02 WR426F Stumbles & Hesitates

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02 WR426F Stumbles & Hesitates

Post by 4Strokes » Sun Jun 05, 2016 8:41 am

Topic: My prison is a WR426F carburettor
Author: dr.taco
Posted: 02/10/2006 4:32:07 PM

It seems that carburetion woes are common to the WR426Fs. Mine stumbles and hesitates at even throttle only, at 1/4 to 3/4 throttle openings. The plug insulator is light, almost white. I have spent a good deal of time trying to isolate the problem to no avail. Any suggestions would be greatly appreciated!

Here's the scenario: 2002 WR426F, 6000' elevation, 45-55 degree f. air temp. Best jetting so far: 165 main; "R" needle (stock), 4th position; 48 pilot; "16" accelerator pump diaphragm (leaner); 7 mm float height (1 mm higher fuel level than stock).

Going by the stock settings, this should be rich for 6000' elevation, and yet the plug is white and I get popping on deceleration, all indicating lean mixture. Yet moving the needle position to 5 only makes the problem worse. 3 is worse also. What the heck is going on here? Thanks in advance for the info -Adam

Reply by Wild Bill on 02/12/2006 7:40:18 PM
I had a similar problem on my '02 YZ426F. It was an air leak at the neoprene sealing ring between the carburetor body and the rear bell to the air box boot. The vacuum in these bikes can suck that o-ring out of its groove if the screws aren't tight and everything in its place. Drove me crazy for 2 weeks before I disassembled that piece and found the o-ring stretched out of its groove.

White plug and popping even with the choke on. Check all the boots, front and back or the carburetor, and their clamps for seating and tightness. Then look for pin hole punctures or tears in the air box boot. Best of luck, this is a pretty common thing to happen. Taught me a lesson on bike cleaning and preparation being done right, every time. Bill

Reply by dr.taco on 02/15/2006 7:04:12 PM
Thanks, Wild Bill. I will be sure to double check everything for air leaks. I have disassembled it several times looking for holes or tears but have found none. I also tried spraying carb cleaner all around the air box at idle, but I could not detect a change in engine speed. Oh well, I'll keep at it. AK

Reply by Wild Bill on 02/16/2006 10:44:50 AM
Doc Tac, Check the neoprene o-ring at the bell on the intake side of the carburetor. That bell is attached by three screws. The vacuum can unseat the o-ring and cause a leak. Bill

Reply by dr.taco on 02/19/2006 1:11:48 PM
W. Bill, The neoprene seal is okay. But just for your own info: I finally discovered the problem -- it was related to ignition. The Throttle Position Sensor (TPS) connected to the carburetor does some strange things. Several people recommended disconnecting it. I tried it and *WHAMO* the problem was completely gone.

Reply by Wild Bill on 02/20/2006 7:44:42 PM
Doc Tac, I have heard of DTX guys doing this. If it worked and you are happy with the performance it is all good. Glad to hear you got things sorted out. First time I have heard of a TPS going bad, never can tell. Bill

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