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They are listed on the charts in the link I provided
and the tuning manual section provides all the possible alternatives available http://www.mikunipower.com/Manuals/VM_Manual.pdf
The manual doesn't tell me how the seller has jetted the set of VM28s to suit that particular bike. What did I miss?

There is a huge range of jets and emulsion tubes available and more than one way to get an air:fuel chart more or less working. Personally, I don't want to have to keep modifying needle shapes the way we had to to jet a Keihin carb to run on methanol on a bike it was never intended to fit, or drilling needle jets or float valves.

I literally have hundreds of main jets, needle jets, pilot jets, air jets and needles and it still takes me a while to get in the ballpark and I'm just in this for fun and I still need more.....

My question was what parts are fitted into the carbs the OP mentioned were pre-jetted for his bike.

Between PJ and XB there's a wealth of knowledge.
 

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The tuning manual states in great detail there are many variables that will alter the jetting and adjustment requirements. Variations in the environment, in the machine itself, in the nature of the fuel plus His intended use of the vehicle all of which affect what would be an ideal combination of carb aperture, replaceable jets, jet tube and cutout on the slide. Assuming he is using gasoline fuel, the ratio of air to fuel is roughly 15:1 and that doesn't change unless you change your fuel, that ratio is based on an approximate stoichiometric ratio of average pump gasoline, or in simple terms the amount of fuel you need to combine with oxygen to completely burn the fuel. The stoichiometric ratio is irrelevant of whatever engine you are using to burn your particular gasoline, that is the specification of the fuel and earths atmosphere.

OP has committed to a 28mm carb set. (that was his aperture selection of choice) He now has a set maximum volume of air to work with, it's a 28mm hole and that won't be changing unless he wants to buy multiple carburetors. All of the parts that can be changed to alter the fuel air ratio are listed as "selection of" in the tuning manual and the rest are adjustments, (like turning a screw or moving the cir-clip.) The tuning manual tells you how to interpret your real live results, but you have to start somewhere and logically that start point will be whatever Mikuni has determined to be the 'standard' or average setup expected for a 28mm carburetor using regular gasoline.

I can almost guarantee his jet needle cir-clip will be in the middle slot of 5 slots available but beyond that, his performance and milage will literally vary dependent on the combination of absolutely everything. He has to start somewhere and that start point has yet to be determined until he buys the carbs, sticks them on his bike, fills it with whatever his local gas station sells him as resembling gasoline and test rides it.
"pre-jetted for his bike" Does not yet exist! He's not working with an OEM carburetor or OEM motorcycle setup, he's way out there on his own because of all the variables.
( unless somebody out there is a wizard of the clairvoyant magical mythical kind in which case I humbly bow to your superior abilities ) :cool:
 

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BTW: Dime city sells the 28mm carb as (Standard Jetting) -> https://www.dimecitycycles.com/vintage-cafe-racer-caferacer-bobber-brat-chopper-custom-motorcycle-carburetors-mikuni-28mm-001-022.html
and they sell the 30mm VM as being "pre-jetted" but include extra jets ->https://www.dimecitycycles.com/the-coveted-mikuni-vm30-carburetor-complete-kit-for-honda-cb-cl-350-360s.html

... but the OP hasn't even said where he ordered them from :rolleyes:
What he did say is " I will start with the info on the 32's with the understanding that it is only a starting point." which totally will not work.
 

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.......... Assuming he is using gasoline fuel, the ratio of air to fuel is roughly 15:1 and that doesn't change unless you change your fuel, that ratio is based on an approximate stoichiometric ratio of average pump gasoline, or in simple terms the amount of fuel you need to combine with oxygen to completely burn the fuel.... :cool:
With all due respect, that's nonsense. Under what circumstances does the ratio need to be 15:1? That would be steady state. Accelerating needs to be much richer, as is peak power. Highest economy is leaner than that.

That manual is fine as a education tool, but doesn't help anyone to select a jet or needle to start with. And as PJ pointed out in the other thread, that bike will need a very different main jet on short pipes than it will with long pipes, so there are many variables and not all of them are obvious. Jetting is hard enough with a dyno and really hard without one. I had a highly tuned RD350 to jet recently and changed needles, needle jets, main jets and pilot jets to get a baseline and fortunately only needed to change one jet at the dyno. I got lucky.

If his VM28s were ebay Chinese copies, then there is no baseline to work from. That's why I asked what jets and needles they came with. Cheap carbs are incredibly expensive.

PJ or CXman (Murrays carbs) could probably set up a pair of genuine Mikunis to suit that bike, and maybe Ivan and a few others.
 

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With all due respect, that's nonsense. Under what circumstances does the ratio need to be 15:1? That would be steady state. Accelerating needs to be much richer, as is peak power. Highest economy is leaner than that.

That manual is fine as a education tool, but doesn't help anyone to select a jet or needle to start with. And as PJ pointed out in the other thread, that bike will need a very different main jet on short pipes than it will with long pipes, so there are many variables and not all of them are obvious. Jetting is hard enough with a dyno and really hard without one. I had a highly tuned RD350 to jet recently and changed needles, needle jets, main jets and pilot jets to get a baseline and fortunately only needed to change one jet at the dyno. I got lucky.

If his VM28s were ebay Chinese copies, then there is no baseline to work from. That's why I asked what jets and needles they came with. Cheap carbs are incredibly expensive.

PJ or CXman (Murrays carbs) could probably set up a pair of genuine Mikunis to suit that bike, and maybe Ivan and a few others.
Is not nonsense and I didn't make it up, it's chemistry and physics. If you add more fuel it will not completely burn because of a lack of oxygen and if you reduce the fuel it might not even fire during the combustion stroke, then you will be discharging unburnt fuel into the exhaust system. The carburetor is attempting to maintain that fuel/air ratio through its operating range.
I don't make the rules of nature I just try to follow them :cool:
Google up: "stoichiometric ratio of gasoline"

If he buys knockoff carbs they won't say Mikuni on them they will say Minkuni on them -> Sudco Intl. Corp. - Dealer Info <- pretty sure I posted that link too.
 

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On the CB250 I race, I have 30mm VM Mikunis. Most people I know racing CB350 have 32mm VM.
Regarding Air-Fuel Ratio. If you search Wikipedia you'll get a fairly accurate explanation. From a couple of other sources, I've concluded the following.
A lean mixture (high A/F ratio) will give higher top end temperature. No good in an aircooled engine.
Maximum power will be obtained with A/F around 12.5 to 13 on ethanol free petrol.
My CB250 has moderate tuning, only a fairly hot cam but still standard springs. Redline a mere 12500rpm. Still capable for podium places.
For last practice I fitted a wideband lambda sensor to it. Readings in race pace mostly 12.5-13, so it seems jetting is OK.
Happy with that.
 

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Discussion Starter #27
I now have the 28mm Mikuni's (and yes they are real Mikunis not Chinese knock offs) set up as follows for a starting point ONLY. Emulsion tube is a 176 P8 (bleed type), jet needle is a 5F3 with clip in middle slot, Main jet is a 200, Pilot jet is a 30, air screw is a 2.0, Slide is a 2.5. These carbs came with a 200 main, 60 pilot and a non bleed type emulsion tube. I will start from there and post when I have them running right.
 

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I now have the 28mm Mikuni's (and yes they are real Mikunis not Chinese knock offs) set up as follows for a starting point ONLY. Emulsion tube is a 176 P8 (bleed type), jet needle is a 5F3 with clip in middle slot, Main jet is a 200, Pilot jet is a 30, air screw is a 2.0, Slide is a 2.5. These carbs came with a 200 main, 60 pilot and a non bleed type emulsion tube. I will start from there and post when I have them running right.
Good place to start. Main jet sounds way too large though and the needle jets might be a little large, but try them and see how it runs. That Needle is slightly leaner at the top end than the ones they usually come with and is about the middle of the lean-rich range, so I'd run them and see how they work.

Slides might be a touch rich at 2.5, so you may end up going down to 2.0 but that's probably the last thing you'll change.
 

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Got a little bit of time before Jaxon decides he needs to find me. The jetting is based on AMAL cc/min flow at 19" head (or maybe 21", I forget) Even so, 200 is going to be way too rich.
I remember seeing a comment by the guy who did the CB350 VM conversion in Cycle World in 1970's.
He said they had a bucket full of slides, needles, emulsion tubed, etc and final cost was about 5~6 times the original carb cost. As fuel is 'different' today compared to 1970's, the information isn't even a good starting point for today's fuel (no 'lead', mostly E10, various cleaners and additives, etc)
Personally I would go to a 1.0 or 1.5 slide as it will help acceleration.
BTW, Max power FA is around 12.5~13:1 as previously stated by several people. Max rich before miss-fire is around 8~9:1 (depends how good ignition system is )
Max lean before lean misfire is around 18:1. Steady state cruise FA will be around 15:1 on majority of air-cooled engines (fan cooled and water cooled can run a little leaner 16~17:1)
 

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Discussion Starter #30
Just a quick progress report: Started the bike and it was WAY too rich at idle, went to 25 pilots and still too rich. Waiting on more jets and will try 22.5 pilots next. I am trying to get a decent idle before putting the tank back on and trying to get the mains right. I did have one problem though. I think the air jets are going to be too small (they are 0.5's) and trying to get one of the air jets out it was so tight, I stripped the screwdriver slot out of one of them. Guess if I need to change the jet orifice, I will have to carefully drill the hole out according to the hole size of a larger size. Suggestions?
 

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Just a quick progress report: Started the bike and it was WAY too rich at idle, went to 25 pilots and still too rich. Waiting on more jets and will try 22.5 pilots next. I am trying to get a decent idle before putting the tank back on and trying to get the mains right. I did have one problem though. I think the air jets are going to be too small (they are 0.5's) and trying to get one of the air jets out it was so tight, I stripped the screwdriver slot out of one of them. Guess if I need to change the jet orifice, I will have to carefully drill the hole out according to the hole size of a larger size. Suggestions?
Ok hold on here a little; first off did the carb change cure the problem of both cylinders not firing?
Some of the others are making suggestions based on yielding maximum power, but I am under the impression this is just a street bike and you just want to make it ride well on public streets. Pretty sure the goal is not to set any land speed records, maximize horsepower, spew raw fuel out the tailpipe, or break fuel range and emission economy records here, let's just start by getting the thing running decent for you and then move on from there.

The low speed adjustment I would think is a screw adjustment and roughly how many turns out is the screw now? Typically it would be 3/4 to 1-1/4 turns out for almost any carburetor, (1 turn out is a good start point) and when you determined it was rich at idle (which by the way is the opposite of what almost everybody else was expecting or suggesting would happen) What did it do at higher throttle operation, did it bog out, stall or run just fine? ... and have you tried moving the cir-clip on the main jet needle to see what that does? I see no mention of that and was the idle high or low before you started changing out jets, the idle is set by altering the main jet slide and needle, something just doesn't add up here.

Stripped the brass screw slot! ooooo that's not good, pictures of what you are now dealing with might be in order for clarity.
 

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Just a quick progress report: Started the bike and it was WAY too rich at idle, went to 25 pilots and still too rich. Waiting on more jets and will try 22.5 pilots next. I am trying to get a decent idle before putting the tank back on and trying to get the mains right. I did have one problem though. I think the air jets are going to be too small (they are 0.5's) and trying to get one of the air jets out it was so tight, I stripped the screwdriver slot out of one of them. Guess if I need to change the jet orifice, I will have to carefully drill the hole out according to the hole size of a larger size. Suggestions?
The air jets are probably OK for now. They are there to change the atomization through the emulsion tube and if they are much bigger you lean out the top end more than anything else.

As someone already suggested, try to close (or open) the slow speed adjusting screws and see what difference that makes. On those carbs, the screw adjusts the amount of air, so out=leaner and in=richer. Is the problem a lumpy idle or is the problem when you start to open the throttle? If the latter, it's probably the slide or needle jet that's too rich.
 

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Um guys, it is fuel that goes through the pilot jet. In reality if you want to restrict fuel flow through the pilot jet you would actually want a smaller hole through the jet, or reduce the air flow using the pointy screw thing which limits the air flow that makes the jet suck fuel through that circuit :| go back and read the manual chapter 3-1

Bottom line is; there are 2 brass things with a hole in the middle, fuel travels through both of them, that is why they hang down into the float bowl and are accessed from below, air flow is restricted by the position of the slide, or by the little passage way in the pilot jet circuit that bypasses the slide and that air flow is limited by the low speed screw, hence you screw it in to make the pilot jet circuit work less efficient, or wind it out to make the pilot jet circuit draw more fuel up through the little hole.
 

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Discussion Starter #34
Yes, putting the new carbs on both cylinders were running. Both cyls were blowing raw fuel out pipes with the 30 pilot jet. Adjusting the air screw out and end did not make any difference. Put the 25's in and ran better but still ran rich. From what the Mikuni manual says, the pilot jet and air screw control the idle mixture, not the main jet. I have the 22.5 pilot jets in now, waiting on the crappy weather to break so I can try it out. Also have dropped the main jet down to 180. We'll see what it does. And yes, I am just trying to get a clean running bike for the street.
 

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Um guys, it is fuel that goes through the pilot jet. And air......


In reality if you want to restrict fuel flow through the pilot jet you would actually want a smaller hole through the jet - agreed,
or reduce the air flow using the pointy screw thing which limits the air flow that makes the jet suck fuel through that circuit. Not exactly.....

........
Carb 101. Carbs are more complex than they appear and it's easy to get confused. In addition to the main body of air through the carb, air travels through the air jets and mixes with the fuel in either the pilot jet or the needle jet. The primary air jet is often a simple drilling and sometimes it's a bleed from the main air jet passage. Some carbs such as the BS series actually have a pressed in air jet for both circuits.

On a CV carb, typically the slow speed screw adjusts the flow of air/fuel mix after the pilot jet and so on those, screwing it in, reduces total flow and so makes the bike run leaner because there is less air/fuel mix to go with the same amount of air under the slide. On most slide carbs, such as the VM series, the air is metered by that pointy screw before it mixes with fuel at the pilot jet and so screwing the screw out leans out the mixture as was mentioned earlier. That 'pointy screw' changes the amount of air flowing into/past the pilot jet and therefore changes the ratio of fuel:air. So to be clear on a VM carb, opening that screw increases the air flow and makes the mixture more lean.

Just to make things more complicated, there is considerable overlap between circuits and sometimes it's necessary to adjust one slightly off perfect to get optimum transition and overall results. But lets' get the idle smooth first and then transition to the slide and needle/needle jet combos so it can be ridden.

Once the needle/needle jet combo is rideable, focus on the main jet. Don't try to get the needle/needle jet combo perfect until you have the appropriate main jet. Then go back and tweak the needle or needle jet.
 

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.... From what the Mikuni manual says, the pilot jet and air screw control the idle mixture, not the main jet...
Your carburetors have an idle adjustment screw, turn that screw in and out and watch what moves when you do that ;)

These are not CV carbs.
 

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Discussion Starter #37
I appreciate the answer. That is exactly the sequence I am trying to follow. Once I get the engine to idle correctly when warmed up, then I will try riding it and tweak it from there.
 

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By the way, I'm not suggesting that your bike is not running rich, it could well be!
Only advising that changing your pilot jet is not the correct way to adjust the idle speed. If your idle adjustment does not allow you to set the engine speed lower then you actually want it then check your cable adjustment, the slide Should drop down low enough just on the idle adjustment screw to make it stall out. You are closing off the air and that should kill your intake vacuum to a point where neither of the fuel jets will draw fuel up and out of the float bowl.

... if it revs like shit at idle make sure you don't have one or both carb slides installed incorrectly (rotated 180 degrees) the idle adjustment screw should be pushing against a ramp that is machined in the slide, not against the side where the little round pin goes into the groove which stops the slide from rotating.


 

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I am not trying to set my idle speed-I am trying to set my idle mixture.
:cool: then what we have here is a failure to communicate.
Can you properly test ride the bike as it is and how does it perform under all throttle operations, you really have not described that very well.
Are you trying to adjust the carbs while the bike is stationary? Because there is no way you can adjust and trouble-shoot the carbs unless the motor is at operating temperature and under load.
add: are you certain the source of said exhaust smoke is rich fuel and not carbon or water condensation in the engine or engine oil burning off? Does the exhaust stink of raw fuel?
I would be surprised if it does unless the fuel air mix is incredibly rich and given that todays fuel has barely any scent compared to what previous generation fuel smelled like. Adding an O2 sensor and gauge is still the best easy way to test your fuel to air ratio.

... what happens when you apply the choke? (which is actually a fuel enrichment circuit and not a choke plate)


... son of a gun lookie there they show all these gizmos reading 14.7 ;) I wonder where they came up with that number.
 
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