Fuel screw carb adjustment...

[Ride]

I installed a dynojet carb kit and 4-1 header. Bike runs good but is a little lean down low. The kit says to run the adjustment screw at three turns out. Is this a fuel or air screw. Do I turn it in our out to make it richer off the bottom?

- this is the brass adjustment screw on the top front of the stock carb.

thanks.

[Flying Scotsman]

Air fuel mix.More turns out = richer mixture.

[Ride]

Confused. On dirt bike 4 strokes have "fuel" screws so out is richer. 2 strokes have air screws so out is leaner. Confused by the "air fuel" title. You know for sure it is richer as you turn it out?

thanks

[Flying Scotsman]

As you turn it out it lets more fuel through.I have a dinojet kit in my 85 with a kerker 4-1-2 pipe.I was super rich with there settings.3 1/2 turns was to much for my bike and I had to drop my needles as well.I think I at 1 1/2 turns now and needles all the way down.

[SlowOldGuy]

The mixture screw is beyond both the air intake and the fuel intake of the idle circuit.  Therefore it controls the amount of an air/fuel mixture that is allowed into the intake tract.  It is neither an air nor fuel screw, it is a mixture screw.  This is not a dirt bike.

Start at 2-1/2 turns out and do the blip the throttle test.
If you don't know what that is, then do some reading in the files section.

DavidR.

[Ride]

The mixture screw is beyond both the air intake and the fuel intake of the idle circuit.  Therefore it controls the amount of an air/fuel mixture that is allowed into the intake tract.  It is neither an air nor fuel screw, it is a mixture screw.  This is not a dirt bike.

Start at 2-1/2 turns out and do the blip the throttle test.
If you don't know what that is, then do some reading in the files section.

DavidR.

I know it is not a dirtbike... Duh but thanks for that. I have jetted and tuned MANY bikes and am quit good at it thanks. It was a simple question. thanks Scotsman for a good answer. And actually I believe it is a FUEL screw NOT a mixer SCREW. There is no MIXTURE at that point it is fuel and the changes the air fuel mixture after you add or subtract more fuel which this is controlling. Some bikes have air screws some have fuel, this appears to be fuel. Mine needs to be richer as it has a slight ping and hesitation off the bottom. I have found blip tests not under load do little but will indicate lean spots in the pilot jet or needle position. Mine does fine when blipped not under load but is for sure running lean in roll on situations. Want to play with the screw first then the needles if need be.

Anyone know the working range for this fuel screw? Dynojet has you start with three turns out with pod filters and exhaust like I have. Used there settings / slide springs for this setup and other than the slight lean off the bottom runs great.

[the fan]

HA! told you Dave.


It is indeed a mixture screw (or fuel/air, Yamaha refers to it as a pilot screw set) It controls an air bleed connected to the pilot circuit. This bleed controls the amount of fuel the pilot circuit is able to deliver at closed throttle and partial throttle. Turning the screw out opens the circuit allows a larger amount of air to flow creating a lower pressure and lifting more fuel. (Correct me if I am wrong Dr Raforth) In a CV carb both the pilot and main circuits have air jets located in the carb throat. In the case of the pilot circuit this is fine tuned by the mixture screw. Changing the size of the air jet alters the amount of air available for fine tuning. In many cases a larger air jet will help cure a lean part throttle condition.

Adjustment range of the mixture screw is usually 1-4 turns out from seat.

[Dan Filetti]

The mixture screw is beyond both the air intake and the fuel intake of the idle circuit.  Therefore it controls the amount of an air/fuel mixture that is allowed into the intake tract.  It is neither an air nor fuel screw, it is a mixture screw.  This is not a dirt bike.

Start at 2-1/2 turns out and do the blip the throttle test.
If you don't know what that is, then do some reading in the files section.

DavidR.

I know it is not a dirtbike... Duh but thanks for that. I have jetted and tuned MANY bikes and am quit good at it thanks. It was a simple question. thanks Scotsman for a good answer. And actually I believe it is a FUEL screw NOT a mixer SCREW. There is no MIXTURE at that point it is fuel and the changes the air fuel mixture after you add or subtract more fuel which this is controlling. Some bikes have air screws some have fuel, this appears to be fuel.

[andyb]

It's a fuel screw.  What it actually controls and how it works matters not so much, but by giving it a simple name where it fundamentally makes sense makes communication clearer.  Out is more fuel and richer, and v/v.  Calling it a slow pilot mixture idle controlling adjusting screw is being pedantic.

According to the factory, I suppose we could argue that they're emmissions adjusters that are then sealed

[Travis398]

 

[SlowOldGuy]

Gee, I guess you can re-write all the information in the files section since you're such an expert.  If you post an OPINION, you're going to be called on it if it's incorrect.

A FUEL screw controls the amount of FUEL allowed into the circuit.  An AIR screw controls the amount of AIR allowed into the circuit.  On the FJ carb, it's a MIXTURE screw.  At that point in the circuit, it is controlling the amount of an air/fuel MIXTURE allowed into the intake.  That A/F MIXTURE is a function of the size of the idle jet and the air pilot jet.

Take a look at the schematic of the FJ carbs.  I can probably draw it with my eyes closed.

But you can believe what you want.  I've tuned MANY FJs and am quite good at it.  And how exactly would you go about performing a "blip the throttle test"  "under load?"  I have "found" that this test is very good at getting idle mixtures spot on for the FJ.  But you just go on "believing" what you want to.

DavidR.

[SlowOldGuy]

HA! told you Dave.

Yeah.  Man, I hate being wrong for all these years.  Nevermind everyone! Ignore whatever advice I've given you over the years.  It was all BS.  Sorry about that.

It is indeed a mixture screw (or fuel/air, Yamaha refers to it as a pilot screw set) It controls an air bleed connected to the pilot circuit. This bleed controls the amount of fuel the pilot circuit is able to deliver at closed throttle and partial throttle. Turning the screw out opens the circuit allows a larger amount of air to flow creating a lower pressure and lifting more fuel. (Correct me if I am wrong Dr Raforth)

Just a few small corrections, Bill.
The mixture screw is not an air bleed.  It's just an adjustable orifice controlling the amount of the idle circuit A/F mixture allowed into the intake.  In a CV carb, the "amount" of air has no affect on the mixture, it's the velocity of the air that matters. 

In a CV carb both the pilot and main circuits have air jets located in the carb throat. In the case of the pilot circuit this is fine tuned by the mixture screw. Changing the size of the air jet alters the amount of air available for fine tuning. In many cases a larger air jet will help cure a lean part throttle condition.

True, there are air bleeds for the idle and main circuit.  But, if you're lean, a smaller air pilot jet will richen the circuit.

[SlowOldGuy]

Did you install the 144 air pilot jets that came with the DJ kit?  That is supposed to help the lean spot caused by a 4:1 exhaust.

In the olden days, the V&H pipe was known for a bad lean surge.  I think the recommended cure was exhaust restrictors.  

If it's not a bad stumble, you should be able to get there with idle circuit tuning.  A 140 idle jet is another option.

DavidR.

[Ride]

Thanks for the info. Sorry i pissed people off. Seemed like i was being talked down to like i knew nothing because i come from a dirt bike back ground. CV carbs are somewhat new to me. There is no stumble, bike runs good, idles perfect. There is a slight hesitation and I can hear pinging under load right off the bottom some times. I am running the 144. Pipe is a Kerker header with a muffler i graphed on.

[Klavdy]

Nah, you were being talked down to because you come across as a dumb fuck who won't listen to blokes who've given you exactly the right advice.
Now harden the fuck up, this is FJ.
You listen to what these blokes tell you and you cannot go wrong.
Dr Ratfart in particular, why do you reckon he is known as the carb Doctor?