Hello all, I am after some information about modifying the middle box on my Sherpa as I want to cut it open and repack it anyway. I have seen some mention of modifications to the middle box to improve the exhaust but no one has said what to do to them so if anyone knows could they post it on here. Photo`s would be nice. Also would anyone have the measurements and photo`s for a Comerfords/ Reg May rear engine mount for the 199B. The off season is almost here. The swingarm extension mods would be nice as well but I can work that out as I go. The engine in my 199B is a 151 engine. I just replaced the fork springs with some from Bultaco UK and the difference is unbelievable. The new ones were about two inches longer than the old ones and one of the old ones was a bit shorter than the other. It let me throw all of the spacers I had in the forks out as well. Very good improvement. So if anyone can help I would really appreciate it. Thank you. Bully Lover.

The "mid-section" of the exhaust, mmh ... as I have seen from the inside there is just a hole tube with insulation inside the "mid-section", the diameter is a little bit smaller as the manifold, so it's just an absorber, reducing the noise and the speed of the exhaust gas flow, like a diffusor for a classical 2-stroke exhaust.

The gas fumes will then go further in the "rear section" of the exhaust system where there is again a hole tube with insulation slowing down the gas speed.

Then starting whith the boardening of the club-foot design of the rear exhaust comes a plate or "plate-wall" closing the opening but with a small opening where the exhaust fumes have to go through, while also beeing reflected in the opposite direction. The reflected gas will recharge the pistonchamber with unburned gas-air mixture from the exhaust side which have in the meantime got out the outlet port due to the flushing loss.

Then follows some chambers that are connected again by small openings or tubes leading the exhaust gases through the muffler. This system of tubes and chambers works as an Helmholts resistor in physical terms. The resistor is again for slowing down the exhaust gas speed and noise.

If you will do any change to the mid section you have to mdify the rear section of exhaust too, both parts depend on each other.

There are some very good articles that where published in the German Trialsport magazine in the begining 80's about these exhaust systems, how they work and how to set them up if you are interested PM, (warning contains a lot of formulars). I personal have got through them too, but decided the effort would not be worth my riding skills.

You can minimze the Helmholtz resistor system a little bit at the end section (only the outer side!) which will result in a better air flow (= more engine live/response) but you have to compensate it with an adiitional absorber to get the noise as low as before.

To the Reg May modifications to the swing arm you can look up here: works Bultaco

Here a cut through the rear part of the silencer:

The lower picture shows the outer side where changes can result to a positiv effect if you know what you are doing, (just calculating might not result in a positiv effect you need experience too.)

Maybe some other user might post more useful information, anyway this is an very interesting topic!

Thank you for that PShrauber. My Sherpa does`nt have the original clubfoot end on it. It is homemade by someone who knows what they are doing. It has an aluminium end on it that weighs about a quarter of the weight of a standard one. I am not sure what is inside it as I have not opened it up yet. It needs repacking as well. Going from memory does`nt the front box have two perforated tubes in it. I have not pulled a front box apart for about 28 or nine years so I probably am wrong. I will PM you for the articles as they do sound interesting. I was after the measurements of the rear engine mount as that would make it a lot easier to modify the frame as my rear frame already has the top shock mounts moved and the rear frame uprights moved forward at the top. As you can probably tell I think I have to much time on my hands. Thank you. Bully Lover.

Edited October 25, 201114 yr by bullylover

The middle box mod, at most, would be fitting a slightly bigger diameter core. It is straight through already, one perforated tube about 35mm diameter (from memory)

The later clubfoot back box, as shown in the pictures, are half perforated tube/packing, half baffled. The bottom half is perforated tube with packing up to the shock. After the shock it is baffled. They are modified by removing all the baffles and making them straight through also - like the older 'boomerang' silencer fitted to the '73, '74 era bikes. The clubfoot is essentially one of those anyway but with the baffled box added as a spark arrestor for USA market. The twin outlets are also replaced with a single outlet of around 25 - 30mm diameter. I'd assume this is the diameter of the new perforated tube inside.

With the swingarm mod, it was essentially moved as close as possible to the engine. It is self governing really as you casn only go so far before th spindle is up against the back of the engine. Unless you copy JR's bike where they did away with the swingarm spindle and just bolted the swingarm either side to extended frame plates. You had to take the engine out to tak the clutch cover off....

It's debatable these days how much benfit it would give as with modern shocks, the standard set up probably works just as well. I have Magicals on mine and it is very good with loads of feel and normally, the Sherpas feel 'dead' at the back, even when they are working. You will definitely need a longer swingarm if you move the spindle forward. They also tipped the engine up at the rear (or lowered the front) A lot of work for maybe little or no perceptible gain.

Early version of Vesty's bike top photo, his last bike bottom photo

JR bike

Thanks Woody, the photo`s are really good. They are the best ones I have seen of Vesty`s bike. You can see the rear engine mount in the last one of Vesty`s bike. I am still not sure if I should do that. The fact that my bike is already very modified is leaning me towards doing the mod.Decisions, decisions. Some more opinions would be good. As far as I know there is only five 198/199B`s over here so there is not much information on them. I can remember reading that it is supposed to give them a lot more traction. Bully Lover.

To repack middle muffler -

http://i589.photobucket.com/albums/ss338/BULTACOBRIS/Muff2.jpg

That isn't really an effective way of doing it. The only way is to cut the entire back off so that it is split in half. That way the core is exposed and you can see what you're dealing with. The core may be clogged up, broken or even rotted and require cleaning or replacing. If it is clogged, you have to bodge or drill out the caked on carbon from all the blocked holes.

Cuttin a small window isn't the way to go as it doesn't reveal the true state of what's inside. Neither can you effectivly remove all the old packing and replace with new.

Hello all. Today I cut my exhaust open,the whole inside off, and cleaned it all out. The perforated tube in it was the same size as the header pipe. Tomorrow I will repack it and weld it up and see how it goes. Unfortunatly I can`t ride at the moment due to dropping a piece of ply on my big toe and breaking the end bone in three places, OUCH! Thank you for the help. Bully Lover. PS. Would still like to hear opinions on the swingarm mod though.

Edited October 27, 201114 yr by bullylover

That isn't really an effective way of doing it. The only way is to cut the entire back off so that it is split in half. That way the core is exposed and you can see what you're dealing with. The core may be clogged up, broken or even rotted and require cleaning or replacing. If it is clogged, you have to bodge or drill out the caked on carbon from all the blocked holes.

Cuttin a small window isn't the way to go as it doesn't reveal the true state of what's inside. Neither can you effectivly remove all the old packing and replace with new.

I always think that as you weld it back up (gas)at least half the packing gets burnt and your back where you started !

I always think that as you weld it back up (gas)at least half the packing gets burnt and your back where you started !

What we used to do was weld a bit up and pack a bit more, as the packing always seems to get blown out over packing seems ok.

Without a doubt any bully lover will be able to tell one that sounds sharpe and one thats repacked and softer on the ear.

I had a standard 340 modified by reg back then and I am sure it was better.there is definately more movement at the back and as woody says with todays shocks they are better anyway.

There is better clearence for muddy trials and the steering is sharpened a tad so yes if time permits its worthwhile.

However as far as bultacos and 340's perhaps more than any other its meticulous and sensible upgrades which make a good bike rather than just a better swinging arm set up.

Stupid little things like reducing steering stops to the max, shortening the clutch cable to tangent pull etc will make the bike more rideable.