pilot needle question

[sonevdesertrat]

I have a 99 big bear YFM350 with a Mikuni 34 carb.
I am rejetting it in preparation of a new Pro Circuit pipe.
I have an airbox mod already.
I am going up 2 steps on the main jet from 125 to 130 and one step up on the pilot jet to a 45 from 42.5.
Now the pilot screw is on the inside of the carb and you can only get to it by removing the float bowl unlike some carbs that you can get to it from the outside.
The manual stated that the stock setting is 2 turns.
The pilot screw protrudes into a hole in the throat of the carb which means it regulates air flow.
My question is this...
since it is so hard to adjust it I would like to know how much adjustment it needs to make a noticeable difference.
Stock is 2 turns out,and since I want to richen the mix, should I start at 1 3/4, 1 1/2, or 1 turn?

 

[jasonp]

To richen the mixture on a pilot screw on a 4 stroke you back out the fuel screw. I'm not sure how much you will have to go as every mod is different. You shouldn't have to pull the float bowl off to do this. Pilot jet and fuel screw only effects 0 to 1/4 throttle.

 

[sonevdesertrat]

To richen the mixture on a pilot screw on a 4 stroke you back out the fuel screw. I'm not sure how much you will have to go as every mod is different. You should have to pull the float bowl off to do this. Pilot jet and fuel screw only effects 0 to 1/4 throttle.

I dont think 4 stroke has much to do with it, i think this is a carb issue. As I stated before the pilot screw protrudes into a hole in the carburetor throat. This means that it regulates air not fuel. Backing the screw out is going to let more air through the pilot hole therefore leaning the mixture.
Orr.. maybe the air flowing past the little hole at a high velocity sucks fuel through past the pilot screw... then backing it out would richen it....

 

[jasonp]

[quote:fh3t902l]To richen the mixture on a pilot screw on a 4 stroke you back out the fuel screw. I'm not sure how much you will have to go as every mod is different. You should have to pull the float bowl off to do this. Pilot jet and fuel screw only effects 0 to 1/4 throttle.

I dont think 4 stroke has much to do with it, i think this is a carb issue. As I stated before the pilot screw protrudes into a hole in the carburetor throat. This means that it regulates air not fuel. Backing the screw out is going to let more air through the pilot hole therefore leaning the mixture.
Orr.. maybe the air flowing past the little hole at a high velocity sucks fuel through past the pilot screw... then backing it out would richen it....
[/quote:fh3t902l]

Well a couple of things to understand here. When the throttle is closed the pilot block is being used because there is a very high pressure area at that point in the carb right behind the throttle plate. Air is trying to push past that blockage. So fuel is being sucked out of the bowl of the carb through the pilot circuit because of where it's placed inside the carb.
2 and 4 stroke carbs are different. 2 strokes are using a air screw. 4 strokes use a fuel or pilot screw. Basically both are jet blocks are used the same to adjust the 0-1/4 throttle mixture it's just that they use different ways to do it. A 2 stroke uses an air screw and as you back the screw out more air mixes with the fuel as you adjusting the actual air to the jet block and the pilot is fixed, so you get a leaner mixture. So this is why it's called an air screw.
4 strokes use a pilot screw or fuel screw as some call it. They use this screw to adjust the mixture of fuel through the pilot jet block and the air jet stays constant. So as you adjust that screw out the mixture is becoming richer as more fuel is being pulled from the jet block and air mixing with it is saying the same.

 

[mbogosia]

I have been messing around with my jetting after an airbox mod and exhaust and the pilot screw seems to be right at home a 2 turns out. It really never seemed to effect it whether I had it 2-3 turns out it ran the same. I would keep yours at stock and see how it runs like that.

 

[sonevdesertrat]

Ok Jason,, I see what you mean now,, as a matter of fact when I was typing a reply to you that thought crossed my mind,, as you can see at the end of my last post.
I took the carb apart last night and didnt pay close attention but the adjustment is right next to the float bowl and you actually can get to it with the bowl on.. my bad.
Now that that is the case I will do you as you state mbogosia, and start at the factory default of 2 turns out and go from there.
Thanx guys..