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CB350 stock mufflers - rot, rust, and racket

lxhws

Member
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Nov 1, 2021
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Location
MA US
Hey folks - was hoping to solicit some advice from those out there who might be in the know, so that I might make an informed decision regarding how to proceed with my CB's exhaust as I work on the bike this winter.

Is anyone out there intimately familiar with these original Honda (HM317 stamped) mufflers, more specifically the interiors? I ask because this bike is equipped with said mufflers, and both sides have developed some gnarly rattling. Easily heard under acceleration when installed on the bike, or if you shake it around in your hand.

I've got the whole system off the bike currently (pulled engine for another unrelated project) and I'm able to get a peek at what's going on in there. Pretty rusty, but it appears to be mostly surface rust; no really bad spots, or any rotted sections are visible. My viewing angel is looking down from where the muffler slips over the header pipe (so from 'front' towards the 'rear' orientation if they were installed); about halfway down the length of the muffler in the chamber where the connection from the header pipe opens up, there's a loose piece of metal just hangin' out, not secured or attached to anything.

---

It's hard to really get a good look (a boroscope would be nice here) but as best I can discern, the loose piece is circular, fairly thin but almost as wide as the muffler itself, and looks to have flared edges. My assumption is that it's somehow meant to attach or sit on the piece below it (a metal grate with small, circular holes) which I assume is part of the filtering/baffle setup. I tried to fish it out with a extendable magnet tool, but it's far too wide to come out the header side intact. I removed the baffle/silencer that's bolted in to the the rear of the muffler and attempted to go in that way, but there's too many other internal pieces between the baffle end and the middle where the noisy culprit lives.

So, short of cutting these things open, I'm doubtful there's any way to remove or repair these pieces (whatever they may be). I tried to take some pictures to show here, but with a phone camera and that angle, I wasn't able to get a properly illuminated and focused image worth sharing.

I'd be thankful for any info or opinions here, whether it's to repair, replace, or rock em as is till they fall apart. I've looked around the web for something like a cross section or a picture of these mufflers being dissected, but haven't been able to find anything. I'm curious what these pieces are, and how the fit in to or might affect the rest of the exhaust system.

I'm fully aware these original Honda muffler sets aren't exactly survivors, and the rust+rot will eventually one day claim them. I'm hesitant to give up on anything 'original' that might have a chance to be saved. Also,for what it's worth, the exteriors are in pretty good shape, and with some elbow grease would clean up nice. Not perfect, but not complete garbage.


Thanks for looking, appreciate all/any replies. Have a good one!
 
Like you mentioned, you can run them as is, repair or replace... just depends what you're looking for.

Repair is probably Least desirable. You will have to cut it and weld or braze it back together and it still may never be as good as original, and cosmetically will never be as original. Repairing the outer metal from the operation will be all distorted or patched and ugly and may crack later down the road.

I have a 250cc scooter with a busted muffler... like someone dropped it and the inner baffles broke. cutting it open and repairing it wasn't difficult but it sure is ugly. I used spare scrap metal to fill in spots and patch things up.
 
Yes, probably your only option to save them is to cut them open from the back/underside to get at and weld or braze the loose baffle back in place (if you can still see where it broke loose from) and then carefully weld or braze the outer shell section back together. Of course the heat will discolor the chrome, which is why back/underside would be best, and a little bright silver or "chrome" paint will cover it up. As long as you don't get it any hotter than necessary (read: quick and competent welding or brazing) you'll limit the discoloration to a minimum. After 5 decades most of these exhausts are rusted out and/or have been repaired already.
 
Like you mentioned, you can run them as is, repair or replace... just depends what you're looking for.

Repair is probably Least desirable. You will have to cut it and weld or braze it back together and it still may never be as good as original, and cosmetically will never be as original. Repairing the outer metal from the operation will be all distorted or patched and ugly and may crack later down the road.

I have a 250cc scooter with a busted muffler... like someone dropped it and the inner baffles broke. cutting it open and repairing it wasn't difficult but it sure is ugly. I used spare scrap metal to fill in spots and patch things up.


Yes, probably your only option to save them is to cut them open from the back/underside to get at and weld or braze the loose baffle back in place (if you can still see where it broke loose from) and then carefully weld or braze the outer shell section back together. Of course the heat will discolor the chrome, which is why back/underside would be best, and a little bright silver or "chrome" paint will cover it up. As long as you don't get it any hotter than necessary (read: quick and competent welding or brazing) you'll limit the discoloration to a minimum. After 5 decades most of these exhausts are rusted out and/or have been repaired already.

Thank you guys, this was what I suspected would be the case. Metalworking is not one of my strong skills (though I do outsource from time to time), so I'll probably just leave them be for now and consider replacements down the line.

However, on that topic, I'm not too familiar with many aftermarket muffler or exhaust system solutions outside of some of the bigger brands, so I'd be interested in hearing any recommendations. If I did end up replacing them, my gut tells me I'd probably try stay close to the original aesthetics and get another set of cone style mufflers.


Cheers.
 
Curious what outcome you decided to go with on this?

I just picked up a CL175 and just discovered the baffle in the muffler is loose and rolling around. I have no clue how to weld so I was hoping to avoid that route but I have a welder friend so that might be an option.

Curious if you cut it open? Or just replaced the exhaust?


Sent from my iPhone using Tapatalk
 
The HM317 has a removable baffle which is held-in with a 6mm bolt/screw and accessed from the rear;I suspect your baffle has rusted apart inside there and may be bouncing around toward the rear.
Do your mufflers still have a solid 'back plate' which the baffle is bolted to ?
The original ones are very difficult to source and I've looked everywhere online;I'm looking to replace mine out of my HM317's.
The part number for them are #18311-286-610 ,in case you happen to find them.
They started installing that particular baffle starting with the CB350K2 and continued using it all the way up to the CB350G/K5.
I do have an original one which is complete(good condition)out of mine that is getting thin;I know it's possible to make a custom repro of it using this one as a guide,which I'm considering.
 
The HM317 has a removable baffle which is held-in with a 6mm bolt/screw and accessed from the rear;I suspect your baffle has rusted apart inside there and may be bouncing around toward the rear.
Do your mufflers still have a solid 'back plate' which the baffle is bolted to ?
The original ones are very difficult to source and I've looked everywhere online;I'm looking to replace mine out of my HM317's.
The part number for them are #18311-286-610 ,in case you happen to find them.
They started installing that particular baffle starting with the CB350K2 and continued using it all the way up to the CB350G/K5.
I do have an original one which is complete(good condition)out of mine that is getting thin;I know it's possible to make a custom repro of it using this one as a guide,which I'm considering.


Yep, these are on a K4. I removed the 6mm bolt and discovered the same thing. I fiddled with them a bit, but then just reassembled and decided to live with it.
 
Yep, these are on a K4. I removed the 6mm bolt and discovered the same thing. I fiddled with them a bit, but then just reassembled and decided to live with it.

I would like to get some sheet Stainless Steel and make some repros.
The outer case on these mufflers are usually very solid and will last for decades.

What's the condition of the baffles & muffler back plates on your HM317's ?
 
The HM317 has a removable baffle which is held-in with a 6mm bolt/screw and accessed from the rear;I suspect your baffle has rusted apart inside there and may be bouncing around toward the rear.
Do your mufflers still have a solid 'back plate' which the baffle is bolted to ?
The original ones are very difficult to source and I've looked everywhere online;I'm looking to replace mine out of my HM317's.
The part number for them are #18311-286-610 ,in case you happen to find them.
They started installing that particular baffle starting with the CB350K2 and continued using it all the way up to the CB350G/K5.
I do have an original one which is complete(good condition)out of mine that is getting thin;I know it's possible to make a custom repro of it using this one as a guide,which I'm considering.

Here’s what the end of my muffler looks like as well as a couple holes and the baffle that’s rolling around.
4358e8818247fec8710038ac3c33db0f.jpg


389b3d40c8ed277ddc7a301ed5515247.jpg


cda84a5ec3059f608caaf163daafa65b.jpg



Sent from my iPhone using Tapatalk
 
Here’s what the end of my muffler looks like as well as a couple holes and the baffle that’s rolling around.

That level of rust is beyond any welding repair, it will fall apart at almost every area.

To continue the discussion you should start a new exhaust thread in the 175 section since this is lxhws's CB350 exhaust thread.
 
That level of rust is beyond any welding repair, it will fall apart at almost every area.

To continue the discussion you should start a new exhaust thread in the 175 section since this is lxhws's CB350 exhaust thread.

Sorry, didn’t mean to hijack this, just curious about the melding part. I’ll start a new thread. Thanks!


Sent from my iPhone using Tapatalk
 
I would like to get some sheet Stainless Steel and make some repros.
The outer case on these mufflers are usually very solid and will last for decades.

What's the condition of the baffles & muffler back plates on your HM317's ?

Can I make a recommendation?

If you have access to a lathe, it might be easier (depending on your ability with sheet metal) to make it out of as much raw stock as possible (inner and outer tubes) and then machine the other three parts.

You could probably do this all out of aluminum but I did mine with steel because I’m a glutton for punishment, and it’s a teensy bit easier to weld.

Part D in my (very rough) sketch below is probably easy to make from sheet metal as well, so maybe it’s simplest to make A, D, and E out of sheet metal and machine the other two.

Tack weld it all up at the end and you’re good to go. It’s how I made the repro versions of mine.

d7563424f416ecab8d88d27e740e0578.jpg
 
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