MK 6 Bultaco 125

[jerry]

Im on to one before I commit can anyone give me some history. On them. I think they appeared just before the Japanese onslaught and were outdated overnight. Is that correct? Did Gary Flood race one with some success? Thanks Jerry

[JC]

They were a bit dated when they came out- mainly cos of weight. The Mk5 before it was fast & 1 of the 1st 125s to have little/no flywheel, but they were unreliable. Their claimed max power was at 10,500rpm & at that rpm rear piston skirts broke - probably when you missed a gear. So for Mk 6 they 'solved the problem' by putting large (for a 125) flywheels on it which made them feel slow.

But I'm lead to believe the problem was the bottom of the Mk5 inlet port was flat, & even tho bridged it caught the skirt at very hi rpm. They changed the inlet port design on the Mk6 to an unbridged one with 'dished' (concave) bottom but also took the extra precaution of adding considerable flywheel weight.

The Mk6 actually had much better porting as they also added an extra pair of transfers & they make decent power but cos of the flywheels they were considered rather slow & by then were also overweight for a 125 so they were not very competitive. The Mk7 used the same barrel as Mk6, but lost a lot of weight, had closer ratio g'box & I believe less flywheel weight than Mk6 (but more than Mk5) & were almost competitive, so they can be made to work. The Mk8, tho an entirely diff barrel, had similar inlet port to Mk6 &7, & none of the MK6,7 or 8 had the piston skirt probs of the Mk5 IIRC.

Gary Flood probably did race one, no doubt modified, but that seems to be about the time he ceased racing 125s.

A 250 top end bolts onto the 125 pursang bottom end & is a very competitive bike if you wanted to go in that dir'n..
 

[jerry]

Your comments are much appreciated JC thanks for taking the time! I picked up on the 250 top end thing. Are you saying head, barrel, piston will work on the 125 lower end? What about gear ratio's are they the same for either bottom end? If that's true it makes the exercise much more appealing as I believe all the running frame, wheels, suspension are the same and then I just have to chase a 250 top end and put green plates on her! Jerry

[Moto]

Hi Jerry,
           I just had a look at my numbers book and found that from Mod 30 Sherpa-S till the Mk 6 Mod 100, 125's Pursangs all had the same gearbox ratio's, (and the 250's and 360's till Mk10) the Mk 7 Pursang 125 was where the ratio's changed for the 125.
          There was no Mk 8 125 and the Mk 9+10 125 Pursang had a gear primary and a six speed box.
          The torque/power of these engines was also varied as John says by using different ignition flywheel and primary drive weights.
           The Mk 7 was the first with chromoly frame ,hence the weight difference over the Mk 6.

[jerry]

Moto did I read your post correctly. The bottom end (ie same gearbox) is the same for 125, 250 and 325 for MK6? Cheers Jerry

[huskibul]

    Very intesting ! Without doing the homework are the rod /bigend/ little end the same on all size's ?  on MK 5/6 would you need to change rod or crank (or both) for each size 125/250/325 ? cheers

[Stan S]

Gday Jerry,
I certainly remember Gary racing a Mk6 125 and he was very fast on it, but in saying that he was fast on anything that he rode and also Bert had a way of making the slowest bikes very fast. I had a new mk7 125 and 250 in 74. Bert did my 125 motor the same as Gary's and he also put it on methanol the same as his but the thing that we couldn't buy for mine was his tallent.  My understanding was that all the mk6 bottom ends were the same but i can't swear to that. I reckon you should grab it if it's reasonably priced Jerry. The mk6 may not be the most collectable model but their not making any more of them.

p.s "Moto", what did you mean when you said there was no Mk8 125? I certainly bow to your knowledge of all things Bultaco but I thought I bought one on the way to CD7 which you would have looked at up there.
Also "Moto" your crankshaft jig is being picked up from me today.

Regards Stan

[number8]

Gary Flood did ride a Mk7 125,that my Dad brought off Bert in 74 I raced it in Juniors and we had to Lend it back to Gary for the last round of the Vic championships so that he could win the series,the bike was as you would expect very quick and easy to ride,had a lot of extra's in the barrel like transfers and holes in the side and rear of the piston.had a GP alloy tank because it was on methanol,and the rear shocks moved forward,they use to use the original rear shocks that were in the "conventional"position but would cut the shock bodies to make the shock shorter for the new positioning,I had previously been on a Husky 125 so once I got on this my results improved significantly,

Another good story of the 125 is the Mk8 that Bert built for Vic Allan to ride in 75 when Gaston Rahier came out we still had the Mk7 and Bert wanted to borrow the alloy tank so that he could put it on methanol for the Mk8 from memory Vic beat Gaston in one moto at the Aust titles in Tassy on that bike,Stan mat know a bit more about this,

#8

[SAABCOMBI]

last year l sold a beautiful complete 125 mk 8. and l was very silly in selling, l bought the bike off a fellow in Boronia Melbourne.

[Moto]

Hi Stan,
           You are right ,I missed the Model 144 Mk 8,It was a late night sleepy post,  I appologise for that. Is your Mk8 going to be ready for CD?
           Thanks for the news on the jig.I will buy you those beers when I see you !  I hope you were kind to Matcho Mick.

          Huskibul and Jerry,
           The bottom ends are similiar for all Mk 6 sizes 125,175,200,250.The parts book shows the conrod kit 125 to 250 all using the same part no.They all use 60mm stroke,just different bore sizes.The little ends all use a 16mm diameter pin but with different lengths.The crankshafts wheels have different numbers which I think relates to the different drillings for balance on the different size engines.
          People used to just drop the 250 cylinder/piston and head on the smaller engines in the old days and I don't think the balance factor caused too many problems as the bike had just doubled its power so there were smiles all round,who cares about a few extra vibes!!
The Mk5 125 and 250 parts book shows a mixture of crank parts used,It was a changeover model and a lot of the part numbers are from older models(use up old stock?)The Mk 5 250 can use the same con rod as the Mk 6.
           The MK 5+6  325 also have the 60mm stroke,but the cases are  bored out more at the base of the cylinder to allow for the 83mm bore.They also have a larger big/little end bearing.Not sure about the size of the crankshaft wheels.This capacity engine is a bit of a different animal again.

           I guess to summarise you should be able to drop any 125 to 250 cylinder/head combo onto any Mk 5 or Mk 6 (not 325) possibly with a few extra vibes!!

           Jerry,
                  The gearbox was the same for all the engine capacities in the Mk 6 Models. Only the countershaft/rear wheel sprocket size was different.

         Cheers.

[huskibul]

    Thanks for that info moto!  saves a hell of a lot of legwork chasing it up and gives a lot food for thought ! cheers   ;)and good luck jerry with the 125/250 if you decide to get it ! imo it was an great period for the iconic bike's

[firko]

I fitted what I think was a Metralla 250 top end to my dreaded 125 Lobito and it made a reasonable difference but it didn't turn the puss heap into a monster. It turned an inadequate little bike into an almost adequate one.

[jerry]

Thanks Moto your the man! The Mk5 "250" is going to complement my MK3. Stay tuned for developments as they come to hand. PS Your spot on Huskibul it was a magic era to be sure! Cheers Jerry

[Moto]

Firko,
             I still hear the salesman at Frasers laughing.
He sold you a bike that was designed to be ridden by a 15 year old. Then you came back and bought more parts to try and get it to perform like one of the adult models that you were too tight to buy in the first place! 
             Forty odd years later you still haven't got over this fact and its still the bikes fault .The Lobito story is getting tiresome.

            Let it Goooooooooooooooooooooooooooooooooooooo

          Cheers.

[JC]

Moto is correct w his pursang info (except the Mk8 M144 oversight)

However some 125 bottom ends can't take the 250 top end without opening the spigot in the crancases, but I think its only the early 125s (Sherpa S's etc). The 250 spigot is 80dia, whereas some 125s are only 75dia. However I just checked my Mk6 M100 barrell & crankcases & its 80mm.

There are definitely diff holes in the 125, 175 & 250 cranks to balance diff piston weights, as I have checked before. Not much diff between 125 & 175 but a fair bit between 125 & 250, as you would expect.

Altho the 325 uses same 60mm stroke it uses slightly larger dia crank halves & 20mm little-end (vs 16 for 125 & 250) & larger bigend (22 or 24mm IIRC vs 18 for 125 & early 250). Despite that, Bert Flood used to sell a kit to convert 125s all the way to 325s (still using the 125 rod!!) but its a LOT of work & not very effective. There's an article on it in an early Trail & Track.

Tony, you may remember Neil Harnsworth racing a SherpaS w such a conversion at the Six Mile track in Rocky back in the day. The conversion was done by Greg Day in Gladstone (to Floods instructions) where I used to work in the school holidays.

The std Mk6 125 was also 'strangled' w a 27mm carb, whereas the earlier 125 Buls had 30mm. Popular Cycling (PC) ran an article on modifying the Mk6 125 & they got over 20hp on the dyno from 7500 to 10,00rpm, peaking at 22hp. Pretty impressive. The changes weren't all that radical either. My friend in Bris had his barrel ported to the same specs on his Mk7 (same barrel) & won the Qld 125 VMX title on it several yrs ago. I rode it & it was pretty quick - comparable to an Elsie & much sweeter handling. He had a pipe built to the PC spec, but found a stock Mk6 pipe better than the PC or Mk7 pipes.

One thing to remember if you go the 250 top end route. Even tho the M100 had 4 transfers, it was a diff layout to the 250s 4 transfers. The M100s transfers are only about 27mm wide at base gasket/crankcases whereas the Mk6-8 etc 250s are 48/50mm. (IIRC, Mk6-8 all used same M103 Mk6 250 barrell. Moto could probably check that in parts books.) However all is not lost. The M100 125 crankcases have the larger castings to take the wider transfer ports, so they can be opened up w a dremel or die grinder.