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

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

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

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

Click to expand...

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?

Mart UK said:

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

Click to expand...

Always a good point.

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.

Somebody should list what works with a single Mikuni on a stock 850 without any other weird issues and have that list put in the tech section so all the contradictory guessing could be reduced.

Run better spark plugs. standard NGK plugs are not better spark plugs. Merely adequate if everything else is perfect. There is a reason why modern cars run $10 iridium plugs. The reason is because they work with today's pump gas and last a long time.

Here's a monkey wrench tossed in for fun: If you run your finger around the inside of the tip of your exhaust, does you finger have black soot on it? If so you could be burning oil and your engine is not fully broken in yet. You did recently rebuild it right?

Anywho, I'm just here to keep the confusion at a high level. lol

Right now I have three turns out of the screw with a 35 idle jet.
If I go to a 40 idle jet, would that not mean more fuel is passing through the idle jet therefore I would need to unscrew the air corrector screw past 3 turns out?

A bigger number jet means it flows more fuel, if the air corrector screws regulates the air to be mixed with the fuel then you will need more air so it needs to be screwed out more. Same effect on an amal, go from a 17 to 19 and the pilot jet flows more and the pilot mixture screw which regulates air is unscrewed more to get the good idle.

kommando said:

A bigger number jet means it flows more fuel, if the air corrector screws regulates the air to be mixed with the fuel then you will need more air so it needs to be screwed out more. Same effect on an amal, go from a 17 to 19 and the pilot jet flows more and the pilot mixture screw which regulates air is unscrewed more to get the good idle.

Click to expand...

Be careful using the term 'air corrector' with Mikunis. The Mikuni has a jet that the Amal doesn't have. It is called the 'air jet', or the 'air corrector jet'.

It does regulate the air into the mixing chamber, but, it feeds to the needle jet, not the pilot circuit. The OP will be able to get the bike running well enough without changing it. If he wants a correct mixture in all conditions he needs to change it.

The original 'Fullauto' in Australia, who no longer posts here, spent a lot of effort getting his air jet correct with a single Mikuni on an 850 and reported it on this forum. He sought and got advice from Comnoz. I think he end up in the 0.8 range, personally I would start at 1.0 on a road bike, and probably leave it at that.

(I have been as low as 0.5 on my twin 36mm installation and come back to 0.7, further tweaking is possible, but ends up also needing to tweak main jets. And with at least a month between race meetings and very different environmentall conditions is likely to send you chasing your tail and losing track time.)

Factory fit is a size 2.0, and most Mikunis get left with this size, ignoring the effect it has. 2.0 is too large for most if not all 4 stroke applications and leads to too much air mixing at the needle jet at high rpm, subsequently the mixture will a lean out as rpm rises. In most road bikes, riders do not use the rpm range affected for more than a few seconds! An incorrect large size air jet is often compensated for by fitting a larger main jet. A Commando will most likely need a 1.0. Smallest available is 0.5. Too small an air jet will cause a rich condition.

I think the OP should change the air jet, but it won't solve his current problem.

The Mikuni 'mixture screw' is actually called the 'air adjusting screw' and works on the pilot circuit exactly as the OP understands it.

The Victory Manual has very clear guidance starting from 1.5 turns out and states that if you need to wind the screw out more than 2 turns, the pilot jet is too large and should be reduced in size.

I have Mikunis installed on all of my bikes and have found the mixture screw adjustment within close range of 1.5 turn out and fuel enriched startup procedure can get you to a workable pilot jet size very quickly.

(I can't find a reference to confirm it, but I though Amal and Mikuni 'mixture screws' worked in the opposite sense? Or, one meters air and one meters fuel?)

DennisMo said:

Right now I have three turns out of the screw with a 35 idle jet.
If I go to a 40 idle jet, would that not mean more fuel is passing through the idle jet therefore I would need to unscrew the air corrector screw past 3 turns out?

Click to expand...

Yes, but this is used with the leaner #3 slide, a must do. This combination is the only way I can get reasonable? plug life.

While I feel the #3 slide is a tad lean, I know the 2.5 slide is too rich and I have not found a way around it. What works well is the JS exhaust inserts and a THUNDER PRODUCTS torque wing. The torque wing looks gimmicky but it really works. Also requires no jetting.

Mikunis come out of the box jetted air screw for 2 stroke applications . Size 2 .
A Norton Commando is 4 Stroke and should fit a 1.1 . Air jet .
This is not your issue though , the goal with today's fuels is a light grey electrode and a fairly clean ceramic insulator . Brown is no longer the goal .
Enjoy .

Torontonian said:

Mikunis come out of the box jetted air screw for 2 stroke applications . Size 2 .
A Norton Commando is 4 Stroke and should fit a 1.1 . Air jet .
This is not your issue though , the goal with today's fuels is a light grey electrode and a fairly clean ceramic insulator . Brown is no longer the goal .
Enjoy .

Click to expand...

Air jet , not screw. The hand adjusted screw for richness , lean is the goal of 1 1/2 turns out .