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Yet another XT 350 jetting thread

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Hi, I am in the process of jetting my '99 XT350, wich have an opened airbox and a supertrapp slip-on with 8 disks, and as it is pretty long, I thought about making this thread as a kind of database of jetting. So anyone who succesfully rejetted can post theire jetting here.

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Good idea.

I'm just going to coppy what I posted in another thread.

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I've done quite a bit of work to my carb to solve a mysterious lean condition (after I replaced the intake boots) after it was stored for several years.

I have a slightly opened up airbox, Uni filter and a slightly opened up stock exhaust.

Here is a list of things I did along with some basic instructions.

1. Adjust the Pilot screw to 3 1/2 - 4 turns out. The screw is under a brass cap on the front of the float bowl. Just drill the cap out to get to it.

-you should be able to fix the idle and it will start MUCH easier. It will also help off idle response.

2. Shim the needles. I used the eyelets off of small electrical connectors. 2 on the primary, 1 on the secondary.

3. Replace the main jet on the primary carb with one from an XT600. I think it was a 140. If I remember correctly there are 2 sizes available for the 600 that are bigger than the 350

4. Installed home made Power Now inserts.

-I did this instead of trying to find a Pilot jet or drilling mine out.

I messed with adjusting the linkage to the cv carb, but ended up setting it back to stock because I didn't notice any differance. This was before I discovered how these carbs were actually were designed to work so I may try again.

Y.D.I.S Carb theory -from 1985 XT350 Yamaha service guide

"From idle to half throttle, primary carburetor supplies air-fuel mixture and it operates with secondary carburator at high speed and full throttle.

Due to two venturies, intake port area of each valve is smaller than in conventional engine, their combined area is about 20 percent greater than single.Since 4-valve layout offsets intake port relative to cylinder-bore axis, strong swirl is produced along cylinder wall in low-to-middle range.

Combustion efficiency is significantly enhanced due to this swirl, resulting in improved fuel economy at usefull engine speed."

So basicly you have a small mechanical carb for down low where you need the response a higher volocity intake charge will give you and then at full throttle you get both carbs open to give you a bunch of air flow when you need it. Great idea, too bad it's such a pain to tune.

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As far as it goes, left the primary main jet, 2 shims on the needle of the primary carb, redrilled the secondary main and 1 shim on the secondary needle. Fuel screw 3 1/2 turns out. I'll post any update if there are

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I installed the 6sigmaracing jetting kit I bought off Ebay. It came with a #54 drill bit for slide lift hole of the CV (primary) carb, but mine was already that size stock. I went with 2 shims under each needle (Not sure the exact thickness, but I'll try to measure once it's out again). I installed a 130 main in the slide (secondary) carb (The kit came with a 135 as well). I drilled the CV (primary) main with the included #58 drill bit.

I have the fuel screw at 3.5 turns out. I have the snorkle removed, stock airbox, Uni Filter.

What I found:

Idle up to 3/4 throttle was excellent, tons of pull (for this bike), and not a hick-up.

Wide open throttle, especially from low RPM, and the bike hick-uped and hesitated before pulling very well wide open at higher RPM.

So, I pulled the side cover off the airbox, and the hesitation was better, but didn't go away.

So, I put a stock main back in the primary (CV) carb, and it got better, but hesitation still there with airbox side cover on.

Next step is to put the stock main back in the secondary carb, and see if it's cleaned up. If so, I could have saved a bit of money and time by just shimming the needles. We'll seein the next week or so once I get the time to pull the little monster of a carb system off again.

I think with a more open exaust, the complete kit would work beautifully. I just don't want to throw down $200 on a slip-on exhaust that's noisy as hell.

Psycho

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I installed an original style Supertrapp with flexible SS midpipe. With 3 discs it was louder than stock but the backpressure matched the original jetting. The pilot air screw was 4 turns open. I adjusted it to 2 turns and the exhaust is quieter. I am not sure why it is quieter but it is. It is just slightly louder than stock. The low response is better but I have to open the throttle a bit to start it. I bought a jet kit and plan to go larger. I previously put a washer under the primary needle which improved mid range power.

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Keep in mind I did this quite a while back:

1987 XT.

Shimmed both needles. I used very small SS washers from an electronics hardware bin at work. I ended up with perhaps .020-.030" worth of shims on both. I also used mains from the TT version, and having a complete set of submicroscopic drills, "leap frog- drilled" them: I put in what I thought I needed, if still too lean, drilled the smaller one I had, tried it. If still lean, repeated again. That way I could go back to the last smaller sized jets I had. I had modified the original silencer within the pipe, as this was before SuperTrapp made anything. I believe that White Brothers did in those days, but I couldn't afford it. I had some sort of aftermarket air filter, and two fairly large holes cut into the side of the airbox with foam glued on and oiled with air filter oil. I ended up slightly richer than the TT jets, and the idle mixture screw was turned out in typical four stroke fashion: turn out until engine speed stops increasing, lower idle speed, fine tune from there. Find "perfect" spot, turn back in about 1/8 turn. Too long ago, can't tell you what that was, although I had it written down then.

I will tell you it made a completely different bike out of it: it ran very well, started (usually) first kick and didn't get appreciably different gas milage. The vibration was down slightly, and the engine was not only more responsive but smoother all across the power band. Uncorking the airbox made a large difference, and by only messing with the airbox itself, the outer side panels kept any splashes from easily getting in. The pipe mod was drilling additional holes into the baffle, and putting it back in. Not much louder, but you could tell the flow had increased. Nice, reliable, fun bike. Once the carb was sorted out, it was almost as much fun as my old two stroke that it replaced. (but the XT was almost 60 pounds heavier and down about 15-20 HP) can't have everything.

Hope this helps: I know it isn't current, but I spent a lot of time on it, and I was quite successful. I had a lot of experience tuning old air cooled two strokes, and this was my first venture into four stroke singles. I did have experience tuning several XS-650s and their CV carbs, but as you know, this is a combo carb setup. Be really careful about getting too lean: if it spits back out the intakes, it will kill off the diaphragm on the right side carb.

And when you've really had enough of the TK carbs, several outfits, including Sudco, used to sell a set of carbs already set up that bolted in place.

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As of now, The secondary carb is stock, on the primary, I have a 130 main, two shims under the needle, pilot screw is 3 turns out, can't remember if I messed with the pilot jet.

Oh and I have 12 discs on my supertrapp.

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Know this old but anyone know if there is multiple positions for the main needle?

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