B31 valve timing

[Janacek]

Checking the valve timing on re-assembly,is the timing checked when the valves start to open,or when they are just fully open?
There is quite a difference between the two.
Lining up the dot and dash on the cam wheels,the inlet starts to open at the book figure,but there is 20 degree difference when fully open.
The exhaust starts to open at the book figure when it's cam wheel is advanced one tooth,but again a 20 degree difference when just fully open.
Are the figures given in the handbook with the valves just starting to open,or with the valves fully open.
If the latter,then the marks on the cam wheels are meaningless.
Any advice appreciated.  Thanks,Terry.

[Pete C]

this might help to clarify the valve timing

[JulianS]

Valve timing figures given are usually for tappets set at a different setting to the running settings. Gold Star figures are for tappets set at 0.018 inch and A10 figures for tappets set at 0.015 inch. This from factory data books. I have not seen a setting figure for the B31.
It can be very difficult to establish exact opening and closing figures, but if the timing marks line up it is very unlikely that the valve timing is out.

[Klaus Jung]

As you can see at the diagram, both valves are in TDC (Exhaust Stroke) the same amount open.

[Janacek]

Thanks for the replies.Checked again the opening and closing of the valves,and the inlet starts to open and close at the book figure with the dots aligned ( 25 and 65 degrees.
The exhaust to get the book figure needs the cam wheel advancing.
A bit puzzling,but although the cam and follower show little signs of wear,I am guessing that the cam may have been re-ground or re-profiled at some time.As the cam wheel turns at half engine speed,it would take very little wear or grinding to affect the valve opening.
I am going to assemble it and see how it runs.It was running fine when I got the bike,untill the valve timing slipped due to a wrong too small woodruff key being fitted to the engine pinion.I didn't notice the cam pinions position when I took it apart.It may be that a previous owner found the same thing that I have and altered the valve timing to suit.It did run fine when I got it,but ran like a dog when I put the cam wheels to the dot and dash.

[limeyrob]

You may have had "2 wrongs making a right".  There's really no alternative to getting it all how it should be.  Worst case you'll need a set of cams.  What numbers are they?

[Derek996]

The standard cams for the B31. B33 and M20/21 are 65-2420, both inlet and exhaust. I've got 6, all in good condition, and would be happy to sell a pair.

[scifi]

There are a few cams available on Ebay,   The Inlet and Exhaust look similar, but they could be slightly different as they sit on the left and right of the engine.  That could cause a half tooth difference if they have been installed the wrong way around.  Ebay listing.....
 
https://www.ebay.co.uk/itm/286799783300?_trkparms=itmf%3D1%26aid%3D1110006%26rkt%3D10%26algo%3DHOMESPLICE.SIM%26mech%3D1%26algv%3DDefaultOrganicWebWithV11WebTrimmedV3VisualRankerWithKnnV3AndUltBRecallAndCassiniEmbRecall%26pmt%3D0%26amclksrc%3DITM%26sd%3D127642521718%26sid%3DAQALAAAAEJAGpYMn8jjaP%2FkTlm%2FXI1Q%3D%26itm%3D286799783300%26noa%3D1%26brand%3DBSA%26asc%3D20220405142716%26ao%3D1%26rk%3D3%26pid%3D101506%26b%3D1%26mehot%3Dnone%26lsid%3D3%26meid%3Dacf4cb8783214d5384fd6d4345a20d2e%26pg%3D4481478&_trksid=p4481478.c101506.m1851
.

[limeyrob]

The 65-2420 inlet and exhaust cams are the same, they both have timing marks for inlet and exhaust.

[Janacek]

You may have had "2 wrongs making a right".  There's really no alternative to getting it all how it should be.  Worst case you'll need a set of cams.  What numbers are they?

There are no numbers on the cam wheels,but both are embossed with the letter A.

[limeyrob]

There's a good set of cams on e-bay at the moment for £30, just type in BSA 65-2420

They should both have a dot and dash on the other side.

[Pete C]

You may have had "2 wrongs making a right".  There's really no alternative to getting it all how it should be.  Worst case you'll need a set of cams.  What numbers are they?

There are no numbers on the cam wheels,but both are embossed with the letter A.

the standard cams do not have the number stamped on them

[Derek996]

Has this issue been solved yet?

[Janacek]

Has this issue been solved yet?
[/qu

Mainly yes thanks Derek.Starts easily and ticks over except when cold it spits back through the carb,whether the ignition is advanced or retarded.
Tickover is steady ignition retarded, but at full advance the tickover is not steady but misses a beat frequently.I fitted a new oil seal to the mag pinion and checked the ignition timing several times before re-assembly.
Lapped the carb flange so I am pretty sure there are no air leaks to cause erratic tickover at full ignition advance.
I had the same year and model B31 when I was 19,(a long time ago) and I don't remember it having an erratic tickover on full advance.All the carb jets are as per the book and blipping the throttle the response is good.
Running out of ideas,but wondering if the rubbish E5 petrol is responsible.

[Derek996]

Good to hear you have it running well now apart from the slightly erratic ticker. I doubt if it's the E5 petrol as mine runs well enough on it although it definitely ticks over steadier with a bit of retard. At full retard I can just about count the firing pulses, but I don't like to run it like that for too long as the exhaust gets very hot and doubtless the exhaust valve and piston too.