LCGB Forums

Just running in & hopefully dynoing an RB20 engine for a Bristol LC clubmate who's got his hands full, both with full time work & the even fuller time job of a young son. The second I took the engine for a spin it was obvious that the clutch wasn't coping. gently rolling on of the throttle worked up to quarter(ish) throttle, after that it was straight into auto engine sounding squeal zone, ie. screaming like a b*stard & going nowhere fast/at all. The 4 plate, easily compressed between thumb & forefinger springs, came out & a Readspeed 'Red Devil' (nice bit of kit! ) 5 plate clutch, assembled in the same order it came in, fitted. Thing is even though everything feels right at the handlebars, biting point, cable routing was fine on the RT lump that came out I've got exactly the same issues at the same throttle openings. Obviously the clutch is coming straight out & everything will be examined with a microscope. I'm just wondering whether there's something I may be missing, or if anything else (highly unlikely as far as can see) might be the cause of these symptoms...? With many thanks in advance!

Check the spring and the ramps on the front sprocket too.

rossclark wrote:Check the spring and the ramps on the front sprocket too.

Good call. I have had this a couple of times. I notice that Casa Performance have now produced a 10% uprated front sprocket spring too.

Thanks gents! I do remember looking at the cush end of things, without knowing why, when I was fitting the new clutch & everything looking fine... Progress so far? Zorst & sidecasing off again, the what I assumed to be & still believe to be historical (lack of ali glitter in the oil) gashes in the crank end of the chaincase still look suspect but tomorrow will tell... In the meantime hypothetically, how might a weak/possibly broken cush spring/any other weak links at the front drive end give the symptoms being experienced...?

The sliding dog and front sprocket ramps are supposed to be sufficiently compressed by the spring to stop them passing over each other, ie slipping. But springs weaken over time and they were never made to deal with the power of such engines in the first place.

When this happens, you get “ The second I took the engine for a spin it was obvious that the clutch wasn't coping. gently rolling on of the throttle worked up to quarter(ish) throttle, after that it was straight into auto engine sounding squeal zone, ie. screaming like a b*stard & going nowhere fast/at all.“ in my experience.

If you have a new sliding dog, compare the height of the ramp with the one on the engine. This may indicate whether it is worn out.

Having said all this, it may be the clutch. Do Reedspeed do different strength clutch springs? It may be worth giving them a bell.

Is the Red Devil the one with capacity for up to 10 springs? I was freading about on on the SKUK site recently.

Nope, just the 5 spring unit Colin.

UPDATE! Well, well, well no sign of operator error but the sliding dog is indeed more f*cked than the local crack whores (& that's saying something...). I can't believe that issues weren't manifesting themselves in the engine's previous Mugello incarnation

Someone of a less neo Luddite inclination will be along at some point to post some less than pretty pix. With thanks again

Nice one. If it was my RB, I’d try the Casa spring too.

I often wondered why no one had made an uprated front sprocket spring.

Posted on behalf of Andy BLC. Sorry I couldn't put them up earlier mate, Tess and Issy's birthday so been busy with them.

paste image host

Er that's your problem!

Having had a bit of experience with RB's, particularly RB20's as long as they're in good order, standard cush/springs are fine, if it is slipping it'll be down to the fact the sprocket/sliding cush surfaces are worn/damaged, which yours clearly are.

dscscotty wrote:Er that's your problem!

Innum... That be West C*ntry talk for the perhaps more universally understood 'innit'

Just to reiterate, this engine was not built by me, attention to detail issues like the lack of a circlip at the bottom end of the sidecasing clutch rod etc... wouldn't ever (!) pass...

Working backwards... The top end was built by someone whose work I respect, the leak-down test kinda proved it..., he however had no hand in previous work on this engine, nor the quality control involved...

With many thanks to Solid Air Mark for continuing to put up with me & hoping that this post helps a bunch of peeps in the future, as surely that's why we do this... Right?!

Right!

The fivers 'in the post' m8

Having subjected the cush drives to the rigours of racing, I've never had OEM's ride over the cams like the one depicted. That said, I loaned my good 14T sprocket out so bought another non OEM for Three Sisters (when it was a shorter track) & lost drive after a few laps of practice. The cam of the sprocket was worn away so badly the drive cam just rode over it. The gearbox oil was metallic soup.

BTW, the 14T I eventually got back, was made from an OEM 18T @ Apple in Thrapston.

A useful trick to preload the cush is to use the S1 top dished washer which is shallower.

It might be useful to mention that there is no need to faff around trying to align the dished washer splines. Just tighten up until you meet a little resistance, then a copper faced mallet on the dished washer will align it, which you can tell when the mallet starts bouncing. Then torque up the bolt.

A useful trick to preload the cush is to use the S1 top dished washer which is shallower.

We cured a gearbox breaking problem by reverting from S1 type back standard well matched cush drives, our 40bhp group 6 runs a standard cush and checked regularly to ensure good matching of surfaces,

dscscotty wrote:

A useful trick to preload the cush is to use the S1 top dished washer which is shallower.

We cured a gearbox breaking problem by reverting from S1 type back standard well matched cush drives, our 40bhp group 6 runs a standard cush and checked regularly to ensure good matching of surfaces,

That's good to know. Just in case I ever build another......

Just because a procedure I adopted worked for me, doesn't make it the best....

I can't help but notice that a clutch based cush drive has become available.....@ some considerable cost. I had thoughts on a hydraulically damped slipper on the DRIVE side of the chain. I used to run a roller tensioner on the top run on occasion, dependent on the sprocket/chainwheel arrangement. Pull down as well. 30 odd years ago! This last decade I knocked up an all alloy (apart from the plain plates) clutch six plater from a Japanese bike, for less inertia, to help with the breakage of gears.

Racing was very innovative & secretive then

Lol, it still is,

Evenin' all. Well I wasn't expecting to be back on this topic, thinking that fitting all new components (bar the cush drive sleeve, the splines were A1 OK, I imagine if this were totalled so would the crank) would have resolved the problem WRONG!

To be fair I'm not sure about the heritage of the components or where they came from apart from the sliding dog which was an out of the packet Scooter Restorations S.I.L. item of mine. The front sprocket was a good fit, spinning freely with no play, the spring was sitting proud of the sleeve, so theoretically fine, while the Innocenti 'etched' top plate looked like the Innocenti lettering had been applied with letraset...

What I am sure of is that the first test ride resulted in the same symptoms at the same throttle openings, this is starting to get a tad tedious... I truely don't see the clutch being the problem, a lovely smooth feel to it that makes me less worried about my long term riding career (Taunton station 1 , Andy 0 - different story )

After another strip down the Casa uprated spring seems to be the next option, though obviously Scotty's above & beyond experience of RB's would make this an overreaction... So 1st step another strip down to examine the components YAWN... This was only supposed to be a run in & dyno kinda fing....