Pilot screws...please explain this if you can.

I have read a lot on carb theory and spent a bit of time working on them (my XJ, my Harley and my dirt bikes). On my XJ I am trying to understand the theory behind why the pilot mixture screw works the way it does.

As I understand, it is an "air" screw. To me it would seem that turning it IN would reduce the air and make the mixture rich. However, everything I've read says just the opposite...IN for lean, OUT for rich. I'm at the "fine tune" phase and would like to understand why this is.

Is it because the pilot air passage is used to "pull up" extra fuel at idle and, the less air you allow through that passage the less "additional" fuel is introduced into the stream?? Am I even close?

Any help in explaining this would be appreciated. I HAVE read the two articles that have been posted on here (I have them bookmarked at home, can't remember the links or names). I just don't find a clear explanation, just "IN is lean, OUT is rich". :?:

 

The pilot mixture screw is a "fuel" screw in the path of the idle air stream. Opening it or turning counter-clockwise increasing the amount of fuel that can enter the air stream. .....you were so close!

 

Thank you!

Guess I have to look at those "cut away" drawings of the passages again.

 

The Pilot Mixture Screw is an AIR Screw.
The Pilot AIR Passage is married to the Pilot Fuel Jet.

The AIR Speed regulates the amount of Fuel Drawn-up from the Pilot Jet.

Out = More AIR = More Fuel Drawn-up.
In = Less AIR = Less Fuel Drawn-up.

The Critical Adjustment for the Pilot Mixture Screw IS NOT measured in Fractions of a Turn.
Rather, the Critical Adjustment is just a matter of Degrees (2~3)

When you have the Cylinder firing and the Pilot Mixture will sustain Idle ... you are near the "Window" of Fine-tuning.

The Mixture will change to too Rich or too Lean within the width of a Nickel.

You can PLAY with that adjustment.
Within those 3-Degrees is:

Lean for Maximun acceleration with heavy engine braking ... or,
Rich for smooth application of power with "Coasting" when the Throttles are closed.

It all happens within a few degrees. The Air~Fuel Ratio changes that precisely ... this is why the Pilot Mixture Screw has such Ultra-fine thread!

 

Okay, then. That's a better explanation of what I was TRYING to say in my original post. So it does work the way I was thinking. Thanks.

 

hey fellas just have a few questions about the carbs. I just cleaned them with ricks manual....THUMBS UP RICK!.....and bench synced them as well. i installed them and the bike fires right up and it revvs with no hesitation and little popping. my question is via ricks instructions i removed the mixture screw and innards and didnt take note of how many turns they were out i am at a loss of where to start. If its just the width of a nickel that can ruin things and its about 7 turns till they bottom out thats a lot to play with is there something im missing? my plugs when i run it now after i scrub them out they are black as night should i bottom them all out and slowly work them out? lemme know thanks in advance!! Oh


PS. 82 xj750 24k

 

Is the the screw that willierides is referring too?

 

Mike that's the pilot jet. The adjustment screw is right next to the enrichment valve (or "choke") on top of the carb.

 

ericstanicki,

Black plugs=rich

there is a baseline some folks use of ~2-3 turns out.
try turning 1/4 of a turn and checking plugs again, repeat if nessisary
once they are turning white you have to go back OUT
ONCE your closer, work in DEGREES as rick explained.

ps dont run lean for extended periods of time, unless you feel like replacing your engine in the near future!!!!

 

Ok, so is screw B in this photo the screw that willierides is talking about?

 

they might be invisible to you right now. like they were to me. due to the fact that in some cases they are covered with a cover/cap. the pilot screw is located near the cold start valves on the topside of the carbs. one per carb. kind of under the skin...

search the FAQ section for ricks old school carb routine. it will lead you to where you need to go and then some. i had a haynes manual once. now i just come here instead.

actually, i still use my haynes manual alot but parts of it are useless without this site...

 

Turtlejoint - Thanks for the info, I will take a closer look around the cold start valves to find this god forsaken screw!

 

Don't get Fuel Jets confused with the Pilot Mixture Screws.
Tighten all the Fuel Jets. You don't want them coming loose.

Now, lets go looking for the Pilot Mixture Screw.
Look on the Number-1 Carb.
On the top. Put your Finger on the Chrome Dome.
Move your finger toward the Cylinder Head.
Now, forward until you touch the "Choke" Valve.
Right next to the Choke Valve, to it's left, is a small circular opening.
Well, ... maybe it's NOT Open.
There could be a Safety Plug in it.
Either way ... there's a small, round opening at the base of the Choke Valve that encloses the Pilot Mixture Screw.

If it's Plugged with the Safety Cap. You have to remove the Safety Plug.
If it's not Plugged ... you'll see the Top of the Pilot Mixture Screw.
First, locate the Top of the Screw of it's hiding place and we'll go from there.



Right there at the edge of the shadow. Directly under the left hose clamp on the body of the Carb. Just to the right of the Choke Rod Pillar is where we hid the Pilot Mixture Screws on you.

If you look closely enough at this photo you can see the Slot of the Screw.

 

my next question is that i have finally gotten a good color on the plugs somwehre along the lines of hot chocoloate shade but i have had to turn the mixture screws almost all the way out. Am i asking for trouble down the road or is it legit....... i just put new plugs in and revved it at 3.5k for 15-20 secs then check the plugs then adjust then repeat.....thanks in advance

 

The pilot screws have little impact above 1500rpm or so. You should be setting up your mixtures at about 1000 rpm. Darn tough to do without a colortune plug (or exhaust gas analyzer). Hard to note much difference tweaking a 4 banger.

Having them turned way out may allow them to shake loose as the spring won't have enough tension to resist them turning.

My plugs soot up at idle. They don't lighten until I run at speed, then they're mostly white with just a shade of tan on one side.

 

ok good tip on the rpms

how long should i let it idle before i go back and check the color of the plugs....

I have taken peoples suggestion and started with say 3 turns out and ridden it for a day and it looks like i just took it out of the box so ive kept backing it out.

 

I found them hiding under a nice layer of black goop, and cleaned them up good. I would have never known they were there being as dirty as they were! Thanks turtlejoint and Rickcomatic.

 

Make yourself a Screwdriver that fits the SLOT on the Pilot Mixture Screw perfectly.
If they are stuck and you attempt to get them to move with a tip thats a perfect (NO Endplay) fit ... The "Wings" either-side of the Slot will move WITHOUT the Body of the Screw moving and get deformed or snapped right off.

If they move ... don't try to bring them OUT.
Oil the Hole with some Vegetable Oil that won't flame-up and apply some heat.
Then, turn them IN and Down toward clean threads.
"Exercise" them Down
Back up into the Oil
Back down
Up into the Oil
Until you get the Oil working for you.