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

Members Login
Post Info TOPIC: How does the pilot screw on the TTR250 OEM carb work?


Super Guru

Status: Offline
Posts: 7160
Date:
How does the pilot screw on the TTR250 OEM carb work?
Permalink  
 


Jarrah called it a "fuel metering screw" but I have also seen it called the "air" screw.

Pilot_screw_new.jpg

I am trying to resolve an issue on a carb I am testing and it has made me question how this screw actually works. I could turn the screw all the way in and many turns out and it made no difference to the slow running confuse

Does it meter air or fuel????

If air, then I guess screwing it in richens and, if fuel, that would lean the mixture out?

Brian



__________________

Exeter, Devon, UK

http://www.ttr250.com  - The one and only dedicated TTR250 FAQ! 
 

TIP: For easy viewing bookmark the "Recent Posts" view - http://ttr250.activeboard.com/p/recent/ 

Moo


Senior Member

Status: Offline
Posts: 367
Date:
Permalink  
 

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...

__________________

Malcolm, Northamptonshire. West Anglia TRF

1996 TTR 250 Raid

2005 TTR 250 Bluey



Super Guru

Status: Offline
Posts: 7160
Date:
Permalink  
 

Hi Malcolm. I spent quite a while trawling through the dozens of threads on the Carb forum but was none the wiser hence my question here. I hate mysteries or just not knowing stuff for sure no

My problem is that the TTR is running very rich at the bottom - it sort of blubbers and there is a strong smell of petrol from the exhaust fumes.

Once the throttle is cracked open it goes off like a scalded cat and easily tops 70mph. I replaced the 50 pilot jet with a 48 and it made no difference. I removed the air filter cover and air filter - no difference.

After looking back at my float level photo I am suspicious that the fuel level in the float bowl is too high and is flooding the engine at low revs.....

Brian



__________________

Exeter, Devon, UK

http://www.ttr250.com  - The one and only dedicated TTR250 FAQ! 
 

TIP: For easy viewing bookmark the "Recent Posts" view - http://ttr250.activeboard.com/p/recent/ 



Super Guru

Status: Offline
Posts: 7160
Date:
Permalink  
 

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:

Pilot screw operation2.jpg

 

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. 

Pilot screw operation1.jpg

 

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.

Pilot screw operation3.jpg

 

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

Pilot screw operation4.jpg

 

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?

Pilot screw operation5.jpg

 

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 biggrinbiggrin

Brian



__________________

Exeter, Devon, UK

http://www.ttr250.com  - The one and only dedicated TTR250 FAQ! 
 

TIP: For easy viewing bookmark the "Recent Posts" view - http://ttr250.activeboard.com/p/recent/ 



Senior Member

Status: Offline
Posts: 103
Date:
Permalink  
 

I missed the holes directly above the pilot. Doh. Might be my cold start issue. Well spotted.

__________________


Guru

Status: Offline
Posts: 824
Date:
Permalink  
 

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 biggrin



__________________


Super Guru

Status: Offline
Posts: 7160
Date:
Permalink  
 

Hi Les and thanks for the response. Hope you had a good ride!

No hurry on this one. I just want to get it right wink

The slide in its fully closed position doesn't cover the holes in the base of the carb. There is a cut out that allows mixture to flow from the pilot jet.

Pilot screw operation 6.jpg

The reason that I had come to the conclusion that the pilot screw allowed in extra mixture rather than air is that it is on the "suck" side of the carb which I think means that it wouldn't allow airflow back to the pilot jet. What do you think?

Brian



__________________

Exeter, Devon, UK

http://www.ttr250.com  - The one and only dedicated TTR250 FAQ! 
 

TIP: For easy viewing bookmark the "Recent Posts" view - http://ttr250.activeboard.com/p/recent/ 



Guru

Status: Offline
Posts: 824
Date:
Permalink  
 

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 



__________________
Page 1 of 1  sorted by
 
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard