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.
this might help to clarify the valve timing
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.
As you can see at the diagram, both valves are in TDC (Exhaust Stroke) the same amount open.
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.
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?
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.
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
.
The 65-2420 inlet and exhaust cams are the same, they both have timing marks for inlet and exhaust.
Quote from: limeyrob on 26 Jan, 2026, 10:15 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.
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.
Quote from: Janacek on 27 Jan, 2026, 11:34 Quote from: limeyrob on 26 Jan, 2026, 10:15 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
Has this issue been solved yet?
Quote from: Derek996 on 11 Feb, 2026, 19:51 Has this issue been solved yet?
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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.
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.
Not sure if the old wives tale is true (sorry if that offends any old wives here😉) but I have understood not to leave these engines (or at least the Gold Star variants) ticking over too slowly for too long as there is insufficient oil flowing at these speeds and you risk overheating with a piston face picking up or big end getting 'dry'.
Any knowledge of this out there?
Quote from: neil1964 on 18 Feb, 2026, 08:23 Not sure if the old wives tale is true (sorry if that offends any old wives here😉) but I have understood not to leave these engines (or at least the Gold Star variants) ticking over too slowly for too long as there is insufficient oil flowing at these speeds and you risk overheating with a piston face picking up or big end getting 'dry'.
Any knowledge of this out there?
I'm not sure that that would be the case with a roller bearing big end. It would to be a
very low flow indeed, but piston face overheating may be a possibility.
In Ducati circles I have certainly heard this mentioned but that was concerning the engines with plain bearing big ends which depend on a good oil pressure.
Looking at the back of the inlet timing sprocket/cam on my 1950 B33, there is a number stamped which could be 12-1 or possibly A2-1. Could anyone advise whether these are standard for this model or quieter cams that were fitted later?
Cheers
Quote from: Greenfield on 22 May, 2026, 15:04 Looking at the back of the inlet timing sprocket/cam on my 1950 B33, there is a number stamped which could be 12-1 or possibly A2-1. Could anyone advise whether these are standard for this model or quieter cams that were fitted later?
Cheers
The A2 is commonly seen on 65-2420 cams which are the standard type for B31/B33 and the B32/4 competition models post war until end in early 60s. These all had quietening ramps and 3 thou clearances. The Gold star type 65-134* and 65-243* (known as type A) pre 52 did not have ramps and need close tappets at 1 thou. Cams with 65-244* (type B) had ramps and wider clearances (6-8 thou).
Thanks Neil, any thoughts on a cam stamped 'D'?
Quote from: Greenfield on 24 May, 2026, 16:30 Thanks Neil, any thoughts on a cam stamped 'D'?
Sorry, I do not know. I've got a few Goldie standard cams across the era but just a D means nothing to me.
Can you do quick measurement of base circle and lift?
Otherwise you'll need to install and measure lift in situ and measure the opening times with degree disc on the crank.
As Neil says, the standard 65-2420 is often marked A-2 but just as often have no markings or sometines some other letters. The biggest give away as to whether a cam is a gold star one or not is the base circle diameter, which is smaller on a Goldie cam effectively giving more lift.
The cam on the left in the attached picture is a standard cam and the one on the right is a 65-2446. Note the wider diameter at the bottom and the more pointed peak.
is that base circle measurement dependent on the intended use for said Goldie cam? Fast road work maybe, but trials etc too?
Quote from: V500 on 25 May, 2026, 11:17 is that base circle measurement dependent on the intended use for said Goldie cam? Fast road work maybe, but trials etc too?
BSA produced cams for trials, scrambles, touring and clubmans and racing. Various combinations were used depending on the use, and whether 350 or 500. The ZB series used the same cams for tourers and racing yet had a different pair of cams for clubmans! The 1949 Scrambles used the same cams as the 350 tourer. It seems there was a constant development going on.
The trials Goldies used the standard 2420 cams. All the other Goldie cams have higher lift and reduced base circle diameter. I suspect that the base circle diameter may be different for different cams. I have a few that I could measure to get an idea.
The measurement across the base circle of the standard 2420 cam is 25mm as near as damn. The Goldie cams that I had to hand measured 22.5mm, 22.5mm and 22.3mm.That's an increase in lift of 2.5mm and 2.7mm. The cam profile itself will set the timings and opening durations.
A couple of my bikes had a mix of 'A2' and 'D' stamped cams. Both are 25mm diam across the round and about 32mm at the cam peak, so doing the same job. One or both could well have been replaced over the years.