Possilby rich pilot?

6 replies to this topic
  • yamaha426

Posted April 27, 2002 - 11:59 AM


I just picked up my shiny new 2002 426 about a week ago and have just had it on one ride after breaking it in. Most of the time it runs really well, but sometimes if you snap the throttle when the revs are down it trips over itself, puts out a small puff of black smoke and then runs fine. I ride at about 3000' I believe and the weather is now about 60-70 but it will get up to over 100 in aobut a month. Is there any way to fix it without taking the carb apart....I'm not real familiar with that part of the bike. :)

  • skthom2320

Posted April 27, 2002 - 12:22 PM


Probably just need a fuel screw adjustment.

Get a nylon spacer about 3/4 inch long and a flat blade drill insert, drill out the spacer so you can tap in the drill insert and epoxy it in. That makes a nice fuel screw adjuster.

Then reach in underneath the carb on the cylinder head side and tweak the fuel screw in a bit (in to lean, out to richen). Go 1/4-1/2 turn and see how that works. Record the stock number of turns in case you screw yourself up. Don't be afraid to keep going, you won't hurt the bike.

If that doesn't help - repost. Or read this link http://www.thumperta...c&f=10&t=004190

[ April 27, 2002: Message edited by: skthom2320 ]

  • Mark_Cantrell

Posted April 27, 2002 - 01:40 PM


You are describing a too rich symptom here but depending on how much you crack the throttle it could be on the pilot, needle or main. The Jetting 101 is probably a good post to read in general if you are new to this.

Your problem is transient. Since it 'runs fine most of the time' my guess is you have too much accelerator pump output. The AP squirts a thin stream of gas into the intake, and unlike the other jets, it does it regardless of the amount of air going through carb. It is activated by the movement of the throttle, not the position of the throttle. If you twisted the throttle three or four times when the bike isn't running, you would just be squirting raw gas into the engine and would flood the h311 out of it. All the early model bikes came with too much AP squirt. The Taffy mod, the BK mod, the P38, and leak jets are ways people have coped with too much AP. Some of the new ones have too much, some too little, and some are just right. You probably have too much.

If you are brand new to the 426 and brand new to carbs, this may seem a little daunting. On a 2002 (or even a 2001) you can get a large leak jet to reduce the AP squirt. The leak jet is between the bowl and the body of the AP and 'leaks' AP squirt back into the bowl. A larger jet 'leaks' more of the squirt, reducing the amount of gas into the engine.

To change the squirt, you have to pull the bowl, which can be done fairly easily without removing the carb. There are 4 3mm allen bolts on the four corners of the bowl. You will have to twist the carb slightly to get to all of them (especially the front left one, it's by the timing chain tensioner) to get them out and the bowl drops. You will have to disentangle it from the vent tubes and the idle adjust screw, just remember where they go. The little jet in the bottom of the bowl is the leak jet. There are sizes from 40(?) to 135(?). Look in the manual and see what size you have, and buy one a little bigger, the biggest they make, and one in between. They are about $9 each at the dealer. Sudco doesn't know what they are (neither will the dealer's parts man, but show him the jet in the bowl on the fiche). I can't remember, the YZ has little ones and the WR big ones or vice versa, check both on the fiche and get big ones, a la the Ladies' Man.

If yours is too big (possible?) the bike will quietly die if you whack it from idle. If riding it (not on stand), you can back off on throttle and it will smoothly come back to life. If it feels OK but you never had one big enough for it to die (they all die if not thoroughly warmed up) when hot, you haven't gone big enough. The bike is snappiest with the largest one (least AP) that keeps the bike from dying.

If that isn't enough, or for more control over tuning, look up the BK mod. In fact, it might be the better fix anyway but is more (not that much) involved, take off carb, drill and tap a flange, put in a screw with a spring, not really that much).

Anyway, you can perhaps mask the problem by just leaning out the pilot, see Jetting 101 for fuel screw and pilot jet changes. As you mention, too rich may cure itself when the weather is hot or you get up in the mountains.


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  • Mark_Cantrell

Posted April 27, 2002 - 05:52 PM


Whoa, and I was stone cold stupid when I wrote that last paragraph. Just rechecking TT before bed and the comment about hot should be 'and it will just get worse' about hotter and higher. Many appologies.


  • Taffy

Posted April 28, 2002 - 12:38 PM



you're starting to stumble
you're running rich which means you're gonna run out a gas soon :D (i hope)


you're not offering answers to the poor man you're offering Q's :D :)

yamaha 426. tell us what year and what jetting you already run. the rael answer is tocopy hick's jetting. he knows what he's doing.


  • Mark_Cantrell

Posted April 28, 2002 - 03:26 PM



I defer to Taffy. If you still have problem after his suggestion (unlikely) remember what I was talking about. I only brought it up because of the transient part, i.e., it only happened during or after adding throttle. Taffy has one of the popular mods for reducing the AP if it does come to that.

Good luck,

  • yamaha426

Posted April 29, 2002 - 06:32 AM


Well, I went with my gut feeling on this and turned the fuel screw in about 1/2 a turn and it made it a whole lot better. But as for what I am running it is a 2002 with the stock jetting, I don't have my manual right now so I can't tell you exactly what it is.

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