December 4th, 2008, 04:29 AM | #1 |
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DIY - Accessing and adjusting idle mixture screws
While in the grand scheme of things the idle mixture screws are the last step in carburetor tuning I'm covering it first. Why? Well a lot of people are content with the main jet (Step 1 in carburetor tuning), and a lot of people have already experimented with shimming their needles (Step 2 in the process, and strongly encouraged by me). There's a step 3 (adjusting fuel bowl float levels) which isn't really discussed as its impact is really low and evidently pretty spot on from the factory (although I'm still going to write a DIY up on it ).
So then we get to (in my opinion) the least appreciated and most overlooked bit: adjusting the idle mixture. This roughly affects the RPM from from idle - 5,000 RPM. The relationship to the effectiveness of the idle circuit is linear to the power-band. (The higher the RPM the less effect it carries). What is the idle mixture screw? There are TWO jets in the bowl of the carburetors. The main jet and the pilot jet. The pilot jet is what is 'active' when the throttle blades are mostly closed. There is little reason to have to change it's size as the "idle mixture screw" fine tunes the circuit. With the idle-mixture screw all of the way open fuel can flow to the maximum that the pilot-jet can flow. This isn't really feasible unless you have a heavily modified 250 . So for 99.9% of us the idle mixture screw limits the fuel flow into the idle circuit and there's no need to bother with the actual jet... The catch: From the factory the carburetors have caps preventing the end user from turning this screw . I don't blame them as whenever you give people the option to turn things they DO. So let's begin : Removing the caps: First you need to remove the front fairings to gain easier access to the carburetors. Not too difficult to do since you've done it before to shim the needles RIGHT? The carburetors basically are identical so I'll just show the one (as the other one follows along the same lines... The two circles towards the right of the picture are carburetor 1 and 2's mixture caps. I have the one carburetors fuel bowl off and you can see the pilot jet (not too clearly, but no matter). While you don't need to remove the bowls I did (for a complete different reason actually) to put things into perspective. NORMALLY it is suggested to remove the carburetors to drill these caps out. You don't need to do this and here's how: That little handy devil makes EVERYTHING carburetor-wise on these bikes a cinch. It's a Milwaukee Offset Screwdriver and damn near every hardware store sells them. They have a standard 1/4" hex shaft that in any 1/4" ratcheting handle or drill can attach to (Like I have in the picture). Since these caps are thin aluminum caps I did it by hand... Start with: #8 Sheetmetal screws (Handy to have) and the head fits right into the 1/4" end of the M.O.S. It's so stupid simple with this I didn't get an "action shot" of drilling the caps out as it took seconds. With one hand you hold the M.O.S. and put pressure on the screw. With your other hand crank away at the handle. Once it threads into the cap (Not much, just enough really so that your fully perforate the cap) you can stick a small screwdriver into the hole or just use the screw to jiggle the cap free. Here's one of mine... Do the other side and then you can start playing with the idle mixture screws . Adjusting the idle mixture screws: Like any carburetor tuning it should be done with the bike warmed up! With the fairings off just go for a quick jaunt around the neighborhood. Make sure that afterwards if you touch the side of the block it really hurts! That's a good way to figure if it's warmed up. The way I found to get you into the ballpark is to (one carburetor at a time) tighten down the idle mixture screw. Again the M.O.S. with a flat-blade bit in the end fits perrrrfectly to the idle mixture screw. Start tightening the screw down until you can hear the engine idle suddenly drop. Then back the screw OUT 1/4 of a turn or so until the idle is restored. That is your "base" so-to-say. Now blip the throttle. Nothing crazy, just rev it to 2,500 RPM or so and let go of the throttle. Does the throttle immediately drop to idle? If not: If the throttle hangs - The idle circuit is still too lean. Loosen the screw down a little more If the throttle drops below idle and takes a moment to return - The idle circuit is too rich. Tighten the screw a little bit. Then repeat the process until the idle is restored damn near instantly after you blip the throttle. I try to err on the side of 'rich' as this will help the bike warm up a little quicker and isn't going to REALLY affect fuel economy or anything in the end. Repeat for the other side's carburetor. Once you're satisfied with it take it out for a quick ride to make sure things are O-tay! Like any tuning practice don't make huge jumps in the adjustments. It's really easy especially with 4-cylinder bikes to go nutty turning screws until you don't have a leg to stand on. If you think it's perfect from the get-go, make a mental note at the position of the bit on the idle mixture screw and see if you can mess the idle up and then fix it. If it's back where you had it before, then you truly had it right from the get-go! If you find in the end the bike is stumbling off the line then go back to the drawing board and fiddle some more! Last futzed with by VeX; January 7th, 2009 at 02:31 AM. Reason: updating pictures |
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December 4th, 2008, 08:44 AM | #2 |
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Name: Wayne
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Another great job, VeX. Keep 'em coming!
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December 4th, 2008, 12:14 PM | #3 |
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Excellent info, as usual. I agree, the mixture screw adjustment is the most often overlooked of all the tuning jobs. All the other adjustments are either "on" or not. With the idle mixture, because of this mixture screw, one can fine tune the circuit within a gnat's ass. And as you pointed out, you can't just say... set it to so many turns out and it's done... far from it, that's only your rough starting place to fine tune.
I will add, if you make any changes to your jetting (main or needle clip position), you will most likely need to go back and readjust the mixture screw (which is why they say to do this adjustment last) as the other components will have some affect on the idle mixture... that's what I've found, anyway, with my playing with the carbs. Also, after you're done, check your carb sync.. they may be off due to the rebalancing of the mixtures. some pics of removing the caps for clarity... Great Post, Vex!! Thanks for your contribution. |
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December 4th, 2008, 04:00 PM | #4 |
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VeX,
Thanks for the DIY. You reminded me why I needed to order a MOS. I was hoping to avoid taking the carbs off, in order to remove the plugs, and you have fulfilled that need. If anyone is interested, the MOS can be ordered using the following link: http://www.northerntool.com/webapp/w...574&in_merch=1 Shipping is FREE!!!!!!!!!!!!!!!! |
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December 8th, 2008, 12:35 AM | #5 |
ninjette.org member
Name: Felix
Location: Brisbane, Australia
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Ok, I feel a little dense asking this but...
If I think my bike is running a little lean should I look at doing this? I've shimmed to .6, no snorkal and full 12inch AreaP. I used 3 x .2 shims, I feel like I should consider re-shimming with larger shims, instead of having three on there. Going to up the octane next fill just to see if it is pinging that I can hear and not some other noise. Thoughts? Felix |
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December 8th, 2008, 12:47 AM | #6 |
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At what throttle openings are you hearing the pinging?
have you changed the main jets at all? |
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December 8th, 2008, 01:12 AM | #7 |
ninjette.org member
Name: Felix
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No, pretty much at just under WOT.
Problem is I ride with ear plugs in, and today I didnt and it was the first time I noticed it. Felix |
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December 8th, 2008, 01:19 AM | #8 |
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So you're hearing it only under heavy load conditions?
What octane are you running now? Going up a step next fill up is a good idea if it is pinging. if you have chance, pull the plugs and check the insulator color. |
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December 8th, 2008, 01:25 AM | #9 |
ninjette.org member
Name: Felix
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Yeah, I'll fill up tomorrow... running 91, I'll put in 98, and see what the difference is.
Felix |
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December 8th, 2008, 01:26 AM | #10 |
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Please let us know what you find.
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December 8th, 2008, 03:07 AM | #11 | |
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Name: TJ
Location: Ames, IA
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Quote:
I'm stealing this picture, but the pilot circuit has no effect on the overall fuel delivery at WOT and the needles stop being effective past 3/4 throttle or so. You should consider upping to a 100 main jet AT THE LEAST (DJ #98). It's got to be confusing on the forums with all us Yankees talking about 87 octane gas all the time even though most of the rest of the world goes by RON only |
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December 8th, 2008, 04:29 AM | #12 |
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Name: Felix
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Going to try the fuel tomorrow, and see. You guys have seen my AFR chart and dyno run before, its lean but not MEGA lean. I'm trying to get away from jetting since I'm not planning on keeping the bike long, and the aussie/us exchange rate is still ****.
I guess if I was going to spend money on a last mod it could be a jet kit and carb sync. Felix |
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December 8th, 2008, 04:50 PM | #13 | |
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Quote:
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December 8th, 2008, 05:02 PM | #14 |
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I like that one... see how the different circuits interrelate with one another at some part on that graph? ...which leads me back to my statement about how if you change any part of your jetting setup, it will affect the other settings you have in different circuits. It's not as simple as saying, high speed= the mains, middle= the needle clip position and lows= mixture screws.
I.M.S.??? |
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December 9th, 2008, 11:47 PM | #15 |
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Name: Felix
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Ok loaded the different fuel, had a play. I was wrong in my first statment its not anything to do with throttle position, its just the same lean spot in the 6.5 7.8k range. Kinda annoying... suggestions? Clear as day on the table.
Felix |
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December 10th, 2008, 12:38 AM | #16 |
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try adding another shim.
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December 10th, 2008, 01:39 AM | #17 |
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Name: Felix
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December 10th, 2008, 01:56 AM | #18 |
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December 10th, 2008, 02:18 AM | #19 |
ninjette.org member
Name: Felix
Location: Brisbane, Australia
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Not so much that, yes we have hardwares, just dont have a way to check thickness.
I was under the impression, proper shims was the best way to go. I also need to chase down the external and hole diameters Felix |
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December 10th, 2008, 02:25 AM | #20 | |
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Quote:
beats running lean and ruining the engine. |
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December 10th, 2008, 08:11 AM | #21 |
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Name: Anthony
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I've got my Ninja 250's idle mixture screws set to 2.5 turns out helping with the idle lean settings, but I noticed when I cleaned my original carbs the previous mech.'s had put in a washer on each needle. I removed it and found the reason they had put them in, as I then had a midrange hesitation, re-installed the washer on each needle, problem fixed.
Tony
__________________________________________________
1997 Yamaha Seca II - mostly stock, Racetech upgraded forks, FZ6R rear shock, Oxford Heated Grips, Barkbusters Blizzard Handguards, a Scottoiler vSystem chain oiler. My Mileage Tracker Page. |
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December 10th, 2008, 01:55 PM | #22 |
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Name: TJ
Location: Ames, IA
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That's a pretty decent power curve (minus the aformentioned spot 5500-8500).
It really does wreak hell on a dyno run when the A/FR falls higher than 13:1 and sure enough right where your needles are 'effective' it's high... Someone over on KF was pissing and moaning that removing the air filter box kills horsepower and posted a dyno chart. Their A/FR's were ATROCIOUS and the torque curve followed suit. They didn't fall below 13:1 A/FR until like 9,000 RPM and they blamed it on removal of the air filter box when obviously they didn't touch their needles or idle mixture screws |
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December 14th, 2008, 03:44 PM | #23 |
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Used the MOS and #8-1/2" self-tapping screws to remove the idle mixture screw plugs. My idle screws are now set at 2 1/2 turns as well. Much better idle. Will hopefully get to ride again tomorrow for the after test.
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December 14th, 2008, 04:26 PM | #24 | |
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Quote:
do you know the drill to set the mixtures screws properly with the engine running to fine tune?? |
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December 14th, 2008, 05:39 PM | #25 | |
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Quote:
Also, just an FYI on VeX's MOS technique of drilling the plugs. Initially, I thought I might be able to both carbs from the left side, but no go. Had to do the right from the right. Now that I have removed the covers once, that task will go real quick in the future! Thanks to all you pioneers!!! Even to my nemisis...kkim and the green machine. |
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December 14th, 2008, 07:16 PM | #26 |
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Another tool for adjusting carbs
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December 14th, 2008, 08:11 PM | #27 | |
That's me!
Name: TJ
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Quote:
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December 25th, 2008, 08:16 AM | #28 |
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Results are in
Since I adjusted the idle mixture screws, the temp here has gone into the teens a few times. Prior to the adjustment, I had to open the choke all the way, in order to start my bike. Now it starts with no choke or just a little. I'm able to drive away within a minute or two with no stumbling. I haven't touched (shims) the needles, yet. Still breaking it in, so I haven't had it past 11K RPMs.
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February 8th, 2009, 09:24 PM | #29 |
lolwut
Name: Alex
Location: socal
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FYI Harbor Freight has a offset screwdriver similar to the Milwaukee one, but it costs only $12. This has an extended flexible shaft and a quick release for bits.
http://www.harborfreight.com/cpi/cta...emnumber=96726
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February 8th, 2009, 09:48 PM | #30 |
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good find!
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February 28th, 2010, 09:41 PM | #31 |
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I just installed a Dynojet jet kit on my carbs and noticed that from the factory, the idle mixture screws were significantly off from each other. Meaning, one had much less turns than the other. Is this normal? I set mine to 3 turns per the DJ instructions. Since the original settings were off how will this affect my bike now that they are set equally to 3 turns?
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March 1st, 2010, 08:28 AM | #32 |
self wrencher
Name: john
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Normal
Posted via Mobile Device |
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October 5th, 2010, 10:20 PM | #33 |
Eddie
Name: Eduardo
Location: Colton, CA
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so what happens if someone who tries this happens to drill inner threading and doesn't have full control of these screws? -.-
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October 5th, 2010, 10:33 PM | #34 |
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That depends on how much inner thread you lost. What do you mean exactly by not having "full control"?
Worst case scenario, you're due for some repair or outright replacement of that side your carb.
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October 5th, 2010, 10:43 PM | #35 |
Eddie
Name: Eduardo
Location: Colton, CA
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well what i meant by no "full control" was the most you can turn the screw outward (screwed in all the way as your starting base) is say 1.5 turns out, if i remember right..
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October 25th, 2010, 02:16 PM | #36 |
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Name: Armando
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Hey guys I just got done with shimming my needles 3 washers each and added hmf slip-on and snorkel removed. Now when I blip the throttle it kind of floats for like a second at 2000rpm before it goes back down to idle. But when I'm riding and say I'm stopped at a stop light the idle like drops to 1000rpm then back to idle like it bounces when I'm stopped.
Will doing this help my idle? |
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October 27th, 2010, 04:33 PM | #37 |
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Removing the plugs in order to access the idle mixture screws
kkim provided excellent pictures on how to remove the idle mixture screw caps in this thread. I'm repeating one of the pictures here:
The next picture was taken from the '88 Service manual and illustrates the carb plug (item #2) which must be drilled, prior to using a screw as in the above picture. Notice that there is not a lot of clearance between the cap and the idle mixture screw. |
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October 27th, 2010, 04:39 PM | #38 |
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yes, be very aware of how deep you're going when removing that cap/plug. Here's a picture of the cap removed. Noticed I nicked the mixture screw with the point of the screw in the previous picture. Guess it comes from being a noob... learn from my mistakes.
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October 28th, 2010, 04:20 AM | #39 |
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When I did mine, I used the MOS and a self-tapping 1/2" screw. The force of the screw tapping the plug spun the plug out before the screw penetrated more than a 1/16"! And this was done without removing the carbs from the bike.
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November 5th, 2010, 03:47 PM | #40 |
ninjette.org member
Name: Armando
Location: So Cal
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So i have been trying to adjust my screws and i was trying to use the alternate method and turning it down but i turn my screws down all the way and the rpm doesnt drop. When i Turn them all the way down is that the stock setting?
I turned them down all the way and turned the left one at 2.5 turns out and the rght 1.75 out and im still getting it rich. Like the rpm dips when i blip it. it doesnt do it if the screws are closer to being turned all the way. |
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