Bigger Mufflers

[xsbean]

Hey fellas, I just finished retro fitting some short screamin eagle mufflers for a Harley to my xs. It’s just the tailpipe and the header pipe is still stock standard apart from chopping about 70mm off the last internal portion of the header. But the ID is still the same. I’m curious as to whether I’ll need to adjust jetting specs? My first thought would be maybe not so much but then again it is a shorter length of pipe and to that affect, slightly more free flowing. What are your guys thoughts? Cheers!

 

[grizld1]

If you relieve exhaust pressure, you alter not only A/F ratio but the whole rhythm of the way the exhaust behaves (and enough on that!) Yes, you need to rejet. No, I have no recipe for you. Yes, there's how-to help on the board already, and if you're willing to put in a little work of your own to serve your own self you'll find it. You got on that horse. Get off it or learn how to ride, your call for your self.

 

[xsbean]

Yeah I have the carb guide so I’ll follow that, wasn’t sure whether to bother or not just because I wasn’t sure whether it actually would alter back pressure as the exhaust slims at that point anyway

 

[grizld1]

"Back pressure" is a tricky little term. Most use it to mean restriction in the exhaust system. Some, better informed, use it to mean the upstream beat of the resonant pulse. Some just sigh when they read/hear it....

Anyway, best of good wishes to you in your tuning.

 

[650Skull]

have you clamped the muffler over the external pipe of the header.............The internal pipe has to be sealed off to the muffler, so the exhaust fumes won't travel back between the inner pipe and the outer pipe on the header.

scroll down to Post #19. is is one way to solve the issue.
https://www.xs650.com/threads/after-market-exhaust.276/

This is another way. A shop done this to fit aftermarket mufflers

 

[xsbean]

Circling back to exhaust back pressure, am I right in believing that by shortening the restricted length of the header pipe to fit the new mufflers, the negative and positive waves in the larger section of the muffler are reversed back to the exhaust valves at an altered the time at which negative or positive back pressure flows back into the cylinder and perhaps out of the intake? I’ve been reading a lot and now I’m unsure whether to actually start my tuning by leaning out or richening. Generally larger pipes mean richening a mixture but I’ve also read differing articles

 

[arcticXS]

You are correct in assuming that altering the header length will change the pressure wave timing. Generally, shorter headers tend to reduce low end power, and improve high end.
As for your jetting, exhaust changes normally require less jetting changes than air filter mods.
I would recommend slightly richer as a starting point. That is one or two steps up on the main, and maybe one position richer on the needle (either by clip position or shimming) And test from there. You may well end up where you started .

 

[xsbean]

Right so I’m back on here to bounce of the experts again. So I’m running bigger mufflers and unifilters. Pipes are welded and sealed correctly. I’ve synced carbs, timed the bike, checked valve clearances etc. I’m now at a point where my jetting is nearly right. I’m running standard bs38s. My main jet is 160, biggest you can get. I’m also running a richer needle, and in the lowest clip position and I have only just eliminated the flat spot that I had off idle. However, if I flick the choke on and hit the throttle, it runs even better. Where do I go now, I’ve got the richest needle at the richest position and the richest main. I have a 35 pilot set at 2 3/4 out which is richer than it needs to be. I set the air fuel mixture using the dead cylinder method and got 1 3/4 turns out with the 35 pilot.

Do I go a richer pilot to eliminate flat spotting, running lean off the bottom of the throttle? The bike already has trouble idling, probably because it is so rich. However if I wind the idle adjustment screw in a bit, the idle hangs a bit, which indicates to me that it is lean? Also on deceleration, I get very very minor popping and backfire, but when I flick the choke on in decel, this is eliminated.

Bike is still good and rideable and runs pretty well up in the revs, just the fine tuning I can’t get a grip on!! Cheers blokes.

 

[gggGary]

Man that all seems like you should be sputtering rich. Spray throttle shafts and around the intake side with carb cleaner, while idling, that change anything?
you getting a decent grade of gas?

recheck the synch? especially with carbs off, using the eye-crometer. a running check should need VERY little adjustment.
100% certain strong spark, correct timing at idle and advance, smooth between?
cam crank are aligned properly valves set compression strong?
remove headers and confirm you have a clear exhaust path all the way through (tie a socket on a string?
internal collapse of an exhaust header can happen.

 

[jpdevol]

I don't know what series (model yr) carbs you're running, but I can't imagine 160 main jets running right without something wrong somewhere - air leaks, etc. - and 35 is a big pilot jet for later model BS38's. What needle is in it? What needle jet? What is the float height when measured from the gasket surface (bare) to top of float (upside down)?

 

[bosco659]

Man that all seems like you should be sputtering rich. Spray throttle shafts and around the intake side with carb cleaner, while idling, that change anything?
you getting a decent grade of gas?

recheck the synch? especially with carbs off, using the eye-crometer. a running check should need VERY little adjustment.
100% certain strong spark, correct timing at idle and advance, smooth between?
cam crank are aligned properly valves set compression strong?
remove headers and confirm you have a clear exhaust path all the way through (tie a socket on a string?
internal collapse of an exhaust header can happen.

Eye-crometer - gotta get me one of those. . I’d imagine they’d be tough to keep calibrated.

All good advice. I agree that sounds like a super rich set up. Something else must be off, start with checking the basics if you haven’t done so already.

 

[xsbean]

I’ve got late model bs38s. I’m running early model diaphragms and slides. I don’t think that would make a difference, I’ve measured needle lengths and cutaway also.

I’ve got the z2 long needle jet and the just needle I’ve got is a 502 but it comes in the keyster kit, it’s called a 502 RR which is an even richer needle, had a steeper needle angle and thinner point.

Float heights are 24.5 mm. Also, these are a completely rebuilt set of carbys, with new throttle shaft seals. I thought I’d covered it all.

I’ve tried spraying the intake side and from memory, no change. But I will try again.

I’ll check the sync again and I’ll double check the exhaust, I’m fairly certain they’re all clear as they’re relatively new.

I’ll recheck all the above mentioned things. Compression however, I don’t have a reading on. I just go off the fact that it starts easy hot or cold and rides hard. But having said that, I do get a bit of blowby, both from the oil filler hole (if I take the dipstick out) and out of the crankcase vent.

Very confusing! Because when I’m up on it, from about 4K to redline it screams! I was running a 142 main jet prior and I had some surging and dying out before redline, I would assume that is too lean.

 

[gggGary]

late model bs38s. I’m running early model diaphragms and slides. I don’t think that would make a difference, I’ve measured needle lengths and cutaway also.

I’ve got the z2 long needle jet and the just needle I’ve got is a 502 but it comes in the keyster kit, it’s called a 502 RR

Oh man, you are a brave soul.
Yeah my list is kind of clutching at straws, looking for the odd gotcha that might have been missed.

How late, how early?
bike looks to be 78 or 79 but straight up, I couldn't help with deciphering those parts together.

 

[5twins]

You can't run 5O2 needles in the early slides, they're too long. That's probably why you have lean running issues and are having to jet so rich to try and compensate. Here's a pic of the BS38 needles. You would want a 4 series needle for the early type slide .....

The '78-'79 slide that uses the 5O2 needle mounts it up higher in the slide. That's why it's longer than the earlier needles. If you measure the amount of needle hanging out of the bottom of the slide, a 5O2 in the later (correct) slide is about the same as the 4 series needles in the earlier slides.

 

[bosco659]

You can't run 5O2 needles in the early slides, they're too long. That's probably why you have lean running issues and are having to jet so rich to try and compensate. Here's a pic of the BS38 needles. You would want a 4 series needle for the early type slide .....

View attachment 237984

The '78-'79 slide that uses the 5O2 needle mounts it up higher in the slide. That's why it's longer than the earlier needles. If you measure the amount of needle hanging out of the bottom of the slide, a 5O2 in the later (correct) slide is about the same as the 4 series needles in the earlier slides.

We’re so lucky to have this knowledge on this forum. Looks like 5T has a solution for you and it makes sense.

 

[jpdevol]

Carbs are very precise buggers and a slight parts mismatch can really foul things up. The carb body is actually different between early (306) carbs and later (2F0). It might be possible to find a needle that would work, perhaps a 4M1 or even 4JN to match the slides you have, but IDK. Here's a source if ya wanna try:
http://www.nichecycle.com/ncs/categ...ng-tuning/needles/102-3890-needle-by-ncs.html

http://www.nichecycle.com/ncs/categ...ng-tuning/needles/102-3650-needle-by-ncs.html

I suppose the best solution is to find a set of late model ('78-'79) slides to match the rest of the set-up and run the 5O2 needles - perhaps @ Dogbunny has something used.

Once you get it right, you'll be back to near stock jetting - even with your pipes - perhaps a 30 pilot and 140 main.

 

[gggGary]

There may be other issues with Frankenstein carbs. The late 78-79 body's vent the float bowls through the intake bell. Earlier parts (float bowls) will allow outside air in also. Results? We don't know. The method of retaining the needle in the slide is different in those late carbs also.
Pilot jet series need to be matched to year/type of float bowls also.

 

[5twins]

The '70-'73 4JN19 needles won't work either, they're too short. The '70-'73 carb sets had a raised floor and a longer needle jet. The jet sat up higher and that's why the needle was shorter. The '78-'79 carbs had the lower floor and used a shorter needle jet .....

I'm thinking the 4M1 needles may be best but combined with your lean Z-2 needle jet, mains will need to be around 140. You're kind of in uncharted waters here as there hasn't been a lot reported about mixing and matching carb parts like you're doing.

 

[xsbean]

Yeah that’s good I do actually have 4M1 needles also. I measured them all up with verneers and the lengths seemed exactly the same between the early and late model slides. Unless I got mixed up! But I do have the late model 78-79 slides too, the diaphragm in one of them is holed but, hence why I used the other early model slides, as I could by replacements. I think I’ll give the 4M1s a go for sure, I neglected to try simply because they were so much shorter but essentially I’m getting close to that short running my needles right at the bottom clip position anyway. Thank you all for you knowledge, some proper wizards on here!!