feetupfun

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.

Mmmmmm...you are probably right to share it

Might not work that well once someone from BOC reads your post

Easy fix - tighten the bolt Slightly less easy fix - weld up worn part of hole and file back to size/position

Mark if you pull the flywheel off we should be able to see if it is the same ignition as something else contemporaneous for which there are modern ignitions available And yes, fitting an electronic ignition does away with the need for a condensor Regards David