B31 Poor Performance

[martinh10]

I am in need of advice/opinion for my B31 problem. Can you help?
I recently completed a full restoration of a plunger B31 including rebore, new piston and rings (NOS Hepolite +.030), new valves, springs and guides but on my first few test runs found acceleration and top speed (55 mph) to be very disappointing.
My B31 is a stripped down job – no lights/electrics – just a magneto which has been fully reconditioned by a reputable auto-electrics company in Preston. I took it back to them for retesting at the start of my problem and it was found to be performing very well with an unsuppressed plug cap, albeit not under compression.
I’ve fitted a brand new Amal pre-Monobloc 276 carb made and jetted for a standard B31 and the plug looks correct so the air/petrol mix must be about right.
I’ve checked the ignition timing half a dozen times and it’s bang on 7/16 BTDC fully advanced. I’ve also checked a cam is not a tooth out. It ticks over and runs very smoothly but something is holding it back. The high level exhaust and silencer is new and unblocked.
The bike (a non-runner as purchased) came with rocker oil feed banjo bolts with 3/32” dia oil holes which I reused. On removing the head it was evident that excess oil was getting into the combustion chamber. I have since fitted the correct 65-317 items which have a 1/64” dia hole (one thirtysixth the area of a 3/32” hole if my sums are correct!). I really thought this might be the problem but alas it was not to be.
I have fitted a new Triumph-type clutch and have not detected any slipping. Nor with the engine sprocket cush drive. Oh, and the brakes are not binding.
I had two B31’s as a youngster, a road bike and a field bike, and both were pretty clapped out but performed far better than this one. I don’t know what to try next – if any of you BSA single specialists out there have any ideas I’d love to hear from you.

[idie]

You should be able to get about 70 mph from a B31. You are asking a lot if it has just been rebored, have you run it in yet? The tappets aren't set to tight, they should be able to rotate when turned. Have you checked the gearing? Some people think that by raising the gearing the bike will go faster. But with only 17 bhp on tap rather difficult. I think your engine sprocket should be a 17 tooth. It is easy to get the valve timing out by a tooth, I have done that on my B33. Check that the choke is off and that the carb jetting is correct.

[martinh10]

Thanks idie, Yes I think I've covered all the things you've raised. Tappets can be quite difficult. Mine vary by up to .003" when rotated but I always make sure they are .003" at the tightest point. All gearing is standard. Cheers.

[Gadorey]

Shall be following with interest. I‘m having a very similar problem with a 500cc OHV Sloper.

Here‘s a link to the thread. There might be some ideas in there you can try.
https://www.bsaownersclub.co.uk/bsaoc_forum/index.php?topic=10083.0

I would check if your fuel flow is ok. If there‘s a partial blockage ( for ex debris in the tank plugging the petrol tap‘s mesh filter) your carb might not be getting enough fuel at high speeds.

With mine, the only thing left to check is the valve timing - with a timing disc. Who knows if the cam wheels are really the originals?  Maybe the timing marks don‘t match correctly…

[MGI]

Check the sprockets as Idie suggested, not just the engine sprocket, the easiest one to check is obviously the rear wheel sprocket. I am having similar problems with a B33 and I am now about to check the revs when flat out as I think it may be overgeared!.

[idie]

Fuel flow may be a problem. I had a problem with my Shooting Star wouldn't go above 60 mph. I t was the float needle hanging up I changed the needle and seat which solved the problem. Not much help to you as you are running a 276 and not a monobloc.Check the filters on the taps, is it better with both the taps open. I the mag giving a good spark at speed.

[Pete C]

I have a B31,  B33 and M21 and the B31 is the only one that really seems to thrive on revs, and needs them to get the best out of the performance . Until your's is run in and the engine spins freely the performance may be a bit lacklustre. Your previous clapped out bikes would not have had the running in requirements 
Not sure where your oil is coming from. As you have new guides etc it may just clear up when the new rings have bedded in fully.

I hope you get it sorted, B31s are brilliant bikes and a lot of fun.

Cheers
Pete

[Bess]

Hi,
    Sounds like the timing needs slight a slight retard due to modern fuels, heres a disc (CD size) with the timings for you if it helps.

Best wishes...

[idie]

I have found that with modern fuel my B33 runs better with slightly more advance. By retarding it you run the risk of less power and overheating. If the advance is to much you will know by the tinkling sound as it pinks.

[martinh10]

Many thanks for all your suggestions. The only thing I've not tried so far is advancing the ignition a tad more. I'll do that next. I'm thinking that whatever is causing the problem can't be something major given the amount of work that has been done and the inherent simplicity of the B31 engine and ignition system. I'll keep you posted with any progress.

[Bess]

Hi,
     Here's my experience albeit with a twin, hope it helps someone.

I was running in a newly restored A65 using 99 octane petrol with slightly larger jets, needle raised 1 notch to keep it slightly rich, the timing spot. The engine was performing as expected but getting hot, plugs showed a lean mixture, exhausts deep yellowing, no pinking, the oil thinning so much I couldn't keep the oil pressure. The performance was good.

I decided to return the carbs back to specification, gave them a clean, balanced them and checked the timing which was still spot on. After riding the bike warming the oil I drove up a 2 mile hill and couldn't get the power as before, changing up and down the gears experiencing similar to what you described. At the top of the hill on the flat the bike sounded like a tin of nails and the oil pressure low so I took it easy coming home. It didn't sound like it was pinking more like the top end was starved of oil.

Next day checked the plugs a nice brown colour and the timing spot on. I checked the oil feed to the top end and changed the oil as it was multigrade, the engine perhaps destroyed the polymers. Having read SRM recommended a 2 degree retard on the A65 , I retarded the engine 2 degrees. (In the past I believe SRM recommended different timings to BSA on the introduction of unleaded, I have only found the settings for the twins).

Bingo, bike performs like a dream, not hot, oil pressure staying as expected and the engine sounds sweet.

Best wishes...

[martinh10]

Taking note of the above advice from both Bess and Idie I decided to set the recommended full advance setting of 7/16” BTDC midway on the handlebar advance/retard lever using a magneto static timing light (www.themagnetoguys.co.uk). This gave me about 11/16” BTDC with the lever fully advanced and 1/8” BTDC fully retarded. I’m not sure what this is in degees as I didn’t use a disc and it’s difficult to calculate (I tried and failed a while back).
I’ve been out for a test ride and would say 7/16 seemed best (+ or – 1/16 I suppose as the lever positioning was not very precise). Performance is still very disappointing.
The plug colour looks good but within 20 miles with new oil there is an accumulation of oily sludge on the piston crown and the oil has become dirty. On rebuilding the engine all parts were meticulously cleaned, particularly the oilways. I thought changing the rocker oil feed banjos to the correct 1/64” hole size parts might have solved this problem but evidently not. I’ve checked that the oil return pipe from rocker to crankcase is not blocked and that the timing case breather is working correctly.
Given that the engine has had a rebore, new piston and rings, new valves, guides and springs I am inclined to think that the oil problem is due to incomplete combustion rather than oil getting to where it shouldn’t. Also, there’s no smoke from the exhaust under acceleration. I also replaced the cams with NOS genuine BSA items because I thought that the original cams were not standard (they were as it happens but hey ho). There was no change in performance.
Which brings me back to the magneto. Whilst it looks like a good spark with the kickstart is it struggling under higher revs/compression – how can you tell?
I’ve checked the gearing again. It’s all standard: engine 17t, clutch 43t, gearbox 19t, rear wheel 42t.
I’m still somewhat mystified by this but I’ll get there eventually. Any advice/suggestions/comment/expressions of sympathy welcome.

[Bess]

Hi,
     Doesn't sound like the mag. Have you checked the needle position, it maybe worth trying with the needle lower 1 notch and test and then raise 1 and test.

Best wishes...

[martinh10]

Hi Bess, Yes I've tried the needle either side of middle position which seems about right. The standard main jet is a 150. I've also tried a 160 and a 170 but reverted to the 150. At the moment I'm really struggling to see what it could possibly be.

[idie]

It could be the mag. As the revs increase so will the compression this can blow out the spark or at least weaken it so you don't get complete combustion. I have played this game with my B33 in the past. Getting back to basics. You have a non resistive plug. the same with the cap, also copper cored HT lead. If all this is correct try closing the plug gap down to about 12 thou and try. This makes it easier for the mag. If this works then a mag overhaul may be needed. With an old mag it is not just the windings and condenser that are the problem sometimes it is loss of magnetism that can be a problem.