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Old 10-12-2012, 12:42 PM   #1
ejute OP
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Mikuni OBI WAN'S

I’m looking for some help/input on tuning some Mikuni VM36 carbs I’ve fitted to my 82’ R100
Some background-my bike is stock apart from Dual plugged heads 9.5:1 comp pistons and epco sports mufflers.
I decided after a lot of research and procrastination to go with the Mikuni’s..they are relatively well priced, airhead simple and a common conversion. I went with the VM36’s because like the 3 little bears story I thought the 34’s might be a bit small(plenty of low/midrange not enough top end) 38’s might be a bit big (less low/midrange but better top end) and the 36’s might be just right?
I searched the web and the only supplier I found that made a kit for VM36’s was Allens Performance in the UK after trying several times to communicate ( exactly what does the kit include etc?) with them via email I gave up and searched locally in Australia for a Mikuni specialist ( http://www.mikunioz.com/index.html ) who could supply me a set of carbs with basic jetting to suit the R100 the rest of the bits I got from Rockypoint Cycles(cables) and Allens Performance ( adaptor manifold) some S/H carb to airbox elbows to suit 32mm Bings, a silicon rubber reducer (63-50mm) for a turbo from a local auto parts chain here in OZ (Supercheap) and stainless hose clamps to suit (choke cable conversion kits on order).
Pictures here http://www.flickr.com/photos/29497288@N00/
Carb specs are:
VM36-4 -
Main Jet- 230 (supplied with one size jet up 240 and down 220 for tuning purposes)
Pilot Jet- 35
Needle Jet- Q5
Jet Needle- 6FJ6
Throttle Valve- 2.5
Main Air Jet- 2.0
Needle Valve- 3.3
I fitted everything up and the bike started straight away with some choke…good start.
I have read the tuning guides and set the pilot air mixture to 1 1/2 turns out and went for a ride to warm things up, first thing I noticed was a big stumble/flat spot when trying to take off, once over this it took off with good acceleration….no worries it just needs some adjustment?
I balanced cables etc. adjusted pilot air with the idle as low as I could manage…still has the same stumble/flat spot….mmmm so I lowered the needle 1 notch ( it was in the middle to start with)….bad move worse that before, raised it 1 notch…big improvement but it still has a stumble/flat spot when you blip the throttle from idle, contact mikunioz and he tells me that when adjusting the pilot air screw you should be able to get it to die when turned in and die when turned out I could only get it die when turned in ½ a turn but not die when turned out so we decide it needs a smaller pilot jet 30 instead of 35. I fitted the new pilot jets and went through the adjustment procedure…same problem, I even found a couple of size 25 pilot jets I had in some smaller Vm mikuni’s I had from some old 2 stroke dirt bikes and fitted those…same problem.
I have reread the tuning guides and none of them say, when adjusting the pilot air, it should die when adjusting in both directions?
So that’s where I’m at, at the moment, my next move is to refit the 30 pilot jets and lift the needle another notch and see what happens? I’m also wondering if the throttle valve is the correct cut away, maybe a 2 (richer) would be better or perhaps it’s the needle jet/jet needle combo?
Looking at the Carb specs it seems to me all I got is a standard VM36 carb with smaller main jet!

So Mikuni Carb Guru’s out there please take pity on a bloke from down under who at the end of the day is looking for a more responsive carb/bike and some help to get it sorted before the MotoGP at Phillip Island.

Thanks in advance OBI WAN’S

Mark
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Old 10-12-2012, 02:03 PM   #2
JonnyCash
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It's hard to jet over the internet, but it sounds to me like you went in the wrong direction with both the needle and the pilot jet change. My experience with Mikuni VM's is on an RD350, but I am continually amazed with how sensitive the air screw is. I find that within 1/4 turn, I can go from bucking lean to too rich.
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Old 10-12-2012, 02:40 PM   #3
concours
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It needs a LARGER pilot jet. 40 years of tuning, repairing carburetors. Mikuni's a specialty.
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Old 10-13-2012, 08:09 AM   #4
anotherguy
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Yeah what he said. 40 pilot,maybe 45. Start at 1 1/2 turns on the screw. Warm bike up and turn screw in 1/4 turn at a time (keep track of total) until cylinder stumbles then back it out until it stumbles. Split the difference. If it's less than 1 turn not enough pilot,more than 3 too much. Anywhere in between 1 and 3 turns and you're good. That is only for carbs (VM is one) that use an air screw,fuel screw is the opposite. You can tell what kind of screw you have by what side of the slide the screw is on. Air filter side=air. Cylinder/head side=fuel.

Tune from the pilot up to the main. One circuit at a time. Post up the pilot results. Until you have the overall tuning done leave the needle jet/slide cutaway/air jet as is. They appear to be good for a 500CC hole.
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Old 10-16-2012, 06:43 AM   #5
Middleweightboxer
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Changes

I took a look at your carb stats and have a couple of comments:

The 6FJ6 needle that you are using has a taper that starts at 35.2mm down the needle. I have always used a 6DH3 needle with good results. The taper on the 6DH3 needle starts at 22mm down the needle. This will give you added fuel to the mix much earlier in the throttle movement. This should eliminate the lean spot just off idle.

My second thought is that a Q5 needle jet is a bit large for the application. My guess is that you will end up with a mid to lower P range needle jet. The needle/needle jet control the mix for most of the commonly used highway speed throttle range of 1/8 to 5/8 throttle, so it is most important to get these correct. Then move on the the main jets. I think you'll find you need to go smaller on the main jets too.

What ever pilot jet makes it idle well in the 1 - 2 turn range of the air screw is the correct setting.

Good luck.
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Old 10-16-2012, 06:53 AM   #6
Middleweightboxer
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Mikuni Jet Chart

Here is my carb tuning chart for keeping a record of your settings. Understanding the dimensions of the jets and needles really helps the jetting process. The D1 through D6 measurements are needle diameters at equal increments as you go down the needle. Sorry, the column spacing gets messed up in the post. PM me if you want an excel sheet.


Bike Date

Carb Mileage Spark Plug

Settings: Air Screw In is richer, out is leaner

Pilot Needle Jet

Main Needle

Slide Needle Clip 1 highest, 5 lowest

Mikuni VM Needle Jet Orifice Diamaters
Leaner <----------------------------------------> Richer
0 1 2 3 4 5 6 7 8 9
N 2.550 2.555 2.560 2.565 2.570 2.575 2.580 2.585 2.590 2.595
O 2.600 2.605 2.610 2.615 2.620 2.625 2.630 2.635 2.640 2.645
P 2.650 2.655 2.660 2.665 2.670 2.675 2.680 2.685 2.690 2.695
Q 2.700 2.705 2.710 2.715 2.720 2.725 2.730 2.735 2.740 2.745

Needle Taper Diameter Dimension Chart
Needle Taper
Length Starts
a b D1 D2 D3 D4 D5 D6
6H1 62.300 37.500 2.510 2.510 2.510 2.412 2.041 1.696
6DH2 62.300 28.000 2.511 2.511 2.466 2.295 2.000 1.660
6F9 62.300 28.900 2.516 2.516 2.475 2.210 1.949 1.678
6CF1 61.500 29.500 2.512 2.512 2.429 2.240 1.974 1.710
6FJ6 62.300 35.200 2.505 2.505 2.505 2.376 2.040 1.606
6DH3 62.300 22.000 2.512 2.512 2.458 2.286 1.948 1.607
6J3 62.300 36.700 2.515 2.515 2.515 2.369 1.912 1.456
6L1 62.300 37.000 2.512 2.512 2.512 2.335 1.826 1.313
6DP5 62.300 32.100 2.518 2.518 2.518 2.372 1.834 1.141
6N1 62.300 37.000 2.514 2.514 2.514 2.278 1.672 1.058
6DP1 62.300 28.900 2.511 2.511 2.476 2.312 1.748 1.075
6F3 60.500 34.200 2.512 2.512 2.512 2.313 2.050
6DH4 62.300 25.500 2.520 2.520 2.440 2.258 1.915 1.575
6J1 64.000 36.200 2.517 2.517 2.517 2.339 1.919 1.495
6DH7 62.200 28.500 2.516 2.516 2.505 2.316 2.009 1.688
6DH8 62.300 22.700 2.522 2.522 2.395 2.160 1.828 1.505

MPG:

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Middleweightboxer screwed with this post 10-16-2012 at 07:02 AM
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Old 10-17-2012, 07:51 AM   #7
Uncle Pollo
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I have same ttansition hiccup. Will report when all smoothed out. Its the needle jet combo
I tried every pj size and still does it
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Old 10-17-2012, 03:06 PM   #8
tradern
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I've had three bikes with mikunis on them now and have yet to be able to rid myself of the stumble/die with a hard blip from idle. If progressively smoothed out by the throttle hand its no dramas. This doesn't present itself as a problem with my riding usually. Although occasionally during slow speed riding, off road with a quick blip of power required has resulted in almost going over the bars from the rev/die/lurch re-action.

After a month of riding, the throttle hand seems to adjust and after a number of months the brain helps with the slow speed stuff - being ready to get on the throttle with care and a bit more progressive.

Otherwise I've lived with it because after some time trying to remedy it I've had no luck. Would be interested to see if any others have the same woes. This has been on a R100GSPD (stock), twin plugged 650 and twin plugged 1000cc.

I'm certainly no demon tuner from hell and only give my limited experience but its been impossible for me to eliminate. It seems to me that cracking (quickly) the throttle from idle, with snappy 450 motorcross bike response, is far from my mortal grasp.

Hope I've added something as I'm reading your post as my feelings when I first entered the magical land of mikuni.

Handy hint though - if the rubber boot on the choke shaft pops off the bike will run like hell. Myself and my girlfriend have both managed to kick this off dismounting/riding somehow. We are a bit special though
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Old 10-17-2012, 03:58 PM   #9
Airhead Wrangler
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Quote:
Originally Posted by tradern View Post
I've had three bikes with mikunis on them now and have yet to be able to rid myself of the stumble/die with a hard blip from idle.
That's the nature of the beast, unfortunately. When you crack the throttle open hard you're allowing more air into the engine than the cylinder is "asking for." This kills vacuum and when vacuum disappears, so does fuel. The carb depends on vacuum on the head side of the slide to suck fuel up through the jets. When that vacuum disappears you get a sudden lean condition and the cylinder stops firing. This is what accelerator pumps were designed to fix. They squirt an extra shot of gas down the throat as the slide opens which counters a loss of fuel flowing through the jets resulting from the momentary vacuum loss when the slide opens. Without accelerator pumps, the only cure is a slow, steady, measured hand.
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Old 10-17-2012, 05:15 PM   #10
tradern
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Thats certainly been the feeling since the beginning but wasn't 100% sure if the logic was correct. Thanks for confirmation Mr. Wrangler.

I've a set of FCR carbs there but there's no way they would get a place on anything but a street/race bike unfortunately.

The mikunis VM's are little bloody wonders of things they are. Simple, robust and should last our RTW trip, if not the bikes lifetime in my hands.
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Old 10-17-2012, 07:08 PM   #11
JonnyCash
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Quote:
Originally Posted by Airhead Wrangler View Post
That's the nature of the beast, unfortunately. When you crack the throttle open hard you're allowing more air into the engine than the cylinder is "asking for." This kills vacuum and when vacuum disappears, so does fuel. The carb depends on vacuum on the head side of the slide to suck fuel up through the jets. When that vacuum disappears you get a sudden lean condition and the cylinder stops firing. This is what accelerator pumps were designed to fix. They squirt an extra shot of gas down the throat as the slide opens which counters a loss of fuel flowing through the jets resulting from the momentary vacuum loss when the slide opens. Without accelerator pumps, the only cure is a slow, steady, measured hand.

My thoughts exactly. This problem is why BMW fitted CV carbs, which basically keeps the engine from getting too much air too fast, regardless of what your wrist tells it. If I were going to go to the trouble of a carb swap, I'd go for the flat slide TM or TMX series Mikunis. Or a set of Dellortos with accel pumps.
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Old 10-17-2012, 08:19 PM   #12
Kai Ju
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Not to be argumentative...........

........but I'm running VM38s on my R100/7 that exhibit none of these problems.
I twist the throttle no different than I did with the 32 Bings and get clean, stumble free acceleration, just more of it
I also get better fuel mileage to boot. Having my cake and eating it, yeah.

I'm still running the stock clamshell airbox and tubes with a stock air filter and stock exhaust.

FWIW, here are my jetting specs:

Description Spec
Main Jet 200
Pilot Jet 25 but will go to a 30 this weekend to shorten warm up time.
Needle Jet 166 P-8
Jet Needle 6DH3
Slide Cut Away 2
Main Air Jet 0.5
Needle Valve Ass'y 3.3
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Old 10-18-2012, 05:26 AM   #13
Uncle Pollo
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I dropped the needle one notch and stopped whacking the throttle loke a baboon. Happy joy joy joy. Pollobusa lifts a wheel now smoothlt
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Old 10-09-2013, 12:18 PM   #14
Andyvh1959
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Here is what I have on my Suzuki GN400 ice bike:

Supertrapp muffler on stock header, with eight plates installed.

VM36 Round slide Mikuni carb: #20 pilot jet, stock needle on bottom notch, #250 main jet. #3.0 slide cutaway.

Unifilter foam air filter with no air box. No air leaks at intake manifold or header. Honda CH250 Elite scooter CDI and coil, with NGK plug cap, Iridium plug gapped at 0.025”. Trigger coil at 10 degrees BTDC.



Pops a lot on decel so I may go to a 25 pilot and 3.5 slide, raise the needle with shims. Plug color is light brown on mid-range. Seems to falter at wide open so I may jet up the main to a 270.
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Old 10-09-2013, 01:09 PM   #15
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I didn't read through all the posts. It sounds like you need richer slides to me. Slide angle and pilot jet combo is critical. For the most part, don't try to tune around those jets with the pilots. Guess what? Jetting takes jets! That includes slides too! At least if you need richer slides.

Slide carbs are not CV's! CV's will make your wrist stupid. To some degree, however small, you can't yank the throttle open and expect your carb to make up for your mistake like you can with CV's. Don't depend on a slide carb to do that. Look within and use The Force!!!
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