fuel screw



20 replies to this topic
  • ricky1

Posted May 29, 2001 - 06:28 AM

#1

While TRYING to get my carb right I remembered a post saying someone had turned there fuel screw all the way in and the engine would still idle. I tried this and it worked. I rode for 20 min. this way, and it ran fine. This makes me think I should go to a smaller pilot jet. Am I correct?

------------------
Rick Fuller
'01 YZ426
'00 YZ426 destroyed by fire in desert race
'81 490 Maico

[This message has been edited by ricky1 (edited 05-30-2001).]

  • James_Dean

Posted May 29, 2001 - 10:34 AM

#2

Contrary to that, if the pilot jet is too small the idle will not respond to turning the screw. I think you want to increase the pilot jet.

  • ricky1

Posted May 29, 2001 - 12:51 PM

#3

Thanks James, I will try it this afternoon.

------------------
Rick Fuller
'01 YZ426
'00 YZ426 destroyed by fire in desert race
'81 490 Maico

[This message has been edited by ricky1 (edited 05-30-2001).]

  • mikeolichney

Posted May 29, 2001 - 01:06 PM

#4

James Dean, I am curious, is the fuel screw in series with the pilot jet, or in parallel (fuel flow wise)? I have not seen a diagram of the FCR carb. It seems like it must be in series, that way, turning the fuel screw all the way closed shuts off all fuel at idle. And then a too small pilot screw will limit fuel, even when you open the screw up all the way. Is this true?

  • Hick

Posted May 29, 2001 - 03:31 PM

#5

Originally posted by James Dean:
..if the pilot jet is too small the idle will not respond to turning the screw.


Please explain, oh bringer of counterintuitive information, so that I may continue to impress my riding buddies with my ever growing jetting prowess.

If the screw is all in, where is the fuel coming from? Another circuit, or do I not get how the idle circuit works (prob. both)?

I’ve almost got my friend convinced to let me rejet his ’01 WR, he thinks Yamaha knows more about jetting than we do here at Thumpertalk. Unbelievable.

Thank you :)

  • Boit

Posted May 29, 2001 - 06:08 PM

#6

Ricky: Did you also try turning the fuel screw out(counterclockwise) to see what effect this had on the engine? Also, was your engine up to operating temperature and after having put a load on it? From my research on this carb, some of them are defective and have leaking internal circuits. I can't remember who...but someone had problems with the bottom half mating to the top half and the pilot circuit was bleeding into the main circuit...or vice versa.The thing about these carbs...and carbs in general...is that if they aren't working properly mechanically, you can jet til the cows come home and just end up frustrating yourself. All circuits must be operating reliably or you just won't be able to get it right. I chased a gremlin til I found an air leak where the air box boot attaches to the stack.....drove me nuts for a while. Many guys have fought the carb woes with these machines. I was one of the lucky ones and got mine dialed in eventually. It's really hard to appreciate this machine until you get it working correctly. Good luck.

  • ricky1

Posted May 30, 2001 - 04:56 PM

#7

Boit or James,
Can you tell me the best way to check for air leaks? Have not tried the richer pilot yet but soon. Thank you.

------------------
Rick Fuller
'01 YZ426
'00 YZ426 destroyed by fire in desert race 5/01
'81 490 Maico

  • Boit

Posted May 30, 2001 - 08:57 PM

#8

The most effective way would be to use an acetylene rig. Turn JUST the oxygen on low and move the wand tip all around the intake tract area slowly with the engine running at a quick idle. The vacuum will suck in the oxygen at the leak and the engine will pick up speed suddenly.
If you don't have access to an acetylene torch, Wal-Mart carries the small Bernzomatic oxygen bottles and wand.

  • sirthumpalot

Posted May 31, 2001 - 02:11 AM

#9

Or a slightly less expensive (perhaps more messy and a tad more dangerous) is to use a spray can of something flamable, such as wd-40. Silicone would be better for the rubber parts but I've never tried this to see if it works, I assume it would. This will have the same effect as the oxygen (faster idle if you find a leak). If you have very good life insurance you can try a spray can of starting fluid or ether. :)

  • Boit

Posted May 31, 2001 - 12:56 PM

#10

That starting fluid would probably work, but it would be EXTREMELY dangerous as Sirthumpalot mentions. If you decided to go that route, at least get out in the middle of your driveway or yard and have an extinguisher handy. ...wouldn't want the bike to go up in flames....you either. Might be wise to have your sweetheart or a friend handy with a water hose.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • Guest_Guest_*

Posted May 31, 2001 - 01:03 PM

#11

On my 66' Stang' I always used carb cleaner.
worked pretty good!

  • mikeolichney

Posted May 31, 2001 - 01:07 PM

#12

ricky1: Considering your history with burning up bikes, I advise you to avoid some of these methods.

  • ricky1

Posted May 31, 2001 - 03:04 PM

#13

Boit & Mikeo
Don't worry, I will be trying the oxygen method.

------------------
Rick Fuller
'01 YZ426
'00 YZ426 destroyed by fire in desert race 5/01
'81 490 Maico

  • ricky1

Posted May 31, 2001 - 03:08 PM

#14

By the way, I have a picture of the well done 426 in my E-mail. Can anyone tell me how to post it?

------------------
Rick Fuller
'01 YZ426
'00 YZ426 destroyed by fire in desert race 5/01
'81 490 Maico

  • Hick

Posted May 31, 2001 - 03:49 PM

#15

Originally posted by ricky1:
By the way, I have a picture of the well done 426 in my E-mail. Can anyone tell me how to post it?


I’d love to see that. Click here for picture posting instructions.

  • KasperMX

Posted May 31, 2001 - 06:26 PM

#16

Thanks Hick
http://albums.photop...9230&p=49576391

[This message has been edited by KasperMX (edited 05-31-2001).]

  • James_Dean

Posted June 01, 2001 - 11:23 AM

#17

Mike and Hick,

The pilot circuit has a branch after the pilot jet from the cross-section picture. A portion is metered by the pilot screw (in parallel). Turning in the screw all the way closes off the portion on the downstream side of the slide. Fuel from the pilot jet will still enter nearly under the slide.

A small pilot jet can become unstable much like running low on fuel. Keihin recommends increasing the pilot jet and pilot air together at the same time. From my testing the #42-45 pilot jets work better with a #75 pilot air and the #45-48 better suit the #100 pilot air.

Why the instability, is uncertain. I keep seeing it on both 2 and 4-strokes. It is almost like the fuel gets "over" vaporized and draws air in uncontrolled from within the pilot circuit. This gets worse as the engine gets hot. It has symptoms much like an air leak or low fuel. I also recall the discussion Boit refers to about possible internal leaks on the FCR carbs. It would not take much on the pilot circuit to cause problems. In either case a step richer on the pilot jet will help add some stability. Having the pilot screw between 1 - 1 1/2 turns would be ideal for a balanced transition. IMHO :)

You won't find much about pilot circuit tuning in most literature. It is referred to in the Patrick burns article about the Fuel (pilot)screw: www.lifenet.com/brm/carbkei.htm

James

  • ricky1

Posted June 05, 2001 - 05:20 PM

#18

James ,

I tried a #42 pilot (was running #40)and it is set at 1/8 out. If I go to 1/4, 3/8, 1/2, etc,(with #42)idle SEEMS (no tach) to idle rougher. Starting seems normal. I am going to go to a #40 and try to get into the 1-2 turns out range. My big problem now is when I accelerate hard through all gears, ( 1/2 throttle, shift, 1/2 throttle, shift, 1/2 throttle, shift, there is a major stumble every time I get back in the throttle. After stumble (1-2) seconds it comes on hard and smooth.
The 'strange to me' part is that if I go through the gears FULL throttle, shift, etc,
through all gears, I get no stumble.
I am trying to shift at the same point ~9000rmm.
I was at #2 needle clip, went to #3, then #4. No apparent change. I only did the full throttle tests at #4. Based on the thought that full throttle =more acc. pump squirt=richer I am tempted to try #5 clip on needle. My goal is to stop/reduce stumble between 1/8-1/2 throttle acceleration.

Side notes,
tests done at 6500ft (AVG riding elevation)
Big Gun SDS pipe with race core + S/A screen
fresh oil, fresh gas,(pump, 93 oct.)
clean air filter
Main jet 158, 1 below stock

ALSO, Printed and read your article on carb needle profiles. Do you think a #75 Pilot air jet would be appropriate? (with #40 or 42 pilot) at this altitude?

I am frustrated, almost enough to drive the 775 mines to Auburn and beg on your doorstep.

Could I Interest you in a Lake Tahoe Vacation?

Sincerly,

------------------
Rick Fuller
'01 YZ426
'00 YZ426 destroyed by fire in desert race 4/22/01
'81 490 Maico

  • ricky1

Posted June 06, 2001 - 12:44 PM

#19

I said I would try to post pic's of burned bike. Posted Image

------------------
Rick Fuller
'01 YZ426
'00 YZ426 destroyed by fire in desert race 4/22/01
'81 490 Maico

  • ricky1

Posted June 06, 2001 - 12:46 PM

#20

looks like I did something wrong. Help, Please?

------------------
Rick Fuller
'01 YZ426
'00 YZ426 destroyed by fire in desert race 4/22/01
'81 490 Maico





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