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Old 01-22-2008, 04:49 PM   #1 (permalink)
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Setting Mikuni Carbs

My ’97 MXZ 670 is running a little rich, is setting the carbs a big deal or should I get a dealer to do it? I am mechanically inclined but I have never set Mikuni carbs before and I have no any factory settings to go off. If anyone has any advice or any spare manuals, they do not need anymore that would be great.
P.S It has twin carbs.
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Old 01-22-2008, 07:10 PM   #2 (permalink)
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The first thing to find out is it running rich at idle, mid range or full throttle? There are different adjustments you can do for each. Some reading for you:

http://www.motorcyclecarbs.com/carbs101.pdf
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Old 01-22-2008, 07:59 PM   #3 (permalink)
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Thanks for the article –smallengineguy- that really helped me out. Just a few extra questions:
* How do I set only one carb at a time?
* There is one adjustment screw on the belt side of each carb, where is the jet needle?
* How do I set the idle? It is currently idling at 1100 and should be running around 1800 from what I am told, correct?

* Finally, would anyone have factory settings I can try for basis point, to get me on the right track?
Thanks again.
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Old 01-23-2008, 06:43 AM   #4 (permalink)
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Idle is set with the LARGE brass screw on the side of the carb. Turn BOTH carbs equally so idles are the same. The slide needle is the long needle when you remove the slide from the carb. There is a clip that should be in the middle slot on the needle. The air bleed screw is on the side of the carb, kinda diagonal, toward the engine. it had a slot for a screwdriver to go in. That should be one turn out from closed, but you'll need to adjust it a quarter either way to get the best results.
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Old 01-23-2008, 08:18 PM   #5 (permalink)
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Thanks again for the helpful info, still running rough at idle to ¼ throttle though. I am going to clean everything again and make sure it is for sure the pilot jet. If I have to change it, how do I know what size to get?
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Old 01-24-2008, 08:47 PM   #6 (permalink)
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what makes you believe your motor is running rich?
have you done any throttle chops and checked plug color or piston wash?
if your motor is stock, i'd leave the stock jetting in it.
your carbs are clean with clear pilot and main jets, correct?
make sure your air screws are in the correct position first, then set your idle to spec.
then make sure each carb is opening at the same time when you push the throttle (sync). if they aren't in sync with each other, adjust the cable accordingly to raise or lower the slide.
then make sure each carb slide is open all the way at WOT, if they are not open all the way, adjust the cable accordingly.
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Old 10-04-2009, 09:34 PM   #7 (permalink)
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i have a 800 triple polaras need carb slider were the best place to get them from i here the black is better ad i hope that the name of the piece call
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Old 10-05-2009, 02:23 PM   #8 (permalink)
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The pilot air jet has a screw that you adjust to let in more or less air for the fuel to mix with. To find what you need to set your carbs to, turn it in all the way and turn it out exactly 1 or 2 turns, and see how she runs. Keep adjusting it as needed until you get it where you want it. It's better to do this during winter because it will change when it gets warmer.
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Old 10-05-2009, 06:07 PM   #9 (permalink)
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*lurking*

Good Info I've also been needing.
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Old 10-10-2009, 03:50 PM   #10 (permalink)
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don't mean to jack, but I can't get this darn sled to idle any lower than 2500. I took the slide out and looked at the needle and they had the clip all the way at the bottom, wide open, and there were puddles of fuel in the carb (on top of the slide)..

I dropped the needle down to the middle clip, and set the idle screw all the way to -throttle closed- but not all the way closed, obviously..

WTF is going on here?? it's smoking pretty good also... Pilot jet too big?
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Old 10-10-2009, 04:55 PM   #11 (permalink)
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Have you tried to adjust the idle stop screws?
Those are the ones with the springs on them.
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Old 10-10-2009, 06:02 PM   #12 (permalink)
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^ yes I did .. those, apparently, open the throttle when screwed in. I loosened them, and screwed them in until the throttle was closed as much as it could be. It ran like that, idled at 1000, then started gradually idling a little higher until it settled at 2500.

fuel mixture is 50:1.
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Old 10-11-2009, 09:17 AM   #13 (permalink)
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Sounds to me like the crankseal is leaking.Smoking and high idle??When you say 50:1 are you pre mixing the gas?Is the oil pump not working?
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Old 10-11-2009, 10:53 AM   #14 (permalink)
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It is obvious that there is an air leak somewhere and you must find it and fix it right away.
It will either be crank seals or carb mounting flanges that are leaking air inti the system.
To test for this, take a can of spray starting fluid or carb cleaner and spray it at the crank seal behind the clutch and around the rubber flanges that the carbs mount to the engine by. If the rpms go up, you have found the problem and then can fix it.
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Old 10-13-2009, 08:39 PM   #15 (permalink)
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^ Would this seal also cause an oil leak? Because there is a considerable 'mess' around the T-stat housing, between the heads and slightly below the exhaust. The bottom end uses gear oil? I've got an oil can on it and it is fed to to bottom end.

There is no oil injection system, I am premixing the oil. (as far as I know, no OI system.)

Can this be fixed with gasket paper or RTV? Or would it be advised to buy a gasket for it? This is a hard engine to find parts for .. it's a 524 or a 535... It originally came with a 440, apparently. this is not the original engine and the exhaust doesn't fit exactly right, it hits the brake. it's also hitting a part of the frame pretty good. I'm gonna have to dimple that, come to think of it.
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Old 10-14-2009, 07:25 AM   #16 (permalink)
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There is no oil in the crank case. The oil it uses is either injected in by a pump, or mixed in the gas.
That little oil tank is to provide lubrication for the rotary valve and the level should not be going down as it is a closed system.
No, the seal on the crank ends would not cause an oil leak, it would allow excess air into the base causing a lean condition (high idle).
It is normal for there to be some seepage at the exhaust manifold and joints until the parts warm up and seal.
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Old 10-14-2009, 06:10 PM   #17 (permalink)
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^ I'll spray it this weekend and look for a leak.. Do you think the 2nd clip from the top on the needle is a bit rich, still? My buddy's clip was on the top and his didn't smoke too too much.

I really don't think it's running lean, because it's smoking a considerable amount.. a Bit more than I'm used to seeing on a 2 stroke, but I'll spray it anyway to check for a leak..
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Old 10-14-2009, 10:40 PM   #18 (permalink)
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i posted a detailed message on this subject for you but the message went into oblivion. email me and i will tell you how to set your carbs up. look in my file
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Old 10-14-2009, 10:49 PM   #19 (permalink)
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It will smoke some even when you have a lean condition.
the normal setting is for the clip to be on the middle notch.
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Old 10-14-2009, 11:20 PM   #20 (permalink)
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as i said look in my file and i will coach you on setting your carbs
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