BS34 Carbs with open pipes ?

Grewth

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I'm thinking of running high level open pipes on one of my 650 Specials.
I'm going for the "Street Scrambler" look like the old Triumph TR6C.
I'm just wondering if the standard BS34 carbs will be able to cope with the lack of back pressure.
I don't want loads of popping and backfiring on the overun.
At a push I could stretch to a set of slider carbs, but I quite like the BS34's, I'd rather keep them if possible
 
It's not really back pressure, it's the speed to the exhaust flow. When you run the garden hose without a nozzle, the water flows out slowly. Put your thumb over the end of the hose and the flow speeds up. To get the water to flow fast without the nozzle, we need a much higher pressure and volume pump.
What I'm saying is, the carburetors will probably work fine. Your performance will probably fall off.
 
No personal experience with open pipes but if I recall, folks running open pipes/pod filters with bs34s usually jet up. 2 sizes on mains and one size up on the pilot. Not sure about torque inserts, short inserts of smaller diameter that fit inside the head pipes for a few inches at the head end. It would be cool to run "snuffers" like the old Honda CL77. Just some general ideas. Hopefully others can help out.
 
I've run "overly open" exhausts a few times. Basically the XS motor will let some mix straight through the cylinder into the exhaust. The headers will glow red (visible at night) and a tuning sweet spot will be nearly impossible to find. Not to mention a straight exhaust ends up with a higher pitch valve ring that is grating not pleasing.
I ran snuff r nots for a week or two before re-installing the muffler on my 305 scrambler in oh about 1971. (high school)
Aiding my decision; I got pulled over for loud exhaust, the cop reached down turned the snuff r nots shut and said that seems better before walking back to his car. :yikes: :cool:
 
"Snuff or nots"
I've got to admit that I'd never heard of them before, and I've been pottering around with motorcycles for fifty years now.
I don't think that I've ever seen one here in the UK, I guess that they just didn't catch on here.

So, the concensus of opinion seems to be that zero back pressure is not desirable.
Well I suppose that I could always create some, with a perforated glass pack type assembly stuffed up the end of each pipe.
If I drill the fixing holes at the rear, it won't affect the appearance of the pipes.
Obviously some experimentation is going to be required (as always)
Re - jetting the BS34's is no problem, but I don't like the idea of unburnt fuel burning up in the header pipes - that wastes money and also wastes the power that fuel could have generated.
Pipes look pretty good I think, and made this side of the pond too, so they don't cost the earth
Screenshot_20240212_180049_eBay~2.jpg
 
Don't forget, you also have pilot circuit air jets you can change. They don't have as big of an effect as changing the pilot jet itself so with them, you can attain richness levels in between pilot jet sizes. For example, if one up on the pilots isn't quite rich enough but two sizes up is too rich, one size up and reducing the air jets by a size or two may be what you need.
 
"Snuff or nots"
I've got to admit that I'd never heard of them before, and I've been pottering around with motorcycles for fifty years now.
I don't think that I've ever seen one here in the UK, I guess that they just didn't catch on here.

So, the concensus of opinion seems to be that zero back pressure is not desirable.
Well I suppose that I could always create some, with a perforated glass pack type assembly stuffed up the end of each pipe.
If I drill the fixing holes at the rear, it won't affect the appearance of the pipes.
Obviously some experimentation is going to be required (as always)
Re - jetting the BS34's is no problem, but I don't like the idea of unburnt fuel burning up in the header pipes - that wastes money and also wastes the power that fuel could have generated.
Pipes look pretty good I think, and made this side of the pond too, so they don't cost the earthView attachment 261615
It’s not back pressure that you want. It’s proper scavenging. Big open pipes need a big pump or more RPM than we can make to get the flow going. It’s the diameter of the pipe that you have to get right. 2” pipes won’t work well with a stock XS650 running on the street. The short of it is stuffing fiberglass in the pipe won’t make it go.
 
Carbs dont care about what pipes you run. Your engine does. Your engine will require more fuel to deal with the open pipes. You just have to rejet your carbs to accomodate. You have fuel paths in a Mikuni CVK carb. You have the idle jet, needle and main jet. Idle jet will need to be bigger but only about 2 sizes. Idle jet is used up to about 2000 rpm then the tranistion to your needle happens and you may have to raise it or shim it notch. Your main jet sits under your needle so when you come off your needle and it is all the way up the taper no longer comes into affect and you are onto your main jet to WOT. It will have to be significantly bigger, propbably 4 sizes bigger depending on if you have pods or not.
 
My experience with open pipes is mostly on Evo Sportsters and a road race XS. On my Sportster, bottom end grunt was greatly enhanced with the installation of 'lollipops' a few inches from the ends of the pipes. On my old XS stocker road race bike, open megaphones and jetted up BS38s made great top end power but not much else. If I was going to run open scrambler pipes, I'd make up a set of lollipops for them.
1000001404.jpg
1000001403.jpg
 
Yes, they don't call straight, open pipes "drag" pipes for nothing lol. They may make more power at very high RPMs but hurt performance everywhere else (low end and midrange). Unfortunately, that's where a street bike spends most of it's time.
 
My experience with open pipes is mostly on Evo Sportsters and a road race XS. On my Sportster, bottom end grunt was greatly enhanced with the installation of 'lollipops' a few inches from the ends of the pipes. On my old XS stocker road race bike, open megaphones and jetted up BS38s made great top end power but not much else. If I was going to run open scrambler pipes, I'd make up a set of lollipops for them. View attachment 261987View attachment 261988
Somewhere here @ forum I posted pic of my XS. Those included the snuff-or-not lollipop thingies. They work. I never bother to turn 'em open though. Make nice bark, and cops doanbother.
 
If you have straight through pipes with a lengthy straight section at the tail end devoid of baffles it will snap, crackle and pop on overrun.
 
Thanks for all the input, guys.
I'm afraid that I've been tied up with something else the last few days.
I've got two sets of those pipes in bare steel, and I was going to grab another set in chrome (the price seems to go up with each new batch) but now I'm having second thoughts.
I'm thinking about fitting torque inserts.
Probably home made to start with, but I'm assuming that they need to be placed at the same distance from the exhaust port on each pipe.
The one set of pipes I picked up for a song because someone had already hacked them around.
I'll use them to experiment with to get the system (hopefully) right.
Interesting that Triumph just fitted straight pipes on the West Coast TR6C, but I suppose that engine tuning was a lot less sophisticated back then.
 
I think you "tune" straight pipes by altering the length. You start with a long pipe and cut one inch at a time off it until you find the length that runs best. For "AR" inserts at the head, you can use the ends of old stock headpipes .....

650 Headpipe Tops.jpg


You'll need to cut and peel the outer layer of pipe off, then do some filing and sanding until they'll slip into your new headpipe .....

Homemade ARs.jpg


Inserts.jpg


The 650 supposedly likes a small diameter headpipe, at least for the first few inches coming out of the head, and these inserts "fool" the motor into thinking that's what it's got. Then you still get the benefit of a larger diameter pipe with more flow farther down the line.
 
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