Rebuild Issues


9 replies to this topic
  • YZ450F4

Posted March 06, 2011 - 06:15 PM

#1

today i put my bike back together with new valves, cylinder, and piston. put it in the bike and it will not run. it rarely even acts like it wants to hit. help please:bonk:

  • Smokeslider

Posted March 06, 2011 - 06:22 PM

#2

I would start with checking everything over to include valve timing and carb settings. then double check the little stuff like the rag you stuck in the airbox and things like that.

  • YZ450F4

Posted March 06, 2011 - 06:33 PM

#3

ive been trying to research and i read something about a flywheel key? could this be it? i did take the flywheel off the put a new timing chain in it also

  • Mr. Neutron

Posted March 06, 2011 - 06:48 PM

#4

I would start with checking everything over to include valve timing and carb settings. then double check the little stuff like the rag you stuck in the airbox and things like that.


Hey!!!! I wasn't the one that rebuilt his engine!!! :bonk: (guess who's done that before?)

YZ450F4,

Chad above here gave some good advice. :thumbsup: I will usually fall back on my "Motors only need spark, fuel, & compression" deal, and try to cover each one of those individually when a motor won't start. But I do modify it somewhat.... If the bike was running alright before I worked on it, I will normally go right back to whatever I just worked on, and see what I screwed up this time.... :busted:

Have you checked for spark? That's an easy thing to look into, so I will check that first in a "no-start" situation. If you're getting a good blue spark at the plug, I would look into fuel next, most likely.

Did you do any carb work while you had the bike apart? I'm going on the assumption here that most everyone here is a better wrench than I am, but we all mess up from time to time. (I'm still trying to figure out an off idle bog I "fixed" into my carburetor when I took it apart for cleaning a few weeks ago.... :lol:) Is it possible something might have gotten done to the carb that needs "tweaking"??? You can pull your subframe/airbox off, and whack your throttle open a couple of times, while looking for the squirt from the accelerator pump. (It's a good time to look for that rag Chad mentioned, as well.... How he knew I did that is a mystery to me.... :smirk: ) If it's squirting a decent, miniature Super Soaker stream, you're definitely getting fuel. Is your pilot screw adjusted right? Mine gets really hard to start when the weather is cold, and it's not screwed out enough (2 & 1/2 turns seems to work for the cooler temps we had last week)...... Double check that all your carb settings are good....

Finally, like Chad said, double check that all your things like cam timing, valve clearances, and critical things like that are all good. You'll have covered your "All a motor needs to run is spark at the right time, the right fuel mixture, and compression" triangle by now, and hopefully, you figured it out, and got it lit off .... Please keep us posted!

Jimmie

  • Mr. Neutron

Posted March 06, 2011 - 06:51 PM

#5

ive been trying to research and i read something about a flywheel key? could this be it? i did take the flywheel off the put a new timing chain in it also


I had a flywheel key shear on one of my bikes when I was younger, and it most definitely caused a no-start deal. Pretty easy thing to check for. Lay your bike over on the rt. side (so you don't lose any oil when you remove the ignition side cover), remove the left side case, pull the flywheel. That should let ya know if that's good or not.....

Jimmie

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • YZ450F4

Posted March 06, 2011 - 07:03 PM

#6

thanks for the info guys. yes the bike does have a stream of fuel going into the motor from the carb and it did run until it was tore apart. it also has a new plug. il recheck my timing for a 4th time lol but im pretty sure its on right. Some people with similar problems i have read, one said he kicked the hell outta the bike and it finally fired and one guy said he roll started it and its been good ever since...i try and find it tomorrow or the next day. im going to check timing, fly wheel key, and spark. it has good compession. all i can get it to do now is backfire rarely so i think its getting spark.
thanks for the help. really appreciate it

  • Aka.Goose

Posted March 06, 2011 - 07:03 PM

#7

ive been trying to research and i read something about a flywheel key? could this be it? i did take the flywheel off the put a new timing chain in it also

Sure could...The woodruff key is a very small half moon chunk of metal that can be easily lost when taking off the flywheel...It just sits there freely in a little groove...You did put this back onto the shaft and align it with the groove in the flywheel, yes? (part #20 in this 07 parts diagram: http://shop.thumpert...007&model=12915 )
My other guess would be that you did not set the timing correctly...

EDIT: oops, sorry typed that out while you guys posted the last few comments...

  • YZ450F4

Posted March 07, 2011 - 03:47 PM

#8

looked at it again today. its timmed right, has spark, has compression and squirts fuel into the head from carb. fuel runs outta the overflow quite a bit while starting it but plug doesnt get fauled? i guess it has a clogged pilot jet or something or its the flywheel key. my dad swears he put it in but im not sure of that. anyone else have any suggestions? tryed to roll start it and it ran for about 3 sec. then choked out

  • YZ450F4

Posted March 07, 2011 - 03:57 PM

#9

could the cam caps be too tight? they are torqued to 14 foot pounds

  • grayracer513

Posted March 07, 2011 - 04:28 PM

#10

could the cam caps be too tight? they are torqued to 14 foot pounds

Tell me you're just kidding :thumbsup:

The manual clearly says 7.2 ft/pound (which I think is higher than necessary), and frankly, I don't believe you can torque them to 14 without the bolts or threads failing. If you did this, I will almost guaranty you've distorted the caps to the point where they will seize.

Regarding your timing, if the Woodruff key is out of place, it can appear to be timed when it isn't. If you suspect this, check the indicated TDC mark with the actual position of the piston by probing down the plug hole with a screwdriver. If the piston is at TDC when the marks say it is, then the key is probably OK.

If the engine was apart for a while, the pilot jet may be clogged with varnish, causing the starting problem.





Related Content

Reviews

Yamaha YZ450F 2017 by Chris.GVS


Yamaha YZ450F 2017
  • - - - - -
  • 0 reviews
Forums
Photo

YZ450F 03 Sparks driving me crazy by SirAttard


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

Is it worth it? (Cam and High compression piston) by macgi77


Dirt Bike   Make / Model Specific   Honda   CRF 150/230 F/L
  • Hot  36 replies
Forums
Photo

Megabomb Fitment by 288yz450


Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply
Forums
Photo
Motocross

Thinking about Yamaha 250... by Arctic Pride


Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  491 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.