taffmeister Posted March 23, 2015 Report Share Posted March 23, 2015 (edited) Hello I run a sammy miller framed Bultaco which has a 1975 PM124 engine fitted. it came with a 1971 front wheel and probably the 1971 forks and clamps as well also a Montesa rear. So, I've managed to grab a barn find: a 1975 PM125 350 and will be using the forks and wheels off this to get the age of it right. alas the offset is no different between the two bikes and already the front is a nightmare when I trial the bike. the front drives on or tucks under so I reckon I need less offset (and more trail) in the clamps. yesterday at a Dabbers trial, after much measuring the nearest i can get is a set of SWMs or Honda TLRs. the Honda clamps look as if I'll have far too little offset. is there anyone who knows what to do here? I believe Sammy is meant to have had the right clamps made at one time? if true I'd like to find a set but if not justy knowing what the flip I need for the right offset would be a huge help! thanks Taffy Edited March 23, 2015 by taffmeister Quote Link to comment Share on other sites More sharing options...
woody Posted March 23, 2015 Report Share Posted March 23, 2015 I don't think there is any difference in offset / rake between any year of Bultaco yokes and if so, changing them won't alter anything with the steering. With your Miller frame, it would be hard to know whether it had '71 parts originally and had a '74 engine fitted or other way around. It could have had anything in reality I've never heard of Miller doing a yoke set, his frames just used the standard Bultaco parts. He used to do a top yoke to replace the earlier original Bultaco item that was considered weak, but no full set as far as I know I've never ridden a Miller frame but heard people say they're an acquired taste. I could never understand why, having declared a Bultaco steering to be the optimum for trials, he produced his own frame with a different head angle... Your front end problem is more likely due to 40 year old springs not being up to the job any more. With correct rate springs and damping working properly, the forks shouldn't dive and cause the problems you're having. If you fit yokes with less offset the chances are the front mudguard will foul the exhaust on compression as they come pretty close as standard. The other problem with older Betor foks is that the piston on the damper rod is just plain alloy and doesn't have the fibre ring that acts as a seal like later forks do. If the piston wears oil can just bypass it and travel between the piston and inner wall of the stanchion instead of being forced up the damper rod and through the valve. Your M150 forks may have the fibre ring, not sure when it was first fitted. If so you can replace this by cutting new strips from the correct thickness PTFE card available on ebay. This will improve damping effect. You can judge the effectiveness of the piston by placing the damper rod in the top of the stanchion and just letting it go into the tube. If it falls freely to the bottom you can be pretty sure there is room for oil to pass between the bush and the inner wall of the stanchion and that damping effect is being lost. With new seals fitted you have to p[hysically push them down the tube, the difference is marked. If your pistons don't have the fibre ring you could get new ones machined with a groove in them to take a seal. Easy job for someone with a lathe. Picture below shos two damper rods, one with, one without fibre seal on piston 1 Quote Link to comment Share on other sites More sharing options...
jc2 Posted March 24, 2015 Report Share Posted March 24, 2015 (edited) Taffy, it's not entirely clear from your post exactly what the the front end is doing - "driving on" or "tucking under" could be two different things ie have diff causes, or one could be causing the other. "Driving on" can be caused by wrong weight distribution (eg incorrect footpeg location) or poor front tyre. I've never ridden an SM framed Bul but the early one's were said to have been improved by relocating the footpegs. "Tucking under" is usually associated with steering geometry, rake & trail etc which you are onto. Most Bultaco pursang/frontera/matador/alpina triples have about 20mm less total offset* than Sherpa T ones since they don't have the angular offset built in. But as Woody says, you'd likely need a new header pipe fabricated to give clearance, & they will also reduce your wheelbase nearly 20mm too. Thats a big difference in both trail & wheelbase. Sometimes tucking under can be 'cured' or at least reduced by fitting shorter shocks or lowering the ride height at the rear (less preload or softer springs). That's perhaps what I'd be trying first. Conversely if your bike has been fitted with longer shocks that often worsens tucking under. Or if your front end is sagging that can worsen it too. * Edit: total offset is offset at the axle, taking into account the extra angle built into the Sherpa T triples/yokes Edited March 25, 2015 by jc2 Quote Link to comment Share on other sites More sharing options...
taffmeister Posted March 24, 2015 Author Report Share Posted March 24, 2015 (edited) well oddly enough the rear shocks are shot and I can't speak lowly enough about the present forks. there's no oil in the rear and not only that but someone welded eyelets above the top of the std fitment thus jacking the rear. fact is, if the headstock has been sharppened 4d then the offset MUST be reduced accordingly. it's about 2mm to each degree and so I need to lose 8mm and I figure the offset right now is 36.5mm give or take with the 1971 set. '71s don't have a top pinch bolt and have a taper to the top of the legs. the 1975 set that I'm preparing to fit seem to have an offset of 30mm. this 30mm therefore needs to become 22mm give or take. it should explain that I'm getting the double whammy of a jacked rear end AND on a bike that needed 8mm less offset before that happened! I'd have liked to try a set of SWMs if I could get them measured more accurately than running around the paddock poking a vernier where it doesn't want to fit! as for space to the zorst well I check that everytime I pull the front end down on the trailer and at full travel there's 10mm still so I fear nothing there. thanks for the advice on the internals, i'll have a look at that? being 15 stone do I want to lose 25% travel stood or 33% on the forks? new rear shocks coming up.... but I daren't buy them till the offset and feel at the front is partially sorted. I've got re chromed legs, steel sleeved brake hubs, new spokes, new wheel bearings and seals and I'm going to try SKF seals as I sell them and they are sooo fab on everything else! the quality is tops..... new IRC tyres and shocks will finish the job nicely. it looked so pretty on ebay................ regards Taffy Edited March 24, 2015 by taffmeister Quote Link to comment Share on other sites More sharing options...
taffmeister Posted July 2, 2015 Author Report Share Posted July 2, 2015 hello Woody et al I'd like to get the pistons off the fork legs but I can't seem to get the alloy tubes off one end ot the steel top hats off the other. I have WP fork rings here and will utilise them on this job. the bikes handling isn't as bad as I thought I guess.... getting really good results now - well for a beginner. I dropped 8 marks for the whole day last Sunday (3 x 15) with Dabbers MCC. I'd like to get these forks done and also can anyone identify the pair of levers on the front? they were really nice and matching but one broke? I have had to spend good money on the Mikuni VM26. new atomiser, needle, slide, choke button, slosh cap, PJ, MJ, fuel valve. you name it, it's had it! now I'm told to go for a 130MJ but several have said it seems too lean? they reckon a TY175 needs a 150MJ so these shouldn't be less? the electrex world electronic ignition works well but we have no real idea of the correct ignition timing so i want to try and set this using a strobe at 5d BTDC? anywadvances on this? at the moment we're rich with a 180MJ and RETARDED at something like 15d ATDC. i'm surprised it's running but better than that she's excellent! I had a tough clutch so backed all the springs off. been running a 30 year old rear tyre but am building up wheels with steel liners, new wheel bearings and seals, spokes, rim tape, tubes and tyres. there's a few bob! the front end easily twists with the old '71 yokes in - but what can you do hey? more to follow. regards Taffy Quote Link to comment Share on other sites More sharing options...
woody Posted July 2, 2015 Report Share Posted July 2, 2015 The alloy tubes at the bottom of the damper rods are just a push fit. If they've not been removed for years they will be properly stuck on. A couple of things to try. Heat them and use a blunt screwdriver or such like to gently drift them off. Or put the damper rod in the vice vertically, upside down, resting the top side of the alloy tube on the jaws of the vice. Screw a much longer allen bolt into the damper rod a few turns and then after heating the alloy tube tap the allen bolt to push the rod out of the tube. There is no standard jetting for the Bulto on the VM Mikuni so it's trial and error unless someone with the same carb can provide the size they use. No use comparing to the TY175 as it is a different type of Mikuni. A 180 main jet is too big. 130 could be right but I'd buy a 130, 140 and 150 and see which it runs best with. Not all bikes run the same with the same set up. Can't help with timing the Electrex, but it should have a standard timing mark? Standard Sherpa timing on points can be anywhere between 2 - 3.5mm BTDC depending on whether you want sharper or softer throttle response off idle. No idea what that equates to in degrees. Obviously there is no advance with the points and I'm not sure what a modern electronic advances to from what. Advancing to 5mm BTDC sounds a bit extreme to me? Be careful backing the clutch springs off as they will foul the inside of the case if you go too far, although the '74 models usually had a pin retainer for the springs rather than nuts. The later bottom yoke won't make any difference to the fork twisting as it's more to do with the mudguard brace which doesn't look as though it could provide much rigidity on yours. However, the aftermarket braces available these days don't provide that much either. Quote Link to comment Share on other sites More sharing options...
taffmeister Posted July 2, 2015 Author Report Share Posted July 2, 2015 whato woody i will now try your idea now I know that the top alloy parts CAN be removed! I did say "5d BTDC" that was degrees mate! I believe this ignition system has 22d of advance someone said if I recall. so 5d plus 22d = maximum advance of 27d BTDC. this is a guestimate as whoever said it - I can't recall. the clutch on my PM124 has nuts. I also have a duplex primary chain. crossover gearchange shaft for gears on the left which I'm told in 1975 was rare. the crossover shaft has smaller splines than later Bultacos so that a Bultaco left-side gearlever can't be used. I need a pair of rear shocks now. can't decide what to get. jetting and ignition will be modified this Saturday. thanks again for the advice. regards Taffy I need to go buy the SM chrome toast rack I guess for rigid forks. Quote Link to comment Share on other sites More sharing options...
woody Posted July 2, 2015 Report Share Posted July 2, 2015 Not sure why I read that as 5mm BTDC.... The 124 / 125 models are from '74, not '75 and the crossover gear shaft wasn't introduced until about '77 on Sherpas. Yours will have been modified which is a common mod on the Pursangs. The clutch is usually a single row chain on Sherpas with pin spring retainers until around '77, so sounds as though someone has changed the clutch too - although not inconceivable that Bultaco did some Sherpas with duplex row clutches. Not sure which shaft has been used for the gear change though, as the splines on all Sherpas are the same in my experience, the same gear lever fits the splines on all of my bikes Even with new mudguard braces, you will still get some twisting on the front - all of mine do, regardless of model Quote Link to comment Share on other sites More sharing options...
taffmeister Posted July 17, 2015 Author Report Share Posted July 17, 2015 here is the result of the testing to this point. Mikuni VM28mm carb. electrex world ignition fitted 'in the middle' as requested. total wan&^%$. just been jetting my Bultaco 250 Sherpa for the last 6-weeks. fantastic voyage! 290 MJ 260MJ 230Mj 200MJ 180MJ ignition retarded 160MJ C3 to c5 c5 to c1 c3 grade 6 plug 145Mj 45 to 30PJ c2 c1 retarded the ignition (now the correct timing using strobe 6d BTDC at idle) grade 7 plug 130mj needle c4 c2 c1 c3 25PJ 45PJ (all I had) 30PJ slide cutaway 3.15 down to 2.25. skimmed slide (by someone else) so needle in c4 is infact in c3. call it where it is here though. 30PJ 25 17.5 22.5 still miles to go but already a totally different bike. try a 20PJ and then go to: need to try a 2.5mm cutaway and a 25-35PJ get an anti-slosh cup design a MAJ test MJ test again with a 125 and a 135MJ. needle up and down regards Taffy Quote Link to comment Share on other sites More sharing options...
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