How to tune carb for small dia.(1.250 OD)open headers???

[RCGRT]

How to tune carb for small dia.(1.250 OD)open headers??? I finally took some pictures

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Hello to all. I've finally registered and inspected my 1980 XS650 (hard-tailed, somewhat bobber-ish), and it's been fun proving out all the little bugs as I've actually taken it down the road, and up to highway speed. In any event, I was hoping you might be able to help with some tunning issues, or possibly you have some friends who will be able to. I made myself some header pipes from 1.250 OD x .063 wall tubing. You may realize this already, but that puts the ID (1.125) a little smaller than the factory originals. The inner tubes on those were 1.250 OD as well, but have a thinner wall section, about .035. Anyway, I'm running into "Break-Up / Stumble" at 3/4 to full throttle, definitely at full throttle. From what I understand this is from a "rich" condition, so I have dropped from a 145 main, all the way down to a 135. I'm still running into the break-up at full throttle. I've got BS34's with pod filters, and they have the upgraded needles from Mikes XS, clip set at mid position (where he recommends for starters). My guess at this point is the header ID is just too small??? Should I lower the needles? Keep going smaller on my mains? Or do I need to up the size of my exhaust headers? Or am I going the wrong direction all together? Any suggestions would be great. I'm just looking for it to run "good", I'm not really looking for high performance, as I have done no other engine mods at this point. I'll post some pictures of it when I get a chance. Thanks in advance.

 

[XSLeo]

I test the main jet size by finding a slight uphill section of road as long as I can. Get the bike into 4th gear at the start of the test. rollon the throttle to wide open hold for a few seconds. Back off the throttle 1/8 turn. If it pulls stronger when you do this the main can go up one size, retest until it doesn't pull harder when you back off 1/8 turn.
If it stumbles or breakes up then the jet needs to go down one size.
It will take some experimenting to get it right.
Are you just running open headers? How long are they? If I recall, they should be 36-39 inches long for best performance.
If you can read plugs well, plug chops help tell about the jetting.

 

[RCGRT]

Thanks for the input, my headers are right at 29" in length. I'll give it a test run, and try out what you suggested. Also I was curious could this be caused by valve float?

 

[XSLeo]

I doubt you will float the valves in a stock engine. At least at any sane rpm. 10k might.
Once you have the mains and pilots pretty close you can try moving the needle to fix the midrange. Sometimes moving the needle can effect the pilots and the mains. So if changing the needle effects the mains you can try upor down one size to check. On the pilots if the needle change effects it, adjust the mix screw to see if that helps.
www.amckayltd.com/carbguide.pdf is a guide to clean and tune your carbs, reading that may help youunderstand your carbs better.

 

[RCGRT]

Yeah, I do have that Crab Guide, it's a very good reference, learned a lot from it. Ok, so yesterday I had a chance to take it out for another spin. Another guy had told me that the "pod type filters" which I am running sometimes cause a problem with the air intake. After taking them off I could see what he was talking about. The filters have a C-Bore face on the rubber boot that gets pretty darn close to the face of the carbs, if it were touching the face of the carbs, I'd see a problem for sure, but there is a small gap, something like an 1/8 of an inch or maybe a little bigger (I didn't bother to measure). I'm sure however this is less than ideal for airflow, but that's besides the point. I went for a spin without the pods, and there was NO change, if anything it was worse. My next try was to go back to the 145 mains, and I slapped on a set of 1.5 open headers that I got with another parts bike. Took it for a spin, and wow, completely different, it opened all the way up, with very little sputter or break up. So that was obviously a large part of the issue. The only thing I can think of is that the 1.25 headers I made were just restricting the top end way too much. I took a quick measurement of the exhaust ports coming out of the head this morning and it looks like it's right at about 1.5 ish. So my question at this point would be, two things...Do you think the bike can run properly with the 1.25 headers, giving some more tunning? Should I make a new set at 1.375? or 1.5? If the OEM pipes were 1.25, it just seems like 1.5 is overkill, considering I'm not running any other engine mods/upgrades...
Any info would be greatly appreciated! Thanks...

 

[XSLeo]

On the filters you can trim the lip out where the holes in the carb bell are. What I might do at this point is go back to stock 132.5 mains, 42.5 pilots, and see what the engine likes from there. Maybe a 137.5 main and a 45 pilot.
I think the 145 main is too big. That's 5 steps up. Most use 2 or 3 over and run good.
These carb are vacuum operated and easy to over jet. If over jetted the vacuum tries to compensate by not opening the slides fully.
One my 76-77 carbs, with 74 internals. Stock was 127.5 mains, 42.5 pilots. I have a 47.5 pilot with a 132.5 main, if I put in the 130's it runs lean, power boost at the WOT test. A 135 bogs on the WOT test.
So testing the jets is important. When you think they are right test one size both ways to be sure.
I have Uni-Filters, 1.5 inch head pipes and straight through Emgo shorty mufflers. Open headers are harder to tune to. The bigger and shorter the harder to tune.
The 1.5 inch pipe is a good choice. It frees up the stock engine and works great up to an 800+ cc engine. Any bigger and it moves the power band up to high to use well.
If you build a new set of pipes use 1.5 pipe and have them in the 36-39 inch length.

 

[RCGRT]

Ok, all of that sounds pretty good. I'm glad you mentioned using 1.5 OD tubing, because I was thinking 1.375 would possibly do. When I make my new headers, do you happen to know what "shape" I should make for the flange part that will seat into the head? The 1.25 ones I made, I pretty much copied the OEM pipe flange, with the exception, the OEM flange had more of a radius lead in, from about 1.5 down to a little over 1.15 ish, where the one I made was just an angle from about 1.5 down to the inner tubing ID. It was about 1/2 inch long, I don't remember off hand, but something like 20 or 30 degrees, with a pretty much sharp intersection from the angle to the ID diameter. I guess the 1.5 tubing won't need anything like that as it is nearly the same size as exhaust port. I have seen some "torque inserts" I think, on Mikes. Do you know anything about these? Would it do me any good to just build something like that into the flange of the new header?

How critical is it that I make them to that suggested length? I'm assuming the shorter I go, the less performance and harder to tune is going to be?

Is it true if I weld in, say a 3/8 dowel at the same distance from the head on each pipe, (inside of them), that will create some back pressure, giving a little better performance?

The main reason I was running the 145, is because that was the suggested starting point with the jet and needle kit I got from Mikes, for open exhaust and pod filters.

Where can I get UNI filters for these carbs, that's what I used to always run on all my dirt bikes?

Sorry for all of the questions, just trying to learn a little something. Thanks again..

 

[5twins]

I made my own "torque inserts" using the ends of old stock headpipes .....

I think they helped. The bike seems to rev harder and faster now.

You can get the UNI filters at just about any bike shop or order them on-line. I got mine here .....

http://www.motorcycle-superstore.co...ght-Mount-Clamp-On-Dual-Layer-Pod-Filter.aspx

Here's the size chart from the UNI web site .....

http://www.unifilter.com/online catalog/universal.html

For your BS34s, you'll want part #UP4229 in the black pod, #UP4229ST in the dual layer. These are 2 1/4" I.D. x 4" long.

 

[RCGRT]

Cool, thanks for the link. And yeah I see what you did there. Just cut the flange off of a stock pipe basically huh? Were the stock studs long enough after adding the extra flange?

 

[5twins]

Yes, the studs were long enough but I could only use a lock washer under the acorn nut, not a lock + flat as it originally was. I also fabbed a filler ring out of pipe to fill the rest of the step in the port .....

If I had access to a lathe, I'd turn that flange down to maybe half it's original thickness. If you look at the one Mike sells, you'll see it has a very thin flange.

 

[CDNTX650]

145 is way to big, youll be washing the rings i bet.

 

[XSLeo]

I have some 1.5 inch bends I'm going to use to build a set of head pipes from.
I have the ends cut from an old set of stock pipes. I have them cut a bit longer than 5twins shows in his pic. I have mine cut to 3 inches from end to end.
I will use the stock clamp and this section of old headpipe at the head and weld my 1.5 inch pipe to that. It will act as an anti-reversion cone, Torque tubes, torque insetrs what ever you want to call it.
The 1.5 inch pipe is about as big as you can go without some sort of anti-reversion cone. The reason you need the AR cone is to keep the exhaust velocity up at the head pipe connection. This high exhaust velocity help scavenge the burnt exhaust from the cylinder and help with cylinder fill. Also as the exhuast leaves the head it creates two pressure waves, one is sound and the other is the exhaust gases. As these pressure waves move down the headpipe they travel at different speeds. When they reach the end of the head pipe these pressure waves reflect off the end of the pipe and travel back up the pipe. If the valve is open at the time these pressure waves reach the head they can push burnt exhaust gases back into the cylinder. The AR cones stop the pressure waves from reaching the exhaust valves. Helping to keep the exhaust from getting back in the cylinder.
In the early days of drag racing they would start with a too long pipe and run the bike down the track, cut an inch off the pipe, run it down the track. They did this and the times would get better and better until the pipe got to short and the times would fall off. They then made a new set to the best length for the best time.
This worked well at high rpm, wot applications. On the street running at wot throttle doesn't happen often. If it does your local LE's will see a lot of you.
This kind of testing has shown that the longer pipes bring the effective power output down into a more street usable range.
If you want you can try a too long pipe, and keep cutting a bit off and see just were the power feels right to you.
A large pipe, 1.6 or bigger slows the exhaust velocity. The torque cones help bring the velocity back up. Kinda makes the engine think the pipes are the right size.
If you have a stock set of pipes I would cut the ends off and use them. I think this would work much better than a dowel welded in. The dowel will give more resistance to flow and might increase exhaust velocity. I think the turbulance the dowel will create will counter the the increase in velocity.
This exhaust reversion is why the length and size of pipe is important.
Lots of people like the look of large diameter short head pipes. You can get Drag pipes for Harley's up to 3 inches. An open pipe that big kills engine power. Many of the lagre pipe headers for Harleys are being made like the stock XS header pipes. A lagre outside pipe for the look and a 1.75 inner pipe to get it to run right. Many have a conventional muffler inside the large pipe hooked to the inner pipe. Very expensive though.
On Mike's jetting suggestions, many have tried them then came here for good advice.
You can order the Uni-Filters off line or order them from a local bike shop. Local is a bit more but most don't charge shipping, so it evens out.
I got the dual layer filter. A black inner filter and a red cover filter. They cost a few bucks more but I think on my dusty roads out here in the boonies they will be better.

 

[RCGRT]

What is "washing the rings" exactly? I was under the impression running richer vs leaner would run a little cooler, a little less power, but a happier engine overall? Please correct me if I'm way off.

Wow, that's a lot of info! Awesome! I'll have to read that a few times to absorb all of it, but I certainly will. The whole velocity thing and the "large pipe OD" is exactly why I chose to make my headers from 1.25 OD tubing, that is what the bike came with, the only difference is that I got .063 wall, which puts the ID at basically 1.125. Well the OEM pipes are thinner walled, like .04 ish so the ID on the factory pipes is just a little bit bigger. However, I did make the flange piece quite a bit different than the OEM one, it was essentially the same size, but a little different shape. It ran great in the neighborhood, but once I got it out on the highway, it just started like breaking up pretty bad at 3/4 to WOT...Those headers measured about 29 inches in length by the way.

I do have access to a lathe, I run CNCs all day, so I can pretty much make whatever I want, the only problem is I'm not exactly sure what shape or size to make it, haha. So I'm planning on getting some 1.5 OD bends, and making a new set of headers, so, since I can, it would be nice to make whatever will work the best...

Thanks for all of the info on the UNI filters, I'll definitely look them up.

I'll take some pictures of the 1.25 ones tonight and post them tomorrow so you guys can see them, (and the rest of the bike, it's just a rat bike/bar hopper) I'll try to take pictures of the inside of the flange, it's pretty black in there though.

Thanks again..

 

[RCGRT]

I finally had a chance to take some pictures.

 

[XSLeo]

Washing the rings means the mix is so rich that the oil gets washed off the cylinder walls. This removes the lubricant from the walls and rings. This wears the rings out very fast. Not a good thing.
I don't think I could enjoy riding your bike. Open pipes are too loud. Having the empty out so close to my ears would not be pleasant.

 

[RCGRT]

Ok, I see, so then going too rich could potentially be a bad thing. Yeah, it is very loud, definitely not for the long road thats for sure. It's actually a little louder than I care for my self. I've actually been thinking, the more I read on exhaust, I'm wondering how much more performance I would get out of it, by just making new pipes to fit an aftermarket exhaust system...??

 

[XSLeo]

It may not make more power, just bring it into a place on the rpm range where you can better use it.