The Official Mikuni Flatslide Carb Thread

ttmaniac · Full Member Joined: Dec 22, 2006 Posts: 468

Ok, ok....it's not so "official" but I figured it would make a great title for anyone wishing to know about flatslide mikuni carbs and/or those wishing to share tips, tricks, and experiments.

So here goes....

I got one of my flatslides in the mail today. The other one is a used one that should show up soon. I'll probably then need to order a matching needle for the used carb as well a a pilot jet of similar sizes.

I'm gonna start with jetting/needles as follows and will tweak from there on.

ain jet: 195
Pilot jet: 50
Needle jet: Q6
Needle: 6FJ40-3

These are 36mm carbs. They are non accelerator pump models. They swap directly with the 36mm roundslides.

So far, I've noticed they are a bit more work to install than the roundslide. It almost requires two people. The throttle spring seems much more rigid than my roundslides, though it could just be the newness. It's trickier to get the carb top on using the flatslide's screws as opposed to the roundslide's easy and convenient screw on cap. But, all said and done, it wasn't that much more inconvenient to install.

From the pictures you should be able to gain an idea of the clearance you will have using these carbs. Hope this helps. If anyone has some jetting guidelines for these carbs on a 650 please post em.

81XSChopper

Yup I have the exact same carbs. Thought they were oldest ones and weren't available new -- are yours new? If you decide to drill for carb equalization barbs I put mine right near the 36mm stamping which means that fitting a guage is a pain on the left carb given the proximity to the frame.

It took me 3-4x before I could do a pretty quick & seamless throttle cable install but as you said no big deal.

I will email you the section for that carb out of the Sudco manual when I get home in a few weeks.

Overall I really like 'em, although as performance oriented carbs they seem to suck down more gas than the stock set I was used to. Will be interested in your jetting experiences!

greg

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

The jettings you have will most likely be on the rich side,the pilots are way off. When properly jetting flatslide fuel comsumption in no worse that the VMs....................Jack

ttmaniac · Full Member Joined: Dec 22, 2006 Posts: 468

The 195's worked well in the roundslide so I'm just swappin' em.

The rounds had 25's for pilot jets. I closed the air screws for proper performance. They seemed a bit lean.

Only reason why I went 50's for pilots is cause that is what the flats came with. Good thing I ditched the 280 main eh?

I'm thinkin' I might need around 30-35 pilots all said and done but we'll see.

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

Been using 38mm flatslides on my BMW 1000cc motor for the past 17 years and the jetting for the NJs and N appear to be on the rich side for the displacement of your XS. You might consider ording the 6FJ41 jet needle to use with the rich Q6 NJ which is the the leanest JN for that series. Good luck on jetting and I think you'll be impressed with the smooth power transition of the flatslide offer over the VMs.................Jack

grizld1

Jack, has availability changed? The TM36 takes a series 389 NJ, and my Sudco manual lists 389 NJ's as lean as O-0, but the book's a few years old. I'd be inclined to try P-6 or P-8 if they're still offered.

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

Griz.........I believe the the 34 to 38 F/S come with a 4.0 cut in the throttle valves and with the 10% incresed flow volume require higher jetting settings than the VMs. The 389 series is for the 32 to 40 MM F/S and the 36 to 40MMs require the jet needle starting with the first digit of 6 due to body structural differences in length to mate with the needle jet. Even with 6FJ41 using the Q6,the mid section will indicate an overly rich condition,might need to drop down to a Q2 or Q0,then work out the fuel ratio using either of the two JNs. With no base line jetting for references,it's all trial and error reaching to the deep end of the pockets. When tuning the mid section,I always tuned the F/R that allowed me to run the jet clip in the middle to allow some custion for future mods to adjust out the F/R.................Jack

grizld1

I hear ya re. body design and flow! Have a look-see at the TM36-68 pumper specs (just RS36's set up to mount twin and single instead of in banks of 4, and they do flow the mix!); generic NJ is P-8 in those, likely a tad fat for our twins. Can't generalize too much from one design to the other, but right, I doubt a guy could ever clean up the midrange on Q-6's.

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

The RS series are no doubt a different breed than the TXs that might offer substantial increased homogenization of fuel particals by the effects of the smooth bore and metering passages there-by reducing fuel requirements for complete combustion resulting to a N/J reduction,don't know. At the moment I'm running P8s dual plugged and running one plug I need to jump back -up to Q8s,so either the Q0 or the P8 might get TT in the ball park for the mid range.........Jack

ttmaniac · Full Member Joined: Dec 22, 2006 Posts: 468

Hey, thanks for all the responses and tips guys.

I just got the other carb yesterday. It was used sooo.....Got it cleaned up but I am gonna have to order a couple internal parts. I think I'll go ahead and order a set of the P8's or Q0's and give em a try. A few more bucks but it'll make a nice experiment.

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

TT.............those damn needle jets are expensive,order just one for testing,get the Q-0 first,if if the motor runs lean,I;ll buy those off you for half price ,being they'd be a used product :lol:...................Jack

grizld1

Good luck, tt; getting the NJ/JN combination right is the touchiest part of the job--and the most damn expensive! ('Course not getting it right is even more expensive, bleeds the nickels and dimes out of your pocket a little more slowly but lots longer.)

81XSChopper

Hey, ttmaniac. I just got back in town and have found the Sudco guide I promised to send you. I'd would be easier for me to just copy the relevant pages & snail mail 'em to you. If you want to get me that info I'll put it in the mail. (email: greg @ leroy.com)

greg

ttmaniac · Full Member Joined: Dec 22, 2006 Posts: 468

Ok, so.....I finally got off my rear and got the other carb cleaned up.

The "other" carb is a used and fairly rough looking flatslide. It had the usual mushed out screws. Worst part is, it is missing the idle adjustment. Inside the carb, there is a rod that runs to the top of the carb. Well, mine was missing. Good part is it only cost me 36 bucks instead of 160.

To correct the problem, I set the carbs even and now adjust my idle up at the throttle where the screw setup allows you to take up slack. Guess what? It works better than dinking around down below with screwdrivers. I can totally adjust it while seated on the bike.

Anyhow, on to the performance aspect of the carbs.

I got it running pretty good with those massive size 50 pilots. I went to a bp6es plug in order to heat it up and burn some of that extra fuel.

What can I say, it goes like crazy and sounds mean as hell. I know I'm pretty rich on the bottom. My plan is to throw some size 35 pilots in there. As it sits now it just blows a bit of black when you crack it but is totally run-able.

As for the needles and needle jets, I dunno if they are even worth bothering to change. Am I gonna gain that much from it? Jack? Grizld1? Do you guys think it will boost performance to a noticeable degree? Will it be worth the time and money to change em out?

Oh yeah, for the others that haven't used a flatslide, the needles ain't quite as easy to get in and out. A little more effort is required to put em on, but nothing to get apprehensive about. An extra set of hands is nice though.

I'm gonna go ahead and comment now on the performance difference. If you want FAST throttle response, these are your carb.

To me, the difference was as follows.....roundslide, snap throttle.....a brief lag before it really hauls...mind you, it's a very short lag time.

The flatslides......near instantaneous response. I'm very impressed. No lag. Just gets up and goes.

Flatslides are worth the few extra dollars and the little bit of extra time required to set them up. If you are looking for a new carb to replace your stockers, I'd recommend flatslides over the rounds any day now. No wonder they are banned from the flat track!

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

Hey TT.............You'll never think twice about switching back either once you get'm dialed in,trust me, and they will hit hard off idle to full RPM with no hesitation,sortta like adding a a mini super charger Wink

When I put my Dellortos to rest with the 38 flats I had to replace my clutch diaphram spring with HD unit along with a better quality clutch material due to excessive clutch slippage from increased torque output and that's no bench racing BS either Wink

All my friends who have road tested my BMW can't belive how responsive those carbs are and the smooth power delivery surge

What size did you end up with TT? The Q-6 are more appropriate for the volume flow of the 38MMs not 36MMs. You might wanna try Q-0, 25 pilot jet,keep the jet needle for the time being and play with clip settings. But first change out those pilots,clean the plugs and road test to check plug coloration.....................Jack

grizld1

You'll definitely gain from spending a little time and money on the needles and needle jets, TT. The atomizer/needle combination is the heart of the carbie. You have to be spending just a LITTLE time off WOT, and one thing you'll notice in a hurry with a well-tuned midrange is that roll-ons are stronger while fuel consumption's lower. The brass will pay for itself.

81XSChopper

Prodded by this thread I decided to get off my ass & rejet my flatsides (the 220 mains & 27.5 pilots weren't cutting it).

Tried to order them from my usual supplier of jets (cyclerecycle) but they didn't have some and the ones that weren't in stock I would have had to order in sets of 4. They referred me to another company that had all the different jets I wanted in stock, and tend to specialize in this sort of thing for old dirt bikes. Really good to deal with. Small company, open 7 days a week, friendly people, decent prices & only $7 shipping -- Got to me in 3 working days after I ordered. If you can not locate parts for your flatsides give them a yell:

www.cycleoutfitters.com/

greg

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

You want some real good deals,try this place...............Jack

www.craftstrading.com/...ystems.htm

81XSChopper

Thanks, jack. Much better prices. I will say, however, that cycleoutfitters have been using & building flatsides for decades. I told them what I had and they took the time to send my info to some "expert" there and he confirmed that my choices were sound. Worth the $3 extra I spent for 4 jets to get knowledgeable input. Still, either way, both these companies are great sources.

greg

daddyk · New Member Joined: Jul 18, 2005 Posts: 9

Hi I've been watching this posting for a while. I mounted a couple of RS 34 mikunis on my bike and have been really impressed with the performance. It starts easy,idles smooth and runs extremely well through the entire range,with no popping sputtering or hesitations, with a really big kick on the top end. You people here seem to be running much larger jets in the "standard" flatslide than me. I'm running the stock 17.5 pilot,130 main, P4 NJ,and 9DZH05. Needle 1 click rich. Does anyone know if the RS's are that much different or is their a lot more to be gained with some jet changes. Compared to my 33 year old BS38s this is night and day different. The jetting I have is what they send out for bikes up to GS1150 and FJ1200. Just asking for opinions or experience. Like I said it's running great just wondering if there might be a little more I don't know about.

grizld1

Daddyk, so far you have the only direct experience on this board with the RS carbie, and it's a completely different design from the VM's and nonpumper TM's. I'm a bit surprised--make that real surprised--at the NJ/MJ combination; but there's no arguing with success, and if the scoot's giving good throttle response, pulls hard at WOT, and plug readings off a throttle chop don't raise any red flags, you can't be far off!

daddyk · New Member Joined: Jul 18, 2005 Posts: 9

I haven't had a chance to do a proper throttle chop ,but I did go pull a plug and they're lean. I guess I'll have to do some swapping. I hate the try, take apart, try stuff. I was real happy with the improvement now I wonder how much more might be available . I was already getting clutch slip. I think I'll try the P-8 NJ that's the top of the P series and get 135 and 140 mains. At least these Carbs are easy to access I should be able to unscrew the Main/NJ through the drain plug to get to every thing with them still mounted. You can adjust the needle clip with just three screws. I'll look around local and see if I can find these NJ I'm not sure how common they are. The www.craftstrading.com/...ystems.htm above has them on the web site for a good price but I'd rather not wait.

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

The above jetting combinations are for a 36mm not 34s. You have to remember the RS and TMs aren't exact when it comes to flow metering and the RSs will flow more air than a round slide or TM therefore improving fuel homogenization to prevent partical seperation improving combustion efficiency which will result in less fuel requirements for the exact output. If it's running perfect leave it alone in my book or you could try changing the main jets. What year did you get the RSs off of? They look awesome mounted on the XS...................Jack

daddyk · New Member Joined: Jul 18, 2005 Posts: 9

Thanks. They were an EBay find $56 shipped to my door for the bank of 4. The PO said he thought they'd come off a Katana 750. They came in an RS box RS34-D21-K on the side. They were gunked up from sitting around and one of the float supports had been broken and spliced back together, one of the nicest jobs of doing that I've every seen,but all in all I felt kinda guilty cutting them up. As far as the jetting it starts on one or two half hearted kicks,it will set and idle all day none of the hunting and surging I was getting.It pulls cleanly away from idle perhaps not with as big a surge as others have described. I do have some burbling when I go into a corner chopping the throttle from higher revs which would indicate lean. Top end is much improved I now have clutch slip when on the throttle, 4 to 5 shift that I didn't have before. I haven't changed the plugs and they are noticeably whiter than they were so I'd better richen it up some. It's just a guess but I'll start with going from P-4 to P-8 NJ and 2 to 4 steps up on the main. I'll leave the needle,pilot and air jets alone for now. I'll let you know how it works after trying it out.

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