Mikuni on Norton

Hi again,i have Mikuni VM36 on my MKIII,i thought first that bike need dual set but problem was clogged fuel oil filter(new one)
Before i buy dual carburetors set i try find optimal main jet for bike(PW3 cams,ported cylinder head).
I started size 280-after fourth kick engine was fired,then i chanced 270-after 3 kikcs engine fired.
Next day 240-much better second kick-engine started running,that was yestarday.
Today i chanced over 225-first kick and engine running.
So it seems that between 220-240 could be right main jet size in my Norton.
I can not make any test run outside beacause here are mid winter but couple of month later time will tell how engine run,
before that i think chance over exhaust pipe 2:1 set(Paul Bryant).

Cheers

Hello!

I belive that the mainjet inluence is in the range of 3/4 to full throttle, when driving.
Therefore it will have little or no influence on the startingprocess that rely on other
systems in your carb .Those systems would be coldstartcircuit , airscrewsetting and
lowspeed/idlejet.
I would wait till drivingseason to decide jets and settings,since it
involves lot of driving at various speeds using different throttleopenings, to get it right !!


Benton

Absolutely correct
That bike will start and idle nicely with no main jet in the carb.

Benton said:

Hello!

I belive that the mainjet inluence is in the range of 3/4 to full throttle, when driving.
Therefore it will have little or no influence on the startingprocess that rely on other
systems in your carb .Those systems would be coldstartcircuit , airscrewsetting and
lowspeed/idlejet.

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Agreed that main jet will have little or no effect on starting and idling. However, I think he will find that those latter main jets will be too lean. I am running a 280 main jet in a 36 mm Mikuni on my 74 850 Commando. Also running a 45 pilot jet, 6DH3 needle at clip position 2 and 159 needle jet. Originally had a 40 pilot jet but had to have the pilot air screw fully seated for a good idle. So I went one size larger and the air screw is now about 2 turns out I think. Most of my driving is at 1000 metres elevation.

I run whatever it came with jetted for normal sea level use I guess , an interesting experiment is to mark your throttle grip in 4 quarters (I used typo correction fluid) and go for a run, unless you are a complete mental case or racing I doubt that you will be over half throttle ever which suggests that main jet size isn't that critical really , you'll soon know if its too lean when you roadtest it, suggest you start rich and come down the way but agreed main jet size shouldn't affect starting.

Gino Rondelli said:

I run whatever it came with jetted for normal sea level use I guess , an interesting experiment is to mark your throttle grip in 4 quarters (I used typo correction fluid) and go for a run, unless you are a complete mental case or racing I doubt that you will be over half throttle ever which suggests that main jet size isn't that critical really , you'll soon know if its too lean when you roadtest it, suggest you start rich and come down the way but agreed main jet size shouldn't affect starting.

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It's quite surprising, I found on highway cruising, 70mph, light load, it hardly gets off the slide.

It's quite surprising, I found on highway cruising, 70mph, light load, it hardly gets off the slide.

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Yes I done a 600 mile round trip one weekend and paid attention to what was happening, even passing cars i rarely needed half the throttle travel.

Many years ago I was racing in the IOM at the Manx on a P&M Yoshi Suzuki, first practice session taking my time I realised that the bike was a bit flat at the top end, stll lapped at around 95mph (this was around 25 years ago) I was very embarrassed to find out I'd been out with NO main jets in the carbs. Funnily I wasn't a hell of a lot faster the next few sessions as the thing scared the S*it out of me :lol: