Setting fuel mix screws

Y

Yamablue

Member
Joined
Mar 7, 2004
Messages
221
Location
Ham Lake, Mn
How am I suppose to set my fuel mix screws when I can adjust them all over the board and it doesn't effect the throttle response or idle quality. THe only way I can tell I'm doing anything is to look at my EGTs at idle. I have them set pretty close on temp, but the screws are not close to the same turns out. If anybody wants to know pipes and probes. 02 Viper, CPR pipes with the probes set where Jeff speced them, 7 or 8" fro the piston. Mountain Perf head with 20cc inserts. Lake race ported cylinders. 38mm XCR 800 carbs. I have the EGTs running around 400 at idle, and if I shut the motor down and check the plugs they are wet. I have 45 pilots in them right now, down from 50s last year.
 
MR. sled,
those are air screws, they adjust idle, slightly off idle. make sure you slide heights are all exactlly the same! try around 3 turns out. clean out the motor and warm it up first.
 
those are fuel screws, not air screws.

sorry triple xxx, they are NOT air screws, but are indeed fuel screws. They control the amount of fuel for the idle circuit and partial pilot circuit as the 2 overlap each other. If you remember, the air screws such as those on the round slide mikuni's, were located on the rear of the bell, thus into the airjet passage, letting more air be mixed with pilot jet fuel supply. They were adjusted by turning the screw out was leaner= more air. The flatslide carbs on the yamaha's are fuel screws, located on the front of the throat of the carb, tied directly to the pilot jet circuit, and thru the pilot feed hole in carb throat, the air is now regulated by the air jet, located in the rear bell and those jets can be changed as well. The further out the screw from lightly seated, the richer the mixture is.

sxviperfan, turning those out that small amount does not make the sled use alot more gas, it was probally wet heavy snow, alot of track spinning, and or the clutching.

yamablue: are the carbs you have mikuni or kehin? it matters as some are air screws, and some are fuel screws, this may be what triple xxx was trying to say????? let us know.
 
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700,
He is not running the yami 33mm carbs, he has 38mm polaris carbs. the 33mm carbs are the only ones with fuel screws, the other TM flatslides use air screws. Sounds like his carbs are out of sync a little. To sync the carbs, remove the top plates, and there is an eccentric bolt with a lock screw. I use a drill bit to get them even. you need to remove the air box, and set the center carb first. Don't use the sharp side of the bit! ;):D
 
Yamablue said:
How am I suppose to set my fuel mix screws when I can adjust them all over the board and it doesn't effect the throttle response or idle quality.
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be sure to clear it out between adjustments, they have a tendancy to build up fuel in the crankcase and that makes it a bastard to "tune on the fly"

vipertriplexxx said:
MR. sled,
those are air screws, they adjust idle, slightly off idle. make sure you slide heights are all exactlly the same! try around 3 turns out. clean out the motor and warm it up first.
Click to expand...

bingo

vipertriplexxx said:
700,
He is not running the yami 33mm carbs, he has 38mm polaris carbs. the 33mm carbs are the only ones with fuel screws, the other TM flatslides use air screws.
Click to expand...

from my knowledge this is accurate
 
well WTF...the air jets are in the bell of the carbs but I suppose you could call them air screws in the bottom as they adjust the air mix with the fuel.....am I wrong on this too??
 
MrSled said:
well WTF...the air jets are in the bell of the carbs but I suppose you could call them air screws in the bottom as they adjust the air mix with the fuel.....am I wrong on this too??
Click to expand...

on the 33mm carbs the one in the bell I call the "air screw restrictor" dunno where I picked that up from, I think from bender, not 100% sure tho.

the 38mm's are backwards to that tho, no air screw restrictor, just an air screw on the side.
 
screws

No Sled, your not wrong, I think triple xxx was trying to say that about the polaris carbs, thats why I posted that last line of my reply, wasnt real clear to what he was referring to. Mikuni makes the carbs both ways, air and fuel screws.

Jr.......On the yamaha carbs, they are air jets, I thought you worked at a yamaha dealer.......
 
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bahhhh I'm done playing with you man, I have a picture in my head of how it works that I know is accurate, if I'm mislabeling stuff so be it.
 
they are called idle mixture screws in the book..I would say they mix fuel and air...and can affect all aspects but most often the idle and low end more then any other area.
 
sorry triple xxx, they are NOT air screws, but are indeed fuel screws.
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Sorry 700 you are indeed wrong! I don't post sh*t I don't know for shure, and if it's an opinion I'll tell you. If your going to tell someone they are wrong, you better be dam shure of it! We are NOT talking about yami carbs as stated in the begining of this thread. Sled your right about TM33's, but they are totally different than TM38's.
The 38's have a air pilot jet in the bell of the carb. making this larger, leans the pilot circuit. kinda like making the pilot jets bigger. You might want to try a larger pilot air jet in that carb, it will help bring the idle leaner, along with the pilot circuit, it supplies air to both idle and pilot circuits. you may want to install the original pilot jets for testing. heres a link to buy them. get the 1.2 size, cause i believe you have 1.0. but please check, the size you have it may be a .7
http://psndemo1.com/eshopprod_cat_1286-10275-10277_product_125300.MIKUNI_AIR_JETS.htm
 
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tripple XXX...thanks for that info, I was not aware of the 38 being like that...I must have not read that post completely and for that I appolagize.....
 
Those dam polaris's are all feak'in backwards Ehh? LOL. Sorry Mrviper, I flew off the handle, easy to miss that info at bottom of post.
 
Thanks for the info. The Polaris carbs do indeed have fuel screws, along with a pilot air jet in the front airbell. As far as my carb sync, in the numbered drillbit set One slide was off a drill bit number size. Very small difference. Anyway, the stock carb specs were 440 main, 50 pilot, .75 fuel screw, .9 air jet, 145 starter jet, P-4 fixed needle jet. I currently have them set at 470 mains, 45 pilot, 145 starter, .9 air jet, fuel screw ?? turns out. Correct me if I am wrong. The pilot jet does not affect the mixture from .5 throttle and up. My EGTs are around 1230-1250 at .75 throttle, and 1150-1170 at WOT, with air temp around 25-29. I am going to drop the mains down to 460s and turn my fuel screws out another .5 turns. Since I installed uni filters my carbs are vented to the underhood. Does anybody think there would be any difference in the high speed mixture if I ran some vent lines up into my handlebars?
This thing really goes through fuel. I ran 10.5 gallons of gas through the motor in about 60 miles. But at least I getting a good base on the pistons to some piston wash.
 
Nope, your totally wrong on this, call your local polaris dealer and ask them what kinda screws are on the carbs! 1999-2000 38mm tm flatslides used on the xcrs and ultras are air screws. Please someone besides me CALL AND POST! Here's a link to hot seats pipe install pages, it tells you that they are air screws. The polaris twins with the 40mm tm's had fuel screws, as do the 33mm tm's on yami's. Not the 38's. You guys are driving me to ;):D LOL. :o| While your at it, ask them why your to rich at idle.


http://www.hotseatperformance.com/Instructions/2000 XCR 800 Triple Pipes.doc
 
triplexxx..

I dont think your reputation is gonna be hurt here...lol, dont sweat it.

BTW_ on the mikuni 38mm flatslides on the arc. cats triples from late 99-02 have BOTH fuel screws and air screws. I just did a full service on a 2000 thunderkitten. Never really payed attention to them before, this may be the same on the polaris rack????
 
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