38 Mikuni single help..
- Thread starter scarter
- Start date
- Status
- Joined
- Jan 12, 2011
- Messages
- 1,723
wow, a 38?
Normally, a 34 Mikuni is quite sufficient for even an 850, I ran one for 15 years
and 36mm is less common but certainly works well, but a 38?
anyway, you say yours is running rich so the solutions to make it flow leaner are easy enough: reduce the size of the pilot or idle jet, reduce the size of the main jet, and/or
drop the needle by putting the circlip in the next higher notch on the needle
Normally, a 34 Mikuni is quite sufficient for even an 850, I ran one for 15 years
and 36mm is less common but certainly works well, but a 38?
anyway, you say yours is running rich so the solutions to make it flow leaner are easy enough: reduce the size of the pilot or idle jet, reduce the size of the main jet, and/or
drop the needle by putting the circlip in the next higher notch on the needle
Which VM38 are you running?
The VM38-9 ( 2.5 slide, Q2 needle jet, 6DP1 needle, 330 main)
or the vm38-9-yam500 (3.0 slide, P6 needle, 6F8 needle, 260 main)
both have the 30 pilot jet and .5 airjet.
The VM38-9 is some what rich across the board with the slide, needle jet, and main.
Scarter, are you able to define where in the range you are richer.
What is your jetting now? You really need to ID these things.
For what it's worth the 330 main is way out there and a 260 would be more applicable, maybe even a 240 or less.
Someone gave me a swinging deal on the vm38-9-yam500 racing carb. I am truly a sucker for this stuff and will be testing in the next couple weeks.
The VM38-9 ( 2.5 slide, Q2 needle jet, 6DP1 needle, 330 main)
or the vm38-9-yam500 (3.0 slide, P6 needle, 6F8 needle, 260 main)
both have the 30 pilot jet and .5 airjet.
The VM38-9 is some what rich across the board with the slide, needle jet, and main.
Scarter, are you able to define where in the range you are richer.
What is your jetting now? You really need to ID these things.
For what it's worth the 330 main is way out there and a 260 would be more applicable, maybe even a 240 or less.
Someone gave me a swinging deal on the vm38-9-yam500 racing carb. I am truly a sucker for this stuff and will be testing in the next couple weeks.
concours
VIP MEMBER
- Joined
- Dec 29, 2011
- Messages
- 9,439
How old is the carb? The enrichener plunger seal on Miks can harden up, seal poorly and richen the idle/low unexplicably. Pics please. If the PO is to be beleived, troubleshooting may be in order versus tuning.scarter said:Pete thanks, I'll report back this eve after work...rich on idle, for sure. This carb was on bike when I bought this one 2-3 months ago. PO states good running, for me it's heavy fouling quickly.
- Joined
- May 23, 2014
- Messages
- 10
OK, this carb has 38L stamped over
9
below the enrichment lever. It is jetted 230 main, 32.5 pilot, P-5 needle jet, 2.5 slide, 6DH3 needle, .5 airjet. The carb's age is unknown, the rubber seal at the top of the plunger seems pliable and tight on the plunger shaft. I removed and cleaned with cleaner and air the passageway below the plunger leading to the emulsion tube. Seems clear. Thanks for input.
9
below the enrichment lever. It is jetted 230 main, 32.5 pilot, P-5 needle jet, 2.5 slide, 6DH3 needle, .5 airjet. The carb's age is unknown, the rubber seal at the top of the plunger seems pliable and tight on the plunger shaft. I removed and cleaned with cleaner and air the passageway below the plunger leading to the emulsion tube. Seems clear. Thanks for input.
scarter said:
Those number don't seem to bad. You better check the pilot screw settings.
The pilot screw on the MIk woks differently than the Amal. Not to confuse but the AMAL screw allow for more air when unscrewed whereas the Mikuni screw allow for more gas.
That being said, find out how many turns that pilot screw is turned out. Somewhere between 1/2 and 3 turns is what is suggested where 1 1/2 turns is ideal. Now, if the best running is 1/2 turns or less and your pilot jet is too rich and you may need a 30, 27.5 or 25.
Then again,you may just have to adjust what you have. Turn the screw till it just touches bottom, be carefull not to jam it in there. Then 1 1/2 turns out. With some clean plugs, start it up. If possible, turn the screw in until it fauters, then turn it out until it faulters at the other end. Take note of how many turn it took, then split the difference.
The itty bitty air jet is what sets the function of the pilot system and determines the amout of air that the pilot uses. Clean it and the passage. THIS IS KEY! You may need to go up to a .7 or maybe a 1 to get enough available air for the pilot system to function. These are really finite setting that you shouldn't have to go there, for the carb was working well, right?
- Joined
- May 23, 2014
- Messages
- 10
This engine and carb setup hasn't run cleanly or properly since I purchased, however the PO wasn't at issue with it. I've made no changes and found the pilot screw at 1 3/4 out from stop. Carb had not been off for cleaning with carb cleaner and compressed air, but after performing a strip/clean I still have black plugs. Now initially I bypassed removing the choke mechanism, but have now removed and thoroughly cleaned that as well. Going to have another good look at that air jet..Thanks for your help.
- Joined
- Jan 12, 2011
- Messages
- 1,723
guessing you are going to be at this for awhile, getting the jetting right at each stage
I would order in a couple more sets of new spark plugs for this kind of work as it will make reading the mixture a lot easier as you progress through the throttle opening reading stage
what kind of plugs are in there, maybe that is part of the problem?
have you pulled and cleaned them good with brake cleaner and soft brass brush, etc
most of us use NGK BP7ES or Champion N7Y, you using something like that now?
I would order in a couple more sets of new spark plugs for this kind of work as it will make reading the mixture a lot easier as you progress through the throttle opening reading stage
what kind of plugs are in there, maybe that is part of the problem?
have you pulled and cleaned them good with brake cleaner and soft brass brush, etc
most of us use NGK BP7ES or Champion N7Y, you using something like that now?
- Joined
- May 23, 2014
- Messages
- 10
Thinking the same thing with testing. BPR7ES it's been, before and after. She'll carbon up a new set in the 8 mile run I've got from the farm out a 40 mph gravel then tar and chip (more gravel) road to reach a highway. Spluttering at that point= fouled up. Seems issue's in idle circuit and low throttle. Needle was in mid-position (3 of 5). Lowered a notch. We'll see. :roll:
- Joined
- Jan 12, 2011
- Messages
- 1,723
why do you have an "r" in your spark plug designation, that means it is a resister plug
what kind of ignition is you bike fitted with, is it all stock with points, etc
even old analog Boyers did not call for resister plugs, which are specified more for the newer digital type electronic ignitions
it is possible your resister plugs are the wrong plugs and are contributing to the lack of clean burning in the combustion chamber or something?
just guessing here but lets at least make sure you have the right plugs for your ignition, and
that you have checked the plug gaps with your feeler gage, what is the gap read at?
what kind of ignition is you bike fitted with, is it all stock with points, etc
even old analog Boyers did not call for resister plugs, which are specified more for the newer digital type electronic ignitions
it is possible your resister plugs are the wrong plugs and are contributing to the lack of clean burning in the combustion chamber or something?
just guessing here but lets at least make sure you have the right plugs for your ignition, and
that you have checked the plug gaps with your feeler gage, what is the gap read at?
With the opening caveat that I do not have a VM38-9, thus am not intimately familiar with how the pilot or main circuit operates on this particular carb, I am familiar with other VM series carb pilot and main circuits and therefore don't understand your descriptions. Hopefully by making the following comments and being corrected or accepted I'll learn more about the operation of a VM-38-9.
Regarding the Amal and Mikuni pilot circuits, aren't they essentially identical in function? They both have a fixed pilot jet to limit fuel supply, and a pilot air screw to adjust the amount of air introduced to the pilot circuit. I believe that in both cases opening the screw leans the mixture by introducing more air to a fixed amount of fuel, and closing the pilot air screw richens the mixture by reducing the amount of air introduced with the fuel.
In light of the above comment, if you need the pilot air screw set at 1/2 turn for best idle, doesn't this indicate that you are near the capacity of the pilot jet (you've almost shut the air to the circuit off) and you need a larger pilot jet if you intend to run with the air screw at 1-1/2 turns out?
Regarding the air jet (or air correction jet as it is sometimes called), isn't this is a component of the main circuit and has nothing to do with metering air to the pilot circuit, other than the air correction galley is the available air source for the tiny amount of air metered by the pilot air screw? Of course the air jet and galley do need to be open for the pilot circuit to function, but it does not meter air to the pilot circuit.
Anyway, that's what I'm thinking. If I'm wrong please help me see the shortcomings of my thought process.
- Joined
- Sep 26, 2009
- Messages
- 2,210
All the last post is wrong, amal- VM mikuni are opposite in pilot adjustment, 1/2 turn out and still rich requires a smaller[not larger] slow running jet
BUT only if the pilot screw is out board..which i assume all VM's are?
BUT only if the pilot screw is out board..which i assume all VM's are?
No. Pilot on the Amal adjusts the amount of air. The pilot on the mikuni adjusts the amount of fuel.WZ507 said:
No. However I may have misspoken on its true nature. What the air jet does do is send a vacuum signal to the pilot. So in effect a smaller (restricted) Airjet will richen by sending a stronger vacuum signal to pull more fuel throught the pilot.
From Mikuni tuning manual
IDLE CIRCUIT (PILOT SYSTEM)
The idle circuit supplies fuel at idle speeds and has
a major influence on fuel flow up to 1/4 throttle. There are
three tunable parts in the idle circuit:
1) PILOT JET --- controls maximum fuel flow through the
idle circuit.
2) PILOT AIR JET--- controls the maximum amount of fuel
that will flow through the pilot jet by allowing a higher
(smaller air jet) or lower (large air jet) vacuum signal at the
pilot jet.
3) PILOT SCREW --- controls how much fuel is allowed to
enter the carburetor venturi.
The pilot screw is used to control idle mixture. Turn
the screw out to richen the idle mixture. Turn it in to lean
the mixture. The engine should have a smooth, steady idle
with the screw between 1/4 and 3-1/2 turns out from fully
bottomed (gently!). If the engine requires more than three
turns out, the pilot or pilot air jet may be too lean. If it
requires less that 1/4 turn, it may be too rich.
As the throttle is opened the pilot screw’s position
becomes less important than the sizes of the pilot and pilot
air jets. A larger pilot jet richens the mixture from just off-idle
to 1/4 throttle. A smaller one leans it. A change in pilot air jet
has the reverse effect. A larger pilot air jet leans the mixture
and a smaller one richens it. The pilot jet and pilot air jet
have slightly different effects on mixture strengths. These
effects are discussed in the “General Tuning Procedure”
portion of this manual.
The idle circuit can be adjusted by changing either
the pilot or the pilot air jet. A one-size larger pilot jet will
have nearly the same effect as a one-size smaller pilot airjet.
It is generally easier to change the air jet since it is
more accessible.
After changing either jet, it is necessary to re-adjust
the pilot screw for best idle.
The preceding 2 posts are both wrong as they incorrectly describe the idle/pilot circuit on a Mikuni VM. The 2 subject posts describe the idle circuit of Mikuni HS series carbs which is completely different than the idle circuit on VM series carbs. Please see the above post by WZ507 or the subsequent posts by L.A.B. for the correct description of a Mikuni VM idle circuit.
- Joined
- Nov 20, 2004
- Messages
- 20,415
pete.v said:
Not if it's a VM. In which case the (HS40) tuning information you have given would be wrong.
http://www.mikuni.com/pdf/vmmanual.pdf
concours
VIP MEMBER
- Joined
- Dec 29, 2011
- Messages
- 9,439
L.A.B. said:pete.v said:
Not if it's a VM. In which case the (HS40) tuning information you have given would be wrong.
http://www.mikuni.com/pdf/vmmanual.pdf
It's not quite as simple as the drawing/verbiage suggests. Pete is correct. Turning the screw CCW ( ACW) allows more air/fuel previously mixed at the pilot jet, which, at small throttle settings, makes up a significant amount of the overall mixture. Therefore, reverse from Amal mixture screw.
- Joined
- Nov 20, 2004
- Messages
- 20,415
concours said:
He quite clearly stated, "Pilot [presumably meaning screw] on the Amal adjusts the amount of air. The pilot on the mikuni adjusts the amount of fuel."which is not what you are saying and which I can't say I would agree with.
http://www.iwt.com.au/mikunicarb.htm
- Status
Similar threads
