Mikuni again

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Dec 26, 2007
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Hi there,i have next kind of problem in my MK3:(I have installed 36mm Mikuni VM(round slide)corburetor on the engine,i have also installed PW3 camsaft and cylinder head both ports are made bigger)i can´t take out enough power,so it seems that the engine can´t get fuel oil so much as it need.Starting the engine is very easy,1/3 part of kickstart range is enough to start engine and it´s running idle speed very nice,but when i start increase power it will "turn down" very soon.Have anyone any idea is one carburetor enough or does it need two carburettors?.
Hi , try to buy the Victory library book about mikuni for British twins you will have all the tuning inside for your Norton.
Happy new year to all the Norton world............and to the other too.
You are aware that the choke operates the opposite of Amals ? You relax the cable to turn them off. Tighten to turn them on. A single Mikuni 36MM shouldn't give out till 7500 or so. Now will you be a man and admit to using the choke wrong? Or are you using the lever choke?
MrNorton said:
is one carburetor enough or does it need two carburettors?.

Welcome MrNorton,

The single Amal or Mikuni VM carburettor conversion is a popular modification, and should work very well once the fuel mixture has been correctly set.

Unfortunately, you have not given any technical detail about the carb settings.

Did you buy the carb as a complete kit already set up for a Norton Commando?

Have you tried altering the needle clip position?

What size is the main jet?

What is the throttle slide number?

What is the needle jet number?

What is the jet needle number?

Can you give this information please?

Mikuni VM tuning info = http://www.mikunioz.com/download/pdf_files/vmmanual.pdf

Hi again

Thank´s for many good answers,when i bought that bike(she came from USA)there was already installed that Mikuni and settings was:
main jet:280
air jet:2.0
idle jet:40
needle jet size i don´t know,i did it open yet.

the last one my settings was
main jet:165
air jet:2.0
idle jet:40
needle jet:same as before.

I had tryed many different main jets and needles between 135-600
and 5DP7-6FJ6,always same problem-does not working...
Carb settings recommended for a VM24 from information included on the NOC website are:

Main jet = 230/240

Idle jet = 35

Slide = 2.5

Air jet = 2.0

Needle jet = P-0

Needle = 6DH3

So your original VM28 carb settings would seem to have been fairly good I think, taking into account the PW3 cam and enlarged intakes?

Have you tried raising the float level?

As you own two 850 Commandos, have you tried swapping the carb/s from the other bike to see if that makes it run any better?

Yes,i tried raise that float level but i think that reason must be sowhere else!!,the other Norton have Amal and i want´not mixed those together.

I found that kind of settings from "our" forum.

#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 look like very interesting,i try these next ....

#You are facing the same carb troubles most of us had with the Norton when first starting to get it properly running. The Mikuni is not a bad carb and will most likely be less trouble than 2 amals. Something is not properly set up in the carb for you to have that much trouble and it might be something as simple as your idlescrew setting or air mixture screw adjustment point. If cold, may be necessary to turn up the idle screw until warm and then reduce it after the engine is hot.
On mine, it is necessary to use the choke and then starting is extremely easy with no throttle applied at the handgrip when kicking. Are you using an air filter? Mine is a K&N filter. Here are some guidelines for Mikuni settings:

750 with a single 34mm Mikuni,
main jet 220 needle jet 159 po needle 6DH3 idle jet 35
Slide # 3 idle air jet 2.0

750 with dual 34mm Mikuni's
main jet 230 needle jet 159 p8 needle 6DH2 idle jet 35
slide # 2.5 idle air jet 2.0

main 240, needle jet 159, p2 needle 6DH3, idle jet 40
slide # 2.5, idle air jet 2.0

main jet 240, needle jet 159, p2 needle 6DH4, idle jet 45
slide # 2.5, idle air jet 2.0#

E. Spaghetti
Have you used the choke as a tool when it gets to this point of not running? As LAB says the base line set up should have worked better than what you are seeing. Is it possible you are putting out the fire? What are the coils plug wires and spark plugs?
I would start looking at the ignition even a poorly set up carb will run off load. But a weak spark can be easily snuffed out, and usually at 2500 to 3000 rpm.
Best of Luck,

If the float is set right and you put this stuff in it will run:
main jet 240, needle jet 159, p2 needle 6DH4, idle jet 45
slide # 2.5, idle air jet 2.0#
I may have got off on the wrong foot with you but refusing to answer questions will make your bike hard to fix using writing.
Can you list your Coils used. spark plug wires used, what spark plugs, points or EI? Air cleaners? Have you strobed the timing? Have you checked your strobe gage inside the cover? Have you checked the cam timing? Have you checked the petcock flow? Gas cap vent? All the fuel screens?
Using the choke can help to determine if it's rich or lean. How do the plugs read? What is your gas like?
You see two 850's same equipment everything, jetting is not the same just a bit different.
Can you explain a little better what happens?
Are you riding the bike, does the motor just stop reving, does it misfire or backfire, at what RPM or when does the fault happen? I don't understand your words "turn down" .


my language men(i´m writing from Finland),#turn down#-it´s mean for example after 4000rpm engine loose power,so when i start increase power everything goes ok,engine run very well but after that 4000 rpm power disapeared.

i tryed several times small needles and many main jets and there was sometimes area(engine rounds)when Norton run like hell and suddenly the power disappeared.So when i mixed thos needles and main jets i can chanced that point up or down when engine running well but always end of the rpm range was not give enough power out.

Next time i will stard those basic settings and try that way.
Spark plugs are champion,timing about 32° before TDC ann Boyer ignition,two 6 volts coils.
It is very easy to start,even cold,running idle speed very well and choke is lever not cabel and it works really good,only problem is that power.
Change your carb settings back to what norbsa suggested and then retard the timing back to 31 at 5,000 rpm, Does the bike ping at higher rpm's? It almost sounds like the timing is too far advanced. It will take two people to check the timing, Don't ask how I know. Remember 31 at 5,000 rpm, Good luck from Chuck Horton.
I think it might be time to check the cam timing, you've only got to miss a tooth and you'll get that exact problem.


I will check those cam and ignition timing,sometimes engine ping when i tryed higher rounds.
I will try those basic settings again.
Other things

Gas tank capworking okay(breathing),fuel flow also is okay.I can´t remember anymore (i´m at work now-i´m a sailor)sparkplugs type,NYc something...but they are also ok.air filter is KN and i beleive that every other things are okay,only those carburetor settings are missing now.
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