2000 yz426f wont start but backfires through carb


24 replies to this topic
  • ct44

Posted November 19, 2011 - 01:03 PM

#1

Picked up this bike for a couple hundred bucks, first 4 stroke. Guy swears it would start after adjusting valves, carb, and oil. He has FMF pipe, new top end, new jets, new spark plug, coil, and has gone through the transmission.

After adjusting valves. cleaning carb, adjusting floats, rebuilding carb(bad o rings). Learning starting procedure, learnig bumb starting procedure i have

Fuel
Air(clean filter with oil)
new Plugs with spark(good crisp blue spark, and gapped)
Can find tdc and can get it to backfire through the carb?????
This happens when bumb starting and kickstarting.
I changed to race fuel, checked all the connections. checked valves again. two outside are tight but are good clearance.
Compression is good but i am not sure he rebuilt entire top end. i can still push it though the cyclel but its not easy or fast.
I took it by a shop and they told me to forget the decompression lever all together(contradicting all I have read)
still backfires through carb, oh and no throttle what so ever. with and without choke. if i use throttle it floods.
What would cause the backfire through the carb,
as i have cleaned, checked, and changed now it actually blows the carb off the boot to the engine.

could he have put the lower case together wrong or switched some electrical part i am just missing? I would think tdc, chain, cams all mechanical is in line. but why the spark on the intake stroke?

Please help!!!!

  • gscx

Posted November 19, 2011 - 01:18 PM

#2

You need the decomp lever to start these things. The shop might think its a hot start? Did you check the timing when you checked the valves?

  • ct44

Posted November 19, 2011 - 01:28 PM

#3

yes, he had it all put together but timing was way out. I checked it again with the valves for fear i had checked it withought the tensioner in last time. possibly a bent intake valve??

  • RiderX

Posted November 19, 2011 - 01:41 PM

#4

Sounds timing related. I would make sure the cams line up at TDC. If they do then make sure the TDC mark is actually TDC. Maybe the woodruff key sheered off and the flywheel moved?

  • ct44

Posted November 19, 2011 - 01:51 PM

#5

Ok, cams line up according to manual(10 and 2). and piston is at tdc(via screwdriver). could anything else casue the timing to advance? when it backfires there is no kickback on the kickstart either. I may end up tearing the entire top end apart to verify what he did and didn't do. oh just noticed my spark plug is sparking 2 times per kick??? or is this just a long spark and i am mistaking it for two.

  • bonehead7101

Posted November 19, 2011 - 02:25 PM

#6

+1 on the woodruff key. Either that or valve is stuck open. You may be seeing 2 sparks, I believe there is a low speed and high speed source coil in these for the spark make sure they are not shorted

  • ct44

Posted November 19, 2011 - 02:42 PM

#7

ok so if i am advanced 1 on woodruff key how could i verify and repair, well i have compression and decompression so i was guessin valves are in working order, the clearances havent changed the past two times since i placed new shimms in as well.

  • ct44

Posted November 19, 2011 - 03:54 PM

#8

Also update on spark, problem fixed. cleaned and checked all connections from coil back and down to single spark again. thanks. tearing motor back down to check the woodruff key

  • ct44

Posted November 19, 2011 - 05:03 PM

#9

ok as i have skimmed throught the Clymer manual and searched the webb for the Woodruff key i guess i am having trouble with the name or as to where i should be looking. any help please?

  • RiderX

Posted November 19, 2011 - 05:31 PM

#10

ok as i have skimmed throught the Clymer manual and searched the webb for the Woodruff key i guess i am having trouble with the name or as to where i should be looking. any help please?


We were referring to the key that aligns the flywheel. If you verified that the TDC mark is right at the point where the piston reaches the top of the travel I wouldn't worry about it. Did you notice anything unusual when adjusting the valves? Did you have to use large shims on any of the valves?

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • ct44

Posted November 19, 2011 - 05:48 PM

#11

no,i changed only three of them and it was only one size larger on two intake and one exauhst. the two intake were the tightest of the three and were only out of specs by one size on the feeler gauge. cant remember the specifics, but i know i debated on changing them but figured they would only get further out of adjustment. i don't have any slack in the timing chain but could the chain be worn possibly. i was guessing it wouldn't be advancing on the timing and would be retarding it. but I am deffinately up for sugestions and or corrections. thanks

  • ct44

Posted November 19, 2011 - 06:13 PM

#12

I stand corrected. When first placing bike together i checked tdc with a screwdriver. upon disasembly this last time i checked again. I was almost an 1/8 of a turn out. Deffinately the "woodruff key" greatly appriciated. also noticed when taking apart none of the nuts in the lower motor have locktite and some were only fingertight. thinking i should have torn into this thing as soon as i got it. any other recomendations that i take a look at before getting ahead of myself again.

  • bonehead7101

Posted November 19, 2011 - 09:02 PM

#13

Pull left hand timing cover, pull rotor (puller is required), there should be a small metal block stuck in either the grove of the shaft or the grove on the inside of the rotor if it is z shaped its bad. It should be a rectangle or half moon shape. It will make sense when you take it apart. Be careful whither the threads on the end of the crank shaft, do not strike them what so ever or you will be replacing a crank shaft. The stator and pickup coils can be tested without taking the cover off at the plug under the tank, read the manual should just be an ohms test

  • ct44

Posted November 20, 2011 - 09:33 AM

#14

Thanks, I am down to the rotor and waiting on ebay for the puller. pretty sure i follow throught the book now. The help is amazing, greatly appreciated!

  • RiderX

Posted November 20, 2011 - 10:36 AM

#15

Thanks, I am down to the rotor and waiting on ebay for the puller. pretty sure i follow throught the book now. The help is amazing, greatly appreciated!


I think if you take the nut off and look down the shaft you can see if the key-ways are aligned.

  • grayracer513

Posted November 21, 2011 - 08:20 AM

#16

We were referring to the key that aligns the flywheel. If you verified that the TDC mark is right at the point where the piston reaches the top of the travel I wouldn't worry about it.

I would.

The key that locates the flywheel on the crankshaft is not important to valve timing at all when the valve timing is done as it was here, by probing the piston for TDC. It is, however, critical to IGNITION timing, which the symptoms pointed to right off the bat. The keys don't shear very often at all, but they aren't that hard to push out of the keyway in the crank if the flywheel is misaligned during assembly. Then, since the CDI gets its information about when to trigger the spark from the position of the flywheel relative to the stator, the spark occurs in the wrong place.

  • bonehead7101

Posted November 22, 2011 - 03:08 PM

#17

I would.

The key that locates the flywheel on the crankshaft is not important to valve timing at all when the valve timing is done as it was here, by probing the piston for TDC. It is, however, critical to IGNITION timing, which the symptoms pointed to right off the bat. The keys don't shear very often at all, but they aren't that hard to push out of the keyway in the crank if the flywheel is misaligned during assembly. Then, since the CDI gets its information about when to trigger the spark from the position of the flywheel relative to the stator, the spark occurs in the wrong place.


I have to agree with gray, the only time I have seen one sheered is when the rotor doesent get installed properly or doesnt get tightened down. but i have seen the stator plate not tightened down or installed improperly and cause timing issues

  • RiderX

Posted November 22, 2011 - 03:09 PM

#18

I would.

The key that locates the flywheel on the crankshaft is not important to valve timing at all when the valve timing is done as it was here, by probing the piston for TDC. It is, however, critical to IGNITION timing, which the symptoms pointed to right off the bat. The keys don't shear very often at all, but they aren't that hard to push out of the keyway in the crank if the flywheel is misaligned during assembly. Then, since the CDI gets its information about when to trigger the spark from the position of the flywheel relative to the stator, the spark occurs in the wrong place.


I guess what I was trying to say is that if the flywheel had rotated you would be able to see that the TDC mark on the flywheel would not line up with the physical TDC of the piston, right?

  • grayracer513

Posted November 22, 2011 - 07:20 PM

#19

I guess what I was trying to say is that if the flywheel had rotated you would be able to see that the TDC mark on the flywheel would not line up with the physical TDC of the piston, right?

If you look for that, sure, it's easy to spot.

  • RiderX

Posted November 22, 2011 - 10:02 PM

#20

If you look for that, sure, it's easy to spot.


That's what I was getting at. If the piston's actual TDC did not correspond with the TDC mark on the flywheel, then bingo.





Related Content

Wiki
WR Camshaft Swap Info - last post by jamesm113

WR Camshaft Swap Info


Articles
  • 0 replies
Forums
Photo

Michigan Motocross Tires by 288yz450


Dirt Bike   Dirt Bike Regional Discussion   North
  • 1 reply
Forums
Photo

YZ450F 03 Sparks driving me crazy by SirAttard


Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 5 replies
Forums
Photo

2016 YZ450 by CaptainKnobby


Dirt Bike   Dirt Bike Technical Forums   Suspension
  • Hot  59 replies
Forums
Photo

Chubby dad, looking at bikes , First trip to the orange/black forum ! by Slow_ride


Dirt Bike   Make / Model Specific   KTM   250-530 EXC/MXC/SXC/XC-W/XCR-W (4-Strokes)
  • Hot  28 replies
 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.