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05-03-2006, 11:13 AM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Mikuni Performance Tuning Attachment: gen_tuning.gif I think this forum may be a good place to discuss and answer questions relating to performance tuning of the stock Mikuni CV carburetor on the Rhino. In my opinion, this carburetor is likely the best application for most of our riding conditions short of pure racing. Given the modification variables that we all face when making decisions to attain better performance on our Rhino's, correct carburetion adjustment is an area that sometimes gets a bit short changed.  Sure, we change jets, springs, needles, etc. to achieve better performance. And we generally get it. But do we really think about the impact on performance that incorrect carburetion has as a result of those other modifications that we do such as cdi's, pistons, cams, filters or even my RAIS power tubes?  Any modification that you do is going to upset the manufacturer's goal of achieving the 'ideal stoichiometric ratio' of  14.7 parts air to 1 part gas at any moment in time once underway. In reality and in various load situations and engine speeds, the actual ratio's probably range a few points to either side of ideal with little fanfare. If you do a modification and that modification pushes that ratio outside of the 'acceptable' range, say 18.0:1 which is on the far side of lean, or on the far side of rich, say 6.0:1, then your Rhino stumbles, spits, coughs and generally proves to do quite well at raising your level of frustration.  Correct jetting and carburetor adjustments can correct these conditions and allow full power realization from any given modification.  For example, put one of my RAIS Power Tubes on and put in a 155 Mikuni main jet and shim up the needle. It's obviously pulling in more air and it pulls top end without blubbering all over itself. But what's really happening? Lets assume that a 14.7:1 ratio is achieved by the RAIS/main jet combo and you are attaining your best top speed. The thing runs great, no problems, you're happy. There could be more and in fact, there is more. You just have to find it.  What about your idle mixture, your slow air jet, your pilot jet, slide action, needle jet (not jet needle), jet needle, jet needle positioning or power jet? Idle performance, mid range performance, stomp it performance? The installation of the RAIS or any other modification impacts all aspects of engine performance in all of the above mentioned circuits whether you feel it or not. There may be additional performance gains achievable that you are simply not able to recognize. Take the time to find them, because it is well worth it. Most of the modifications available either attempt to burn more gas, allow more air, achieve the 4 stroke cycle quicker and/or higher rpm's. Those desired and realized performance gains may be masked within an apparent performance issue resulting from a new modification. Correct it and the only thing you see is those trying to catch up with you.  For example, you put in a new air filter and the thing flies like a banshee but it stumbles off idle when you punch it. This condition can be corrected and I hope that this forum can address some of those issues to achieve the best balance of 14.7:1 that can be attained across the entire operating range. I got the attached image from carbparts.com and felt that it was an appropriate image for this forum as it gives a visual account of the relationship between the different circuits on the CV carburetor. Any change in air or fuel demand effectively causes those shapes to widen or become more narrow. Any change in air or fuel supply essentially lengthens or shortens those shapes. If the transition between circuits becomes to distinct then there may be a pronounced stumble at that transition. If the transition becomes to overlapped then there may be a pronounced stutter at that transition. ww   Roostin' Rhino RAIS Power Tube    Report

03-05-2007, 2:28 PM Badley Joined on 02-06-2007 Spanish Fork, UT Posts 84 Re: Mikuni Performance Tuning William, I installed my RAIS Power Tube, K&N Filter and rejetted. Dynojet kit suggested 150 main (145 stock) and to clip the second groove on their needle, and mixture screw out 1 1/2 turns - for my elevation (approx. 4000') - I decided to start there. Engine runs awesome and I have definately realized low and mid rpm gains. Only problem I have noticed is when I am wide open and up to max speed it seems to "blubber" slightly. Friend said it is likely the rev limiter and common with other 4 strokes with limiters - what is your opinion? I do plan to add a Dynotec CDI, and possibly an exhaust tip (don't want it any louder).    Report

03-05-2007, 3:48 PM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning Couple of things. If you still have your stock cdi installed, you are going to hit the rev limiter around 40 mph and thats all it's going to do. You'll basically get to 40 mph and it will choke all over itself regardless of what kind of tuning or modifications you have done. The aftermarket CDI will give you an additional 10 mph. Secondly, If you have a dynojet kit, realize that the numbering system for dynojets mains is not the same as o.e.m. Mikuni mains. At 4,000 feet elevation, you should be able to run an o.e.m. 150 main which I believe would be relatively close to a dynojet 160 main. Regardless, when your correct main jet is installed, you should realize clean running all the way to maximum top speed. No blubbers, coughs or spits. ww    Roostin' Rhino RAIS Power Tube    Report

03-06-2007, 9:22 AM ProWrench Joined on 11-19-2006 Northwest MO. Posts 2 Re: Mikuni Performance Tuning WW, you certainly know the subject well, good write up. It's 14.7 part's air to 1 part gasoline, not pounds. Just trying to prevent another reader from lots of head scratching trying to figure that out. 06 660 with small changes.    Report

03-06-2007, 11:17 AM Badley Joined on 02-06-2007 Spanish Fork, UT Posts 84 Re: Mikuni Performance Tuning Thanks William. I did not realize that Dynojet mains were not the same. Stock was 145 - I installed DJ 150, likely starving for fuel wide open. I will start with the 160 (DJ sent jets in increments of 5 up to 175) and let you know where I end up.    Report

03-07-2007, 11:32 AM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning Thanks all for corrections and clarifications. I'll make the change. ww Roostin' Rhino RAIS Power Tube    Report

03-08-2007, 9:22 AM ProWrench Joined on 11-19-2006 Northwest MO. Posts 2 Re: Mikuni Performance Tuning http://roadstarclinic.com/content/view/61/96/ The link is to a Roadstar site but it has a Mikuni to Dynojet conversion chart on it that is handy for Dynojet users. I bought the Jet Needle seperately from Dynojet. As a rule I dont care for Dynojet stuff but I wanted an adjustable needle, I'm old school. Yamaha & Mikuni did not stop using adjustable needles because they are somehow better but to comply with tightening EPA regulations. I travel once in a while to different riding areas at different elevations and like the flexability. When my Rhino was stock I had the exact same experience, #155 Mikuni jet and stock pilot jet while using the second clip position on the needle with the RAIS was perfect. But after I added a 10.5/1 piston with a more agressive cam I needed a #165-#170 depending on climate and a #45 pilot jet to get the right mix. My local dealer let me use their EGA machine to dial it in. Winter it runs great with a #170 but when it warms up it's to much and the #165 puts it back to running it's best. Just thought I would post this in the event someone else has made some engine upgrades and having trouble getting dialed in afterwards. 06 660 with small changes.    Report

03-16-2007, 9:38 AM Badley Joined on 02-06-2007 Spanish Fork, UT Posts 84 Re: Mikuni Performance Tuning Thanks for the link ProWrench. I installed a Mountain Performance (Dynatech) CDI and a 160 main jet and it almost runs perfect. I have a slight hesitation from a stop if I stomp on it (I did not pay any atttention to this before mods and evaluating performance) - is this normal? Belt engaging from stop? Should I try to improve low end by piltot jet change and/or moving the needle? Motor sounds great and the power seems to be smooth and predictable all the way to the top. FYI - bought the CDI from a local distributor - they manufacture Superchargers for the Rhino http://www.mountainperformance.com/ not for me but interesting...    Report

03-17-2007, 8:21 PM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning My opinion is that you should not experience a hesitation off idle. It's a bit hard to assess given your description but my guess is that you are experiencing a stumble (something that you could almost time) as you transition to and through the pilot circuit to the mid range circuit. My initial question would be, have you attempted to adjust your a/f mixture screw? If so, what is your current setting? If not, why not? As far as the pilot jet is concerned, from a stock or nearly stock perspective, the pilot jet is rarely going to have to be changed. The condition that warrants changing the pilot jet to a larger size is the inability to achieve consistent, smooth idle performance at something less than 3 turns from the fully seated position. If you can adjust your a/f mixture screw out and reach an overly rich condition (idle performance degrades) within 3 full turns from a seated position, your pilot jet is sized correctly and you will realize no benefit from a change. My opinion is that your lower end mid range circuit is where your stumble is occurring and it is likely a lean stumble. By your description, I would assume that you have not shimmed your jet needle, yes? Pull the top off of the carb and get at the jet needle. I have put together a tuning manual with images as well as descriptions which might assist you if you have never experimented with your jet needle settings. Bill is going to help me get it on this site, so hopefully it will be up before long. Regardless, I think you should try raising the jet needle relative to the slide by shimming it and see if that improves your problem. ww     Roostin' Rhino RAIS Power Tube    Report

03-17-2007, 8:49 PM Badley Joined on 02-06-2007 Spanish Fork, UT Posts 84 Re: Mikuni Performance Tuning Thanks William, look foward to the manual - it will definately help a hack like me :) I set the a/f at 1 1/2 turns as suggested by Dynojet - I will make a stubby (my excuse) tomarrow. I will get the /af right first and if necessay try raising the needle. I have the adjustable needle - set on second groove (from the top). Had it out today and it runs great, except for low end - just off idle. I was climbing steep trail and tried to use the low end torque to crest - no go. Had to back it up and get up to mid-range to make it. Justin had no problem lugging it with stock jetting and RAIS.    Report

03-17-2007, 9:40 PM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning There you go. Give it a try and see what happens. If the conditions worsen, go back the other way and try the first groove from the top. That way, you'll know. ww Roostin' Rhino RAIS Power Tube    Report

03-21-2007, 7:42 PM Badley Joined on 02-06-2007 Spanish Fork, UT Posts 84 Re: Mikuni Performance Tuning I ran across this carb tool kit - takes care of the need for a small stubby srewdriver to adjust A/F mixture screw. http://www.rockymountainatvmc.com/productDetail.do?navType=type&navTitle=Tools%2FShop&webTypeId=140&webCatId=22&vehicleType=&pageLinkUri=&prodFamilyId=1472 I checked it out on my Rhino to make sure it will work - plenty of room and the grip is nice. Didn't have the time to work on my mixture yet... tomarrow.    Report

04-02-2007, 7:12 PM Badley Joined on 02-06-2007 Spanish Fork, UT Posts 84 Re: Mikuni Performance Tuning How are you guys comming on posting the carb tuning manual? I am still fine tuning - but I am closer to having it perfect. Just installed a bigger exhaust tip today, will likely have a little more to do.    Report

04-02-2007, 7:59 PM wwlknsn Joined on 04-09-2006 Memphis, Tn Posts 154 Re: Mikuni Performance Tuning I have completed the manual. I gave it to Bill a couple of days ago so I would imagine that he will get to it when he gets a bit of free time. It addresses the steps of the mechanical process of doing an r & r on your carburetor using photgraphs and text. It does not address the methodology of tuning. At some point in time in the future, I may incorporate that into it but for now I would say it was a major accomplishment for me to get this far with it! stay tuned. ww     Roostin' Rhino RAIS Power Tube    Report

04-03-2007, 6:49 PM kracker Joined on 04-04-2007 sanford,florida Posts 11 Re: Mikuni Performance Tuning i live in central fl...running the dragon fire cdi...k&n...billett tip...and a 2" airbox snorkel...ran like crap once i put the snorkel on...would studder and cut out under stress like coming out of a hole or trying to pull somebody out...got the dj kit and went with the dj155,clip on second notch,and screw out two turns like the instructions said...ran a bit beter but still had the stalling problem...moved the clip to the third notch and a big difference was noticed but the stalling under stress was still there...my question is should i go up a jet size to the 160 or 165???or should i go down another notch on the needle???plug tip is a little white right now....thanks....    Report

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