A forum for owners of Yamaha TTR250 trail and enduro bikes!

This is a pretty vague wavey response, but I'm certain I read it on one of the carb threads (or made it up?) If the fuel screw makes no changes doesn't it mean one of the jets is too big/small?

I'm gunna have to re-read them later to try and find it...

I have a carb in pieces so had a close look to try and determine how the pilot screw works - very interesting!

Air enters the system here on the airbox side of the carb at the top left of the carb throat:

 

At the other end is the pilot screw that regulates the flow of fuel/air mixture into the inlet tract via a small hole in the bottom of the carb throat on the engine side of the carb. In the photo below the pilot screw is fully would in and you can just see it brass tip blocking the hole. 

 

As far as I can see, air enters through the pilot air jet, through the "junction box" and down to the gallery where the tip of the pilot jet sits. Here it atomises the petrol for onward transmission via the pilot screw hole.

 

This is how the flow of air, fuel and mixture enter the carb throat via the pilot screw hole.

 

However that is not the full story as far as the pilot jet is concerned. It also allows a fuel/air mixture to enter the carb through a hole at the base of the throat just under the slide. I haven't managed to work out what the second hole to the left of it does as I can't see its exit but maybe it does the same thing?

 

So, in conclusion, I think I have answered my own question. The pilot screw allows an additional flow of mixture from the pilot jet into the engine but this is a supplementary flow to that directly from the pilot jet hole shown in the photo above.

As it is allowing mixture in that explains why it is called neither an air screw or a fuel screw!

I really would welcome input from any carb experts on the forum as to whether they think I got it right and, if not, correct me please 

Brian

Hi Brian, the screw is basically for fine tuning the idle mixture , the pilot jet has an effect for the first 1/3/ throttle . if the jet is partially blocked or out side the correct size for the engine the screw wont be able to compensate .

a tip to make the the circuit make  sense is the amount of fuel drawn up through the pilot or main jet is dependent on the pressure difference across the carb , the slide causes an air speed increase as well as the  decrease in size of engine side of the carb  both have the effect of  causing a pressure drop across the carb.

its the pressure drop across the carb is what makes  the carb work,  the vacuum from the carb to valves moves the air .

 

moving the air screw slows or speeds up the air flow changing the pressure difference across the jet changing the amount of fuel forced through as the float chamber is the highest pressure in the system (atmosphere) .

 

the extra hole you found does it go through to the slide if so you may find it is open when the slide rises above third throttle which will balance the pressure difference across the pilot jet stopping fuel delivery until the throttle closes again 

above third the main does all the work , 

 

 

just got up to go for a ride so this is a bit rushed 

hi Brian

your on the fight track alright ,  the pilot jet limits the fuel that  is drawn through when there is a low pressure above it , your correct the air screw if screwed out increases the air speed (lower pressure across the top of the jet ) and draws a bit more fuel in , if screwed in it slows the air speed doing the opposite and leaning it a bit , 

the pilot jet does the basic metering of fuel the air screw enables an amount of fine tuning to suit the individual bike eg slight changes in float height require a bit of adjustment  .

 

as the picture show once the slide starts moving up the pressure difference across the pilot jet decreases and the air speed increases across the main jet drawing the fuel up through the main with the fine tuning done with changes to the needle height  height 

Screwing out takes the tip of the needle away from the jet orifice, so the mixture will be richened, I reckon. 

You had this dilemma in January 2018 also CLICKY and Les helped you resolve it - I think!!

 

Happy E-U free day!  

Martyn

Well done Martyn - had forgotten all about that!

I followed the link and the symptoms described are exactly the same as I have with a carb I am rebuilding and testing at the moment. Float height perfect, pilot jet reduced from 50 to 48 but the pilot screw seems to do very little.

The change from 50 to 48 pilot seemed to help but there is still a slight hesitancy on constant third or less throttle - just not as smooth as I would like. Above that throttle opening (i.e. on the main jet) it works fine.

Anyone got any thoughts?

I might have to crack out my AFR 

Brian

PS I have merged the threads so maybe I will find it next time!!!!   

 

 

4 stroke - it's called a fuel screw, and meters the pilot fuel. (engine-side of the carb)
2 smokers call it the air screw. (Often filter-side of the carb body) But they do the same job.
I don't notice a lot of difference when twiddling it at idle , but the afr meter will show that something is actually changing, and blipping the throttle from idle it is possible to find a sweet spot with minimal fluffing, assuming the jets are more or less correct.
I found when tuning the 350 carb that the afr was giving me good readings, but I could feel a flat spot, so did like Les suggests and richened the needle by 1 notch. Runs richer on the afr, but she clearly likes the extra juice!

-- Edited by mossproof on Saturday 1st of February 2020 09:08:57 PM