View Full Version : How to set your pilot screw adjustment.

05-03-2006, 01:34 PM
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. If you're lucky, your fuel screw is easily reached with a normal screw driver.


Here's a couple examples of aftermarket FCR fuel screws:


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:

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:

5 is the fuel screw
6 is the pilot jet
blue area is air
orange area is fuel
green area is air/fuel mixture

05-17-2006, 07:07 PM
great post thank you for taking the time to make it simple to understand

05-22-2006, 08:51 AM
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.

06-07-2006, 11:58 PM
I updated a few things for clarity and added a little more info on fuel vs. air screws.

06-09-2006, 04:10 PM
where can I get a FCR fuel screw?

06-10-2006, 05:47 PM
Originally posted by WickedATVer
where can I get a FCR fuel screw?

many places. i got mine from C&D

11-15-2006, 12:17 PM
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.

12-07-2006, 04:50 PM
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.

01-06-2007, 10:10 AM
How in the world do I loosen the screw on the 400ex, it's shaped like the letter "D"?

01-06-2007, 11:37 PM
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.

01-07-2007, 06:42 AM
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.

02-10-2007, 04:43 PM
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..

02-10-2007, 06:24 PM
U done any airbox mods?

02-10-2007, 07:14 PM
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...

02-10-2007, 07:32 PM
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.

02-11-2007, 07:47 AM
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..

02-11-2007, 08:08 AM
I would, it would definitely be easier to read with the new plug.

02-11-2007, 10:48 AM
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...

02-11-2007, 12:02 PM
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.

02-11-2007, 04:57 PM
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.

02-12-2007, 08:01 AM
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...

02-12-2007, 03:55 PM
I'm not sure on how many sizes there are but I do know they are cheap.

03-03-2007, 07:15 AM
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

03-04-2007, 06:30 PM
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.

09-28-2008, 02:05 AM
OK, I'm really frustrated and desparate, so I'm going to add a question to this how-to thread... because I'm not getting anywhere anywhere else.

I'm like the other guy... I had a black plug on all circuits it seemed on my 400EX... Full Yoshi Exhaust... K&N in stock box w/lid... 1200 ft.

I pulled the airbox lid and that was not enough to lean it out, so I went into the carb. 155 main, needle clip in the 3rd, 38 pilot.

I found the pilot screw to be several turns out... and the needle holder/needle jet not turned in fully, just partially and the locknut set. I also am not sure I was getting in the main due to adjustment.

I set the pilot screw at 2.5 turns out, screwed the needle jet/holder all the way in til it stopped, set the locknut, put the 155 main back and raised the needle clip to the second slot down..... after cleaning everything of course.

Now I seem to have a very light tan plug on all circuits, no popping or anything. I think with proper gas (shortage) and a regular NGK plug I might be more brown (we'll see).

What's driving me nuts is the poor starting. That first start of the day is a pain. It starts, then dies repeatedly. Then once it does run and get hot, the idle RPM increases a lot.

So I know I'm off. The trouble is I do not have the luxury of a cool tool to use to 'hot-tune' the pilot screw and idle screw together. So I have been using one adjustment a day on the pilot screw.

So far I think I have been from 2.5 out to 3.5... no change really other than I could tell the exhaust was rich... then all the way back to 1.5 turns out. I think I've been everywhere between 1.3 and 3.5 except 1.75 and 2.25.

I'm lost.... do these things have to be RICH on the pilot to start well?

09-04-2010, 03:39 PM
Originally posted by AbnMP13
How in the world do I loosen the screw on the 400ex, it's shaped like the letter "D"?
Haha, my thoughts exactly when I was jetting, I heard you can use a .22 shell but tbh I'm just getting one from dynojet like Ray said.

09-04-2010, 03:47 PM
Originally posted by MtnEX
What's driving me nuts is the poor starting. That first start of the day is a pain. It starts, then dies repeatedly. Then once it does run and get hot, the idle RPM increases a lot.

My starting use to be perfect and now it's as you described, weird, seasons mess with it any?

09-05-2010, 01:09 AM
Yeah, season changes will mess with it and so will your filter.

Another thing it can be in compression... although nobody wants to hear that...

I know this now because a guy brought his here for me to work on because after his last ride it would not start for the next.

I went all through it and found a little, but not what I expected to find as far as a clogged pilot or whatever.

Then it would hit, but was way worse than mine. I mean we are talking almost killing a battery on a hot day to finally get it going. Once hot it didn't start back great but was nothing bad like before.

So I sat down and thought about everything. Then I pulled the tank and checked compression. I think the best reading I got on it was 90 PSI cold?

He didn't want to hear me telling him it was compression.... that I should be checking it hot and so forth... so I had to post a thread on here to show him...

I've seen this a few times so far. In this case it was the air filter passing dust and crap. Another was a fresh rebuild that ate the rings in short order. And another was just a guy servicing his bike... and I believe what happened is he changed his plug and stuff laying around it fell in the hole and eat it up.

04-09-2012, 10:31 AM
soon im going to have to adjust my idle speed because im going to be putting in a timing advance key which ive been told raises the idle speed.
however a couple years ago when my bike was stock (ive had the carb jetted by someone who knows what they are doing because of exhaust and intake mods) I tried changing the idle speed by twisting the throttle stop screw (silver one on left side with flat slot) and it just twisted back to where it was before.
i dont remember if the quad was running or not when i did this
but would it being off have caused this? if not how can i get the throttle stop screw to actually work and stay where i want it to?

11-12-2012, 12:31 PM
I've looked everywhere for this but can't find it. Do you happen to know the part# and the site that sells them? As you know there's no way of getting back there when the engine is up to temp unless you have the tool for it so I just set mine @ 2 turns the last time I pulled the carb to clean it. It's got a bit of a hiccup on idle which I'm sure it's due to the need of fine tuning. BTW, great info on jetting. Thank you.