Mikuni Carb

8 replies to this topic
  • Nak

Posted July 28, 2003 - 10:08 AM


Hi Guys, I have been away for awhile, riding of course. Anyway, I would like some input on the Mikuni carb for the XR650. What bugs have you found? Was the out of the box set up correct? I laid out the cash for one and so far I am not happy to say the least. I sent it back to the distributer and got nowhere. So today I have opened it up and started machining the necesary parts to attempt to make this carb work with improvement over the stock set-up. It seems that the suppliers haven't done their homework and have done little or no testing to determine the effectiveness of their product. Any insight, jetting, and needle, needle jet recommendations would be truely appreciated. I would love comparing them to what I have come up with, to possibly get this unit to perform. Thanks up front, Ride On!

Nak :)

  • thmprracer

Posted July 29, 2003 - 05:08 AM


What are you un happy about? idle mid top end where is it not working correctly. mine was a bolt in afair with no dramma what so ever. What are you "machining"??? Ive left everything alone exept the main jet cause ive doen the piston cam and exhast and now on the dyno the best main jet seams to be 145-152 depending on fuel and the temp/altitiude.
This carb has an inrichener circut for easy starting compared to a choke like the stock carb make sure you dont use it unless its nessasary (cold) or you may be fouling the cylinder?
What are the other mods doen it is fully un corked right?
Where did you by it also?

LEt us know and maybe we can help.

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  • Nak

Posted July 29, 2003 - 09:25 PM


The bike is unplugged with the stock carb tuned perfectly. I was hoping the new carb would wake the XR650 up.
With the new carb the bike would not start. I started drilling out the pilot jet .002 thousands at a time and finally found the right size to make the beast start easy and run good.( .020 thousands) This is not how Mikuni measures their jets. Their numbers stand for velocity. This covers the idle circuit. Now the mid it still way lean. I bought the richest needle that they sell supposedly and have the clip on the bottom notch. (# 9DJY03) I will test it this weekend. The biggest problem is that the pump nozzle sprays the back of the slide until almost 1/2 throttle. It is to tall. The pump is adjusted to start just off of idle and finish at just below half throttle. .020 thousands gap between the pump actuater and the lever arm. The bike is totally dead on the bottom. So what am I machining? Shorter nozzles so the nozzle sprays under the slide like it is suppose to do. You would think that the people selling this stuff to us would at least do their homework before selling it to the public. I have the resources to modify this stuff, but what are we paying when we purchase parts for our bikes. I would hope the parts were tested and proved better that stock instead of more junk to throw in the trash pile. I must be dreaming right. Anyway this is the short of it, leaving out the stupid stuff the Distributer suggests. Did you have improvement over your stock carb? What needle are you running?, and what is your pilot size? How tall is the nozzle in yours? Mine was .350 thousands high. The new nozzle is much shorter and sprays under the slide just off of idle. It works like it should on the stand. I will find out more this weekend. The proof is in the seat of the pants and the scourched ground below.

Twist hard and ride light!


  • thmprracer

Posted July 30, 2003 - 04:09 AM


Nak, That sounds crazy as mine works fine, maybe to rich on the bottom end if anything. The idle is set high now (2,100)as it was stalling occasionally ( i think cause its a little rich) at the low idle and im road racing mine and that helps in the slow corners as it keeps the engine reving and the chain slack up ect..
It starts just as good if not better! The only thing I changed is going richer on the main jet 9 152.5 now) after cam and piston change, I never noticed where the accelerator nozzle was or wher it was pionting but now i need to know and also have heard of beiong able to turn them to direct the flow ito the stream better. the carb originally was designed for Harley piles and maybe you got one tuned for that application? Sudco and XRs only sell this carb setup for the XR supposedly. Right now im getting ready to install a 45mm mikuki for more flow. this thing wants to breath i think and hopefully theres more power in it.
do you still have the stock carb? if not and after I get the 45 on maybe you can try mine and see if it works better i have no problem letting you borrow it or whatever?

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  • Nak

Posted July 30, 2003 - 05:03 AM


Hey Jim,
That is a very nice offer. I should get this thing to work. It is interesting to hear you are putting on a larger size carb. From the beginning I have always thought that this carb was to small, and I still believe that it is.
Yes you can rotate the nozzle. It is held in place with an o'ring and the float bowl. It should be pointing directly at the needle, not down the throat. This helps atomize the fuel.
I sent it back to one of those places you mentioned as I suspected as you said, was set up for a different bike. I got nowhere, just double talk. Like I said, I will check out this new setup this weekend. Let me know how the 45mm carb works out and where you bought it from. I may be interested in the bigger option.


  • motart

Posted August 01, 2003 - 02:23 AM


NAK, I have a TM40 on my 650. I bought it from sudco complete w/ jets, inlet & outlet spigots, fuel line & cables. Took some playing around to get it set up to where I like it, but definately an improvement and I never drilled anything or spun my pump piston. My engine is stock except for the uncorking and full Moriwaki exhaust-head tubes are bigger, and stock air filter and backfire screen. I'm close to sea level, currently running 142.5 or 145 main, 17.5 or 20 pilot, and set the pump to come as late as the linkage will allow. dmv123 on xr650r yahoo group suggested that and it works a lot better than the pumper coming on sooner. Hopefully this will give you some baseline settings. Good Luck, Jeff

  • Nak

Posted August 01, 2003 - 04:51 AM


Thanks for the post Jeff. I have tried it with the pnmp coming on late and got a big kick way up on top. Not neccesarily where it is needed. I will know more by Monday. I can't test it in my neighborhood, plus there is a brand new Knob on the rear that I know I would shred on the street, and the neighbors hate when I use there yards and little kickers on there driveways to have some fun while testing. So off to the dirt I go. It is good to here that it is an improvement to the bike. That does help my morale.

Thanks for the jetting tips too.


  • Dutch

Posted August 01, 2003 - 12:51 PM


This is something copied from "the other" BRP forum. I do not know the author and have not tried these settings - just thought this would add, (confuse?), to this discussion.---------
For 2500-3500 ft, the main jet should be 130-135, 145 is too rich, even for sea level. I run a 135 right now and will drop it to a 132.5. The pilot I'm running is a 12.5 and the fuel screw is out 3 turns. The largest pilot you should run is a 17.5. With a 30, you must be pouring raw fuel out of the exhaust at idle. How many turns on the fuel screw? If the bike is crisp and responsive immediately after starting without a warm-up, you're too rich. That goes for any carbureted bike IMHO.

The accelerator pump adjustment is critical to making this carb work on our bikes. The way it comes, it shoots WAY too much fuel and the pump shot is too long in duration. Also make sure that the pump nozzle is shooting at the needle, not missing it or hitting the sides of the carb throat. You can aim it by grasping it with a needle nose pliers and turning it in the O ring. Be gentle, it turns easily. My pump is setup to start shooting fuel from just above idle to 1/2 open. You adjust the starting point with the Phillips screw and end point with the 1.5 mm Allen on the top of the carb. I'd make these changes, put the needle back to the middle position and fine tune the pump shot.

Properly jetted, the bike should be very responsive and run as hard wide open as it did with the Kehin or the QS, and return over 30 mpg. That's my experience so far. I'm not done fiddling....

  • Nak

Posted August 04, 2003 - 06:15 AM


Thanks Dutch, Can you tell me what needle you have? Here is what I found. With the 17.5 pilot the bike won't start. Going larger on the pilot it finally started and did not smell to lean. I tested it this weekend, also tore it apart in the field and dropped the needle. I am running the richest needle Sudco has as the one that came in it was way to lean, I thought, and it is not listed in the Mikuni manual. Not sure if it belongs in there or what? I used my custom short spray nozzle and found that it works well over the tall unit supplied. But the lower end is still sluggish.
and doesn't idle all that well but starts well. I rode the tight stuff, it ran just so so. It doesn't loft the front wheel at these slow speeds like the stock unit does. Hit the whoops, sluggish. But at higher RPM's it pulls hard. Flat tracking the graded fire roads, it rips. You are right that there is probably to much fuel at the bottom. I will regroup and try a different approach.


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