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De-snagging an SL125

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Alan Donovan26/06/2021 21:21:03
61 forum posts
29 photos

Hello all.

For those of you that have been following the challenges of renovating a Honda SL125, it is now running, on the road, insured and officially an 'historic vehicle'. I am currently 'de-snagging' the vehicle.

A current 'snag' is that the exhaust is (very) loud.

It has an aftermarket muffler which isn't as well engineered (or expensive as an OEM unit - $2999.95 U.S.) but has the correct contours and is aesthetically correct. I propose to modify the aftermarket unit by putting a diffuser in the exhaust stream to hopefully deaden / reduce the sound. I propose that the diffuser will be similar to the OEM design - see item 6 on the sketch below.


This would require removing the existing outlet pipe and an internal baffle (Rotabroach cutter maybe) to provide room for the diffuser.

I am relatively confident I could make a diffuser to suit from steel, and that the primary design parameter is that the diffusers combined 'flow gas' area should be not less than the open cross sectional area of the current exhaust outlet.

So my queries are -

Is the above design parameter reasonable and do you think a quieter exhaust note would result?

Are there any other issues I should take into consideration?

Should I leave things as they are and claim ' historic vehicle / old technology' if challenged?

Lastly, and this is the most interesting bit,- have any of you any different solutions that could be applied to this to result in a quieter exhaust.

All the best, and thank you in advance for any input / comments


Jon Lawes26/06/2021 21:39:08
872 forum posts

Do you think it is louder than when it was new? I've heard silencers can get louder over time and need repacking as they settle down.

Alan Donovan26/06/2021 21:42:55
61 forum posts
29 photos

Hi John.

In answer to your question, I don't know.

My son and I bought this as five boxes of bits. Even the engine was down to its component parts. So we never heard it running.

It does seem to attract a lot of attention when I am riding it, and my neighbours say it is loud.

It is an all welded unit, and it doesn't feel (?) as though it has any packing inside - I may have to have a 'prod' inside to see its there is anything 'spongy' there.  

The muffler was bought new, by us, as the old one wasn't present when we bought the bike, but it is an aftermarket unit, which I am positive isn't built / manufactured to the same standard as the OEM equipment (for this particular item).


Edited By Alan Donovan on 26/06/2021 21:44:33

Edited By Alan Donovan on 26/06/2021 21:47:38

Edited By Alan Donovan on 26/06/2021 21:53:24

John Olsen27/06/2021 04:45:24
1240 forum posts
94 photos
1 articles

I had the CB100, rather than the SL version, but the muffler was just a large chamber, the only baffle it had was the little unit that appears as part 6 in your picture. It was not excessively loud, even after I converted it to 125 and put a hot cam in it. They never had any packing material like the Dunstall Decibels used. (I had one of those on a Triumph 250, it was reasonably quiet just after it was repacked, and got louder quite quickly as it shredded the packing and blew it out the back.)

I did notice when I had occasion to ask some years back that Honda mufflers were a bit on the pricey side. Bad enough if you just want one, but it would help explain why the fours after a couple of years all had four into one systems fitted. Quieter, lighter, and cheaper as well as more power!


Redsetter27/06/2021 06:40:25
203 forum posts
3 photos

I have not had much to do with bike exhausts but have built a few classic car exhausts when the correct system hasn't been available, I have found it difficult to make anything as quiet as as the stock system, but you can usually produce something acceptable. I have never felt that anything I have built has noticeably restricted the power - in fact the quieter systems have been nicer to drive, and you tend to use more revs because it is not so offensively noisy, so it probably goes better anyway! Usually the outlet is a smaller diameter than the inlet and in one case I remember, reducing the tailpipe diameter by maybe 20 percent by inserting a sleeve had the desired effect.

It seems to me that your internal baffle isn't doing very much, and it is a 4 stroke so the system won't be as critical as a 2 stroke, so you can't make it worse, and there is no harm in a bit of trial and error as the diffuser looks easy enough to make. I would make the "gas flow area" smaller than the inlet area initially and then you can drill a few more holes if you need to. The original part was almost certainly made from a stock size of tube so if you can determine what that is, the internal diameter at least will be correct.

Not very scientific perhaps but that is how I would approach it.


Edited By Redsetter on 27/06/2021 06:42:47

Alan Donovan27/06/2021 08:59:09
61 forum posts
29 photos

Hello again

Thank you all for your comments so far.

The outlet on the aftermarket muffler is smaller than the inlet pipe. The actual exhaust pipe is the inner pipe in the photo below.



The inner pipe is welded into the muffler and has a baffle at the end of it. I think this is supposed to emulate the OEM diffuser, but as you comment the baffle does not appear to be doing much. There is a reasonable gap between the end of the inner pipe and the baffle, so I think it is worth making a diffuser to suit this. As mentioned earlier this pipe is smaller than the inlet end, so If this combined reduction in the 'gas flow area' is detrimental to the bikes performance, then that is the time to consider modify the outlet further.

P.S. Just a thought while writing this. The black box on the exhaust system is described as a 'muffler'. Is a muffler an alternative term for silencer or is a muffler technically different to a silencer?

Best regards to all.


Edited By Alan Donovan on 27/06/2021 09:01:39

Redsetter27/06/2021 09:42:35
203 forum posts
3 photos


Looking at the photo, the enlarged end of the outlet pipe will have a megaphone effect, somewhat like a loudspeaker cone. Extending the inner pipe, or simply inserting a bush to reduce the outer pipe diameter, might be interesting. Whatever you do, you will have to get it very badly wrong to compromise performance. Let us know how you get on.

I think Muffler is just Chinese/American for Silencer.

Emgee27/06/2021 10:12:20
2404 forum posts
285 photos


I believe what you intend to do will reduce the dB level from your exhaust system, provided the combined area of the wall holes are equal to the existing outlet area performance should be similar.
From the sketch it seems the your new diffuser has a closed front, is that correct ?

However if the outlet area is currently larger than optimum reducing the output area may increase power output with less noise.


Clive Foster27/06/2021 10:16:37
3103 forum posts
107 photos

That diffuser is basically a simple annular discharge outlet functionally equivalent to a slot outlet fishtail but significantly less effective because the flow path length is the same all round. Properly made fishtails are better because the path length varies depending on where the gas actually comes out of the fishtail and expands into the atmosphere. Considerable potential for destructive interference reducing sound levels. If you are lucky there may even be a bit of pipework in that diffuser giving some sort of reverse flow effects.

Messing about with different styles of diffuser / absorber bungs is likely to be interesting, frustrating and inconclusive in fairly equal measure.

One things for sure as an aftermarket unit the internals were never designed in any real sense of the word. Usual philosophy seems to be "Just stuff something it that looks like it might work." It is, after all quite hard to make a completely ineffective silencer / muffler. Especially when market demand is for something a bit more rorty than standard.

The aftermarket one on my BSA DB32 Gold Star had a single lonely baffle about 2/3 rds of the way along yet was silly quiet up to around 3,500 or so RPM. Above that standard "Who started World War 3" service was resumed. I felt the characteristics well suited to my youthful use. After all successfully creeping in at silly early hours in the morning to be followed by bleary eyed breakfast time maternal interrogation "And what time did you get in last night? I didn't hear anything." not being a standard Goldie specification requirement.

If you really want something effective its probably best to bite the bullet and accept that the back will have to be cut off and proper innards inserted. After experimentation with a relatively crude but near enough the same shape to fit mock up.

For innards consider a pair of hockey stick or "J" shaped pipes with closed ends, one on the inlet and one on the outlet set back to back and overlapping for most of the length of the silencer. Drill holes in opposite sides of the pipes so the gas can get from one to the other. Holes at top end of inlet and bottom end of outlet to give some element of reverse flow and expansion. Maybe add a longitudinal wall between the pair to make the flow behave better and another transverse one at the top end of the inlet pipe to properly close things off.

A second wall at the top would let you further drill the output pipe and stuff in some wadding to make a short absorption silencer which would take out some high frequency sound removing the hard edge to the exhaust note. That would be similar in principle the the short absorption silencer accessory BSA sold to shove up the tailpipe of a Goldie fitted with the proper, baffle-less, "megaphone" silencer properly intended for racing use. Surprisingly effective at rendering the exhaust less offensive but I never felt the real noise levels were much reduced.

A simple pair of J shape pipes is pretty effective though.


Phil P27/06/2021 10:33:01
802 forum posts
194 photos

Back in my Pre 65 trials riding days, a popular addition was to fit a VW Beetle tail pipe on the exhaust outlet.

That might be an easy option for you to try.


Howard Lewis27/06/2021 14:19:28
6005 forum posts
14 photos

I made silencers for a friend's 250 BSA trials bike.

We made a new central pipe and drilled 8 holes holes along, and around it, equal in area to that of the pipe, each side of a pair of diametrically opposed holes, half way along the pipe. Into the pipe was inserted a snug fitting plain mild steel blank, so that it covered the central holes. These were use as an entry point for the spelter used to braze the blank into position.

The result was a sound reminiscent of a motor cycle of that vintage..


Alan Donovan27/06/2021 15:43:03
61 forum posts
29 photos

Hi all.

Thank you so much for all your input. There are plenty of options for me to try. I have provided a sectional sketch of the outlet end of the muffler below.

I have found a piece of pipe in my 'it may come in useful one day' pile that is a snug fit on the exhaust (diameter A). So first off, I intend to start with just a short extension tube and see what effect that has. I could then try and add some sound insulation on the inside of that and see what effect that has if still noisy. - basically as per Phil P's suggestion.

I will probably try the diffuser style of modification as well just to see how that compares to 'tail pipe' mod.

To answer Emgee's question the diffuser would have a closed end according to the diagram, but my thoughs are to push the diffuser against against the baffle therefore effectively closing off the end.

I will try the easy solutions first before moving on to the more difficult stuff.


Best regards.


Howard Lewis27/06/2021 16:57:16
6005 forum posts
14 photos

You could try having a longer pipe which protrudes into silencer until it butts hard up against the baffle. (Ideally, it should seal against it, but a small leakage will act as a restrictor without much adverse effect. ) The tube should be drilled with enough holes, say 4, 6 or 8, that equate to the cross sectional area of the pipework. In this way, the main body of the silencer will act as an expansion chamber, and the drilled tube will provide a little back pressure, to help to reduce the noise level, by damping the flow and pressure pulses.

It may even improve fuel economy by reducing carry over of the charge, as well.


Gazz01/07/2021 21:37:07
78 forum posts

about 15 years ago i had a cruiser style bike (yamaha virago 535) and i pulled the stock baffles out as i wanted it to sound more like a harley than a sewing machine.

Needless to say it was a little too loud, and being a short stroke much higher reving engine than a harley... it sounded nice at idle, but droned awfully when riding at speed, and would set car alarms off in towns.

So i did what was a common thing back then, mounted a bolt with a washer welded on the head in the end of the pipe, here's a photo i grabbed off google that shows similar to what i did:

Exhaust Drone Update - Aftermarket or Honda fix? | GL1800Riders Forums

It took a little playing about with the size of the washer, the hole size in it, and also the angle you locked the bolt off at... thus angling the washer adjusted the sound,
all playing on reflected sound waves i think,

Sure the exhaust was never as quiet as with the stock baffles in it, but that simple washer in each pipe really helped tone it down a bit.

Rich Griff 230/01/2022 13:56:21
14 forum posts

Hi Alun, just joined this place.

Sl125, split it solid cylinder head ?

I am sure you now know of T, F, O and the rotor ign advance marks.

Might you have any Motor Cycle Mechanics magazines of the 70's.

It was a monthly magazine.

Always in the back of the mag, in the smaller black and white adverts, usually on the RHS page, appearing regularly, an advert for a honda cb125s big bore kit, showing the unmistakable cb125's engine, with a morphed cylinder head artwork.

That art work would look great on a cb125s tank I have in the shed.

I have been looking for years now, for a mag with the advert, or a copy of the advert.

Might you be able to help ?


Rich Griff 230/01/2022 13:58:47
14 forum posts

Sorry Alan, Alun should read Alan.

Cannot find edit button.

Rich Griff 230/01/2022 14:01:02
14 forum posts

Solid or split cylinder head

Alan Donovan30/01/2022 19:37:52
61 forum posts
29 photos

Hi Rich.

The concept of a split cylinder head was totally new to me. A quick check on the internet and (I think) I understand what that concept is now. So to answer your question ...... No, it does not have a split cylinder head, it is a solid head. I have posted an extract from the Honda parts manual to show you the type of cylinder head it has. Just in case I have misunderstood this concept.


To answer your second question, ......

Sorry, I do not have any old ‘Motor cycle mechanics’ magazines from the 70’s. So cannot help with the advertisement or artwork.

I can only suggest that you start a new thread on this forum detailing what you are after. Someone may be able to help.

All the best.


SillyOldDuffer30/01/2022 21:00:03
8469 forum posts
1885 photos

Posted by Rich Griff 2 on 30/01/2022 13:58:47:


Cannot find edit button.

It's only available for about 30 minutes after pressing the Add Posting button. Allows enough time for mistakes to be corrected after a quick proof read, but then the post is locked. It's confusing if someone else comments on a statement that's changed or deleted much later.


Rich Griff 230/01/2022 21:11:56
14 forum posts

Yep, that looks like the one I mean.

You know of CMSNL ?

No doubt you are aware of the "Jesus" clip on the bottom of the cam chain tensioner " bolt" ?

Be very carefully not to over tension the cam chain. I have seen these clips ( E retainers ) lying on the cam chain area floor.

I modified ours ( cb125s ) replacing the Jesus clip with a washer and split pin, but the bolt is very hard in the clip area.

Tax and mot exempt ?

Anyways, enjoy...

Edited By Rich Griff 2 on 30/01/2022 21:14:55

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