VM30 tuning...

FYI, 6F4 is richer than a 6F5 needle all the way through. It has the same taper but the taper starts earlier which is why it's richer.

Thanks Teazer, that's one thing I haven't checked out in the manual yet, haven't been serious about ordering a new one. Once I can get the idle mixture right, I think the jet needle setting will be the next thing I adjust, I'm at the top clip now with the 6F4, so if it's too rich still I'll have to look at a 6F5 and/or a leaner needle jet. The pilots should be here before the end of the week so I'll be able to see how it is soon.

Firebane, what symptoms at what throttle position caused you to order a leaner needle and jet?

thovda said:

Thanks Teazer, that's one thing I haven't checked out in the manual yet, haven't been serious about ordering a new one. Once I can get the idle mixture right, I think the jet needle setting will be the next thing I adjust, I'm at the top clip now with the 6F4, so if it's too rich still I'll have to look at a 6F5 and/or a leaner needle jet. The pilots should be here before the end of the week so I'll be able to see how it is soon.

Firebane, what symptoms at what throttle position caused you to order a leaner needle and jet?

Click to expand...

LOL my bike barely idles with the setup I have now. It seems the setup I am ordering is a fairly common starting ground for our bikes with a VM32 so I'm starting with that.

Well Shitbuckets, 15 Pilot Jet, New Plugs, Idled Ok For Ten Minutes, Stopped It, Pulled plugS, Super Sooty. Air Mixture Screw, Still Dont Know What That Does. I Could See The Exhaust Was Rich, So I Craned It Out To 2.5 Turns During Idle, And Nothing Changed, I Must Have Something Else Going On. If I Go To A 12.5 Pilot My Polaris Buddies R Gonna Give Me Shit About buying A Baby 4 Wheeler. My Valves Must Be Leaking Or Something, but Its bLack Gassy Exhaust, W T F ! Maybe Weak Spark, But It All Checked Out. Any, Any Ideas? F A Fucking A

Idle mixture screws take air in from the hole in the venturi intake side of the carb
and connects with the pilot jet and then to the venturi after the slide.
On the VM series carbs turn in ( clockwise ) will lean out the idle mixture,
turn out will add fuel.
Typical setting is 1 1/8 to 1 1/4 turns out from lightly seated.
As for valves, check your valve lash then do a compression check.
Spark should be white with a blue glow.
Take an old plug and open up the gap to about a 1/4".
Ground it to the engine and give it a spin.

Thanks Nebr. You made me nervous so I had to double check, screwing out leans the mixture which is how I've been tuning, lets more air in.
I'll dig in the manual and find out what the valve lash is, get that checked out.
The one thing I haven't done is a compression check...It has more than enough power and speed but I need to get some numbers and see what's going on.
I guess three turns is the max turnout for the air screw, I'm going to do that with new plugs and see what I get, if the soot amount is different/less, it's telling me I should still lean the pilot some more?

What plug are you using?

No, turning screws out adds fuel.It opens up the hole the more the screw is turned out.
The more air that travels thru that little hole, the more fuel that gets sucked up thru the pilot jet.
If you don't believe me try this,set screws at 1 1/2 turns out from lightly seated and run it until your plugs carbon up.
Then try it at 3 turns out and see which takes longer to foul out.


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Regal, I'm using BR8ES, I tried a litter hotter, BR7ES, but it didn't seem to change anything.
Nebr, I totally believe you, I can't find it in the manual anywhere, so I've read a few threads from googling it on different forums and have gone by what the other people have said. It's good news for the bike, I'm going to close it completely with new plugs and see what happens. Thanks!!!

That's not right. On a VM the screw/passage allows air into the upper part of the slow jet where it mixes with the fuel from the bowl. Open the screw and more air is allowed in. Fuel is pulled in by the difference in pressure between the venturi and float bowl (atmospheric). Air traveling through the air screw/passage does not create that pressure differential and effectively replaces fuel with air as it's opened up.

It's true that air in that system is slightly below atmospheric pressure but it has little effect on pulling up more fuel. The Mikuni manual shows it pretty clearly - Page 10 on my copy Fig 16.

In this particular bike there is something odd happening and it almost suggests that the wrong type of slow jets may have been fitted or there's a blockage in the air side of that circuit. Are you 110% sure that you have VM22/210 slow jets and not BS30/96 or jets without bleed holes?

If you remove the slow jets and air screws and spray carb cleaner down the appropriate air drilling in the bellmouth, does it spray out of the pilot jet position?

Thanks Teazer. I Double Checked The Pilot And Its Right, Was Hoping That I Screwed Up. Thanks For The Idea To Check The Air Passage, Cant This Weekend But Monday. It Sounds And Looks Like Its Choking, The Fuel Is Restricted Enough Now, Definitely Time To Start Checking Air Intake. Is There Any Other Place To Check For Air? Its Gotta Be That, Or The Spark Isnt Burning All The Fuel, But I Have And Always Have Had Strong Spark?

teazer said:

That's not right. On a VM the screw/passage allows air into the upper part of the slow jet where it mixes with the fuel from the bowl. Open the screw and more air is allowed in. Fuel is pulled in by the difference in pressure between the venturi and float bowl (atmospheric). Air traveling through the air screw/passage does not create that pressure differential and effectively replaces fuel with air as it's opened up.

It's true that air in that system is slightly below atmospheric pressure but it has little effect on pulling up more fuel. The Mikuni manual shows it pretty clearly - Page 10 on my copy Fig 16.

In this particular bike there is something odd happening and it almost suggests that the wrong type of slow jets may have been fitted or there's a blockage in the air side of that circuit. Are you 110% sure that you have VM22/210 slow jets and not BS30/96 or jets without bleed holes?

If you remove the slow jets and air screws and spray carb cleaner down the appropriate air drilling in the bellmouth, does it spray out of the pilot jet position?

Click to expand...

Your right, I'm wrong.
I was thinking the air thru the passage would siphon out of the pilot jet.
But that's not the case, the venturi pulls out of the passage.
It will pull air out easier than gasoline but, if the air get restricted it will pull more fuel.
I guess you can teach an old dog new tricks. Thank you.


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Alright, so I had a chance to run through some things.
Pulled off the bowl, pilot jet, air mixture screw, and blew carb cleaner through it all. Everything seemed clean, some small rubber pieces from the air filter have been coming off from taking them on and off so i was hopeful that a piece got stuck in a passage.
Didn't see anything though.

Started well, idled well, the exhaust was a little black, but when i held a white rag at the end of the pipes, it didn't discolor it badly...if that means anything. I was going to pull the plugs after 10 minutes of idling only and take a look, but being it was idling smoothly, and I forgot the choke on for the first two minutes , and I'm sure sooted the plugs, I gave it a bit of throttle, and that's where it stuttered.

So pilot jet is #15, throttle slide is 2.0, needle jet is Q2, jet needle is 6F4 leanest clip, and main is 145. I haven't pulled out the air screw that's in the bellmouth, typically they're a 2.0 so I'm assuming that's what it is, pretty good sized hole. I wonder if I pull it out completely and see what effect I get??

So checking in with the pros, would you change out the throttle slide next to a 3.0 or 2.5, change the needle jet to a P5 or so, or order them both up but change only one thing at a time? Any other ideas, I think I'm going to take out the air screw to see what affect it has? Thanks

I Got A Fresh Set Of Plugs And Let It Idle Only For Ten Minutes. Plugs Are Sooted Up Pretty Bad.

How many turns out are you on the air screws and idle screws? How did you determine these positions?

Can you ride the bike around at about 20mph? If so, does the engine maintain speed or are you constantly giving it throttle?

2.5 Out On The Air Screw. I Set The Idle Using The Drill Bit Method, 13/64Ths, If I Remember Right. I Can Ride It At Any Speed, Up To 70, I Dont Want To Go Any Faster With The74 Rubbers On It. The Problem Is Its Real Jerky, Cuts Out And In. And The Plugs Get So Sooty It Gets Worse And Worse, Wont Idle After A Ride Unless I Keep The Throttle At 3000.
I Just Tried Taking The Air Screw On The BellmouTh outh, But Its Tight Enough I Dont Want To Strip It Out.
Does That Give U Any Ideas, Sure Apprecite The Help.

If you're running a Q2 needle jet, that is beyond big. 15 pilots sound pretty small, too... Weird combo.

First let your engine warm up. Then try setting your idle screws and air screws to 1.5 turns out and restart the engine.

Begin by adjusting the idle to about 1500rpms by turning the idle screws only.

Then turn your air screws 1/2 turn in either direction (and no more) to determine which setting increases engine speed. Leave the air screws set at the position corresponding to the highest engine speed (within 1/2 turn from the starting point).

Adjust the idle to 1200rpms by turning the idle screws only.

Again adjust the air screw 1/2 turn in either direction to see if that yields an increase in engine speed. Readjust idle screws to get you back to 1200rpms.

This is your starting point to determine if the pilot jets are the correct size.

Did you replace the choke (enricher) plungers? If they get hard or don't seat properly it will basically have the choke on part way all the time.

The main air jet on most 4 strokes should be much smaller 0.5-1.0mm but it only changes fueling through the main jet as revs rise and we are not really worried about the main jet just yet

fresh_c said:

First let your engine warm up. Then try setting your idle screws and air screws to 1.5 turns out and restart the engine.

Begin by adjusting the idle to about 1500rpms by turning the idle screws only.

Then turn your air screws 1/2 turn in either direction (and no more) to determine which setting increases engine speed. Leave the air screws set at the position corresponding to the highest engine speed (within 1/2 turn from the starting point).

Adjust the idle to 1200rpms by turning the idle screws only.

Again adjust the air screw 1/2 turn in either direction to see if that yields an increase in engine speed. Readjust idle screws to get you back to 1200rpms.

This is your starting point to determine if the pilot jets are the correct size.

Click to expand...

Well written, thanks! I'll let you know how it goes. I'm going to get a 6F5 jet needle and P5 needle jet ordered today too. I realize I don't need them at moment to get the idle right but anything after idle and it's way too rich, and the needle jet is active immediately after idle.

teazer said:

Did you replace the choke (enricher) plungers? If they get hard or don't seat properly it will basically have the choke on part way all the time.

The main air jet on most 4 strokes should be much smaller 0.5-1.0mm but it only changes fueling through the main jet as revs rise and we are not really worried about the main jet just yet

Click to expand...

I didn't do anything with the choke. It appears the choke is opening and closing completely, and when I've had to use the choke to start it, it wouldn't start without the choke and when used it fired right away and then i'd shut them off, so it seems to work good too. Thanks