Fuel screw carb adjustment...

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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:
Quote from: the fan on November 06, 2010, 09:35:32 AM

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.

Reply with Quote

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. 

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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?

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