Exhaust baffles, sound, and jetting.

[DanielBlack]

Mailman and I met up for the second time this morning and once again he commented on the bassy, throaty sound of Stella's exhaust. Both of us ride 1977D survivors in pretty good shape other than a 40yo worn engine. Stock exhaust, OEM-style K&N air filters in the stock air box. The difference in our exhaust sounds was a bit of a mystery.

Well, not for long. Later in the day, Detective Mailman sent me the following text and picture.

I was thinking about the difference in our exhaust sound and I remembered an old trick that guys sometimes do. They either go into the end of their muffler with a very long drill bit or hole saw or they take a 4' rebar and grind a chisel end on it and knock or drill holes in the baffles of their muffler.
7 inches in from the end of your muffler there is a solid blocked off baffle. The exhaust passes through the drilled holes internally. If you look into the end of your muffler with a good flashlight I'll bet it has holes drilled in it or it has been knocked out entirely. Here's a photo of what it should look like.

On his urging, I went spelunking and Mailman hit it right on the head. Here's Stella:

On another front, I've been slowly working on my jetting.

5twins suggested we both might like 1 or 2 up considering the K&N filters. Following his advice of starting stock and step by step increasing the main until the first sign of stumble, I'm now 3 up and no stumble.

I've been hesitant to go another, knowing that being so far out of the expected range might be a symptom of another problem.

Mailman's thought of open baffles lays my jetting worry to rest. Time for the next step up...

 

[mjpchief]

Mailman and I met up for the second time this morning and once again he commented on the bassy, throaty sound of Stella's exhaust. Both of us ride 1977D survivors in pretty good shape other than a 40yo worn engine. Stock exhaust, OEM-style K&N air filters in the stock air box. The difference in our exhaust sounds was a bit of a mystery.

Well, not for long. Later in the day, Detective Mailman sent me the following text and picture.

I was thinking about the difference in our exhaust sound and I remembered an old trick that guys sometimes do. They either go into the end of their muffler with a very long drill bit or hole saw or they take a 4' rebar and grind a chisel end on it and knock or drill holes in the baffles of their muffler.
7 inches in from the end of your muffler there is a solid blocked off baffle. The exhaust passes through the drilled holes internally. If you look into the end of your muffler with a good flashlight I'll bet it has holes drilled in it or it has been knocked out entirely. Here's a photo of what it should look like.
View attachment 104057

On his urging, I went spelunking and Mailman hit it right on the head. Here's Stella:
View attachment 104058

On another front, I've been slowly working on my jetting.

5twins suggested we both might like 1 or 2 up considering the K&N filters. Following his advice of starting stock and step by step increasing the main until the first sign of stumble, I'm now 3 up and no stumble.

I've been hesitant to go another, knowing that being so far out of the expected range might be a symptom of another problem.

Mailman's thought of open baffles lays my jetting worry to rest. Time for the next step up...

Right after I bought my XS650B in 1975 a Yamaha rep asked me if I wanted my bike to sound better. I said sure I do. He showed me how to take a piece of black pipe and knock the baffle out of the muffler. Sounded real good for a while and then I noticed that when a bunch of went riding everyone wanted me to ride in the back of the pack. Ended up buying a new set of mufflers to make it livable again.

 

[YamadudeXS650C]

WeekendRider educated me about this modification, and I investigated my '75 to find it had been done. I LOVE the sound of these stock mufflers with the baffle plates knocked out !

Stock jetting, stock air boxes, stock everything, and it runs Hunky Dory.

 

[5twins]

The stumbling I spoke of is in the upper midrange, say from 4 to 5K, and is the result of larger mains bleeding over into the upper midrange and making it too rich. But to notice it, you will have to test using full throttle. Due to the forgiving nature of these CV carbs, the stumbling probably won't show up unless you push the carbs hard like this. If you do find it, that doesn't necessarily mean your mains are too big, it just indicates you have to lean the midrange (needle setting) to compensate.

 

[DanielBlack]

I wouldn't have done it myself, but I do like the modified sound. It's lower and throatier, but I wouldn't call it too loud or really much louder than Bob's unmolested baffles. We'll have to do some A/B video next time.

Thanks, 5twins. Yes, pushing it hard from ~2.5k all the way up to 6.5k or so. Still no stumble. Next time I'm near the local genuine jets supply I'll pick up the next two sizes.

That said, it does seem to be running rather well. All signs of leanness went away with 2 ups, other than a slight hanging idle. But as 2M and Robin have discussed, that may be due to my very low compression. 85/90psi.

Yamadude: With your knocked-out baffles, have you tested larger mains and found them too rich?

5twins, anyone else: do you think the OEM-style K&Ns flow well enough to take advantage of the open baffles to their fullest?

 

[YamadudeXS650C]

I wouldn't have done it myself, but I do like the modified sound. It's lower and throatier, but I wouldn't call it too loud or really much louder than Bob's unmolested baffles. We'll have to do some A/B video next time.

Thanks, 5twins. Yes, pushing it hard from ~2.5k all the way up to 6.5k or so. Still no stumble. Next time I'm near the local genuine jets supply I'll pick up the next two sizes.

That said, it does seem to be running rather well. All signs of leanness went away with 2 ups, other than a slight hanging idle. But as 2M and Robin have discussed, that may be due to my very low compression. 85/90psi.

Yamadude: With your knocked-out baffles, have you tested larger mains and found them too rich?

5twins, anyone else: do you think the OEM-style K&Ns flow well enough to take advantage of the open baffles to their fullest?

Dan,
I the case of this '75 of mine, it is accelerating and otherwise running very well, with stock jetting. However, I need to put more miles on it; I've been running it with a "borrowed" license plate (illegal) this season. Just between you and me

Its not totally stock, actually. I installed a PAMCO with Hi output coil, and solid state rectifier and regulator.

 

[weekendrider]

Even with the baffle altered with stock head pipes you still have the same small interior pipe. So is the exhaust really freer flowing?

 

[DanielBlack]

Good point. Perhaps that's why there is a change in tone, but not so much in volume.

 

[5twins]

I've never tried the 650 K&N airbox filters but have used them on other bikes. They did increase air flow on those so I don't see why they wouldn't on the 650 too. On those other bikes, they ran fine with no jetting change but ran better with some minor changes. This didn't amount to much, usually just a size or 2 on the mains. I gotta say, the 650 carbs seem to require more fiddling than any others I've dealt with. I didn't expect this from a basic old fashioned twin. The 650 carbs often require pilot and needle clip position changes too besides the mains. But not always. As long as your main isn't large enough to require a needle clip position change, the stock setting along with the stock pilot can be OK. Lean the needle a step to compensate for larger mains though and that often requires a larger pilot be installed.

The 650 has taught me a lot more about carb tuning. Well, I actually knew a lot of this stuff, I just never had to put it into practice until now. Like I said, main jet changes alone were usually enough on other bikes in the past. Carb jetting for mods is all about experimenting and trying different sizes so, yes, I would try different needle clip settings, sometimes different pilots on those other bikes. But I usually ended up back with stock for that stuff with just larger mains. So, keep experimenting. When your jetting is close, you usually can't tell by the way it's running if the next size up will be better unless you just try it because it's running fine.

 

[5twins]

And Daniel, following along the lines of jet experimenting here, you say you have some idle speed hanging. Maybe the K&Ns are leaning out your idle circuit some. Pop some 27.5 pilots in there (one size above your stock #25), that just may fix the issue.

 

[DanielBlack]

Yes. RPMs will drop pretty quickly down to ~2.25k then sort of float down to idle. I'll give those pilots a try, but I was following the mains, needles, pilots order of operation.

 

[5twins]

Yes, that's the usual routine, but sometimes intake and/or exhaust changes require more than just mains right from the start. That air filter is flowing more all the time, at all speeds, not just at higher RPMs in main jet territory. It's simple and cheap enough to try, and if it's no good, just change it back.

 

[XS650D]

On my 77D I have the pod foam filters and Mikes Commando mufflers and after much experimenting
I ended up with one size up on pilot 27.5 Mains 132.5 and needles dropped 1 clip.I may try 130 mains and shim the needle
1/2 step and see if it smooths things out more like the stock jetting and air boxes.Thats my end goal.
Regarding sound of the mufflers,i like the Commandos but I do find them very quiet and possibly restrictive.
Has anyone experimented with drilling a small hole thru the baffles for a deeper throatier and slightly louder sound.
I know it works well on the stock system but the internals on the Commandos may be different.
I really wanted the original early Muffs that Mikes sells as they sound awesome but fricken expensive.
5T I think you have the same Muffs as me, have u tried modifying your system or know anyone that has and can comment.
I think a small 1/4" hole strait thru may be the ticket and then go from there.Comments

 

[5twins]

The Commandos should already come with a hole through the baffle and include an Allen bolt to plug it .....

I tried mine without it when I got them but prefer them with the Allen bolt in there. I felt they sounded rather raspy or ragged without the bolt plug.

 

[Bob Kelly III]

LOL that reminds me of the 400cc suzuki singles with their stock tuned pipe...everyone always took off the silencer first thing they did when they got'em home , never to touch the bike again and when they ran it was like every firing of the engine was like a shotgun blast to the ears !
super loud ..... painfully so , even ! excessively loud bikes I dislike, there is no sense in that....
another cheap silencer, is a simple cap to a sprey can ( metal small ones ) although their hard to find now... take a nail and fill the end of the cap with holes and then slip it in the exhaust.... you want back pressure ? that will give you back pressure ! LOL and snuff it down to a murmur !
but it will get you by the california sound test ! almost nothing else will !
LOL
Bob......

 

[XS650D]

I have the plugs installed.going to pull out one and go for a ride and compare Thx

 

[Bob Kelly III]

Um don't do that !
pull the spark plug lead and put another plug in it and ground it to the head but don't just pull out the sparkplug fire it up and go for a noisy ride because you'll suck all kinds of crap into the engine ! and that spark plug lead has to be grounded through a plug or you'll fry the coil !
.....
Bob........

 

[weekendrider]

Bob, I think you missed this? Or maybe I just passed another test.

On my 77D I have the pod foam filters and Mikes Commando mufflers

 

[XS650D]

LOL! nOT THE SPARK PLUGS BOB ,THE PIPE PLUGS WERE DISCUSSING.

 

[Bob Kelly III]

Oh.... Well...that's alright then ! LOL
ok so I'm half asleep, only half asleep mind you because I tried to sleep 3 times now and can't seam to go to sleep and I keep getting back up ! AURGH ! but I have a plan 2 beers and I'm out ! soooo time fer another ... Night night !
LOL