Mikuni VM36 tuning

I posted last week about fuel dripping from the float bowl overflow (thread here). I've hopefully rectified this issue by losing the filters off the top of the fuel taps inside the fuel tank & fitting new in-line filters, I removed the float bowl & float needle assembly to blast it through with carb cleaner & then compressed air. I then lapped the inside of the float needle assembly with Autosol & a cotton wool bud on a drill.

I had it out for a run out last week but it didn't run well, she was okay to begin with but once she'd warmed up she started cutting out on idle & was popping & banging a bit. The plugs were black. I realised the needle was on the 2nd clip down so it's now on the 3rd (out of 5).

I'll hopefully get the carb back on this week but wanted to know how I go about getting it set up properly? Where do I start? Air mixture should be 1.5 turns out? Where do I start with the idle? Just kick her over and get it sitting at about 1k rpm?

Details:

1974 MkIIa

It's had a new Boyer Brandsen stator plate, alternator stator plate & I've had the carb rebuilt by Steve at Motorcarb.

VM36
Main jet 290
Pilot 35
Needle P2 6DH3
Slide 2.5 cut around
Air jet No. 2

Needle clip is sat in the middle setting

Boyer electronic ignition
Boyer power box

What plugs and plug caps are you using?
Is the bike charging properly?
What air filter do you have?
When the plugs came out black were you at very small throttle openings?
Lowering the needle clip will richen the mixture
If you really want to do a good job on tuning make sure the engine is fully warmed up when you make any carburettor adjustments
Mark the twist grip and housing in closed /quarter/half/three-quarters and fully open positions then when you ride it take note of what position the bike messes about the most
Have a read of the carb tuning overlaps , take your time

Hi Luke. I suggest you get a copy of the Mikuni tuning book from Sudco or whoever is doing it these days. I learned a lot from it. What I have found over the years is that most Mikuni carb kits come set up very rich. Firstly, you must start on the low speed circuits as these add fuel at all throttle openings. Set your idle settings first and then move up the scale with main jet choice last. I actually fitted an o2 sensor and air/fuel ratio gauge as well, from which I learnt the most. Nortons will run well across a wide range of jetting combinations and the o2 sensor will allow you to fine tune very accurately. I was not aware what the various circuits actually did until I read the book. For example, the air jet doesn't have anything to do with low speed running but is a fine tuning part for the main jet. The number 2 is the largest available and all the Mikuni kits come with this size (that I have noticed). Anyway, study up and when you are confident with your knowledge, start tweaking. To do so first is kind of self defeating.

1. Sounds like the enrichener/Choke is in play.
Make sure it fully disengages.
2. Lowering clip raises the needle which made it richer.
3. Adjust mixture screw to max rpm, if more than 2 1/2 turns, pilot jet too large.
4. Get a Mikuni tuning manual. Victory library has some good ones for Norton / Mikuni.

baz said:

What plugs and plug caps are you using?

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NGK BP7ES & haven't the foggiest on the caps, I didn't fit them & they aren't marked with a brand.

baz said:

Is the bike charging properly?

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It wasn't no, I think I frazzled the new Boyer power box by connecting it up wrong. I've now fitted a new one but haven't yet taken the bike out as I'm sorting the carb & filters out etc.

baz said:

What air filter do you have?

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It's a K&N that looks like this (first picture).

baz said:

When the plugs came out black were you at very small throttle openings?

Click to expand...

Very small throttle openings? They were black after my 30 minute ride out but she cut out 2-3 times & I was fiddling with the mixture so I don't feel like they're a true representation of how she's running. I've got a set of new plugs ready to go in.

Luke Turner said:

Slide 2.5 cut around

Click to expand...

Just in case...

If you are light throttle and it gives a frumpfrumpfrump sound at light cruising speed, and plugs get black, the cutaway is very often too rich. It's fairly common and often undiagnosed. Many times a 3 is better but a custom (hand filed) 3-1/4 is often even better. Becase a 3-1/2 will be too lean.

MichaelB said:

1. Sounds like the enrichener/Choke is in play. Make sure it fully disengages.

Click to expand...

Will check whilst the carb is off the bike.

MichaelB said:

2. Lowering clip raises the needle which made it richer.

Click to expand...

I don't know what's what with how far each screw is in & out so I'll put the needle clip to the middle to try & "reset" things & go from there.

MichaelB said:

3.Adjust mixture screw to max rpm, if more than 2 1/2 turns, pilot jet too large.

Click to expand...

Sorry, can you explain this in a little more detail? Adjust mixture screw to max RPM?

I'll have a look at ordering a tuning guide.

Cheers Michael.

Adjust the mixture screw until to you get the maximum RPMS then adjust the idle screw until you are happy with the idle speed
You may need to adjust the mixture screw back in a little if it dips when you pull away
Read up on the tuning sequence and you will see what circuit you are trying to tune,and how much they overlap each other

Luke Turner said:

Sorry, can you explain this in a little more detail? Adjust mixture screw to max RPM?

Click to expand...

Jumping in on MichaelB's suggestion; with the bike warmed-up, adjust the mixture screw to get the smoothest idle. Bounce back and forth between the mixture and idle speed screws to get the smoothest around 1,000-1,100 rpm. Kill the bike, and carefully run the mixture screw in until closed, all the while counting the number of turns it takes to reach bottom. Mikunis typically run around 1 to 1-1/2 turns if the fixed idle jet is correct. While you can correct for an incorrect jet by deviating from the 1 to 1-1/2, you'll then have transition issues just as you're coming off idle (idle to 1/8 throttle position). 'Makes for a bugger to accelerate smoothly from a dead stop. You'll also find that the idle jet plays a large part with cruise economy.

Nathan

Luke Turner said:

Was it charging correctly?
It wasn't no, I think I frazzled the new Boyer power box by connecting it up wrong. I've now fitted a new one but haven't yet taken the bike out I'm sorting the carb & filters out etc.

Click to expand...

That sir is potentially a VERY important little snippet!

My advice would be to do NOTHING more with the carbs until you’ve ridden it with the new power box and a fully charged / properly checked battery.

A fault here can cause havoc with ignition and easily cause the symptoms you describe. Or at the very least, confuse and mask the effects of the changes you are making.

Similar with the plug caps (although their effect is much less) check what caps your choice of ign and plugs require and buy some so that you know they are correct.

Your main jet seems to large to me; I'm thinking 250 or 260, but your 290 is a safe starting point, plugs are less expensive than the havoc lean carburation can reek. As a personal side note: a 36mm VM must be cocked to the right or left of the frames back bone this usually causes (can cause) issues with the fuel level. I see the 36 as a syndrome of Amal envy and counter to the point of using a single carb. A pair of Amals will out perform a single carb from somewhere in the mid range on up. I am building a '73 750 and will be running a 34 VM; I am doing this because I like to ride in the 40 to 80 MPH range; every component, engine & suspension is or will be tuned to balance the equation. When I want to get a "speed fix" I use a different motorcycle; when I ride my Norton it's like waltzing with my wife, when I ride a Ducati it is like cha-cha dancing with my mistress, if you treat both with due respect they will both perform well off the dance floor...

Best

Back up & running!

- Fitted a new Boyer power box
- Lapped needle float assembly with cotton bud & Autosol
- Blasted it all through with carb cleaner & compressed air
- Removed fuel filters from inside tank
- Fitted inline fuel filters along with new hoses & clips
- New NGK plugs

No drips from overflow at idle but will give it a proper test when the weather clears up.

go to a #3 slide first.