Yamaha SR125 custom exhaust advice?

[dartman5]

Hi guys, I've decided to dedicate this summer to bringing my old SR125 back up to scratch but in terms of mechanics I'm a complete noob so I will be working almost entirely from a Haynes manual!

What I want to understand but am having trouble getting to grips with is exhausts.

A few months ago in a car park I saw a 125 cruiser (it had an L plate) and the owner had fitted some kind of custom exhaust which produced the most incredible throbbing bass note I've ever heard from a 125 and since then I've wanted to recreate that on mine, just haven't had the time or money to think about it.

I'm really fussy about this though, a friend of mine customised his XT by taking out the bafflers and slash cutting the exhaust and it sounded f***ing dreadful - imho there's very few noises worse than a loud, clangy, shouty exhaust on a bike, if you can imagine the noise I'm trying to explain. What I really want is not necessarily ear-bleedingly loud but just very deep in pitch and throbbing rather than bubbly sounding like a Harley or something, and I wouldn't want it to look ridiculous like having an Akrapovic carbon fibre racing exhaust on a 90s cruiser lol!

The thing is, I've spent so long reading about different kinds of exhausts and baffles and everything else that I've kind of made it more confusing for myself. Can anyone basically explain how a slip on exhaust or baffle works and is everything kind of cross compatible? I can't find a single custom exhaust specifically made for an SR125 anywhere so could I technically pick up something like the Norman Hyde Toga and mount it to my bike, and how would I go about doing this?

Sorry if this seems really stupid but I hope someone has the patience to explain it to me!

Cheers

[stevo as b4]

Not so sure about you making an SR/any 125 have a very deep pitch and throbbing exhaust? Have you got an example of this amazing sound to illustrate?

Most 125 aftermarket exhausts are cheaply made and nasty. Often they are bodged on by people crashing and damaging the std pipe.

What you don't want is a very large bore, short exhaust with a tiny under the engine silencer and a superbike size tailpipe.

Keep your new custom pipe std bore size from the cylinder head and make the pipe as long as possible. The baffle should be a well wrapped absorption type in a large steel or alloy skinned silencer. The baffle can actually be a few mm narrower bore than the header pipe, without it really affecting performance.

If building your own exhaust, it might be worth experimenting with different silencer outer skin materials, as there can be a big difference in sound/tone between steel, stainless, aluminium and carbon fibre. You could add an empty chamber inside the silencer to see how that affects the sound.

I'd not bother reading books on exhaust gas flow dynamics and tuned lengths as it won't matter enough to cause problems on a single cylinder 125, or major losses or gains in performance.

Exhaust design is generally a very complex process if your not designing a system based solely on noise limits, or compact fitting in a certain place considerations.

[Teflon-Mike]

A 125cc single is never going to make a beep throbbing exhaust noise... it's closer to a lawn mower engine than a Harley or Ducati, and even there.... the exhaust note they make is more to do withe the valve timing and having two cylinders than the exhaust pipe packing... It's a bit like believing that to sound like Chet Baker, you need the same kind of trumpet he played, not the same sort of skill...

Now, you admit at the beginning you know little or nothing about mechanics; you are sorting out an old SR, which by my reckoning must be pushing at least 20 years old in manufacture and closer to 40 in design, and are essentially following the Haynes to learn a bit about mechanics....

TBH that's not farr off; to write the haynes they start with a bike, pull it to bits and completely rebuild and recondition it as they go... one of the reasons they are such a pain doing scratch repairs where you dont want to pull the whole thing to bits just find the dodgy wire or whatever... but does mean that for a full-one ground up resto they can be pretty handy.... and ALL you need do is treat them like the instructions that come in an Airfix model, and do exactly what it says for each picture.... and you should end up with something that looks like the picture and works like the picture.

Doing 'projects'... bit of advice "Keep-It-STANDARD, and Simple"... work to the book, book has the answers... depart from book, with silly ideas and you are pissing in the dark, where the book has nothing to offer, and you are on a falorn hope, that other folk who have chased probably daft ideas before may be able to help you out, and not lead you even further up the garden path.

Next bit of advice.... Carrol Shelby of Cobra sports-car fame... "If it Aint Broke... DONT FIX IT", as KISS thinking, why give yourself more work you dont ned; if it works, all you are likely to do trying to fix stuff that aint broke is break it, not make it any better.

Back to the manual.... follow the instructions, no ideas required, no special parts required, comes together gives you something that looks like the pictures and aught work like the specs say it should... no imagination required...

As said at the top, the exhaust note you get out of the exhaust starts in the engine... first of all how big the cylinder, and how many, whether it's an over-square short stroke engine or under-square long stroke one, whether it is a two-valve or multi-valve design, whether it has big valves or small, whether the velves open a long distance or short, whether they open early or are held open later, etc.. which is all in the cam-timing, not the pipe-work.... then you get to the pipe-work.... the classic design lore suggests that the header should have an equal volume to the cylinder.... but a low reving engine will work best with a longer narrower header, a high revving one a shorter fatter one.... and after the header, it doesn't really matter.... the silencer is just that, a can that takes the crack out of the gas as it leaves the header.... baffles make the gas change diraction disturb the pressure waves of following pulses and mix up a bit to take out the sting and make gases spend linger mixing, packing is like ribber wall-paper giving the gas something to move to take some energy out of the gas, that's how they work, to 'silence'... all the van does is give the gas space to do that, and might act as a resonance chamber to change the tone....

Did you know, that when Mazda designed the MX5 sports car, they had a team of engineers in an acoustic lab, for a year, with an MGB and a Lotus Elan, just working on getting the exact right exhaust noise? More, that to get it, that team were in more disputes with the engine designers who were trying to meet US Federal emission controls!!

This is no small 'problem' and if a multi-billion-yen company can go to those lengths and still have to compromise their design, to get 'something' sort of acceptable... working in a garden shed with a halfrauds socket set, and looking only at the pipe-work, do you really think you can do anything that sounds 'better' than a fart in a bean-can, which IME is the best most folk effing with old 125 4-stroke learner legals ever achieve?

More stilll... who is this for?

I mean, I like the sound of a rorty motor-bike as much as any-one..... I ride a 750-four with near open 4-1 on it..... and I have had the next door neighbor come round and complain about the exhaust on my V8 Range-Rover..... "Eh? I just replaced the U-Pipe that goes over the back-axle that had rotted out?!" ... "Exacly!" He grumbled... "I have to open the window to hear it now!!" lol.... which Is slightly more perverse than most experience TBH.... most folk, in fact even that neighbour who grumbled at me fixing the V8's exhaust, grumble about the lad a few doors down revving up his Lexmoto with a stubby can on the end.... which I have to say is probably one of the better 125 4C singles I have had to listen too.... but still sounds like some-one mowing thier lawn or farting in a trumpet, and annoys the old folk at the bus stop every time he goes past!

Fact: MOST people who are subjected to your exhaust, will NEVER think it sounds particularly wonderful... no matter how great you think it does..... most will be annoyed by the noise of a motorbike, ANY motorbike, no matter what it sounds like. If any bother to take notice they will merely be annoyed. A few, might find it amusing.. and laugh at the fact that its drawing attention to an otherwise unremarkable and mundane 4C Single 125, and even if they know what they are looking at, they will NOT appreciate the time and effort that has gone into making your exhaust sound laughable; they'll see the L-Plate and just laugh.... they'll wonder why you dont ride a 'propper' bike, etc etc etc.....

You may get attention... but likely NOT the sort you would like.... and when that comes from peed-off-plod, to whom it merely gives good cause to pull you, and start looking closely at the number plate size and spacing, and the tyres and their tread-depth, and anything and anything, even before they get to the potential disputes over L-Plates.... so even if you have a licence... "125 init, should have an L-Plate!".....

Mucking with the pipe, are you really doing yourself many if any favours what so ever?

If you like nice noises in your helmet when you ride? Is a fart-in-a-bean-can 125 exhaust nose really the best you can come up with? Pair of head-phones and an MP3 player!!!! You can even buy sound-tracks of the worlds legendary race bikes or cars if that is what floats your boat.... of you can listen to Justin Tiberlake, or Britney Spears or Mozart, or if you really want to get into charecter the SoA soundtrack, whatever takes your fancy really.... BUT you don't subject the rest of the world to it along the way.. or get that unwanted attension, eitrher from folk being annoyed, having a laugh, or puilling you to take a look at the tread-depth etc!!

Your call.... in the renovation of an old bike, especially a learner bike, is THIS really something to stick at the top of the to-do-list?
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[dartman5]

Thanks very much for your advice guys. I really appreciate it, I've decided to give that dream a miss and just go for a nice new paint job instead, gonna go for Matt black I think.

One other question, my fuel mixture is really rich, where is the fuel/air mixture screw? I know this sounds ridiculous but I can't seem to locate it, do you need to take anything off to access it?

Thanks again.