Can you look at my spark plug colours and suggest a path forward for tuning my Mikuni ?

DennisMo · 2026-08-02T15:18:29-0600

Hello all,

I have a Mikuni VM34 on my 1973 850 with settings at the bottom of this post.
All new are the following;
Choke plunger, float, float needle, needle, needle jet, air corrector, idle jet, main jet ,
inlet manifold (to head) gasket, rubber adaptor between the carb and inlet manifold

Adjusting the idle mixture, it is unscrewed 3 turns to get the plug colour below.
If it is screwed in from 3 turns out, the plugs get sooty.
With the colour below, the cold started engine pops about 5 times in the first 20 seconds and after that settles down, the bike is rideable.
Also, with the spark plug colour below, there is usually a pop in the exhaust on engine overrun and sometimes a pop when the throttle is closed to shift.

So given the cold start exhaust popping and the throttle overrun popping, it feels lean.

I would like to get that idea brown coloured plug but can't see how to do it.
For example, if I go to a larger idle jet (say a 40), I will have to unscrew the idle screw more than 3 turns to air compensate for the bigger jet

I would appreciate your advice.

Thanks
Dennis

Idle jet 35
Air corrector 2.0
Needle 6DH3
Jet 230
Cut away 2.5
Air Corrector 159 P0

Mart UK · 2026-08-02T15:23:59-0600

Perhaps check your ignition timing, it may be that you are adjusting your carbs to compensate?

SteveA · 2026-08-02T15:25:45-0600

DennisMo said:
Hello all,

I have a Mikuni VM34 on my 1973 850 with settings at the bottom of this post.
All new are the following;
Choke plunger, float, float needle, needle, needle jet, air corrector, idle jet, main jet ,
inlet manifold (to head) gasket, rubber adaptor between the carb and inlet manifold

Adjusting the idle mixture, it is unscrewed 3 turns to get the plug colour below.
If it is screwed in from 3 turns out, the plugs get sooty.
With the colour below, the cold started engine pops about 5 times in the first 20 seconds and after that settles down, the bike is rideable.
Also, with the spark plug colour below, there is usually a pop in the exhaust on engine overrun and sometimes a pop when the throttle is closed to shift.

So given the cold start exhaust popping and the throttle overrun popping, it feels lean.

I would like to get that idea brown coloured plug but can't see how to do it.
For example, if I go to a larger idle jet (say a 40), I will have to unscrew the idle screw more than 3 turns to air compensate for the bigger jet

I would appreciate your advice.

Thanks
Dennis

Idle jet 35
Air corrector 2.0
Needle 6DH3
Jet 230
Cut away 2.5
Air Corrector 159 P0

Are you running leaded fuel? Very hard to read if not.

Whichever, these plugs look on the rich side rather than lean. Are these plugs after a full throttle plug chop? If not they are near meaningless. Ideal brown coloured plugs are a thing of the past for most of us. Best power with lead is going to come from a sandy looking plug, without lead, it'll probably look white!

On the speculative side, your pilot may be a size or two too large. I would not go larger, consider 30 or 25.

Air Jet 2.0 is wrong for this set up and should be nearer 1.0. (check the Victory Manual for guidance) However, it won't show much until higher rpm., which with a single carb you may never be getting to.

The last quoted number 159 P0 is a needle jet and that is small, P2 or P4 is more likely needed. You don't mention needle position, if in the bottom slot you might be compensating for a wrong jet size. If you confirm to yourself it is running week, go to P2 with middle slot. 6HD3, or 2 needle is typical.

Popping in the exhaust? Have you eliminated an exhaust leak, near the head?

SteveA · 2026-08-02T15:45:16-0600

Mart UK said:
Perhaps check your ignition timing, it may be that you are adjusting your carbs to compensate?

Always a good point.

seattle##gs · 2026-08-02T16:41:57-0600

Replace the 2.5 slide with a #3.
Replace the #35 pilot jet with a #40...that should get your air screw in the neighborhood of 1 1/2 turns.
The needle jet is correct. The needle clip should be 2 notches from the bottom.
Order 2 boxes (8 plugs) of DENSO W22EP-U 3085 spark plugs from AMAZON. Approx $28...cheap plugs of the proper heat range, good for tuning. When the plugs go black THROW THEM AWAY. Don't try to clean them or reuse them.

Always check ignition timing first.

Can you look at my spark plug colours and suggest a path forward for tuning my Mikuni ?

DennisMo

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