[How to set your pilot screw adjustment.] - ATV Riders Forums - Honda, Yamaha, Suzuki, Kawasaki, Polaris, CanAm, Arctic Cat, Kymco ATVs

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How to set your pilot screw adjustment.
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Posted by: GPracer2500

This is my prefered method of finding the proper setting for the pilot screw (aka fuel screw). This method can also indicate if your pilot jet is too big or too small. Towards the bottom is an explination about the difference between fuel screws and air screws.

You'll need to be able to access the fuel screw while the engine is running. This can be quite difficult on many carbs. FCRs have aftermarket fuel screws that are longer which helps. On a stock 400EX carb (and many others) I use a 90 degree screwdriver. I bit the bullet and bought this one from MotionPro. It works well but I'm sure there are others available.

http://www.azjetski.com/catalog/images/03MotionProPilotAdjustSM.gif

Here's a couple examples of aftermarket FCR fuel screws:
http://www.roostracing.co.uk/images/Pulse/fcrscrew.jpg

http://www.xrsonly.com/content/components/com_phpshop/shop_image/product/3c154d81101dad29998bc1e6e1247c7e.jpg

OK. On to the procedure!

1) Warm up the engine to full operating temp.

2) Turn up your idle a few hundred RPM using the throttle stop screw (basically you want a fast idle). This will make it easier to hear small changes in RPM. Watch for overheating--pointing a big shop fan at your engine will help it from getting too hot during the fast idling. The whole procedure shouldn't take too long though.

Each time you change the screw setting 1/4 or 1/2 turn or so, wait about 5 seconds to let the idle speed normalize. It usually take the carb and engine a moment to react to the change.

3) Turn the fuel screw IN until the idle starts to drop and miss. The engine should die if you bottom out the screw. Your pilot jet is too big if it doesn't die when the screw is bottomed out--it should die before it gets that far in.

4) Then begin turning the fuel screw OUT. The idle should peak and become smooth. Keep going and look for the idle to begin to drop/miss again.

5) The goal is to find the setting that provides the highest and smoothest idle. If it's unclear exactly were that point is then set to the midpoint between step #3 and step #4. For example, if the idle starts to drop at 1 turn out and starts to drop at 2 1/2 turns out then 1 3/4 of a turn out should be the correct setting.

If the peak/smoothest RPM is reached somewhere between 1-3 turns then your pilot jet is correct (the 1-3 turns applies to most carb types). If you end up less than 1 turn out then your pilot jet is too big and you need a smaller one. If you end up more than three turns out or the fuel screw seems to make little difference as you continue turning it out than you need to go up (bigger) on your pilot jet.

To re-emphisize: If the idle never drops when you're turning the fuel screw in, you need a smaller pilot jet. If the idle never drops when you're turning the fuel screw out, you need a bigger pilot jet.

Typical fuel screw settings are in the 1 1/2 to 2 1/2 range.

6) Once you've got the fuel screw set, re-adjust your throttle stop screw (idle screw) to an appropriate idle speed.

And that's it! Your pilot circuit is now VERY close to ideal. From here you can experiment with how small adjustments affect low-end (i.e. small throttle openings) response and make adjustments for weather. The hardest part is usually gaining access to the screw while the engine is running. Also, you may really have to listen carefully to detect the rpm changes in the idle. 100 or 200 rpm differences can be tough to detect when the change happens over several seconds.

And lastely, a little bit about the difference between fuel screws and air screws:

Two stoke carbs normally have air screws and four stroke carbs normally have fuel screws. You can tell if a carb has an air screw or a fuel screw by it's location on the carb. An air screw will be on the intake side of the carb while a fuel screw will be on the engine side of the carb.

They sort of work opposite one another. An air screw adjusts how much air is being delivered thru the pilot circuit: in is rich (less air) and out is lean (more air). A fuel screw adjust how much fuel (or air/fuel mixture) is being delivered from the pilot circuit. In is lean (less fuel) and out is rich (more fuel).

This image depicts a carb (a Dellorto in this case) with an air screw:

http://img230.imageshack.us/img230/3148/fig19gi0.jpg
1 is the air screw
5 is the pilot jet
blue area is air
orange area is fuel
green area is air/fuel mixture

This image depicts another Dellorto carb but this one has a fuel screw instead of an air screw:

http://img230.imageshack.us/img230/7382/fig18hd2.jpg
5 is the fuel screw
6 is the pilot jet
blue area is air
orange area is fuel
green area is air/fuel mixture

Posted by: snox31

great post thank you for taking the time to make it simple to understand

Posted by: ohsobad_chevy

Yeah, I didnt know that the FCR screws existed until now. Of course i never really looked, but o well. Getting to that fuel mix screw is a pain in the arse when it is running. Nice post, thanks for the time and im sure that it will help lots of people with correcting their pilot circuit.

Posted by: GPracer2500

I updated a few things for clarity and added a little more info on fuel vs. air screws.

Posted by: WickedATVer

where can I get a FCR fuel screw?

Posted by: cals400ex

quote:
Originally posted by WickedATVer
where can I get a FCR fuel screw?

many places. i got mine from C&D

Posted by: Brehmmotorsport

I replaced my stock setup with 42 also but now after I hammer it and shut it down I get feul dumping out the over flow line on the bottom of the carb off of the bowl? Any ideas.

Posted by: 400exrider707

quote:
Originally posted by Brehmmotorsport
I replaced my stock setup with 42 also but now after I hammer it and shut it down I get feul dumping out the over flow line on the bottom of the carb off of the bowl? Any ideas.

adjust your float level.

Posted by: AbnMP13

How in the world do I loosen the screw on the 400ex, it's shaped like the letter "D"?

Posted by: suzukiray

The Honda F-2, F-3, & F-4 street bikes had D shaped screw heads also & the Dynojet jet kits came with the tool to adjust them. I used to own a shop in Hawaii & I have a few of them. Find Dynojet's website & get 1 from them. Ray.

Posted by: AbnMP13

That's great info, I appreciate it. I found the tool listed in the Dynojet website (DT008) but I had to email them to see if they sell it seperately.

Posted by: stvemerz

Well....I've read this over and over...I'm still having some problems...Stock 400ex 2007...Sparks 6 degree adv. Hmf Sport slipon with quiet core..Keihn 152 main and 42 pilot...Installed...Fuel screw was at 2.5 turns out...used a 2004 needle and put clip 3 down from top...Checked plug...sooty black...I then turned the fuel screw all the way in... of course this was after a full warm up...It died, but yet I could start it as long as I gave it some throttle...Once I let off..It died...So I figure my jet sizes are ok...plug is still a blackish color....I notice when I rev the throttle...looks like a fine puff of exhaust fumes come out...tells me fuel...still running rich...the fuel screw is now at 1 3/4 turn out...Any suggestions...quad worked great before the changes...and works great now...just a slight gurgle when I let off throttle...It runs good..do not get me wrong...but I do not have that nice brown color plug...that I'm looking for...HELP..

Posted by: AbnMP13

U done any airbox mods?

Posted by: stvemerz

No air box mods..I really do not plan on it at all...since were I ride is in dusty conditions..I also forgot to mention..I installed a uni air filter...anyone have any ideas...

Posted by: AbnMP13

My opinion is to go to a smaller jet like maybe to a 148-150 and put the fuel screw back out where it was. If the bike backfires on decel BUT runs normally, then tweak the fuel screw. It seems easier to me that way as the bike is more likely to run when it's a little lean.

Also not to insult you but you are checking the porcelin portion of the plug by the electrode correct?

Hope this helps.

Posted by: stvemerz

Yes..I'm checking the porcelain area..it is blackish...stock is 148...Maybe I will get a 150 and try it....Just baffling to me that the HMF slipon gives 152 and 42...I will try the 150 and put fuel screw out 20 like 2- maybe 2.5 turns out...Ive been cleaning the old plug..before i try the 150..should I run a new plug in it first..

Posted by: AbnMP13

I would, it would definitely be easier to read with the new plug.

Posted by: stvemerz

Well....I think I'm on the right track...pulled old plug and gapped and put in new one....at 1 and 3/4 turn out..running a a slight bit of tan on porcelain...and very dull white on elctrode but no backfire on letting off throttle....so I backed out the air screw to 2 turns and then checked same plug..little lighter white...went out 2.5 turns and now will go get a new plug and try it...Does it sound like on the right track...I think that I have some bad gas as well..mostiure in exhaust...water dropplets at the tip..not fuel dropleys...will try new plug and let you guys know...

Posted by: stvemerz

Well...here we are....152 main 42 pilot, keihn...new plug and 2 3/4 turn out...no backfire..but on higher revs..fine exhaust fumes out the exhaust...also..plug is a super light tan....I mean super light not like a paper brown bag....Think I'm almost there...any advice or hints....and do I need change the plug after every run...thanks..NEW to this.

Posted by: AbnMP13

Sounds like you're getting there, mine is light tan and I still backfire at times on decel, keep working it, change plugs when you make a major change, that way you'll know which way to go. I know I don't have to tell ya this but keep the plugs that don't foul, they're still good. I just changed mine on major jetting changes to get a true read.

Posted by: stvemerz

I will keep on doing what i've got going...just surprised that even light tan...it does not backfire...Right now I'm at like 2 3/4 turns out...and upon heavy throttle...I get a puff/fume out the exhaust..maybe back it in 1/4 turn....I'm starting to wonder...is the 152 main..to small....is there a 154....I have a 42 pilot...so I know thats good..and 3 down on clip....Not so sure....at least the main is easy to change....Any ideas..still looking for the paper brwon bag color...Will post again when I work on it...

Posted by: AbnMP13

I'm not sure on how many sizes there are but I do know they are cheap.

Posted by: TRAV05

newbie question where is the fuel screw on the 400exs or is it also called the pilot screw i just need to turn my idle up
thanks

Posted by: hornetgod13

The pilot and "fuel" screw are the same thing.
If you need to adjust your idle than there is a different screw for that. The idle adjustment screw on a 400EX is on the left side if the carb. It's in the front side and it's a large straight slot screw. If you turn it clockwise it will increase the idle. Counterclockwise lowers the idle.