Jetting 36 mm Mikuni

Installed a 36mm Mikuni on my 850MK III. Works great but looking for some thoughts as to what others have jetted their carb to and needle position.

Thanks in advance

Hi Ron

I have a mikuni 36mm carb myself fitted to my 850 commando, I have only had this carb for about a year and found that the bike was running rich, I have posted before on this forum about jetting and it appears to be a lot of trying different jets is the answer, as I have been told that no two bikes are the same. My carb came fitted with a 290 main jet and a 35 pilot, I was told by the company that I purchased the carb from that every thing was correct for the 850 commando including the slide and cut away.

My bike had the engine re-built during the winter and again I have found rich running problems, I haven’t reached the main jet yet when out riding as I am running the thing in and still on pilot jet however I changed the main jet to a 260 and the pilot to a 30 as recommend by someone else and did a few plug chops to find that things were better but still a little on the rich side. I have bought myself a 27.5 pilot and will try this next.

Overall, the bike is running very well so I have no regrets of changing to the Mikuni. I still need to sort out the rich running problems out.

Hope this helps.

Neil

Thanks Neil
My carb came with a 280 main and the needle was located 2 up from the bottom. Plugs were a very dry black soot. I changed to a 260 main and raised the needle to the top. Some loss of power and the plug was just a little too light coloured for my liking. Back to the 280 main and set the needle to 2nd from top . So far , so good .

Hi all, only just found this site ...looks interesting. I have a '75 Mk111 with a 36mm Mikuni. It is also running a Megacycle 560-NSS cam ( higher lift, longer overlap) and Boyer.
The jets I'm running are:
Pilot 25
Air Jet 2.0
Main Jet 270
Needle Jet 159
Needle 6DH3 on 2nd clip up
Slide cutaway 2.5
K&N Airfilter ( oval pancake)

This is all as I purchased the bike , with about 20,000 miles on this setup.
Always starts by a half kick with enrichener on, but no ignition, then a full kick with ignition on ( I try to save the starter for hot starts only). In warm weather it doesn't need the half (priming) kick.
It seems there is no fixed formula...... every engine seems to need a slight change of jetting. Hope this is of interest
Jay in NZ

Ron: What pilot jet are you running?

Neil
Not sure on the pilot jet as i have not taken it out.The carb came with two spares , a 3.5 and a 4.5.. The idle is very good and low speed acceleration is fine. If you want i can open it up and find out for you. My biggest issue was black sooty plugs even after some good hard riding . That i believe i have now figured out .

Just did 60 miles of regular riding and pulled the plugs . Things look very good indeed. 280 main , 3rd notch up from bottom on needle.

Most people are finding that the new less volitule gas we buy in the States has a tendency to run rich. Many are running smaller idle jets to stop idle speed plug fouling. This does make it so that you have to run the enrichner to start the bike. The pay off is a bike that will idle down and run right after runnig it hard and than run slow like you have arrived at the camp ground a are searching for a camp site (slow). If you live were it gets cold you may need to flip back to a richer idle jet in the fall it will pop back to let you know. I have seen people run all the way down to a 27 or so idle jet. I run a 15 because I have a UFO on the bike.

Ron Hulton said:

Neil
If you want i can open it up and find out for you. My biggest issue was black sooty plugs even after some good hard riding . That i believe i have now figured out .

Click to expand...

Hi Ron

No need to open your carb for pilot jet size when you have things running ok but thanks anyway.
i have not had time to try my 27.5 pilot jet in yet but if things dont improve after doing this i will try the set up you are using, i have never looked at what position the needle is set at yet.

Thanks
Neil

Hi Neil
The needle position for me was the whole problem. No matter how hard i ran it the plugs would be black. The needle clip was 2nd from the bottom . I went to the extreme and put it at the top but i was nervous about the plugs being too clean. Dropped back to the 3rd from bottom notch and the plugs are perfect

Good Luck

I have been through this & ended up with 27.5 pilot, 3.0 slide. Main jet & needle jet as sent with kit by suppliers & I dont know what they are, needle is on 1 groove lower than centre position.

Slight update. After trying the 27.5 pilot jet and seeing no major improvement, I decided to go to Ron's configuration. I removed the 260 main jet and replaced it with the 280. I also checked the needle position and found it was 3 notches from the top, I moved the "E" ring to the second from top position then turned the air screw two and a half turns out from the closed position.

Gave it a quick blast up the road and checked the plugs to find no sooty deposits whatsoever! They seem very clean so I think, thanks to the input I’ve received, I'm finally getting somewhere.



Thanks,

Neil

neil and ron,


im about to go sort this out and try your config also.. im having some problems on my newly installed system. the only thing is i think my mikuni is a 34mm...

Ok here goes. Been lurking for a bit and I find the info here is great. so thanks guys, you've been a great resource. I just went through my VM34. and the bike is a 750 Commando with a Boyer Mk III ignition. The carb has the stock jets. I've set the clip on the second from the top. The float height is at 23mm and the air jet is about 1 3/4 turns out.
The plugs are that lovely brown colour now.
The float height needs to be between 22 and 24mm. It will have a direct influence on the fuel mixture. With all the bouncing things do over time there is no guarantee the original setting is correct so it's worth a look.
The lower the clip on the main jet needle, the richer the setting. (it raises the needle off the seat letting more fuel through). I'm not racing mine, but if I were looking for more top end I'd be inclined to drop the needle clip a notch to open it up further at the top end.
Most of us think of jets in terms of fuel where turning it in is leaning it. The jet on the rhs of the bike is AIR not fuel, so if you turn it in you're enrichening the mixture.
Hope this helps.
Cheers,

Simon

simon,

thanx for the info! even though this post is about a 36m mikuni (i think the vm34 is not a 36mm), you have answered a question of mine.. now i have one more for ya. on my vm34, how do i check the float height? i do not have a diagram handy, but if you could let me know that would be great!


mike

Driggs,

There's a Mikuni tuning manual pdf here that could help?:

http://www.mikuni.com/fs-carburetor.html
>Manuals exploded Views>VM "Round Slide" Tuning Manual.

To check the float height you should remove the carburettor from the bike. It can be done in situ but its easier on the bench. Remove the float bowl from the carburettor. (4 screws) This will expose the arm that moves against the float needle. The floats themselves will come away with the bowl as they slide up and down on a couple vertically mounted rods. If you're doing this in situ, back off the clamp securing the carburettor to the rubber flange and rotate the carburettor to give you better access. I use a set of calipers to measure mine but a decent small straight edge should suffice. If you do it on the bench turn the carburetor upside down so the float arm is resting on the float needle and the float needle is seated. Measure on the air inlet (filter) side of the carburettor vertically and project square across to the float arm. The VM34 you want 22 to 24mm. The VM36 and bigger you want 17 to 19mm. The bigger the measurement here the leaner the setting. ie. the floats close the float needle sooner, keeping less fuel in the bowl. To explain it, the lower the fuel in the bowl the greater the elevated distance the fuel must be raised by the pressure difference between atmosphere (high) and venturi (low). If yours needs a tweak there is a tab that touches the needle move this tab, NOT the two arms. They should be straight and parallel with each other. Be gentle here. Bits are fragile and malleable. Measure more than once and take your time.
Hope this helps.
P.S. just checked out LAB's link. +1 here!

Simon.

thats why i come here! thanks guys!!



mike

Readig plugs not so easy as it may seem. this is a good read I use to worry about white plugs. http://www.dragstuff.com/techarticles/r ... plugs.html

A lean burn at wide open will hole a piston no one needs this. Most of the time your not running on the main jet. Once you are running on the main you read the plug way up inside and all the color from running the lower circuts will be long gone. Reading the colors visible by useing the eye alone is only good for setting these lower circuts. Since your not going to hole a piston using the lower circuts you need to set these as lean as you can and still have a bike that starts nice and picks up off idle well. When I hear people saying things like "I don't need a choke to start my bike" and "My plugs are a nice brown color" I know that they don't get it. With Mic's you will see a dry black ring at the bottom of the threads from the idle circut on your plugs. But if you have color seen by eye you could go leaner and find more power. A two mile trip around the block won't give you a read you can use. MPG should be your guide to some degree. with a Mic you should get min 45 and on a trip for 300 miles last summer at 55MPH I got 59 MPG.

Hi,

Sorry to revive an old thread but I'm still not convinced the 36mm is running right. Just to recap; when I got the kit in June 05 it came jetted as follows:

290 Main Jet (280 and 300 also provided)
35 Pilot Jet (32.5 also provided)
6DH3 Needle
159 P2 Needle Jet
2.5 Slide
2.0 Airjet

I found this configuration caused the bike to run too rich. After some experimentation, I found this configuration was the best (so far):

Airscrew 2 1/2 turns out
260 Main Jet
27.5 Pilot Jet
Needle one position away from the weakest setting

The bike now appears to be running very hot and occasionally spits back through the carb. I've since ordered a 22.5 and 25 pilot jet - neither of them would allow the bike idle. The plug electrodes seem to be the correct colour (very clean) but still a little sotty around the centre which indicates to me it's still running rich.

Any ideas/ thoughts as to what to try next would be greatly appreciated.

Cheers,

Neil