,but I can't think what.
Any ideas?
Thanks, Terry.
Have you checked the valve timing? BSA service sheet 305. Dash is inlet and dot exhaust.
Quote from: Pete C on 06 Dec, 2025, 15:39 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.
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..?
I would check for a blocked idle jet, common and it can cause spitting back.
Quote from: Janacek on 06 Dec, 2025, 19:39 Quote from: Pete C on 06 Dec, 2025, 15:39 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.
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.
The crank pinion slipping - key sheared is possible and it would explain a lot.
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.
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..!
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.