Sherpa Model 80 Engine Exhaust Query

[triple_x]

Hi

My target was to bring my Sherpa back to original Kit campeon spec.

To this end i have fitted a new middle box and a new triangular back box (both supplied by Butaco UK).

When the back box is fitted it kills all low speed response and it feels like engine gets bogged down until it about 1/8 throttle, also it does not want to start.

Middle box outlet internal dia is 22mm dia and back box inlet and outlet bores are app 19.5mm dia, would a reduction of 3mm cause these problems?

Remove the back box and all is fine, a bit loud (96 decibels) but it really works well, fires second or third kick and runs sweetly.

I do not want to be refused a start because the bike is a tad too loud, so I am wondering where to go next .

It has a model 80 air filter with new element, new Mikuni carb, absolutely no air leaks and the colour of the plug is the right shade of brown, timing is set at around 3mm.

I did a refresher day with Steve Saunders and he told me to get rid of the rear box but at the time it had a Concentric MK1 and I thought the Mikuni may have put it right and let be go back to original complete system.

I tend to believe that it is the increase in back pressure that is causing the problem.

If this is a known problem with this type of back box what is the remedy other than getting a club foot tail pipe.

any comments would be appreciated.

Regards

Martin

[woody]

The bikes run absolutely fine on the Amal (obviously, as long as it is in good condition) but those triangular back boxes were always removed when the bikes were current for the very reason you've experienced. They choked the engine.

If you want to use the bike and want it rideable/competitive, then originality often has to be sacrificed as OEM parts were sometimes a compromise. I'm assuming the triangular box was a spark arrestor for the US market to enable them to sell the bikes there. It's probably full of baffles. They probably knew that most owners would remove them to ride in competition so the choked performance wasn't an issue.

I agree that the bikes sound a bit noisy with just the single silencer, not offensively so but they don't have a nice muted exhaust note.

The options I'd consider are;

- Cut the triangular box open and see if it will take a perforated tube with packing with the baffles removed (deep breath required to cut up something that expensive....). The tube can either be straight between inlet and outlet or it could have a slight S bend in it. This secondary silencer should quieten it down but shouldn't restrict performance at all. I'd match the inlet and outlet to the middle box at 22mm. This is something I will try on my M80 when I finally get around to rebuilding it, so I've no idea of the results if it is tried, just a theory at the moment.

- Fit a 'boomerang' type rear silencer. Not original on the M80 but it will quieten it down effectively and no performance issues.

- Fit the triangular box for any noise tests and them remove it for the event. I doubt it would fail a noise test anyway, as mentioned before, they aren't obtrusive - a TY Yam has a far more 'clacky' note and they have no problem.

[hoodie2]

I've only heard of this being remedied by removing the triangular back box and either living without it or replacing with a clubfoot. In their day I think it was normal to take the triangular box off as it was known to stifle performance. My M92 frame had been modified when I got it, with a lug for a clubfoot added for this reason.

Edited March 20, 2010 by hoodie2

[triple_x]

Thank you, both.

My son took the box to work and they put a miniature camera inside.

All they found apart from swarf, was two diagonal sheets of steel plate spot welded at inclined angles with a small gap at the top rear end of lower one and the front lower edge of the top one.

Thus forcing the gasses to go up, down and up in order to escape out the tail pipe.

Not good for gas flow!

So apart from spending

[majesty320]

Hi before you do anything Id just like to say I restored the same bike a while ago. I also fitted the same exhaust parts from Bultaco Uk, but the power and the way the bike ran after fitting was fine. I also fitted a Mikuni carb maybe this is the difference.

[triple_x]

Hi Majesty

Your comments are encouraging.

I always thought that Bultaco knew what they were doing and I remember the Campeon parts being a very popular upgrade.

I also remember a lot of riders throwing the little back box away because either they did not like the way the bike pulled or because it jammed on the rear tyre if you had an off and dropped the bike on the silencer side.

I am quite prepared to accept that the bike will run with it but it is just knowing where to start, jets or timing.

The bike had a brand new Mikuni fitted two weeks ago (supplied by Motocarb with spare jets) and whilst it starts easier using the cold start lever it still appears strangled pulling from very low revs.

I realise that the pilot jet works to around quarter throttle but I have done the basics.

Marked the twist grip movement into 1st quarter rotation ( Pilot) next half (needle) and final quarter (main).

Then i have run the engine constantly in each range and removed the plug and it is the correct colour (a nice mid brown) so think the jetting is correct.

Timing I believe is correct 2.8 - 3mmBTDC

It has a new air filter.

So i am left with the belief that with the back box fitted it is restricting the breathing.

I will probably rework an old clubfoot tailpipe that I have and give that a go!

What worries me more is that we get enough stick riding off road without being a noise nuisance.

Thank you for your comments much appreciated.

Martin

Edited March 22, 2010 by Triple_X

[majesty320]

Hi I have just had a brain storm, the triangular silencer came from Hughs Bultaco in the USA it was the middle box hat came from Bultaco UK. I dont know if there is a difference between the two as I have since sold the bike.

[triple_x]

Bultaco UK supplied a model 125 rear box which is straight through and quietens it down to acceptable levels.

So off I went to round 1 of the SM Champoinship and apart from entering the Twinshock class instead of the Open class easier route had a smashing time.

The bike lasted and I had a complement from Sammy Miller on the condition of the bike and sound of the engine.

Whilst I was out of my depth on the hard route I found that in certain sections the engine would not pick up cleanly from very low revs.

Now it was so much better than before, but still not quite right.

So I am looking at the easy stuff, one rider said he thought my bike was geared too high for the tighter section of today. Gearing is 11 front 46 rear on a 520 chain.

Another said he thought the carburation was a tad weak at the bottom end (30 pilot jet).

So all in all I was very pleased to go from rebuild to finishing a National trial (all be it last) and to be nearly there with the engine.

So if any of you guys have thought on the gearing or Mikuni settings I would as always appreciate you advice.

Kind regards

Martin

[woody]

So that was you on the Bultaco. I was on the KT and we spoke at the burger van afterwards when you were commenting on how well some of the little TY Yams had gone - sections were ideal for them.

I saw you through some of the sections, although from a distance usually and I'd agree the bike was off at the bottom end, carburation or timing or both, but whatever, I saw it was enough to cause you some problems in some sections. I didn't get close enough to get a real impression of what it could be.

I remember seeing the J reg plate but again, from a distance, the motor looked like a 325 with the squared off fins, either that or a later 250 barrel and head fitted. Is the engine number an 80 prefix?

If you're going to the next round which is the Harry Perry in a couple of weeks, I'll have a ride on it if you like and see how it goes as I'm familiar with the early Bultacos and know what it should go like. I'm hoping to be riding my '72 325 model 92 at that one. It's been a long rebuild but I think it's finally there.

[triple_x]

Hi Woody

I think my main problem was being out of the (trials) saddle since the Perce Simon in 1973.

I am doing the Harry Perrey so i will be pleased to let you have a go.

This time I have entered the Open class.

The barrel was butchered by a previous owner who attempted to take every other fin out!

I think he was using a chainsaw, any way I did my best to tidy up what was left.

The engine is an 80 prefix.

A fellow rider on an 84 Montesa suggested that i take the tickover off and that helped, but as you may of noticed I was not in control!!!

A couple of things, i could not get used to the position of the back brake lever and kept missing it and whilst my previous experience told me what to do I simply could not put it into practice.

But what the hell I had a great time and both bike and I came back in one piece!!!

Look forward to seeing you on the 17th.

Martin

[triple_x]

Hi

Well nearly finished.

It has done two trials and is getting better (I wish I was).

http://lh3.ggpht.com/_BIYAknJT7JM/S9xiSKKS...20/P1010772.JPG

Runs a lot better with the new rear box and have just modified the yokes and fitted a 48 rear sprocket to try out.

Have managed to get hold of a barrel of the correct type but need to heat it up so that I can twist the liner by about 3mm to line up with the ports.

Regards

Martin

C:\Users\Owner\Pictures\bultaco 09\P1010772.JPG

Edited May 1, 2010 by Triple_X