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Old 01-07-2013, 03:24 PM   #1 (permalink)
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Question general carb tuning

It may be hard for you guys to relate well to my verbs but i will try.
I have 2 yamahas with Mikuni carbs. Both have been cleaned, I am trying to fine tune the pilot screw or air screw. they both run well at high rpm.
the 97 Vmax 600 twin liquid cooled idles well but is "gurgly" or slightly "sputtering" sounding when accelerating off the line. then it really takes off. I am thinking this is too rich (screw not out enough). What do you think? They are already out 3.5 turns and book is 1.5 I think.

compare to my 1990 Phazer ll which runs better at low rpm's with sl choke. this is too lean, right? Pilot screw needs to go in?

Or is it something else? Sleds are sure hard to test ride, eh? I'm trying to make some educated guesses in the garage.
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Old 01-07-2013, 05:26 PM   #2 (permalink)
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The more you wind the screw out the leaner it gets because its allowing more air to travel past the taper on the air screw (hence the term "air screw") Turn the screw in would mean richer (less air). 1.5 turns from bottem sounds about right. Make sure the carbs are insync with each other. Thats very important when tuning an engine with 2 or more carbs.
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Old 01-07-2013, 06:10 PM   #3 (permalink)
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Thanks. What I forgot to say was I think my 2 yamahas have the opposite problem. The Phazer bogs and catches especially if you choke it.
the Vmax gurgles while it winds up till mid rpm then smooth.
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Old 01-07-2013, 06:11 PM   #4 (permalink)
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If by in sync, you mean everything is set the same, I can do that.
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Old 01-07-2013, 06:35 PM   #5 (permalink)
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The air screw controls air flow to the pilot jet. If you are having bogging on acceleration, turn it in, as ZRTeeth suggests. If you turn it in all the way and have no changes in performance, it's likely you have clogged air or fuel passages in the pilot jet circuit of the carb and a good cleaning may be in order.

What part of Oregon are you in?
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Old 01-07-2013, 10:48 PM   #6 (permalink)
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I am in Oregon City. There is a discrepency on the shop manual I have on pdf. On the diagram the part we are talking about is called the Pilot Screw. This screw is on the engine side of the carburetor so it must be a rare 2 stroke and have a pilot fuel screw instead of air screw. ??? Then in the specs, it adds another part called Pilot Air Jet. This part has a spec of 0.5 which is preceded by a little symbol o with a slash through it. There is no picture or diagram of this part. The Pilot Screw specs say " 1 1/2 turns out".
So I would like to know what I am doing when turning this screw; putting in more air or more fuel by in or out.
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Old 01-08-2013, 01:37 AM   #7 (permalink)
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I'm definitely not the resident Yamaha expert, but I do know that some Mikuni rack carbs had air screws and fuel screws and pilot air jets that are at the intake of the carbs. If it's a fuel screw, out is more fuel. If it's an air screw, out is more air (less fuel.) Looking at diagrams for your sled, it looks like a Mikuni rack style carburator. If there aren't both air and fuel screws, it will have a fuel screw, so turning out will give more fuel.

I also did a search and found this thread on another forum that goes through the cleaning process, it looks to be very well done. You may want to look at it for reference.

http://www.totallyamaha.com/snowmobi...ith%20pics.htm
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Last edited by BC_Dan; 01-08-2013 at 01:38 AM. Reason: added thread link
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Old 01-08-2013, 07:22 AM   #8 (permalink)
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The Vmax should have Rack style carbs and they would have 'fuel' screws as well as air screws?
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Old 01-08-2013, 08:41 AM   #9 (permalink)
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many different terms for the same thing. air screw, pilot screw, slow adjustment, etc. as a rule of thumb, if the screw is on the airbox side, its an air screw. turning this screw in reduces the amount of air and richens the slow speed circuit. if the screw is on the engine side of the carb, its referred to as a fuel screw but often times still called the pilot screw.

on your carbs, both sleds, your adjusting the air intake at low speed. turning the screw in reduces the amount of air(richens), out increases air(leans). the air jet your referring to is located in the opening of the carb on the airbox side. there are 2 or three holes if you look, and the air jet is usually at 5:30 or 6:00 position. very tiny opening in this and its easily plugged often times creating the issue your having and often overlooked when cleaning carbs. if its blocked or partially blocked, the air screw will have minimal to no effect when adjusting. these air jets are available in different sizes, thats why yours is noted as .5. rarely are these changed except for serious tuning most times in race applications.

to answer your question, if you turn the pilot, air, slow adjustment screw in, your blocking the air jet, reducing the amount of air and richening the low speed circuit. by turning the screw out, your opening this circuit increasing the air flow thus leaning the circuit.

rich=more fuel, lean=more air.

hope that helps a bit to clear the confusion. more info available if needed.
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Old 01-08-2013, 06:08 PM   #10 (permalink)
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I think I got it YamyPops. Your answer was understandable and complete. Here's what I understand specifically for my Phazer ll: The easy to get at screw on top of outside, engine side is an air screw. I turned it in to richen it up.
I only took out one airjet when I cleaned the carbs because the other side looked so pristine that I used it for a model to put the filthy one back together.
I will mess with air screw in the snow and if that doesn't work, I will indeed take both air screws out (ugh. I lost starter plunger under engine last time!)
thanks again. You know your Yammis!
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