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04-03-2007, 8:08 PM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning My opinion is that you should start from the bottom up, identify the richest setting for each circuit and you should be able to find the sweet spot. I should frame it up and say that I have no experience with snorkels, so I'm making a lot of judgements. Will it idle with the a/f screw out 1/4 turn from seated? As you turn the a/f screw out, how many turns do you have to go before you recognize a rich condition (idle speed drops substantially)? If you can idle at 1/4 turn out, go to a 37.5 pilot jet. If you can go out more than 3 turns without affecting idle, go to a 42.5 pilot jet. Ride it around and see how it feels.  Once the above is done, go to work on the mid range circuit. You know where you are today, go down one more notch and see what happens. My guess is that you will end up going the other way,...up to the 2nd notch and possibly the first. Ride it around and see how it runs and feels. Settle on the setting that gives you the best mid range acceleration without stuttering or lethargic performance. Once the above is done, put the 170 dynojet main in and do a full throttle run, extended, say 1/2 mile. if you notice a cough, sputter, whatever, drop down 1 size and repeat until that condition is eliminated. The key is to run it a good half mile at full throttle. This eliminates the question of whether or not the condition noticed is on the mid range circuit or on the main circuit. For what it's worth and that is free, I would typically associate that type of condition as a rich condition on the mid range circuit. More than once have I noticed the same condition (usually shows up in the mud, with a hot motor and you are trying to get to the grunt) and have eliminated it by leaning up the mid range circuit. Should also say that the path to best performance comes from the shortest distance that intake air has to travel to get to the carb. It's the reason why velocity stacks are still around. Extend that path by snorkeling for example and you are going to have to deal with and rethink some of the more conventional methodologies regarding tuning.  To close, a thing or two to consider and they are a bit out there. If you can't get it tuned using the method above consider the following. Plug one of your vacuum ports from the inside on the slide and see what happens. If the condition improves, leave it plugged, if it doesn't, consider opening up both vacuum ports as I mention in the vacuum port modification post. Change back to the stock jet needle and slide spring. If it improves, leave in place, if it doesn't, go back to the dynojet needle. Lastly, there is a balance tube coming off of the carb to the filter boot. Get a 3 or 4 foot tube and run it from the carb outlet all the way up to 3 or 4 inches below the snorkel intake, tie it in and see what happens. Good luck. ww       Roostin' Rhino RAIS Power Tube    Report

04-06-2007, 10:41 PM fasttrax Joined on 08-03-2006 West Jordan Posts 133 Re: Mikuni Performance Tuning Will, Finally got around to attempt jetting. I've never quite understood it. Anyway, I only had a 140 in mine stock and went to the 145. I set my air / fuel screw in and back out two. (It went in around six turns). I put it together and when I apply gas it sputters and dies. WHAT DID I DO WRONG? Also, how are the leg minders fitting?      Report

04-07-2007, 9:24 AM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning Once again, I have no experience with the snorkel setup that you are using, so any opinion I have is pure speculation. I suppose that my first speculation is that the 2" snorkel is the primary culprit for your condition. You primarily ride in Utah which is likely going to enhance the problem due elevation. For example, my RAIS Power Tube has a 2.25" opening which is the same as coming straight off of the airbox. Your snorkel is taking that down to 2" plus it is extending the length that air must flow and gain velocity before it gets to the carb. True that the nature of the CV carb compensates for that somewhat but the carb knows no different regardless. So, you step on it, butterfly opens, slide begins to rise as it attempts to equalize atmospheric pressure. The problem is that fuel is already coming in. To much fuel to fast and not enough air quick enough. Or the other way around,...not sure. My guess is the former. Rest assured that that big piston has some umphhh to it once that butterfly cracks open and yanks that slide open much to quickly and well before the increased air demand reaches the goal of the carb. What happens when you disconnect the snorkle from the airbox? instant improvement? What would happen if you ran a 3" snorkel? What happens if you reduce the length of the snorkel? The solution is to find that point that performance degradation occurs and work from there. Find that point and work both sides of the variable,...wider/narrower, open/closed, larger /smaller...whatever it is. One thing I know is that a Rhino can easily function under a 2.25" air box opening up to and including no cover or canister at all. Atmospheric pressure and air is right at the carb all the time. Bottom line is that I don't think you've done anything wrong, I simply think you haven't done enough to locate, isolate and correct the problem. I hope some of this helps. Before you start anything, make sure to change out your plug if you haven't already. The leg minders came in yesterday so I haven't gotten around to installation but they look like they are going to accomplish the task. Not to keep my legs in but to keep the cottonmouths out. Those are some mean snakes, i have a lot of them on the farm and they don't think twice about sliding right into the floorboard in a foot of water. These things will give me and extra 8 or so inches. ww    Roostin' Rhino RAIS Power Tube    Report

04-07-2007, 9:50 AM fasttrax Joined on 08-03-2006 West Jordan Posts 133 Re: Mikuni Performance Tuning I have the Rais tube and run 5000-6000 elevation. My main was a 140 and I went to a 145. My air /fuel was more than 2 turns out, probably around 6 from factory. Any help would be appreciated.   Ryan    Report

04-07-2007, 1:34 PM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning Attachment: CSC_0071 (Small).JPG I'm sorry. For some reason, I had it in my head that you were running an extended snorkel. Must have been from another post...Sometimes thing just run together! Change your plug first and if you haven't already, pull your carb and blow out all jet circuits and jets with compressed air. No real need to worry about the main right now. However, at your elevation, the 145 should be just about right. You may be able to run a 147.5 or a 150 but the bog/stumble needs to be taken care of first and that is occurring on the idle/pilot/mid range circuit transition. Since someone has been inside your carb before you, I would also determine the size of the pilot jet. It should be a #40. May need a magnifying glass to read it. I assume that you noticed a degradation in idle performance with the a/f mixture screw more than 2 full turns out from seated position, yes? If your initial check of the a/f screw setting was 6 full turns out from the seated position and you have it 2 full turns out now, you should notice improved idle performance. According to Mikuni, if you have to go out more than 3 full turns to get proper idle, then you need to go up a size on your pilot jet. We'll assume that yours was improperly set for whatever reason.  Next thing to do once you have determined pilot jet size is to work the mid range circuit. This assumes you do not determine that you need to change your pilot jet. ww The attached image shows the stock relationship of washers, bushings and circlips on the jet needle. Make sure all are there and in the right order. Is what is in the image what you see when you look at your jet needle assembly, assuming that you have not swapped your thick and thinn washers account the RAIS install?       Roostin' Rhino RAIS Power Tube    Report

04-09-2007, 11:00 PM fasttrax Joined on 08-03-2006 West Jordan Posts 133 Re: Mikuni Performance Tuning Will, Finally got it back together last night. It ran better switching out the pilot to a 145, The main I went to a 142.5. The main at 145 would choke as soon as I gave it a quick throttle. I ran it up to around 45 mph, when I let off quick it backfired and acted like it was going to die. Now what?    Report

04-10-2007, 3:27 PM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning The most efficient way to correct your condition is to take one circuit at a time. Remember that your foot is controlling the position of the butterfly valve, not the slide itself. Proper idle is the first step. You should be within 3 full turns from the seated position with the stock #40 pilot jet on your a/f fuel mixture screw. The only reason you should go to a larger pilot jet at this time would be if you could not achieve best idle within 3 full turns from the seated position. Determine where you stand with this circuit. My opinion is that you should reinstall the #40 pilot jet. These 2 circuits control how your Rhino responds the minute you stomp the gas. Just because your foot is on the floor doesn't mean that you are on the main jet circuit. It takes a bit of time to get there since it is a CV carb. Call it 1/2 second. In other words, your foot is on the floor for 1/2 circuit before your motor transitions through the idle and pilot circuits. It now moves on to the mid range circuit for probably 2 1/2 to 3 seconds  The mid range circuit is where I feel your problem lies. You indicate that it would choke as soon as you gave it a quick throttle. Is quick 1/2 second or between 1/2 and 3 seconds? If it is, your needle adjustment is where you need to work next. Given you are at 5-6m feet, you may not need to swap the thick and thin washers, so put the needle back to the stock configuration as the picture above shows and determine if that improves or worsens the choke after quick throttle. If it gets better,remove the thin washer all together and check again. Eventually, you should get to a point where your cough or choke should improve. If it gets worse, shim it up, richening the mid range circuit. Only after you do the above should you try and dial in your main jet as your main circuit is from 3/4 to full throttle, not measured by your foot on the floor but by the relationship of the slide to the venturi. My opinion would be that your backfire when letting your foot off from full throttle is a result of the 45 pilot jet. The pilot circuit bascially provides fuel on decelleration and in this case, I feel it is providing to much resulting in a backfire.   ww     Roostin' Rhino RAIS Power Tube    Report

04-12-2007, 9:42 AM Badley Joined on 02-06-2007 Spanish Fork, UT Posts 84 Re: Mikuni Performance Tuning FYI Ryan. We should be at about the same elevation - thought I would post specifics of where I am at currently with carb tuning. Keep in mind I am a hack - just following WW's advice. My is running awesome although I will try to get idle-1/4 repsonse dialed in a little better. Mods: RAIS, K&N Filter, Dynatech CDI, 2" exhaust tip, and Dynojet Stage 1 jet kit. Also running 26" tires which could be part of the reason for the sluggish idle to 1/4 performance. Stock main jet was 145 - I am running the DJ 160 which should be = to Mikuni 150. Pilot jet is stock. Mixture screw is at 1 3/4 turns. I installed DJ adjustable needle (second clip as recomended) and I believe this is what has caused most of my problems. I am currently on the first clip with the thick/thin washers reversed - it runs a lot better. I am going to try going back to the stock needle with washers reversed and see if that improves low end response. I am suspect of the DJ needle profile. One other thing. I was running 91 octane fuel, last trip I went 87. I believe it runs better on the 87. Do you have any thoughts on octane William?    Report

04-12-2007, 11:17 AM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning I run everyday regular unleaded with no problem. I don't think it is really necessary to run higher octane until you get into some internal modifications. I'm not saying that it is best for everyones application as yours could very well be different than mine. I might add that swapping the thick and thin washers applies to the stock jet needle components only since you have an adjustable clip on the dynojet needle. In other words, you can accomplish the same by changing the position of the circlip. I think you are right about the 26 inch tires as well. I run stock size tires but would suspect that the different size causes a bit different power delivery profile than that of a Rhino with stock size tires. ww  Roostin' Rhino RAIS Power Tube    Report

04-12-2007, 10:43 PM fasttrax Joined on 08-03-2006 West Jordan Posts 133 Re: Mikuni Performance Tuning I tried the 145 jet in mine up from a 140 before  I had to go down to a 142.5 to get the thing to keep from dying when giving it throttle. I am going to go back to the 40 this weekend to try and eliminate the backfire issue.  Will, do you recommend messing with the needle setting after changing back the pilot? Badley and I are in the same elevation (wasatch front). I have the clutch sheave, rais tube, dynatek cdi and stock air, stock tires as well. Is it best I go to a K&N with outerware first?    Report

04-13-2007, 8:08 AM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning What I would suggest is that you follow some of the steps I have outlined. First thing, change the plug. If you haven't blown out all of the circuits with compressed air, now is the time to do so. Put your #40 pilot jet back in. Set you idle correctly according to the instructions above. Do a run with your needle where it is set now, raise it up (circlip for dynojet needle or shim washers for stock needle) to the point that you notice a change and note the type of change. If it was a change for the worse go the other direction and note the same. At some point, you will determine where you need to be and it will be obvious. Once all of the above is done, then you can change out  your main jet to provide best running and top speed. The main jet only functions (effectively) from 3/4 to full throttle. Makes little difference if or when you add a k & n. The overall process above remains the same. Only difference is that if you do the above without a k & n, when you decide to install one, you will need to repeat above steps (at that point, the process will be easier). ww  Roostin' Rhino RAIS Power Tube    Report

05-06-2007, 2:35 PM kracker Joined on 04-04-2007 sanford,florida Posts 11 Re: Mikuni Performance Tuning will how can i correctly adjust the idle knob(the one you can adjust with your fingers)to where it should be?i messed around with it one day and i want it to be where it should be before i start tuning my carb.i bought the ngk air/fuel sensor so i hope this can get me dialed in on all circuits.    Report

05-06-2007, 4:32 PM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning What you want to do is to turn in the knurled idle speed screw to the point that you achieve a sustained idle speed of approximately 1500 rpm's. This is the point that you want to adjust your a/f mixture. You will likely end up with the screw about 1.5 to 2 turns out from seated position. Back it out until you notice a stuttering idle performance or in your case a richening condition on your a/f gauge. Then screw it back in approximately 1/4 to 1/2 turn to achieve the richest sustainable idle. Then, screw the knurled idle speed screw to allow proper idle at approximately 1500 rpm's. 1,500 is Yamaha's recommendation. Personally, I prefer a lower idle speed but to each his own. ww    Roostin' Rhino RAIS Power Tube    Report

05-07-2007, 11:36 AM kracker Joined on 04-04-2007 sanford,florida Posts 11 Re: Mikuni Performance Tuning so i take it i need to hook up a tach to know where 1500 rpm's are?    Report

05-07-2007, 12:40 PM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning 1,500 is simply the advised rpm for idle. I don't think you need to hook a tach up to it. What you are searching for is a consistent, smooth idle and you may need to have your idle speed high initially to be able to correctly adjust your a/f mixture screw. Once your a/f mixture screw is set, turn the knurled knob to achieve your desired idle speed. ww Roostin' Rhino RAIS Power Tube    Report

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