2001 YZ 426F/ Troubleshooting a weak spark/ possible carb issue

2 replies to this topic
  • el caballo

Posted January 02, 2015 - 03:35 PM


Hello guys and gals, first post hear and here it goes.


I own a 2001 yz 426. The last ride I was on the bike kind of sputtered when it started cold. It didn't really rev up on the choke like it usually does. I gave it a little throttle to help it get warm. Once it was warm choke off it idled fine and off I wen't. About 10 miles in (riding in Gorman) I pulled off trail to take a break. The bike was warm like it sometimes gets pushing hard. There was some fluid coming off the overflow tube. No problem gave it a few min checked the radiator. It was almost full. It took about a cup full from the back pack. Bike started fine, ran it another 20 miles or so..........ok so fast forward.


A couple weeks later I tried to go riding. Bike wouldn't start. I kicked for about 30 min on and off and knew something wasn't right. Loaded up and wen't home. The bike sounded like it wanted to but almost like the kill switch was being pushed.......... 


First I tried a new plug and checked for spark. It has spark but In my opinion it looks weak. My mates are better mechanics then me and I get a whole range of answers on that "if it sparks you have spark" and so on. But I've seen posts and manual comments to "weak spark". For now I move on to the basics. Fuel, air, spark and off I go troubleshooting by the book........I tried kicking it with a new plug with no luck.


1. Air- Changed filter wiped out box.


2. Fuel-

    The bowl had fuel in it so I assume its not the carb. It was rebuilt the off season before. After looking past the carb I pulled it and rebuilt it with a moose rebuild kit. All my        jets are stock and they all matched the kit. I cleaned the carb thoroughly avoidin and rubber bits like the needle seal do hicky. Installed everything just how it was. 4th            ring on the needle, pilot was at 2 turns out, float looked good no holes and so on. Checked the fuel tank, emptied and cleaned it and put new fuel in. It's worth noting the      carb looked good inside. The air jets had some dirt in them. Fuel looked a touch dark but hard to tell because I use stabil on occasion. Tried to start bike and no luck.


    I never touched the TPS sensor while cleaning the carb.


3.Spark- ( I have one)

   So off we go troubleshooting the whole system by the book. Even though I have a (weak) spark. The coil primary,secondary, neutral switch, stator on and on it all comes      up in spec, even the kill switch passes the test with a multi-meter.Things I'm left with are.....


   A. Bad CDI?


   B. Stator and or flywheel? I haven't laid eyes on them but they come up good under the manual specs.


   C. Spark plug boot? Question.... Can the boot be tested with a multi-meter? The book doesn't show this. My resistor showed 10 ohms. I put it back in the boot and                     checked the whole boot. Still got 10 ohms. Am I missing something?


   D. Kill switch? Question....If I unplug it will the engine run without it? If so I'll check it manually. Just to be clear when I checked it to the switch for continuity the multi-meter        doesn't make any noise until I push the kill switch button.


   E. TPS unit?


4. Engine-

    I removed all fluids checked for water in oil and oil in water. All came up well. Threw on some high performance radiators, changed oil and filter. Also checked valve              clearances while it was tore down. All valves were in spec except for one on the intake side. While setting the piston to TDC I checked to see if the cams were off                  indicating a timing chain may have jumped a tooth. That wasn't the case, the cams lined up perfect. So I shimmed it into spec. All looks good. Piston still has strong              compression. Motor turns over freely while compression lever is pulled.


So I put it all back together fingers crossed and no luck. She still sounds like shes being starved or chocked off while trying to start. Plug boots are 100.00, CDI 450.00, Stators are 300.00. I'm kind of at a dead end without just buying all new stuff but damn........


Any thoughts from any mechanics out there would be greatly appreciated. I've ordered a BikeMaster Ignition Spark Tester to check for strength of spark. I've read 10mm is the bench mark


Happy New Year!

Thanks again!

Edited by el caballo, January 02, 2015 - 03:40 PM.

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  • el caballo

Posted January 02, 2015 - 05:52 PM




So I unscrewed the spark plug boot, screwed the plug directly into the plug wire and I'm getting about the same spark that I get with the boot on.


Then I popped my new spark plug out to look at it, it's bone dry. So now I'm onto figuring out why no fuel is getting to the spark plug.


My main question would be after rebuilding a carb what could be left to check to solve this issue?

  • el caballo

Posted January 17, 2015 - 11:38 PM


Ok so bike runs like a champ. Turns out the timing chain was way stretched out and appeared to have jumped a tooth. Timing chain replacement directions were found on another site posted here... http://www.thumperta.../#entry12150021



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