Removing carbon

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David Lahey
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Re: Removing carbon

Postby David Lahey » Mon Dec 26, 2016 4:27 pm

Many 1970s trials bikes have a steel main exhaust chamber that has two walls. The outer wall you can see, and an inner wall that is made from perforated steel sheet, that conforms to the shape of the outer wall. Between the walls there is a layer of mineral fibre packing. It is possible to melt the fibre if you get things too hot when decoking. I don't know which Montesa models have this sort of arrangement, except for a 348 exhaust which I just rebuilt and it does not have the twin-wall design. Bultaco trials bikes do not have the twin-wall design either.
You can tell what design yours is because if you can see the insides of the main chamber and can see one or two perforated steel tubes, then it will be single-wall design, which means you will need to cut it apart to repack it. If it is the twin-wall type and made of steel, you can burn the oil out, but with care not to get the packing too hot.
Another complication with exhausts on old trials bikes is that there are sometimes fatigue failures of the inner wall or the perforated tube that the gooey internals are holding in place, but when you decoke it, the loose/broken pieces come free and will need to be repaired. This happened recently with me with a KT250 exhaust I was cleaning out.
The way the exhaust is packed on an old trials bike does have a significant effect on the way it will run. It mainly affects the smoothness and evenness of the running at low RPM, which is like a holy grail for trials bike tuners. Maybe not so important on smaller motors like the Cota 172 but is a make-or-break on bikes like the big engined Bultacos and Montesas


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Jools
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Re: Removing carbon

Postby Jools » Mon Dec 26, 2016 7:27 pm

But what can you do if the exhaust is badly oiled/coked up then? Do you just put up with it? Surely the efficiency of the exhaust will be VERY compromised anyway if the system is so oiled up.



David Lahey
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Re: Removing carbon

Postby David Lahey » Mon Dec 26, 2016 11:04 pm

Jools wrote:But what can you do if the exhaust is badly oiled/coked up then? Do you just put up with it? Surely the efficiency of the exhaust will be VERY compromised anyway if the system is so oiled up.


I thought that I said in my previous post that you can cut them apart and repack them?

Alternatively you might be able to buy a new ready-made (aftermarket) one from Spain. They are made for some of the 1970s Spanish trials bikes

Before you do anything rash I suggest you find out for certain what, if anything, is wrong with your exhaust. In your first post you said there was a huge amount of carbon build-up in the exhaust but didn't say if it was on the perforated tubes, on the inlet or on the outlet. Does it rattle when you shake it? What does it sound like when you tap the outside? How heavy is it? Can you see the carbon? Do you have access to a fibre-optic camera to look inside with?


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Guy53
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Re: Removing carbon

Postby Guy53 » Tue Dec 27, 2016 1:02 am

The second TY 250 TS exhaust I burn out was proof for me that it made a difference, I did it in the middle of a ride. I like a '' clean '' rev and a predictable throttle response. What I felt : before the burn out, power was good but not steady. I'm not saying it was '' unridable '' but just annoying ; after the burn out, at whatever rev. you crack the throttle and power come on. I mean TY power not jet power. I have to admit I felt more the improvement when riding in the mountain than at trial event. Another thing, I'm a little crazy, I keep record of things, I don't have to change the sparkplug as often as I use to. I wish I could have weighted the exhaust before and after, but I don't believe there was big difference. This spring ( here in Quebec ) a Wes exhaust will be on. I keep my finger cross, I hope it's worth the investment.

Guy



JAM RACING
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Re: Removing carbon

Postby JAM RACING » Tue Dec 27, 2016 6:13 am

yes it will make a difference.......but whats it doing .....Two stroke engines are in effect a two stage compression ......the intake being on the bottom of the piston and the ignition...power stroke on the top.......Any thing that influences this will reduce its performance . once it leaves the exhaust port it goes into the EXPANSION chamber and then through a tortured path muffler....larger restriction of the muffler in effect change the performance of the engine and change the gas/air flow timing as the exhaust doesnt get away......



Stanm
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Re: Removing carbon

Postby Stanm » Tue Dec 27, 2016 9:57 am

Hi Guys
When i burnt out my exhaust I did not realize there was an inner layer of packing. I put it on a raging fire and it burnt it out to much. It run very poorly and needed the main jet reduced. The exhaust felt really light but much noisier. I cut a hole in it and found the packing gone. So I gave this exhaust the flick, used a spare and put it down as a lesson learned.
I then saw Dave Ryan cleaning exhausts using one of those small hiking burners with the blue refills and he lets it run through the pipe for an hour or so. The burner is on the ground and the pipe upwards. It is a much gentler method and would probably take the coke off the baffles. We copied this and used a small hiking metho burner burner we had. It still gets pretty hot once the build up starts to burn and again not sure if it suits all exhausts. My son used this method on his modern alum exhaust with good results. Be prepared for lots of smoke and take the washing in!!!

I am running low on exhausts so I may try cutting the old exhaust open and repack it. David L what packing have you used?
Hope this helps.


Cheers



David Lahey
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Re: Removing carbon

Postby David Lahey » Tue Dec 27, 2016 10:36 am

I haven't repacked a TY yet, have only done Montesa and Bultaco repacking and used the loose wool from the local bike shop wound around the tube/s and held in place by steel wire.
I have seen photos of a twin wall exhaust - I think it was a TY mono exhaust - being repacked and it looked like they used the exhaust packing that comes as a mat, split to get it thin enough.
I have a TY250 exhaust and a KT exhaust that have had the mesh come loose inside and will probably be having a go at fixing them so would be interested in how you go with yours.


relax, nothing is under control

Stanm
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Re: Removing carbon

Postby Stanm » Tue Dec 27, 2016 1:14 pm

Thanks David. I will add this job to my non urgent job list. May never happen but if it does I will let you know the result.
Cheers



JAM RACING
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Re: Removing carbon

Postby JAM RACING » Tue Dec 27, 2016 4:59 pm

stainless brilo pads......steel from the kitchen when shes not looking



Phil 850
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Re: Removing carbon

Postby Phil 850 » Thu Dec 29, 2016 2:00 pm

Here's a photo of the complete exhaust
Image
The first pipe on the right is the header pipe and had about 3mm of oily carbon buildup and should be relatively easy to clean out with a blow torch and wire rope in a drill.

The second part is the main muffler/expansion chamber. This is reasonably heavy and has a perforated inner tube through it. You can see the oily carbon and some of the holes I have uncovered.
Image

The next little box section I can't see into so I'm not sure if it's just a resonator or something. It's fairly light so I guess it's hollow.

The final stinger has a perforated tube through it and has insulation between the outer case and the inner tube. I know this because I had to weld a patch onto it where it rubbed through from a PO.
A blow torch and wire brush should clean this out.

Nothing rattles in any of the pipes :D




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