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Post Info TOPIC: Sanity check! Does screwing out the pilot screw richen or lean the mixture please??


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How does the pilot screw on the TTR250 OEM carb work?
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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



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Moo


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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...

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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



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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



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

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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



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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



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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 



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Sanity check! Does screwing out the pilot screw richen or lean the mixture please??
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Yeah - I know I should know but my brain isn't working this morning disbelief



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

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

 

Happy E-U free day!   biggrinbiggrinbiggrinbiggrinbiggrin

Martyn



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I agree
Now if it was a 2 stroke...

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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 no

Brian

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

 

 



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The screw is really only there to set the mixture at idle.  

With the hesitation at a third throttle maybe try dropping the needle clip one notch and it will ritchen the mixture slightly when transitioning into the main jet only. 

Just a thought 



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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

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Thanks for the feedback guys!

Interestingly, when I first ran the carb it was quite blubbery on constant low throttle openings and at that point the needle was on the 2nd notch from the bottom. I assumed the blubbering was because of too rich a mixture so moved the needle to the middle position which improved things. Changing from a 50 to a 48 pilot jet also seemed to make it run better.

All this made me think it is still running rich.

Just to be perverse I might go to the lean extreme on the needle and put the clip in the top notch. See what happens and go from there wink

There again, I should probably try the carb on a 250 as that will be its eventual home confuse



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