BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: martinh10 on 09 June, 2021, 21:15:38

Title: B31 Poor Performance
Post by: martinh10 on 09 June, 2021, 21:15:38
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.
Title: Re: B31 Poor Performance
Post by: idie on 09 June, 2021, 21:59:58
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.
Title: Re: B31 Poor Performance
Post by: martinh10 on 09 June, 2021, 22:29:44
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.
Title: Re: B31 Poor Performance
Post by: Gadorey on 10 June, 2021, 06:29:32
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…
Title: Re: B31 Poor Performance
Post by: MGI on 10 June, 2021, 09:07:09
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!.
Title: Re: B31 Poor Performance
Post by: idie on 10 June, 2021, 09:18:47
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.
Title: Re: B31 Poor Performance
Post by: Pete C on 10 June, 2021, 09:27:44
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
Title: Re: B31 Poor Performance
Post by: Bess on 10 June, 2021, 11:09:00
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...
Title: Re: B31 Poor Performance
Post by: idie on 10 June, 2021, 11:23:39
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.
Title: Re: B31 Poor Performance
Post by: martinh10 on 11 June, 2021, 08:17:52
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.
Title: Re: B31 Poor Performance
Post by: Bess on 11 June, 2021, 19:51:49
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...

Title: Re: B31 Poor Performance
Post by: martinh10 on 13 June, 2021, 11:58:18
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.
Title: Re: B31 Poor Performance
Post by: Bess on 13 June, 2021, 16:35:12
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...
Title: Re: B31 Poor Performance
Post by: martinh10 on 13 June, 2021, 17:41:28
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.
Title: Re: B31 Poor Performance
Post by: idie on 13 June, 2021, 18:14:36
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. 
Title: Re: B31 Poor Performance
Post by: martinh10 on 13 June, 2021, 22:26:03
Thanks Idie I'll give that a try.
Title: Re: B31 Poor Performance
Post by: Calum on 14 June, 2021, 18:48:40
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.

Have you tried a smaller main? It might be too rich at wide open throttle... for any given speed, a larger bore will be drawing faster across the bridge and so more fuel. I wouldn't have thought a rebore of +.030" to have made much difference really but I know it can do. A friend had to mess about with his S1 Land driver carb jetting after only a .020" over bore (as a percentage it might be greater, 3.046" standard bore on a 2l) and it made a big difference.

I would have thought a mag would only get better with speed, and you'd get erratic missing etc not just like a speed limiting effect?
Title: Re: B31 Poor Performance
Post by: martinh10 on 14 June, 2021, 19:57:01
Hi Calum, I agree with you on the magneto - I now don't believe that's the problem. Also it doesn't appear to be running rich - I think the 150 main jet is right. Lying in bed this morning, wondering whether to bother getting up I developed another theory as to what the problem may be. I'm busy tomorrow so I'll try it out on Wednesday. Wish me luck.
Title: Re: B31 Poor Performance
Post by: tdc on 16 June, 2021, 17:19:38
My1957 BB33 hc. returned from America would die when the mag got hot .so rewound commutator .new valve springs and exhaust valve. Gold star exhaust and silencer and it performs very well indeed .I don't know if anything has been done to the bottom end but the new exhaust has made it very responsive . Good luck .
Title: Re: B31 Poor Performance
Post by: martinh10 on 16 June, 2021, 17:34:07
More disappointment! I had convinced myself (with the help of John Robinson’s “Motorcycle Tuning: Four-Stroke”) that the oil could only be getting so dirty so soon due to exhaust gas being drawn into the cylinder with the fresh gas on the intake stroke, resulting in a significant decrease in volumetric efficiency.
Given that the valve and ignition timing was known to be correct I thought the only other obvious contributory factor was the exhaust system. The B31’s I had in my youth were remarkably insensitive to all sorts of abuse of the exhaust system but I thought the high level system I had fitted to this one (see photo) may be the problem.
I still have and refitted the tatty standard low level exhaust, which at a total length of about 61” is 9” longer than the high level. I have been out on the bike today and regret to report no change in the dismal performance.
I think I’m going to throw a sheet over it for a while and get on with my RD400 restoration.

Title: Re: B31 Poor Performance
Post by: Calum on 18 June, 2021, 13:20:27
I'm running the same pipe as you (an Armours one) and I have to say it seems to go quite well indeed. In fact the bike kooks nearly identical!
I'm currently waiting for my mag to return from overhaul as it was not behaving at all when hot (ruled everything else out)
Title: Re: B31 Poor Performance
Post by: martinh10 on 18 June, 2021, 13:49:49
Cheers Calum, hope you're sorted when you get the mag back.
Title: Re: B31 Poor Performance
Post by: martinh10 on 24 June, 2021, 13:51:57
I’ve had to get back to the B31 - this lack of performance is really bugging me, especially after all the money it’s had spent on it (way more than it’s now worth). I’ve taken the head and barrel off again and again, after less than 50 miles, the piston crown has thick carbon deposits on the spark plug side/middle but nothing on the opposite side (see photo). The scratches are where I have been poking a screwdriver through the plug hole before lifting the head and the loose bits on the farside are where I have scraped some carbon off.
To me it looks like incomplete/poor fuel burn in the combustion chamber but I just can’t see any reason for that. As stated previously everything is bog standard with a new 276 carb, a really strong spark from a newly reconditioned mag, new rebore, piston and rings, new valves, springs and guides. Valve and ignition timing has been rechecked and is correct.
Any thoughts anyone?
Title: Re: B31 Poor Performance
Post by: Pete C on 24 June, 2021, 17:23:19
just out of interest, what sparkplug are you using?
Title: Re: B31 Poor Performance
Post by: Gadorey on 24 June, 2021, 17:59:26
You mention you have a new carb (276). But what size is it?
Also, you’ve made sure the decompressor is not touching the exhaust valve while the engine is running?
Maybe a valve sticking open when running, or catching causing a delay in closing?

Running out of ideas here, but very interested in what you find. 

Gui
Title: Re: B31 Poor Performance
Post by: Gadorey on 24 June, 2021, 18:12:25
Just to make sure everything is correct with the carb.

For the B31 it should be a 1” carb, 150 main jet, nr 4 slider, std pilot jet .106.
Can you confirm this is what you have?

And I know it’s probably a stupid question, but…
You are sure it is a B31 engine with 350cc? And not a B31 bottom end where the previous owner put a B33 top end on it to get the 500cc?  Because that would require a 1 1/8” 276 carb.

Gui
Title: Re: B31 Poor Performance
Post by: DAVE BRADY on 24 June, 2021, 18:24:26
Hi,

I am not a B31 expert but to me it looks like it could also be an oil problem.  The bulk of the carbon is opposite the inlet indicating that the inducted mixture is pushing any oil present in the head away and keeping that part of the piston cleaner.  It is also a bit cleaner near to the plug where the combustion should be most efficient.  Was the engine smoking much and is the exhaust valve a  good fit in the guide?

Dave.
Title: Re: B31 Poor Performance
Post by: martinh10 on 24 June, 2021, 19:09:26
OK, several replies in one here.
I'm using NGK B6HS plugs Pete which I believe are the equivalent to the originally specified champion L10S. I bought two at the same time and have tried them both. I keep thinking of buying another to try but have not got round to it. I'm past caring if I look foolish at the end of this because it was a duff spark plug. If you suggest another plug let me know.
The carb is brand new, made specifically for a B31 with all the bits you specify Gadorey. I've had it in bits and everything is spot on. It's definitely a B31. I had it rebored to 71mm +.030. Decompressor is clear of exhaust valve and valves/guides are now OK. I did have a sticking exhaust valve initially but that has been sorted. I'm keeping my eye on your sloper issue.
I agree with your assessment Dave but I'm damned if I can see how any oil is getting in there. I've lapped and blued the head to the barrel. I've made plug gauges to ensure the new valve guides are the right size and I've now got the right rocker oil feed banjo bolts fitted (1/64th dia. hole). I will try another plug but will be amazed if that fixes it.
Title: Re: B31 Poor Performance
Post by: Gadorey on 24 June, 2021, 19:24:38
The plug you’re using is fine. Even if it was too hot or cold, it would just improve the combustion but not explain the lack of performance like you are having. Even if oil is getting in there it also wouldn’t explain such a lack of performance.

Most things have been covered:
Good spark from magneto
Proper ignition timing
Good and clear fuel flow
Correct valve timing
Valves closing completely
Correct valve timing
Correct carb set up
Correct spark plug and gap
Exhaust is not blocked

Only two other things I can think of
Wrong cams
Slider not opening all the way when you twist the throttle fully open

Is there any change when you’re pulling at constant speed and play with the choke lever?

No more ideas at the moment…
Gui
Title: Re: B31 Poor Performance
Post by: Bess on 24 June, 2021, 19:53:44
Hi,
     Sorry to hear you are still having issues. You are covering much of the usual solutions, as Dave mentioned it looks like an oil issue, are the rings staggered? If so have you tried Shell V power which is 99 RON to see what difference it makes?

Best wishes...
Title: Re: B31 Poor Performance
Post by: DAVE BRADY on 24 June, 2021, 20:28:12
Hi,

A bit of a long shot but what does the bore look like on the plug side of the barrel?  Could it be that it has become glazed and is not allowing the rings to do their job.  Unusual for this to be localised but may be worth a look.  If you have an unused ring check it in the bore.  Slide it down evenly and shine a bright torch from underneath and look for light leakage.  It is possible for a bore to not be totally round or for there to be other irregularities.  Either way oil could be getting past the rings. 
Did you check the compression or did it feel OK?

Dave.
Title: Re: B31 Poor Performance
Post by: martinh10 on 25 June, 2021, 10:03:26
Gadorey/Bess: Thanks for your comments. All the things you mention have been checked out/complied with at some stage.
Dave: You say it’s a long shot but you’ve got me thinking here. The rings on the NOS Hepolite +.030 piston were each slightly pitted in one location and I decided not to use them. At the time I couldn’t find +.030 rings anywhere so on the advice of a couple of trusted suppliers I purchased +.040 rings and gapped them to suit my bore. I gave them the feeler gauge and flashlight check and, if memory serves, I was fairly satisfied that there was good contact with the bore all the way round. It’s just possible I was being a bit optimistic in thinking they would quickly bed into the bore and was reassured by the fact that compression was very good. However, it now seems to be a very possible cause of the oil in the combustion chamber, if not the lack of performance.
My trusty Suffolk supplier now has +.030 rings in stock and I’ve ordered a set – it may make no difference but it’s worth a try. Fingers crossed.
Title: Re: B31 Poor Performance
Post by: Calum on 25 June, 2021, 10:28:40
That sounds very possible. Whilst they may have touched enough to not let a feeler or light pass, enough pressure on the wall to seal is different. I make piston rings from scratch quite a lot at work (from 1" up to around 10" bore) and I tend to 'cheat' and make them bigger on the OD which means they don't quite touch all the way around until well bedded in.  But it does mean as soon as they are ground to thickness they are ready to fit - no hot work necessary. On the applications I use them in it isn't really a problem and they do settle down.
I believe the proper way is to make the ring to the right bore size and then heat them and 'set' to give the correct spring. So your 40 thou over rings won't quite be sealing properly. It should be quite obvious on the ring I should think.
Title: Re: B31 Poor Performance
Post by: Bess on 02 July, 2021, 11:33:00
Hi,
     Did you manage to solve the issue?

Best wishes...
Title: Re: B31 Poor Performance
Post by: martinh10 on 02 July, 2021, 13:20:19
Hi Bess,
On removing the head and barrel it was evident that about a 1.5” section of the +.040 oil-control ring was not making contact with the cylinder wall. So I’m pretty sure this is the cause of the oil in combustion chamber problem. The compression rings were making good, even contact all round.
I’ve now received the +.030 rings but after carefully checking the oil ring in the bore against a light source found that although it is much better than the +.040 one it is not perfect. I have been in contact with both the supplier and the manufacturer and both have been very helpful. I have returned the +.040 ring to the manufacturer for examination and he is dispatching a replacement to me. When received and tested I’ll post an update. I still have my doubts about this being the cause of the performance issue – we’ll find out in due course.
Title: Re: B31 Poor Performance
Post by: martinh10 on 14 July, 2021, 13:49:10
Not making any progress I'm afraid. No change in performance with the new rings. I've tightened the clutch springs a bit just in case there was some slip (not always easy to detect) - no change. If it was ratcheting on the crank cush drive I think I would hear it so have no reason to suspect that it is. The mag is sparking over a 5mm gap, I've rechecked the ignition timing and valve timing. Good fuel feed.
Amal have agreed to test the new 276 carb for me. It's a long shot but I'm running out of ideas.
Title: Re: B31 Poor Performance
Post by: Bernessie on 23 May, 2022, 19:42:23
Chatting to the chaps at Amal they said check the fuel level, I am running a 276 Pre Monobloc standard jets etc and my B31 runs out of puff at about 40-45mph. It was completely rebuilt by the guys at SRM with reconditioned mag, so I am following this thread with interest.