Topic: Question for those with stock 4 into 4 exhaust system,

[DesertKyorugi]

Do you run them with or without the diffusers/baffles? If you don't run them please comment the reason(s). Thanks

[Deltarider]

What's a diffuser?

[DesertKyorugi]

What's a diffuser?

I guess I should edit it to include baffles. Diffuser is the db killer/silencer insert in the muffler

[gtmdriver]

Stock system with baffles. CB 350F.

[PeWe]

Honda call them for diffusers.

I'm sure it depends on which bike and exhaust model.

-My CB750 K2 has Yamiya no number 4-4 which is same as  HM300 (K0-K1).
It has the diffusers in, restrictive sound muffling "wool" removed.
It work fine with nice sound.

(My K6 with loud Hindle 4-2-1 with long pipe sounds less when crusing in 80-120 kmh. Different when open the throttle!)
 
Remove the diffusers completely, too loud. Fun for 15 minute ride.

- Same with Lotus Root pipes (Yamiya replicas of the first pipes on the earlier Sandcasts.)

No diffusers way to loud. Diffusers in which I drilled a ring of 5mm holes in each diffusers bottom to improve flow.

- My CB750 K6 had its original very restrictive HM341,  no end plugs in when I bought the bike January 1979. ( Quickly replaced by 4-1)
I doubt it will affect sound much, just a design thing to look like the earlier bikes. HM341 is so restrictive so needles must be in the middle, main 105 while no numbers ends up with needles one step richer, main 115.
This with stock engine, airbox.

This has been repeated on a friends bike with HM341 that could not use my K2 settings last year.

[Deltarider]

My experience is limited to the OEM 4-4 on the oldstyle CB500 and CB550. These pipes will become louder over time. There's not much you can do about it. To prolong life, avoid short rides and make sure the drain holes at the bottom where the wider mufflers begin, are always free. Owners of K3s may even consider to drill holes in the same spot, since Honda didn't. Realise that most exhausts will rust from the inside! The K3s exhausts were very silent indeed. I love that turbine sound. I hope the repro's, nowadays on offer, are just as good. I hate loud bikes and admire bikes that only have the air intake audible. Yamaha has some fine examples. They hiss. Wonderful! A tip if you - for whatever reason - remove the 4 OEM pipes. Make sure you have pillows, cushions, mattrass or whatever under them, when you do it. I always had at least one exhaust drop to the concrete. There's a serious risk the internal damper, which is no doubt already rusty, falls apart, which will result in a louder pipe.

[evinrude7]

hm341s i used to run them without the diffusers because that's how i got it.  loud as hell.  bought the diffusers a couple years ago and love the sound now. 

[BenelliSEI]

My cb750s are on standard HM300 and HM341 pipes. They all have original baffles. My K0 and K1 sound great on the stock pipes. My K4 (on HM341) sound anemic by comparison.

[PeWe]

My cb750s are on standard HM300 and HM341 pipes. They all have original baffles. My K0 and K1 sound great on the stock pipes. My K4 (on HM341) sound anemic by comparison.

Your HM300 have the padding around the baffles/diffusers?
Really thick restrictive stuff.
White before use
They muffle the sound some extra, bike a little bit slower.

https://www.ebay.de/itm/221569795096

[BenelliSEI]

PeWe..... my KO and K1 have an NOS set of HM300’s I found many years ago, in their original cartons. I had to install a few baffles/diffusers in one set. I bought them from JTMarks and from memory, they were very short, no padding of any kind. Those pipes ar a lot louder than any new replica pipes or HM341’s!

[754]

In 09 I drove all night to pick up a 4 pipe bike , to then ride to Bonneville.. a 300 dollar 750.
Iasked if they had pulled the baffles they  said no.. when I went towee it I had a 10mm wrench in hand, before I rode it, I pulked them out.. Them 341,s  then sounded much better, and never went back in...

[Johnie]

My KO has the stock HM300's as they came from Honda. With already some hearing loss I left them stock and even wear some hearing protection from wind and exhaust noise.

[Don R]

 I have the Yamiya no number pipes on two bikes, run the baffles in both. I tried running without them once and wasn't that impressed with the difference.  I didn't do any drilling like we used to. I did coat the insides with Eastwood inside the pipe exhaust coating, not sure if it has any effect on sound, probably not. I have a spare baffle with a 1/2" hole in the end that I used to insert the probe for an O2 sensor. I'm not sure it made a big difference in sound either.

[DesertKyorugi]

Thanks guys for your answers and opinions. The reason I started this poll is because I am having some doubts about the HM341s that are on my K1.

I purchased my K1 with an almost straight through 4 into 1 exhaust. I added the HM341s and the bike feels like it is holding back when going full throttle from 5k RPM. Since I have never owned or rode any other CB750s, I don't really know how it is supposed to be like.

I have read about the Hondaman mod and have seen most YouTube videos are without baffles/diffusers. I have tested the sound levels with and without diffusers and it is about 2+ DB louder without at idle.

I think at this point I will have to break out the 10mm wrench and take the diffusers off and go for a ride.

[754]

It only takes a few minutes to pull them, won't be as loud as pulling them on 300,s.
What I found with stock pipes is performance  below 3500 rpm , seems better..

[PeWe]

PeWe..... my KO and K1 have an NOS set of HM300’s I found many years ago, in their original cartons. I had to install a few baffles/diffusers in one set. I bought them from JTMarks and from memory, they were very short, no padding of any kind. Those pipes ar a lot louder than any new replica pipes or HM341’s!

That's strange. Yamiya no numbers look exact as the NOS diffusers I linked to, Honda bagged units for HM300.
Sure HM300?
I saw another old open version, stamped CB750 a friend has.
My Diffusers did not fit as I can recall.

My Lotus Root pipes have really fat ones, as small bombs. No padding around them except for a thin fabric not removable .

[DesertKyorugi]

Ig only taies a few minutes to pull them, won't be as loud as pulling them on 300,s.
What I found with stock pipes is performance  below 3500 rpm , seems better..

That's my thinking as well, the straight 4 into 1 is definitely softer under 4000 rpm compared to the HM341s. I regret not riding the 4 into 1 exhaust longer before swapping in the 4 into 4. I never rode it passed 5000 rpm because the bike was still new to me and I was still in the careful first start/run stage after reviving it from 39 years slumber

[MauiK3]

My Yamiya “no number” pipes have the diffusers in. They sound very healthy but not too loud. Best 4cylinder bike sound ever.. but that’s just me.

[PeWe]

Ig only taies a few minutes to pull them, won't be as loud as pulling them on 300,s.
What I found with stock pipes is performance  below 3500 rpm , seems better..

That's my thinking as well, the straight 4 into 1 is definitely softer under 4000 rpm compared to the HM341s. I regret not riding the 4 into 1 exhaust longer before swapping in the 4 into 4. I never rode it passed 5000 rpm because the bike was still new to me and I was still in the careful first start/run stage after reviving it from 39 years slumber

If you did not change jets, richer needle and mains,
a more open exhaust will have bad midrange if carbs are jetted for HM34I that's for sure.

I tried that on my no numbers as initial setup, my old K6 stock setting for 341. Very dull midrange and top.
Same setting as a friends bike with HM341 must have and opposite, he tried my well working setup for my no numbers, too rich for HM341.

4-2-1 often mentioned to give midrange while 4-1 has more at the top.

Later upgrade : (same carb setup)
CI K7 +0.5mm pistons bumps up the compression in a stock K6 head. (K2 1975 need that head design).
K7 cam. Bike is fun to ride and nice sound from the no numbers.
See the graphs, dyno after piston change. Torque horisontal.
The bumpy graphs might have been the dyno that soon after blew a sensor.
My K6 got more stable graphs at higher rpms after Dyno repair.
The torque start at 3500 rpm, 2nd vertical line

[DesertKyorugi]

@PeWe,

I am assuming my K1 bike's carbs were set up for the 4 into 1 exhaust that it came with. I didn't make any changes to it when I swapped in the HM341s.

Two possible scenarios here, one is the carbs were left unchanged when the PO installed the 4 into 1, the other is the carbs were changed from HM300 setup to whatever it is on the carbs now to run the 4 into 1.

I am leaning towards carbs set up unchanged and is the stock set up for HM300. That means the carbs are running richer than what it needs to be for the more restrictive HM341s. I guess I will find out if running without the diffusers would help make it less restrictive and less rich

[PeWe]

My friends K2 running HM341 with my setting for Yamiya No numbers ran obviously rich wich he found out at first ride and changed needles from 4 to 3, main jets from 115 to 105.
Plugs looked sooty too.

[DesertKyorugi]

My friends K2 running HM341 with my setting for Yamiya No numbers ran obviously rich wich he found out at first ride and changed needles from 4 to 3, main jets from 115 to 105.
Plugs looked sooty too.

Well, I just pulled the #3 and #4 plugs from my 30 miles ride the other day and the plugs looked fine. Probably can go richer but I don't really want to pull the carbs at this time.

[ekpent]

 You could easily drop a bowl and unscrew an emulsion tube with the main jet in it and see what size it is and if its a stock Keihin. Every K1 I have always had a stock #120 in it and as they switched up to HM341's it went down to 110 then 105 by 1975-76.

[754]

  IF I saw that , I would try one step up on the mains..    leaving everything else alone to try it.

 I had an original  set of no numbers,  baffles were quite big... Not sure but think they looked like what the early parts book showed.

[DesertKyorugi]

I pulled the main jets and just as I thought they were 120s. What should I go up to if I was going to run the pipes without diffusers? 125? 130? Thanks

[seanbarney41]

don't go up at all until you need to...120's are usually all that is needed

[754]

Give it what it asks for, it looks a tad lean , imo... Each step is 5 .
 What I was trying to say before, if you try it try just that, then if you pull baffles , that US another test..
 Easier one thingar a time..

Back in 75 I had a 72 with a Hooked header and pods. It ran good but my buddy who was a lot lighter had a 74 and he was always a bit ahead of me.... we tried many times. Finally I bumped  up a jet size and it ran stronger, kept up or even pulled on him a bit.. felt good.. About 2 day's later , we totalled both bikes.. hit each other at a stupid speed difference. .
 I think I had gone from 120 to 125..mot sure..it's been 45 years.

They do seem to vary, not all want that much, and big bore...oddly does not always need more.. and after a good bunch of miles, at times we have found we could go down a size....
 There is no formula, trust the plugs...

[DesertKyorugi]

Well, my butt dyno tells me it can run a little richer than 120anove 5000rpm. And this is running with the diffusers in my HM341s.

I imagine if I am running without the diffusers I would definitely need to up the main jets

[754]

Real good chance of that... you probably got a pretty good  one...

[PeWe]

Tricky with A/F check on dyno with diffusers in.
The Dyno guys here stick a copper pipe as deep in as possible. An external device with lambda.

My K2 on dyno with no numbers got no A/F since bike felt OK, needles and mains found by road tests. Dyno guy agreed, bike felt right.
 (Too rich needles or mains can be verified by slowly repeat increasing-decreasing throttle at around 100-140kmh for needles, faster for mains. You hear and feel when slowly decelerate if too rich. Good to repeat that from 100kmh- full speed.

I did that last summer on my K6, often in the 140-200kmh area which sometimes was risky when too focused on engine.

My K6with same No numbers 4-4 a few years ago got A/F check without diffusers. Power checked with and without.
Same with the Lotus Root pipes.

Hindle 4-2-1 does not have such restriction, a wide hole all the way in!   (Easy for Dyno A/F copper pipe.)
It muffles the sound surprisingly well when cruising.

[DesertKyorugi]

Tricky with A/F check on dyno with diffusers in.
The Dyno guys here stick a copper pipe as deep in as possible. An external device with lambda.

My K2 on dyno with no numbers got no A/F since bike felt OK, needles and mains found by road tests. Dyno guy agreed, bike felt right.
 (Too rich needles or mains can be verified by slowly repeat increasing-decreasing throttle at around 100-140kmh for needles, faster for mains. You hear and feel when slowly decelerate if too rich. Good to repeat that from 100kmh- full speed.

I did that last summer on my K6, often in the 140-200kmh area which sometimes was risky when too focused on engine.

My K6with same No numbers 4-4 a few years ago got A/F check without diffusers. Power checked with and without.
Same with the Lotus Root pipes.

Hindle 4-2-1 does not have such restriction, a wide hole all the way in!   (Easy for Dyno A/F copper pipe.)
It muffles the sound surprisingly well when cruising.

Unfortunately, all I have to go by is my butt dyno. It's been decades since my racing days and my Shoei's soundproofing is a little too good for listening to deceleration. The only comparison tool I had was my newer sportbikes.

[PeWe]

You will feel it more than hear it. My helmet also reduce sound rather well. Schuberth C3 Pro.

You'll get the feeling for it when riding that specific bike and exhaust for a longer ride or 3.

[DesertKyorugi]

Is "butt dyno" not a term used for motorcycles as well? I am pretty sure it is not strictly a car term 😅😂

[PeWe]

Butt dyno mentioned on many threads!

[JLeather]

The last bike I had with stock exhaust I ran without the diffusers.  I really liked the sound.  Surprisingly louder than with the diffusers in, also throatier with more of a "pop".  May have cost me a little HP but I didn't care.  I recall having to bump up the needles a click and I think I ended up at 42 idle jets on that bike...

[DesertKyorugi]

The last bike I had with stock exhaust I ran without the diffusers.  I really liked the sound.  Surprisingly louder than with the diffusers in, also throatier with more of a "pop".  May have cost me a little HP but I didn't care.  I recall having to bump up the needles a click and I think I ended up at 42 idle jets on that bike...

What year is your CB? My K1 with HM341 s idling a little rich with the stock 1 turn on the air mixture screw. The bike runs great below 4000 rpm and as you can see from the spark plugs, it is quite acceptable but I am just being picky due to my butt dyno telling me the performance can be better above 5000rpm.

I will be testing for sound difference with or without diffusers soon and will report back

[JLeather]

It was a '76, the most detuned bike to ever run 341 exhaust stock   I also had a K7 with the ends of the pipes cut out (same as removing the diffusers) and it was awesome sounding.

[DesertKyorugi]

Had a little bit of time today so I did a little sound test at idle at 1500rpm. With diffusers, it is averaging 81db. Without diffusers, it is averaging 90db.

A quick spin around the block, my butt dyno says quicker response and more eager to rev. I did not have time to go for a real ride so I will have to wait for another day so I can do a plug chop as well to see how much leaner it is running without diffusers.

I also want to confirm what you guys are saying, all the diffusers were wet with condensations. I decided not to put them back in.