Can you give me advice about tuning/jets etc on my single Mikuni VM34

D

DennisMo

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Jun 6, 2023
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I have had my 1973 850 for a year and refreshed everything except the Mikuni and hope you will share your advice.

I have not had the Mikuni to bits but here is what the old owner says is the setup.
He bought the new carb already set up for the 850
Below the setup list below are my questions

Slide 2.5 cutaway
Needle 6Dh3 (It is set to the middle notch)
Main Jet 250
Pilot jet VM20/210 #35
Air jet BS30/97 #2.0

Idle speed is about 1200 rpm
The plugs are black (not too black) showing richness so I adjusted the pilot
circuit as below. The idle air bleed screw was set at 1 1/2 turns out.
I unscrewed the screw and got fastest idle at about 2 1/2 turns and if if I unscrew it further, the idle speed does not drop off.
I thought that it should drop off as I unscrew further. Does this mean I should reduce the idle pilot jet size
by one ? If yes, can you tell me what that size is?

I have seen on the Forum that others suggest a 240 main jet.
Should I convert to that size, or should I drop the needle a notch (or two) and see what happens.

Lastly, is the slide, the needle etc as listed above correct?

I really appreciate your help. Mikunis are new to me. They look simple to set up. I am familiar
with British SU carbs!!

The choke works well. Bike starts nicely!

Thanks!

Dennis
Vancouver
 
DennisMo said:
I have had my 1973 850 for a year and refreshed everything except the Mikuni and hope you will share your advice.

I have not had the Mikuni to bits but here is what the old owner says is the setup.
He bought the new carb already set up for the 850
Below the setup list below are my questions

Slide 2.5 cutaway
Needle 6Dh3 (It is set to the middle notch)
Main Jet 250
Pilot jet VM20/210 #35
Air jet BS30/97 #2.0

Idle speed is about 1200 rpm
The plugs are black (not too black) showing richness so I adjusted the pilot
circuit as below. The idle air bleed screw was set at 1 1/2 turns out.
I unscrewed the screw and got fastest idle at about 2 1/2 turns and if if I unscrew it further, the idle speed does not drop off.
I thought that it should drop off as I unscrew further. Does this mean I should reduce the idle pilot jet size
by one ? If yes, can you tell me what that size is?

I have seen on the Forum that others suggest a 240 main jet.
Should I convert to that size, or should I drop the needle a notch (or two) and see what happens.

Lastly, is the slide, the needle etc as listed above correct?

I really appreciate your help. Mikunis are new to me. They look simple to set up. I am familiar
with British SU carbs!!

The choke works well. Bike starts nicely!

Thanks!

Dennis
Vancouver
Click to expand...
Dennis , your air jet is twice the size it needs to be . 1.1 .
Also new float needles are good to change for new at the same time , since you will be ordering in parts .
Never leave a Mikuni choke on for more than a few seconds .
 
Forgive my ignorance in asking this, but is the "needle jet" not the "main jet" which the past owner listed as 250 ??
When I start the bike, with Alton starter, I have my left hand on the choke which I turn off as soon as the engine fires.

Should I change the main jet (250) to a 240, or should I first try to drop the needle a notch or two

Thanks
Dennis
 
The last owner did list a jet called 159 PO.
Is this the needle jet you refer to? is it correct?
Thanks

Dennis
 
I just looked at an exploded view of VM parts and now understand what/where the needle jet is.
Thanks
Dennis
 
Your needle jet is correct.
** The clip in the needle should be in the 4th groove...counting down from the top.
** #3 slide
** Currently I run a #40 pilot though I'm at sea level. If you live above sea level ...about 3000 feet...then a 37.5 pilot jet. Buy both and experiment.

Your main is OK.
 
acotrel said:
You have not mentioned the needle jet.
Click to expand...
He hasn't mentioned the air jet either. And the one he has is wrong. Around 1.1 should do it, as noted by Torontoian.

He should try searching for the several threads already on here on 34mm Mikuni jetting.
 
DennisMo said:
Forgive my ignorance in asking this, but is the "needle jet" not the "main jet" which the past owner listed as 250 ??
When I start the bike, with Alton starter, I have my left hand on the choke which I turn off as soon as the engine fires.

Should I change the main jet (250) to a 240, or should I first try to drop the needle a notch or two

Thanks
Dennis
Click to expand...
Drop the needle one notch at a time until you get the miss, then raise it one. As long as the main jet is big enough, you will be OK. I would forget the changing the main jet until you get mid-throttle right. If you get the middle right, then fit a bigger main jet, you will find nothing has changed. The main jet only works when you use full throttle. If you get the midrange right the bike will accelerate much faster. It is normal to wind the throttle on in a controlled fashion if the carburation is right. If 240 is the biggest specified main jet, I would use that and forget it. It won't be lean enough to burn pistons.
 
So I will do the following.

1) Get a 1.1 air jet to replace my 2.0 airjet
with the objective that when I adjust the idle screw from say 1 turn off seat, the engine speeds up at 1 1/2 turns off seat and then as I unscrew further, engine then slows down? (My problem now is that the engine speed peaks at 2 1/2 turns off seat and as I unscrew further, engine does not slow down

Once that is solved, I will put in a 240 jet.

How does that sound?
Thanks.
Dennis
Vancouver
 
I do not know whether your idle screw adjusts fuel or air, but it is usually fuel on more modern carbs. Idle speed is usually adjusted by the throttle stops or the lengths of the cables. With a single carb, the slide should hit bottom when you wind the twist grip forward. You should be able to start the motor on idle. With the idle screw 3 turns out. If it adjusts fuel, the motor should start. Wind the screw in until the motor misses, and then back it off slightly until you get smoothe idle. Then adjust the idle speed with the throttle stop screw, if there is one, or with the cable adjuster, if there is no stop screw.
If your idle screw adjusts air, you wind it out when you want to lean-off the idle mixture. You should probably not need to change the air jet. I think the air jet and the idle screw work together to control the idle mixture, not idle speed.
I have never had a problem with it, but my brother races Kawasaki two-strokes in sidecars on methanol, and he sometimes changes air jets. You would never need to run a four-stroke engine that lean at idle speed. On speedway, rich idle on a two stroke, can make a bike a bit slow at the start of a race.
 
The idle speed screw adjusts the height of the piston in the vertical bore and thus the speed.

The pilot screw adjusts the air bleed into the pilot idle circuit. Screwing it in is less air in thus richer and unscrewing is
more in thus leaned. My problem is that as I unscrew the screw after unscrewing for maximum idle sped, the idle speed does not drop. I am told that I need to reduce the size of the pilot circuit fuel jet.

Dennis
 
The correct fuel/air ratio should be correct at all engine speeds. Idle speed should not be set by adjusting the mixture. The slide is usually adusted by a stop screw or the cable adjuster. If you slacken the cable and back off the stop screw, the motor should probably stop, even when the idle mixture is correct.
With motocross bikes,they are usually tuned to excellent off the start in a race, but the motor stops when the throttle is not held open. The stop screws are often backed right off. The idle mixture is still set , so the throttle works well from bottom to wide open.
 
Last edited:
DennisMo said:
The idle speed screw adjusts the height of the piston in the vertical bore and thus the speed.

The pilot screw adjusts the air bleed into the pilot idle circuit. Screwing it in is less air in thus richer and unscrewing is
more in thus leaned. My problem is that as I unscrew the screw after unscrewing for maximum idle sped, the idle speed does not drop. I am told that I need to reduce the size of the pilot circuit fuel jet.

Dennis
Click to expand...
What ignition?
Points?
Electronic? What brand/model?

Pump gas?
 
I have tri spark and use premium gas without ethanol.

I did some reading and thionk I am more understanding how the Mikuni works.

So I want to first deal with the fact that when warmed up and at idle, I unscrew the pilot air screw and get maximum idle speed at 2 1/2 turns but if I unscrew it further, the idle speed does not drop. From reading, it will not lean so I think this means that I need to get a smaller idle jet.

I have a number 35 idle jet. Might someone suggest what size down idle jet I should try?

Thanks

Dennis
 
DennisMo said:
I have tri spark and use premium gas without ethanol.

I did some reading and thionk I am more understanding how the Mikuni works.

So I want to first deal with the fact that when warmed up and at idle, I unscrew the pilot air screw and get maximum idle speed at 2 1/2 turns but if I unscrew it further, the idle speed does not drop. From reading, it will not lean so I think this means that I need to get a smaller idle jet.

I have a number 35 idle jet. Might someone suggest what size down idle jet I should try?

Thanks

Dennis
Click to expand...
IF, you are willing to entertain the notion, other than what you know to be the problem💡


The Tri-Spark (and others) ignition idle stabilization feature makes tuning idle mixture by the old methods impossible.

Are there drivability issues? Stumble at tip-in?
Strong odor from exhaust?
Dull thudding exhaust note?
Measured fuel consumption?
Power level below expected?


Also, the spark plug coloration that was a mixture tuning tool 50 years ago is no longer valid.


Unleaded (this has nothing to do with ETHANOL BLENDED FUEL) gas mandated 40+ years ago does not color the plugs like "coffee with cream". Only leaded race gas will do that.

A bone white spark plug insulator is expected with a proper mixture.

A brown or black coloration can come from from even a small amount of oil consumption. (I know, I know, the engine does not use oil. Mine didn't either. Then after freshening up the head, BAM, snow white plugs)

Can you give me advice about tuning/jets etc on my single Mikuni VM34
Can you give me advice about tuning/jets etc on my single Mikuni VM34
 
Last edited:
The bike rides well. The plugs are black ie rich.
Trying to adjust the pilot mixture has been the same problem when I had a Boyer on it. I just put the
Trispark on a month ago
No driveability issues.
No unusual exhaust odor.
50 mpg on the highway
There was slight engine surging on overrun with the pilot air screw at 1 1/2 turns out.
At 2 1/2 no surging on overrun

My only concern is that I have maximum engine speed at pilot air screw at 2 1/2 turns out and that if I continue to unscrew the pilot air screw, the engine speed does not drop. From reading, this seems to point to the pilot jet being too big and I need to downsize it. If I do need to downsize it from 35, what size might be suggested?
Thanks

Dennis
 
DennisMo said:
The bike rides well. The plugs are black ie rich.
Trying to adjust the pilot mixture has been the same problem when I had a Boyer on it. I just put the
Trispark on a month ago
No driveability issues.
No unusual exhaust odor.
50 mpg on the highway
There was slight engine surging on overrun with the pilot air screw at 1 1/2 turns out.
At 2 1/2 no surging on overrun

My only concern is that I have maximum engine speed at pilot air screw at 2 1/2 turns out and that if I continue to unscrew the pilot air screw, the engine speed does not drop. From reading, this seems to point to the pilot jet being too big and I need to downsize it. If I do need to downsize it from 35, what size might be suggested?
Thanks

Dennis
Click to expand...
Don't phuck with it.

The end.
 
I have never had a problem with an idle circuit. The adjustment has always been adequate without changing the pilot or air jets. In any case, one of those jets is used to supply a squirt of fuel when the throttle is slammed shut. Two-strokes often seize as riders brake for corners at the ends of straights.
 

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