That annoying off idle hesitation!



12 replies to this topic
  • Matt_W

Posted November 26, 2001 - 04:40 PM

#1

O.K., I've done the BK mod where spray just misses the slide and squirt duration is at about .4 to .5 seconds. Bike responds better over all, but I still get that annoying off idle hesitation when I wick the throttle. Gas is Sunoco 94, jetting is stock with exception of 168 main. As the temperature gets colder my bike begins to pop when I decelerate. Is the popping a sign of lean or rich jetting ?? What circuit causes the popping during deceleration ??

I'm in Pennsylvania at approximately 850 feet above sea level. Would anyone who has done the BK mod with great success care to share there corresponding jetting changes if you are close to the same elevation as I ?

If I've done the accelerator pump mod correctly and I've still got the off idle hesiation, what is causing this problem. Is this caused by the jetting being to lean or too rich, and what circuit would most likely clean this problem up . . . Pilot or Needle ?

  • F-Pilot

Posted November 26, 2001 - 04:53 PM

#2

I had to turn the pump timing screw in 1/2 turn to get rid of the bog. Setting it at just missing the slide must be a little too soon. I think some other TT'rs have found this same problem/fix.
Popping with the throttle off is lean and can be stopped by opening the fuel screw a 1/4 turn at a time till it quits.

[ November 26, 2001: Message edited by: F-Pilot ]

  • Matt_W

Posted November 26, 2001 - 05:11 PM

#3

If my pilot circuit is lean, why not go bigger on the pilot jet as opposed to turning out the pilot screw ?? On a two smoke the rule of thumb is that if the air screw is more than 2 & 1/2 turns out, go bigger on the pilot jet. Is there a similar rule of thumb on the 426 carb ?? How far out is too far on the pilot screw before I should go bigger on the pilot jet if the popping does not cease ?

  • F-Pilot

Posted November 26, 2001 - 05:52 PM

#4

Same rules for the thumpers just opposite to fix. Fuel screw (not air screw as on 2 smoke) should be 1 to 2 turns out.
I run a 45 pilot (stock '00 is 42), 1 1/4 turns out, stock needle in 4th position and 165 main.
I am at 1k-2k elevation about 65f-80f degrees now.
I'll drop to 162 main in the summer.

  • Psycho426

Posted November 26, 2001 - 11:33 PM

#5

After setting the 0.3 BK mod duration, and making it just miss the slide, I would then set the pilot/fuel screw. Make sure the bike is up to operating temp, very important. If you have a tach, set the idle speed to 1800 RPM, otherwise guess! Note the idle RPM at 1/2 turn out, 1 3/4 turns out, and 3 turns out of the fuel screw. Which is fastest? If its 1 3/4, you have the right pilot. If its 3, then go up to a 45. If its 1/2, then go down to a 40. Now fine tune the fuel screw. It helps to have a tach but most people get good at it without one, to find max revs with the screw. You can also turn the fuel screw in until you get decel backfiring, then back it out 1/2 turn from there. I also recommend that Kouba fuel screw adjuster, getting a driver bit into that recess in the carb is a pain.

Next I set the main. I have a temperature vs altitude chart that works for my bike. I can send it to you. But alot of people do uphill WOT runs comparing how well the bike pulls with different jets. Do the same with the needle, looking for how it runs at 1/4 to 3/4 throttle. If you get that gurgling/cracking sound, that is too rich. I never had much luck with looking at the plug. My 250F plug looks totally different than my YZ426, yet both are jetted well.

Good luck. BTW, I had a bad problem this weekend and I could not figure out what was going on. I remembered I had drained my 5 gal gas can into the bikes tank. Water is more dense than gas, so maybe the gas can had some collected at the bottom. I drained about a cup of gas using the 3mm allen bolt on the float bowl. The bike ran great afterward.


The advice above was originally posted by 'mikeolichney' in BK MOD=Jetting help needed!!

Hope this helps you a bit.

[ November 27, 2001: Message edited by: Psycho426 ]

  • Matt_W

Posted November 27, 2001 - 01:58 AM

#6

Psycho,

Where can I get a "Kouba fuel screw adjuster"? I think it is virutally impossible to make an "accurate" adjustment to the fuel screw while it is on the bike. I was pondering that dilema until you made your post.

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  • Psycho426

Posted November 27, 2001 - 03:46 AM

#7

Matt - Kouba Fuelscrew adjuster there is a ordering form on the website.

  • Taffy

Posted November 27, 2001 - 11:52 AM

#8

psycho426

petrol is hydroscopic. there's a long word for you.

there are two types of popping on the decel. both are a lean condition. if you ride flat out and then close the throttle;

1)if at the beginning you get a slow soft popping it is your needle.

2)if it happens much later at lower revs (by now you would have kicked down to 2nd gear) and it's more often-it's the pilot system.

my advice to anyone is to lower the pilot air jet to create a richer set up. this will get rid of the hesitancy. the best way to do this is to buy the pilot air screw which you simply turn in or out to allow more air in or not.

after much experimenting i found that the lower the settings the harder it was to stall, easier it was to start and that in second gear at low speed i could hit the throttle and the engine would just 'spin' instead of all that thrumming as it revs laboriously.

Taffy

  • rmarion

Posted November 27, 2001 - 01:16 PM

#9

I'm Old school, my YZ is a 99'

Every owner had the same problem as you mention.

The B.K. mod? Sorry I have no idea what you guys are speaking?

Here's the low down out in So-Cal.

I had the same problem, I spoke with PC and Ty Davis in Mammoth. No such help.

I ran into a Factory YZ pilot at an OTHG event, did not catch his name, but he gave me some sweet news.

He said all YZ's were tuned Lean for the EPA requirements.(2000, 1999, 1998 models)

He wrote down 182 main, 50 pilot, one needle richer, clip same.

(99 stock set up = 175 main, 45 pilot, OBDVR-4 needle)

I've no hestiation what so ever anymore.

Most Tracks I run are 1500-2500 elevation. (All So-Cal tracks)

Good Luck :)

  • Shawn_Mc

Posted November 27, 2001 - 02:23 PM

#10

Rmarion 180 MAIN! Does your scoot belch black smoke? Im being a jackass in asking this question, im serious. Your jet specs are so much richer than anything ive ever heard. Ive done a lot of jetting and tuning in my day, and have seen plenty of stuff that has burned down as a result of lean mixtures. Ive never seen anything or heard of anything coming out of a factory that lean. :)

  • Shawn_Mc

Posted November 27, 2001 - 02:26 PM

#11

Rmarion 180 MAIN! Does your scoot belch black smoke? Im being a jackass in asking this question, im serious. Your jet specs are so much richer than anything ive ever heard. Ive done a lot of jetting and tuning in my day, and have seen plenty of stuff that has burned down as a result of lean mixtures. Ive never seen anything or heard of anything coming out of a factory that lean. :)

  • rmarion

Posted November 28, 2001 - 03:59 PM

#12

Hey guys,

No B/S here. It's what I run, No smokin here, just kickin ASS.

B/S aside, I told the truth here. I had the same hestiation as everyone, had nothing to lose, changed everything like he said and pressto, no sputering. It'll cost you $20 to try?

Also I'm running local Tracks ay So-Cal.

PS, the bike won't smoke if you don't run premix hehehe.

:)

  • mxcowboy

Posted November 28, 2001 - 04:30 PM

#13

Originally posted by Matt W:
Psycho,

Where can I get a "Kouba fuel screw adjuster"? I think it is virutally impossible to make an "accurate" adjustment to the fuel screw while it is on the bike. I was pondering that dilema until you made your post.


I found this at my local dealer for 12.95..It's a fcr carb tool and it works great, check it out cheaper than the kouba tool..

http://www.motionpro...onvtools_2.html





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