Mikuni tuning

Hi all,

I’ve been working on fine tuning my mikuni vm34 on my MKIII. It’s been running rich and will only start with the choke off, and the air screw isn’t getting me where I need to be. I tried stepping the needle but the spot it’s on seems to be the one where it runs best. Am I correct that the next logical step would be to try sizing down my pilot jet? My Mikuni was probably added in the 80s is equipped with the following:

Slide: 3.0
Pilot jet: 25
Main jet: 200
Needle: 6f4

I know all bikes are different but does my setup seem to at least be in the ballpark of what might be expected?

Thanks,
Anthony

FWIW, I have success with a 20 pilot jet on a single Amal 34, 850 mk3.

Yes, the next logical step would be to try sizing down the pilot jet. One full size should sort it as Mart UK comments.

Question might be why is it now a problem and not before? Unless it has always been difficult to start?

So, what is the condition of the air filter?

Also, a thorough cleaning of all the fuel and airways is in order. And the rubber parts, O rings on the adjuster screws and the rubber on the choke plunger.

And drop off the bowl and inspect the float and needle jet.

Since you are there, check the rubber flange for air leaks through fit and cracks, and when did you last tighten the flange clip?

Though logically you have the opposite problem now would be the time to go through the basics.

The 6HD2 or 6HD3 needles are more typical choices, and occasionally 6F5, but the issue doesn't sound like the needle but check it for ridges and general wear.

I found this a useful reference

You didn't state the needle jet size. It should be a 159 - p6 using that needle. Double check the needle jet. That needle is seldom used.

The standard set up is :

37.5 pilot
#3 slide
159 P-0 needle jet
6DH3 needle
230? main jet

From the Norton Tech Digest:

Failed enrichener plunger seal.
(Or misadjusted/rusty enrichener cable if so equipped.)
Plugged air passage from intake horn.

FWIW - I have an 850 in the shop right now with a 36mm carb on it - jetting was a mess.

I changed it to the Tech Digest recommended jetting, and it runs beautifully now. Only change was to drop the needle one notch from the middle notch - was a bit rich and 8-stroking a bit on neutral throttle in the mid-range.

Agree with @concours comment - all parts need to be in good nick before starting to tune.

Thanks for your replies everyone. The vintage bike builder page on Mikuni tuning was what brought me to the conclusion of stepping down the pilot jet, its a really great resource! This all is good information, it seems my setup may be a bit out of the norm. I will check the needle jet next time I have it apart to see what's installed. In the meantime however, since my initial post I stepped the pilot jet down to a 20 per Mark's recommendation and things seem to be in a pretty good state. The bike now will not run sustained cold unless on the choke circuit and seems to have the correct range of adjustability from the air screw (engine runs worse when screwed too far in and too far out). I need to get a new set of spark plugs to test that I'm not running too lean, but its starting and running pretty predictably now and my pipes aren't showing any new bluing so I may be in ok shape.

FWIW: freshly cleaned air filter, new plunger body and seal, new rubber flange with no leaks, needle seemed to be in good condition when recently inspected. I do have a bit of silt making its way from the tank past the fuel filter, however. I'll likely have to try another round of electrolysis at some point.

Regards,
Anthony

TomU said:

I found this a useful reference

Mikuni Tuning and Jetting Guide

What kind of Mikuni do you have? You may not know it, but not all Mikuni carbs are the same. That 24mm Mikuni that needs replacing on your 2005 Yamaha TTR125 isn't exactly the same as the VM24...

www.vintagebikebuilder.com

Click to expand...

Yes, it looks useful, and worth a read, but....

You won't be able to get good clear plug readings with modern unleaded fuel! Certainly not any 'caramel colour'.....your plug is going to look more like the 'too weak' colour range.

The guy doesn't mention the air jet until the very last 'solution' panel

At least I can agree with his suggestion for changes, but you need a start point, which for any 4 stroke isn't the 2.0 that comes fitted as standard on VM Mikunis and is only right for 2 strokes.

The Victory VM Mikuni manual for British twins, gives more useful start points for jetting for, well, British Twins!

Any jetting suggestions for running original dunstall pipes instead of peashooters?

GavinJuice said:

Any jetting suggestions for running original dunstall pipes instead of peashooters?

Click to expand...

I am actually running an original set of Dunstall Decibels. I didn't consider factoring in those into my math until you brought that up, but in retrospect of course that change could affect carb settings intended for a stock setup. So FWIW, the setup in my initial post with a size 20 pilot jet instead of the size 25 seems to work pretty well. I haven't checked the needle jet yet but as Seattle##ps said, my needle would likely be paired with the 159 - p6 needle jet.

Anthony

For those interested: I called Sudco about my T34 FLAT slide carbs. There is only a 4.0 slide for this carb, but they did give me these suggestions for a 850 Norton. I included prices for shoppers

mean gene said:

For those interested: I called Sudco about my T34 FLAT slide carbs. There is only a 4.0 slide for this carb, but they did give me these suggestions for a 850 Norton. I included prices for shoppers


View attachment 101625

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Sudco seems to remain ignorant of the function of the air jet, the 2.0 is the factory standard fit for replacement carbs. Sudco as far as I have seen from people's post have never quoted anything other than a 2.0 air jet.

2.0 is only a correct fit for a 2 stroke. Too big an air jet will tend to high rpm lean out.

A correct air jet sizing is likely to be between 0.9 and 1.1.

The info they have given you for the flat slide would read across to a VM round slide fairly well!

I had a ton of trouble getting the Mikuni going on my Triumph until I bought a color tune spark plug and changed the air jet. With the color tune I was able to dial in my idle jet and from there all other efforts were in needles and needle jets.

Additionally I found this chart which helped me get in the ballpark of jets to buy. I went 2 up and 2 down on these options when I purchased. I have a bunch of spare brass, but also a well running machine. The air jet made a ton of difference as well, on the Triumph I use a 1.0 air jet this guide suggests a 2.0.