feetupfun

I am in the process of getting my 348 working well and would like some advice about the issue with the plastic bushings in my forks. After reading Martin's posting I wondered of the problem I am having with my 348 forks might be caused by that same issue. My problem is that when the forks bottom they feel quite harsh in comparison with when the forks bottom on my OSSA MAR and Bultaco M49. The action of the 348 forks is otherwise fine. Tonight I tested the action of one 348 fork leg without the spring in it and it did stick a bit at the bottom end of the travel. I pulled that leg apart and the plastic sleeve seemed to be a neat fit in its mating part. I measured the clearance between the parallel sections of the parts at between 0.1mm and 0.2mm and the plastic sleeve is also slightly belled out on the bottom end. I then measured similar parts from a set of Bultaco Betor forks and the clearance between those parts was 0.5mm. I believe the function of these parts is to provide a hydraulic anti-bottoming function that is effectively very heavy compression damping that happens in the last 25mm or so of the fork stroke. Am I right in assuming that the clearance between the Montesa sleeve and mating part should be more like the 0.5mm that I found with the Bultaco forks? Note the Bultaco parts do not appear worn or damaged in any way and are both made from aluminium. The Montesa plastic sleeve mating part is made from steel.

Another thing is the type of riding style you have. If you generally ride gently and want to maximise traction and control, go for the softest springs you can. If this is you, then aim for about 50% of the travel used (front and back) when you do the Glenn test. Some other people like jumping off big things. They are usually willing to compromise a bit of traction and control for the sake of not breaking wheel hubs and spokes, and so they opt for springs on the stiff side to avoid harsh bottoming. If you are like that, then aim for 30% to 40% of the travel used up when you do the Glenn test.

Further to what Glenn said, if when you do what he has said, you end up with the preload set so high that the shocks are topped out with no rider aboard, that is telling you that the springs are not stiff enough. Betors come with 40 or 50 pound per inch springs. If your bike is a standard TY175, then I would suggest as a guide that if the rider is over 90kg to use 50 pound springs and under 90kg to use 40 pound springs. If your TY175 shockie mount locations or footpeg locations have been customised, then that spring rate recommendation may not apply. For bashplate's TLR, the rear suspension leverage ratio and footpeg position is different to a TY175 so may need different springs for the same weight of rider.

I can take a photo of mine but it wont be until the weekend and no it is not for sale

Yes the brake pedal height, and fore/aft distance relative to the footpeg is very important in trials. Reaching either way is a disaster for your position on the bike. Anything is possible when it comes to modifying brake pedals. While you are at it (lowering the pegs) have a think about what you are going to do with the stand mount too. One option for OSSAs is to fit a wheel that has the brake and sprocket on the same side so you can do away with the cross-over rod and bowden cable which makes for a sub-optimal brake action on the standard MAR. That also opens up options for how you make the brake pedal fit in with lowered footpegs.

In support of this, US Dirt Bike magazine did a series of dyno runs with a two stroke MX bike and found that the more oil there was in the premix, the more power it made. They went all the way to a very high oil concentration (something like 15:1 fuel to oil) and the pattern remained

I just measured the dish in an original TY250 sprocket at 4.5mm, which is pretty close to 3/16".

fergie are you saying a 428 sprocket is 3/16" thick? or that the spacer should be 3/16" thick?

I'll have a guess the it means that the handlebar mounts are in front of the steering axis

Guy I can't say I'm any smarter than you but I do remember that the thickness you need for a sprocket spacer on a TY250 is the same thickness as the sprocket which from memory is 1/4" That is based on the offset of an original Yamaha dished sprocket. There are probably lots of TY250s out there with bent swingarms that may need a different thickness spacer. Regards David

A fixed setting is a compromise to suit the whole RPM range. For trials bikes it is usually chosen to provide the best performance at low to mid RPM. On an MX or road race bike it is chosen to provide the best performance at the top end of the RPM range. An ignition that has the correct curve for a motor and application gets rid of the compromise

There was an OSSA MAR bashplate that just finished an auction on eBay in Australia - no-one bid on it Yes there are modern replacement ignitions available ex the UK for a few hundred dollars There should be an engine number stamped into the aluminium on the top side of one of the front motor mount lugs I dont remember where the frame number is but can have a look at mine after work tomorrow if no-one else can help

Very nice Andrew

They will all bend if the bike gets crashed the right way. Another thought is that modern Betors are not that much better in damping action than original OSSA MAR Betor shocks so don't expect a huge improvement. I quite like the OSSA MAR Betor shocks I have on one of my Yamahas Some modern shocks will leak oil if the shockie is put into axial bending as the suspension moves (due to not being mounted properly). Some modern shocks need the gas pressure maintained to make the seals work properly. This has also led to some people having problems with oil leaking from the shocks. I have had a very good run from four sets of Falcon Classic trials shocks. The only problems have have been from bending one (due to crashing), and the bottoming cushions only lasted a couple of years. The damping action of the Falcons is noticably better than the 1970s OSSA MAR Betors.

Yes changing the springs like you said is fine and you shouldn't need to do any adjustment of anything.

My M198 gasket appears the same shape as the upper gaskets in the photo - ie both the inner and outer edges make full circles

The intention of fitting reeds to Sherpa Ts, is not to make the motor more snappy or more powerful. It is to make it run more evenly at low load and low throttle opening combined. Dare I suggest those of you who haven't yet directly compared riding a Sherpa T both with and without reeds, you should have a go and see for yourself before you say how silly the idea is. Bultacos are not the only Spanish trials bikes that can benefit from reed valves. Some OSSA MARs have also been converted to reeds. The benefits are such that that some recently posted new rules I saw for trials bike eligibility in Spain, specifically excludes the use of reed valves in pre-72 class. Having said that, my Bultacos do not have reed valves and I have no intention of fitting reeds (or lightening the flywheels) as I love them just the way they are.

Yes Sherco reeds are used here in some late model Sherpa Ts which have been modified relatively recently. There are also conversions using other reed cages, and I learned recently that converting Sherpa Ts to reed induction was being done here during the 1970s.

Yes my M198 head has no recess and the sleeve end is flush with the barrel.

My M198 came with a copper head gasket. It was the first time I had ever seen a head gasket on a Bultaco and assumed it was standard. What's the go? Is it meant to have it?

Its a pretty common mod on late model Bultacos where I live. Yes it improves the even-ness of the firing.

It should be possible to machine off the existing mounts, leaving flat areas for new upper and lower clamp halves (and spacer blocks) with bolts going right through and nuts underneath. Ive seen this done on a few twinshocks other than TL250s Alternatively you can get bars custom made with a higher rise than 6" Renthals.

I have a DT175E bottom end that has a TY175 flywheel and ignition stator fitted so I guess that is one of the DT175 motors that has the same taper on the LH end. Are you really asking if they are the same crankshaft or if their crankshafts are interchangeable?

Oxy-LPG is used for oxy-cutting, not welding, and welding is what most home welders want to do.