SHERPA+B25 engine

[arthursal]

Hi folks,i'm new to this forum,i have a 1973 sherpa 250,i also have a BSA B25 in bits,what i need to know is can i swap the bultaco engine for a B25 engine.I rebuilt the sherpa engine twice first time fitted a Sammy Miller crank rebuild kit,this lasted 773 miles,then got my brother to machine an exact replica heat treated properly. Low down engine speed is fine but on the road the engine seems to 4 stroke i have the original exhaust pipe (fisrt long & second triangluar)fitted,or is there a way to make the sherpa run smoothly???. Any help will be appreciated. Yours ARTY

[woody]

It's up to you what you do obviously as it is your bike, but if you put a B25 motor in it there are two main consequences. First, the bike's value will probably go down the pan as if you ever wanted to sell it you have to hope there is someone who wants to buy a hybrid like that. Second is the performance. The Sherpa engine is much better than the B25 and although you have had problems with yours, it is much more reliable and requires much less maintenance.

The crank shouldn't have failed because of a quality issue as I'd guess that all the dealers have probably sold the the same kit for years. Perhaps it wasn't assembled corrctly or the engine took in dirt or whatever, but they are reliable as a rule.

Not sure what you mean when you say that you've had an exact replica machined (conrod kit?) so I can't imagine what if anything, that may have to do with the 4 stroking.

The first thing to do is throw away the triangular silencer. They are fine for appearances on museum pieces but strangle the motor if you are using the bike. If you're not bothered about it looking original put it on ebay. Most people threw them away when the bikes were new. Then take it back on the road and find somewhere to run it near flat out for a while to see if it will burn of any gooey residue that may be in the silencer. That sometimes works wonders for a 2 stroke motor that has got choked up.

There are other reasons why it may 4 stroke such as choked up main silencer, choked airfilter, worn carb components or incorrect size jets, points and/or condensor.

Try it without the silencer first and take it from there, checking the carb jets/settings and replacing the points and condensor before looking at other possibilities.

A long shot is that it may be burning clutch oil through the crank seal(you'll know from white smelly exhaust smoke depending on what oil is in there) which could also cause 4 stroking.

Whatever it is, sorting it is going to be a whole lot easier than barstardising the poor thing with a B25 engine.

Edited July 27, 2011 by Woody

[02-apr]

I agree with what was said above, it should be easy to get a Bultaco to run properly.

Or go for broke - back in the 80s a pal put a 500 Triumph in a Bultaco frame and rode it in nationals. Mind you he also put a Saab motor in a BSA frame.

[b40rt]

I agree with what was said above, it should be easy to get a Bultaco to run properly.

Or go for broke - back in the 80s a pal put a 500 Triumph in a Bultaco frame and rode it in nationals. Mind you he also put a Saab motor in a BSA frame.

Had to be Andy ?

[02-apr]

Nope - John Scott.

[sherpa325]

Hi, cranks can easily be bent[put out of round] when the cases are pulled together, if not done correctly. So this may be the cause of your problem, the crank should always be checked for correct running after assembly. As Woody said the original triangular muffler is very restrictive and should be replaced with a later boomerang or clubfoot muffler. The four stroking can be caused by a worn needle and needle jet in the amal carbie. The motor will start and run OK but splutter and carry on from about half throttle upwards and no amount of fiddling will fix it other than replacing the worn parts. Bully motors are very simple, reliable and virtually bulletproof as a trials motor so I would persevere with the original. Hope this helps Greg

[arthursal]

It's up to you what you do obviously as it is your bike, but if you put a B25 motor in it there are two main consequences. First, the bike's value will probably go down the pan as if you ever wanted to sell it you have to hope there is someone who wants to buy a hybrid like that. Second is the performance. The Sherpa engine is much better than the B25 and although you have had problems with yours, it is much more reliable and requires much less maintenance.

The crank shouldn't have failed because of a quality issue as I'd guess that all the dealers have probably sold the the same kit for years. Perhaps it wasn't assembled corrctly or the engine took in dirt or whatever, but they are reliable as a rule.

Not sure what you mean when you say that you've had an exact replica machined (conrod kit?) so I can't imagine what if anything, that may have to do with the 4 stroking.

The first thing to do is throw away the triangular silencer. They are fine for appearances on museum pieces but strangle the motor if you are using the bike. If you're not bothered about it looking original put it on ebay. Most people threw them away when the bikes were new. Then take it back on the road and find somewhere to run it near flat out for a while to see if it will burn of any gooey residue that may be in the silencer. That sometimes works wonders for a 2 stroke motor that has got choked up.

There are other reasons why it may 4 stroke such as choked up main silencer, choked airfilter, worn carb components or incorrect size jets, points and/or condensor.

Try it without the silencer first and take it from there, checking the carb jets/settings and replacing the points and condensor before looking at other possibilities.

A long shot is that it may be burning clutch oil through the crank seal(you'll know from white smelly exhaust smoke depending on what oil is in there) which could also cause 4 stroking.

Whatever it is, sorting it is going to be a whole lot easier than barstardising the poor thing with a B25 engine.

[arthursal]

Thanks Woody,will try removing tail exhaust and giving it a burn out,think i will keep the bully engine in as you advised,will also look at the amal carb as other person said to do. What i ment was the crank journal wore out,it was not properly case hardened to the correct depth also it was made on a centre less lath with no hole going through the centre, the hole helps with the case hardening,so my brother made one to the original sizes and had it hardened where he worked as a tool maker. Thanks for all the help you have given... Arthur Mead (Swindon)

[motovita]

[quote name='Woody'

The first thing to do is throw away the triangular silencer. They are fine for appearances on museum pieces but strangle the motor if you are using the bike. If you're not bothered about it looking original put it on ebay. Most people threw them away when the bikes were new. Then take it back on the road and find somewhere to run it near flat out for a while to see if it will burn of any gooey residue that may be in the silencer. That sometimes works wonders for a 2 stroke motor that has got choked up.

What do you recommend as a replacement for the rear box on a 199A?

Edited August 1, 2011 by motovita

[woody]

What do you recommend as a replacement for the rear box on a 199A?

Nothing to be honest, what's wrong with it? They, or the earlier 'banana' type, are a good replacement for the triangular one on the earlier models.

As long as it isn't clogged up it should work fine. The section that runs up to the shock is perforated tube and the packing can get clogged or just disintegrate and get blown out. It can be cut open and repacked but it's a messy job. From the shock to the outlet (ie; the box section) it is full of baffle plates, presumably for the job of spark arrestor, so there is nothing to repair in there as long as they haven't come loose, I can't see how that chamber can get blocked up. As a mod, the plates can be cut out and the perforated tube extended to the outlet, effectively giving it the same internal arrangement as the banana type which is tube all the way through. The outlet just needs to be changed from 2 to one larger hole, as below

Edited August 1, 2011 by Woody