Mixture screw adjustment w/vacuum gauge??

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Tyboy11

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Anyone here recommend or set their idle mixture screw while the vacuum gauge is hooked up, looking for peak rpm?

I have tried adjusting it listening for stumble and then out for peak RPM, but I think my hearing is shot from being around turbine engines my whole life. I can't hear that stumble or see that RPM rise that people talk about when adjusting.

Would it be more accurate to do it while the vacuum gauge is hooked up and if so, whats you technique?
 
That is typically the preferred method when setting up a car carb and i would think should work just as well for this application! You may be able to use a sync tool as well to get the highest number (which is vacuum based anyway) and already be hooked up to fine tune the sync when done.
 
Bike is stock, except for drilled mufflers.

So should you just adjust each carb for the highest vaccum or just highest vaccum and back in a 1/4 turn or so?
 
'Scuse me if I'm being a bit thick here but...

The sync tool measures the vacuum behind the throttle butterfly.
Open the butterfly the vacuum drops; close it and the vacuum increases. This is how the carbs are sychronised by aiming for a similar manifold depression on each carb by adjusting the relative opening of each butterfly.

The idle mixture is adjusted by its screw.

Am I correct in assuming that what is being suggested is that once balanced when turning the mixture screw you see a will change the manifold depression on an individual cylinder?

I understand that adjustments to an individual idle mixture can raise or lower the revs but would have thought that would affect manifold depression on all cylinders equally. Therefore the sych tool would show a similar rise or fall across all the board.

The factory measures exhaust gas to set the idle mixture and I would have thought that would be the best method....or get the help of an assistant with more acute hearing?

A clarification would be appreciated.
 
Bike is stock, except for drilled mufflers.

So should you just adjust each carb for the highest vacuum or just highest vacuum and back in a 1/4 turn or so?

I tried to set mine one time with a vacuum gauge (a carb sync), and really couldnt see a change in the readings as I was turning the screw in or out.

So, most people will start at 2.5 turns and work their way up. When you blip the throttle, if its too lean, the RPMs will appear to hang up just a bit before settling down to what ever the idle is set at. if the carbs are too rich, you will blip the throttle, and the RPM's will fall pretty quickly, and then dip below where the idle is set at, and then gain back to the idle set point.

Also too (of course this is sound) when you have the A/F screws set to where the bike likes it, the sound should be very crisp, as you blip the throttle.
 
I also suffer from hearing loss due to my time in Navy helicopters and working on target drone J-400 and J-85 Turbojets. A wide-band digital air fuel gauge is the way to go, but that involves some $$. Perhaps a digital tach would work better to witness the RPM fluctuations when adjusting the mixture screws.
From some of my research on the subject of carbs: (Some from this forum) Bold sections were emphasized by me:


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"The pilot circuit is fed into the manifolds below the throttle plate.. For many this is one of the hardest circuits to dial in for off idle or throttle response because the pilot circuit overlaps into the enrichment circuit as soon as the throttle plate is opened... the enrichment and following circuits all overlap to the next circuit and all are sensitive to velocity,not rate of air flow. I myself use the mercury stick because they give me a wide range of revving (blipping) the throttle to the point where velocity creates the vacuum needed to compress my slide springs. This gives me the best WOT right off idle... if I am tuning a bike and it's off on one of the pilots, rich/lean condition will be seen when the cylinder spikes on the mercury. Then I can make the adjustment. But still, we need to watch the tach because although we get it all equal doesn't mean its right... they can all be too rich/lean... at quick blip, the needle should rise up then drop to the set rpm, not float or hang or fall below the set rpm. And like I said in past posts, don't be surprised if one pilot is .5 turns and another is 3 turns, this is normal... only way they will all be the same is if the valves are tuned.-Paul"

************************************************************

"...The idle screws are basically pilot or idle circuit fuel curve adjusters. The idle circuit caters for idle through to about 1/2 throttle or so when the fuel curve is influenced more by the needle and main jet. The idle screws will only have an effect on vacuum as you close them down too much or open them up too much causing excessively lean or rich mixtures respectively. You don't really need a vacuum gauge to measure this; it is very obvious in that the bike starts running rougher when you go too far with any of the individual pilot screws. You need to turn each screw in fully till the engine stumbles and roughens up then back out till it stumbles with too rich a mixture and go back in till you have the smoothest possible running. Do this with each cylinder screw in turn and don't panic if some of the screws have a lesser effect as you go in or out. Just try to end up with the smoothest idle and blipping the throttle should leave the bike with the tach needle dropping back down to a clean idle rather than dropping below idle speed and rolling back up (too rich) or hanging up after blipping the throttle and slowly returning to a good idle (too lean). The only other way to do this is to have an accurate A/F ratio gauge setup in each exhaust header..."

"...Mixture screw settings... As lean as possible. Set up your carbs...then take a ride and adjusting each carb one at a time. With the engine at operating temperature Start with one carb and turn in the mixture screw a very, very small amount at a time ....turn it in a hair. then ride it . Turn it in some more then ride on ....keep doing this until the carb starts to spit back when you accelerate or take off from a stop...then back out the screw a bit at a time till it stops spitting ....When done go to the next carb ....it takes a while to do this..."

*************************************************************

"Setting the pilot screws - Jim Younghusband

For me, the easiest way to set the idle mixture screws, is to run the motor until warm and idle stable. Run each screw in, one at a time, until the engine stumbles, back the screw out until just rich of peak idle (idle drops again). Then run them back in to peak idle. Repeat for each carb. If your screws are out more than 3 turns, I feel the jet needs to be swapped to the next larger. Your final position should be around 2.5 turns out."
 
Sync and clean piolet jets must precede mixture screw adjustment when using the ear method. Or, some carbs will have little or no effect to the screw movement. I end up at about 2 3/4-3 turns out. From the factory, they are not always equal. Like mentioned the factory sets the carbs with a CO meter.
 
After the cleaning/rebuild process we run every set of carbs on a test engine looking at idle, sync, starting response and mixture screw sensitivity. The engine change is usually very noticeable when turning in the mixture screws, even to me and my hearing is very bad.

Granted, the valves have been adjusted to spec on this engine but a proper carb cleaning may make all the difference.
 
Good info guys, Thanks.

Right now on my stock bike (drilled muffs) I have found the best setting on all 4 is 3 turns out. I need to do them individually like said above though. I have been adjusting all equally. It did liven up after a synch with them all at 3.

I am going to do them one at a time turning in and riding till it coughs, back out and move to the next one.

During rebuild I found that PO had installed a stage 1 needle, set to max (rich) with a 177.5 Paj2 and a 152.5 main jet. It actually felt stronget at the top end with this combo, but rough at mid to low.

I moved needle to leanest position and just couldnt get it right. The power curve was really weird.

I ordered a set of stock needles, set of 170 Paj2's and set of 150 mains from Sean Morley. Installed the stock needles and the 170 Paj2's and it just felt flat.

I threw the 177.5 Paj2's back in and it ran a lot better. I'm just trying to get it dialed in. It still doesnt seem to be as strong up top with this combo as opposed to having the stage 1 needle in there.

If after adjusting each individual carb mixture screw with the adjust and ride method doesnt show positive results, I'm either going to (1) pull carbs and install the 150 mains (2) throw the stage 1 needle back in and set it initially to the mid position, or (3) put the 170 Paj2's back in with the 150 mains and try it.

Altitude in DFW area is around 650 feet.

Decisions, Decisions:ummm:
 
Stock exhaust, stock carb set up is best for you. Mains can be reduced, but I'd ditch the stage 1 kit. Things like shorter diaphragm springs aren't going to help you out. And stage 1 needles can make you run rich as hell. Compensating with bigger PJ2's won't work well in this situation.
 
First of all, if the person did the other parts of a stage 1 kit, they probably changed the mains as well. In the stage 1 kit, DJ recommends DJ jets of 165 (which is just slightly smaller than a mikuni jet 155.....remember stock is 152), and thats for a stock exhaust. Most people think that richer is better, and could've put in the 170s, which is equal to a mikuni 160.....WAY TO F**KING rich for a "stock" vmax, hell way to rich for a Vmax with a jet kit and aftermarket exhaust. I'm only running 155's (with an A/F mixture of 13.6 up top.) Thats on the 07, which isnt stock.

So, now you probably know why it feels flat. Put in the MK 150's and that should wake it up.
 

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Stock exhaust, stock carb set up is best for you. Mains can be reduced, but I'd ditch the stage 1 kit. Things like shorter diaphragm springs aren't going to help you out. And stage 1 needles can make you run rich as hell. Compensating with bigger PJ2's won't work well in this situation.

The springs that are installed are 7". I have the carb removal down pretty good now, I'd just hate to throw in the 150 mains, only to go back in and put the 152.5s back in later.
 
First of all, if the person did the other parts of a stage 1 kit, they probably changed the mains as well. In the stage 1 kit, DJ recommends DJ jets of 165 (which is just slightly smaller than a mikuni jet 155.....remember stock is 152), and thats for a stock exhaust. Most people think that richer is better, and could've put in the 170s, which is equal to a mikuni 160.....WAY TO F**KING rich for a "stock" vmax, hell way to rich for a Vmax with a jet kit and aftermarket exhaust. I'm only running 155's (with an A/F mixture of 13.6 up top.) Thats on the 07, which isnt stock.

So, now you probably know why it feels flat. Put in the MK 150's and that should wake it up.

The mains are 152.5 Mikuni's, confirmed during rebuild I did a month ago. The install of the 150's seem to be the way to go. Like I said above, I'd just hate to go in, only to change them back later.

**I did just put a K&N filter in the bike in the past two days. Just the stock looking oval one. From what I understand, this really isn't a big gain in air, so it's not something that would have an impact (leaning out the bike) right?
 
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The mains are 152.5 Mikuni's, confirmed during rebuild I did a month ago. The install of the 150's seem to be the way to go. Like I said above, I'd just hate to go in, only to change them back later.

**I did just put a K&N filter in the bike in the past two days. Just the stock looking oval one. From what I understand, this really isn't a big gain in air, so it's not something that would have an impact (leaning out the bike) right?

Yep, the K&N shouldnt be an impact.

The Vmax likes it lean, so the 150's are the way to go. (I had missed it earlier when you said that you went back to the 152's.

Question, what is your float height?
 
From what I understand, this really isn't a big gain in air, so it's not something that would have an impact (leaning out the bike) right?

It is the Y piece that restricts the air not the filter.

One other thing to consider would be if the PO had drilled the slides - check with a #19 drill.
 
Yep, the K&N shouldnt be an impact.

The Vmax likes it lean, so the 150's are the way to go. (I had missed it earlier when you said that you went back to the 152's.

Question, what is your float height?

I meticulously set it to the lean side. I don't remember the measurement off hand, but it was definitely the LEAN side.

I have yet to change the 152.5 to the 150's.
 
It is the Y piece that restricts the air not the filter.

One other thing to consider would be if the PO had drilled the slides - check with a #19 drill.

I checked the slides before for being drilled, and they looked like they had not been drilled. Looked original, especially when compared to pictures of drilled slides. Next time I pull them, I will verify with a drill bit though.
 

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