40mph in 4th open the throttle and the revs increase but the bike doesn't accelerate.
Slipping clutch? However inserting a piston locking bolt and standing on the kickstart I can't make the clutch slip,it's quite solid.
Is there anything else apart from the clutch that can slip?
Sprockets are all in good order.
Quote from: Janacek on 18 Jul, 2026, 19:07 40mph in 4th open the throttle and the revs increase but the bike doesn't accelerate.
Slipping clutch? However inserting a piston locking bolt and standing on the kickstart I can't make the clutch slip,it's quite solid.
Is there anything else apart from the clutch that can slip?
Sprockets are all in good order.
Most likely clutch.
Odd thing I've experienced twice is sliced woodruff key on the gearbox mainshaft taper. But that is with me giving my ABSAF 600 some stick, but that has aluminium hub.
I doubt that your B31 could be doing that. Thinking laterally your engine shock absorber could spin but the spring would have to be pathetic and you'd feel the ramping of the cam.
Pretty sure your clutch.
I have the same problem Janacek. I fitted a new Triumph-type 4-spring clutch to my B31 during a full restoration and have been troubled ever since with the clear impression that road speed is not proportional to engine speed in any gear, despite none of the usual indications of clutch slip. I cannot induce clutch slip by standing on the kickstart or by giving it full load at low speed in a high gear.
I recently fitted a cheap electronic tacho, the type that clips to the HT lead. It showed that engine revs at 60 (roughly top speed) were more than double revs at 40! Clearly something somewhere is slipping.
I have to agree with Neil that it is unlikely to be the engine shock-absorber, my spring is good and you would feel/hear it ramping. My woodruff key also is intact. I think I can rule out tyre on rim slippage so there is nothing else but clutch.
I have stripped the clutch several times, cleaned it, checked plates for flatness, tried it dry (no oil in chaincase), made sure everything is correctly assembled - no joy.
I'll give it one more go - really wind up the compression in those springs and try it without the chaincase cover.
Last resort may be buying a different clutch or even reverting to 6-spring. Which is yours by the way, 4 or 6?
I Agree with Neil. I had the same issue with my B32 last year, slipping in 4th if I gave it a handful going up hill. The previous winter I'd had the clutch assembly off but when putting it back together it seems that I hadn't tightened the nuts sufficiently. The manual gives no guidance as to how tight they should be.
So this winter past I had to remove the primary chaincase and tighten the nuts a bit more. I tightened them by a further two turns and that seems to have done the trick. I can feel that the lever is now a little harder to pull but no sign of clutch slip.
I should add that my bike is running with a 9:1 compression and I never had any troubles kicking it over.
Quote from: martinh10 on 19 Jul, 2026, 11:59 I recently fitted a cheap electronic tacho, the type that clips to the HT lead.
I'd be interested in detail about that tacho.
p.s If the tyre was slipping in the rim you'd experience an instant flat tyre when the valve got ripped out of the inner tube :o
I wouldn't go too much on trying to co-relate Tacho with Speedo. Both are likely to be only 90% accurate +- 10 mph zero error.
You could compare 60mph with full throttle, compared with 60mph no throttle.. Do the readings change.?
Derek: I tried to put a link or a picture of the tacho on here but failed miserably. They're on eBay listed as "Digital LCD Engine Inductive Tachometer" for around £17. I'm not saying they are any good but the readings corresponded to what I was sensing. Perhaps it wasn't clear but I was not being serious about the tyre slipping by the way!
scifi: No, the readings don't change between full and no throttle, and I would expect they would with clutch slip.
Quote from: martinh10 on 19 Jul, 2026, 13:10 Derek: I tried to put a link or a picture of the tacho on here but failed miserably. They're on eBay listed as "Digital LCD Engine Inductive Tachometer" for around £17. I'm not saying they are any good but the readings corresponded to what I was sensing.
Thanks Martin. I think this is the one you are meaning. LCD Inductive Tachometer (https://www.ebay.co.uk/itm/387204838589?_skw=Digital+LCD+Engine+Inductive+Tachometer&itmmeta=01KXXA9MVVJKZS17DYY9VHT80T&itmprp=enc%3AAQALAAABAGfYFPkwiKCW4ZNSs2u11xBvv9iYIrReFd0ICdDtHuRy2xu0rIx1ZSsAVXc29mzWDjHcBwoKMyitKjLzj%2FpTKqRxHjDYrJHz42GGMcFOcp%2F8QemEGoep2myBAePQhKMTo3IC9yzzAYAIzRQdFL4Aq9RJa6l5qReIkvSZ98Ms74KNVrOvgKBVPfAQcijoOHg6FcacETwx6q8GKtUcnxkuMayAB%2BNC6UuKeGMqNdIjE67d7gXTDjMXetGPJitBdpFDl5HPJp4hspmYCo1wLP1Fx%2Bcdj0S8Z40My2RxIyIlWz0DsoT6IwXkAimQAy6jqaId44UgWXnrloqXFU2eERZL230%3D%7Ctkp%3ABFBMpM6mqu9n).
I'm just looking for something to give me an idea what revs the engine is doing at various speeds and that should do the job. It would be nice to find something looking like the Smiths ones for a more permanent fitment but they don't seem to exist for an inductive attachment.
Quote from: martinh10 on 19 Jul, 2026, 11:59 I have the same problem Janacek. I fitted a new Triumph-type 4-spring clutch to my B31 during a full restoration and have been troubled ever since with the clear impression that road speed is not proportional to engine speed in any gear, despite none of the usual indications of clutch slip. I cannot induce clutch slip by standing on the kickstart or by giving it full load at low speed in a high gear.
I recently fitted a cheap electronic tacho, the type that clips to the HT lead. It showed that engine revs at 60 (roughly top speed) were more than double revs at 40! Clearly something somewhere is slipping.
I have to agree with Neil that it is unlikely to be the engine shock-absorber, my spring is good and you would feel/hear it ramping. My woodruff key also is intact. I think I can rule out tyre on rim slippage so there is nothing else but clutch.
I have stripped the clutch several times, cleaned it, checked plates for flatness, tried it dry (no oil in chaincase), made sure everything is correctly assembled - no joy.
I'll give it one more go - really wind up the compression in those springs and try it without the chaincase cover.
Last resort may be buying a different clutch or even reverting to 6-spring. Which is yours by the way, 4 or 6?
My clutch is the six spring type.
Draganfly have the friction plates but not the inserts for the chain wheel.I will have a search on the web for the inserts.
The six spring clutch is not reckoned to be very good,but has been fine up to now,with no drag or slip.
To add,my six spring clutch has three friction plates and friction inserts in the chair wheel.
Hi,
If say at 40mph in top gear your B31 was doing 2500 rpm but doing more than double at 60mph at say 6000 rpm would it not be very obvious that the clutch was slipping. Do B31s rev that high? I would suspect that the cheap rev counter is not reliable. Also, if this happened several times then it is likely that the clutch plates will be 'toast' and the primary chain case oil will be black and smell burnt.
Dave.
Found only one supplier of clutch inserts on the web.
£8-99+postage for 24 new old stock BSA inserts.
Have ordered them and will look for branded friction plates,as Draganfly only have unbranded ones at present.
One of my pals was plagued with a clutch slipping issue last season - and after multiple strip downs and much fettling it came down to being due to the oil he was using in the primary drive. Not the plates, spring tension etc etc. I think he had heard somewhere to use some modern fancy oil - and this was the cause of his trouble. (Next time I see him I'll ask him for further details...)
Quote from: Tigerfeet on 19 Jul, 2026, 18:46 One of my pals was plagued with a clutch slipping issue last season - and after multiple strip downs and much fettling it came down to being due to the oil he was using in the primary drive. Not the plates, spring tension etc etc. I think he had heard somewhere to use some modern fancy oil - and this was the cause of his trouble. (Next time I see him I'll ask him for further details...)
It's quite possibly the oil.I was using the same stuff in the primary chain case that I used in the engine,(10/40 semi synthetic)Now having stripped the clutch I will replace the friction plates,and when the inserts I ordered for the chain wheel arrive,if they look ok I may order inserts for all the friction plates.It will be about half the price of new friction plates
Hi,
I have used mineral auto transmission, the red stuff, in 3 bikes for many many thousands of miles.
Dave.
Quote from: Janacek on 19 Jul, 2026, 19:50 Quote from: Tigerfeet on 19 Jul, 2026, 18:46 One of my pals was plagued with a clutch slipping issue last season - and after multiple strip downs and much fettling it came down to being due to the oil he was using in the primary drive. Not the plates, spring tension etc etc. I think he had heard somewhere to use some modern fancy oil - and this was the cause of his trouble. (Next time I see him I'll ask him for further details...)
It's quite possibly the oil.I was using the same stuff in the primary chain case that I used in the engine,(10/40 semi synthetic)Now having stripped the clutch I will replace the friction plates,and when the inserts I ordered for the chain wheel arrive,if they look ok I may order inserts for all the friction plates.It will be about half the price of new friction plates
That will be your problem. Only ever use mineral engine oil or ATF trans fluid in the primary. ATF not AQF!
Quote from: Janacek on 19 Jul, 2026, 19:50 ...It's quite possibly the oil.I was using the same stuff in the primary chain case that I used in the engine,(10/40 semi synthetic)...
Be wary of using modern oils in an old engine. These engines have no real oil filter and over time deposits will gather in nooks and crannies within the engine and probably even more so in the bottom of the oil tank. Modern oils have detergents that will loosen off all these accumulated deposits which will then be distributed around the engine and can cause damage to bearing surfaces. Unless the engine was been stripped and the internals thoroughly cleaned, also the oil tank, then detergent oils are best avoided.
Personally I would only ever use Surflex friction plates. Having had 6-spring niggles in the past I rebuilt my clutch with new double row bearings, new plates, new (genuine NOS springs) with a 5-plate basket (Gold Star scrambler). I used new nylon lined cable and new pivot. The clutch is perfect. Light, no slip, no drag always find neutral even when hot. I actually do use modern Castrol 10/40 as used in bikes that share clutch/engine and gearbox oil (GP50 in engine).
If I could only get my NEB in the 600 to work as well without dragging!!
Quote from: neil1964 on 20 Jul, 2026, 10:34 Personally I would only ever use Surflex friction plates. Having had 6-spring niggles in the past I rebuilt my clutch with new double row bearings, new plates, new (genuine NOS springs) with a 5-plate basket (Gold Star scrambler). I used new nylon lined cable and new pivot. The clutch is perfect. Light, no slip, no drag always find neutral even when hot. I actually do use modern Castrol 10/40 as used in bikes that share clutch/engine and gearbox oil (GP50 in engine).
If I could only get my NEB in the 600 to work as well without dragging!!
Looked up Sureflex friction plates,and the price! Almost twice the price of unbranded ones.Having fitted many pattern parts I have not been impressed with the quality and fit of most so I suppose I will have to open my tight Yorkshire wallet and buy decent quality.
Quote from: Derek996 on 20 Jul, 2026, 10:17 Quote from: Janacek on 19 Jul, 2026, 19:50 ...It's quite possibly the oil.I was using the same stuff in the primary chain case that I used in the engine,(10/40 semi synthetic)...
Be wary of using modern oils in an old engine. These engines have no real oil filter and over time deposits will gather in nooks and crannies within the engine and probably even more so in the bottom of the oil tank. Modern oils have detergents that will loosen off all these accumulated deposits which will then be distributed around the engine and can cause damage to bearing surfaces. Unless the engine was been stripped and the internals thoroughly cleaned, also the oil tank, then detergent oils are best avoided.
Thanks for that Derek.I used to have a canal boat and the same reasoning for using modern oils was put forward.Although most newer boats favour Japanese diesels there are many older ones using Lister,Perkins and BMC diesels.Mine was BMC and used any modern 20/50 oil.I did change oil every 50 hours and had no trouble.Others used Morrisons Classic 20/50 or Halfords classic oil.
Thinking of what you posted,I will use Halfords or Morrisons in the bike.
There are different numbers of plates and depths of baskets. The B31 having 3 friction plates and the Gold Star 6.
If your basket will allow maybe go for 4 friction plates (you may need longer springs). Dragnfly have a good page with the various types of
I have priced up replacing my six spring clutch with the later four spring one;more than £385 from Draganfly.
The problem I have with my six spring clutch is the friction inserts for the chain wheel are unavailable.
Rather than stump up £385 I will make my own chain wheel inserts.
Quite difficult finding material thick enough (8mm) but I found a couple of suppliers.
Question is;there seems to be three types, natural cork,nitrile bonded cork and nitrile neoprene rubber bonded cork.
Which to use? anyone know?
This is what an AI response says:
AI Mode conversation: Which is the best motorcycle clutch insert material, natural cork, nitrile bonded cork or nitrile neoprene rubber bonded cork?
Nitrile neoprene rubber bonded cork is widely considered the best choice among the three for motorcycle clutch inserts, particularly for vintage restorations or custom relining.
You can read the whole answer here (https://www.google.com/search?q=Which+is+the+best+motorcycle+clutch+insert+material%2C+natural+cork%2C+nitrile+bonded+cork+or+nitrile+neoprene+rubber+bonded+cork%3F&rlz=1C5CHFA_enGB849GB851&sourceid=chrome&ie=UTF-8&amc=1&aep=42&cud=0&source=chrome.crn.rb&udm=50&mstk=AUtExfBvu0hd69GI9shp0HVt9uhn1GDav7NuAEPVnfEegiR1pTGBkc_1V7-Ln5fQvpVs-Ih6pC86TK2NeV6DCLhQAraIFb1y3X-ST9R2qj9C1T0m9SSwDmEcdY6LM_QnTQy40FVSE8oga-KIKLxan0ESyTfRuJxfk28fSof8v6u-eJDd7-jNy0PzONQkeQqtXpY_SnEscHfiAuOTOo4IgwSp8JAzaI0yn92ZoduYfz6Xsa5tLChQnSyucxuWrw&csuir=1).
I used plain cork in the past, but some used to be woven, perhaps asbestos? So its worth being careful with old stock in case its very old. I suspect eh asbestos were the best of the lot.
Make a jig as pressing them in all the same amount is tricky.
I suspect the whole problem is caused by the oil This is not a "wet clutch" in the true sense as its not submerged, its a dry clutch that gets oily. I don't have the B31 torque curve to hand, but I suspect the sip is at peak torque which would make sense.
I would expect the "original" oil to be SAE 30 filled to just below the clutch basket. Now ATF is a good option as its designed to lubricate gears but also allow the brake friction bands to grip.
More clutch woes! 1957 B31 six spring clutch.
Replaced clutch springs,cleaned clutch plates and road tested. No slipping! However the clutch action is now very stiff and "notchy"I think the cable needs replacing as the plastic outer cover is broken in a few places.
Replacement clutch cables seem to be 2mm inner,but the one on my bike is 3mm,with a 5mm outer.This is 5mm metal outer where the plastic has broken off.
What is the correct diameter inner clutch cable supposed to be?
Looked at a Venhill clutch cable replacement kit and their inner is listed as 2mm.
Quote from: Janacek on 30 Aug, 2026, 12:10 ... However the clutch action is now very stiff and "notchy"...
Have the plates worn grooves in the clutch basket?
Quote from: Janacek on 30 Aug, 2026, 12:10 What is the correct diameter inner clutch cable supposed to be?
Looked at a Venhill clutch cable replacement kit and their inner is listed as 2mm.
I don't know what it should be. Mine is 2mm but I made up the cable myself.