yz450f big bore kit


33 replies to this topic
  • Ray450

Posted July 15, 2009 - 01:52 PM

#21

hey guys
the reason why i rebuilt the motor is becuase the rod got seized to the crank

How did you not need to rebuild the low end then? And my guess would be to not worry about fine tuning yet, even a poorly jetted bike should start and run. What caused the original failure, it very well may be whats still wrong

the stock piston is still in good shape i just wanted to try a big bore i didn't rebuild the bottom end the only thing i have touched is the top end. as for it being on true tdc i looked down the plug hole with the valve cover off and could see the piston reach the top and right when it started going back down i stoped moving the motor and looked to see where the mark was and it all looks ok.

Do like Gray said, use a poor man's dial indicator (screwdriver) and find TDC, then take a picture of the cams and the flywheel and post them here, we can tell you for sure if it's all right.

The bike sat for about a year not running but all of the jets look clean and are not plugged i think i might look more into the valve clearances to see if they are in spec.

This would be very easy to check, and could be the problem. Were you carefull to use the exact same pads that were on each valve. If you had enough heat/wear to seize the crank, theres a good chance you also have valve issues

also how can i tell if the float valve is stuck


If you are seeing the A/C pump squirting every twist of the throttle, then you are getting gas. You can lean it over and some should start running out the overflow as well, but it sounds like your getting gas. I'd be willing to bet that whatever caused your original seizure, is still causing you problems, or you do not have the timing correct. Do not do any further mods or jetting/tuning until you know why it doesn't run.

  • mjw1

Posted July 15, 2009 - 02:42 PM

#22

unplug the tps (throttle positioning sensor) and see if it will fire up my tps on my bike went out a few years ago i unplugged it and started right up still have it uplugged to this day with no problems.

  • kylerf150

Posted July 17, 2009 - 12:45 PM

#23

hey guys
the reason why i rebuilt the motor is becuase the rod got seized to the crank...... i didn't rebuild the bottom end the only thing i have touched is the top end.


sorry for the misunderstanding i didn't rebuild the bottom end my self i had a shop do the repair because i didn't have the tools or know how. and the reason why it seized is becuase i was at wot racing my buddy and well i guess it didn't really like that to much :worthy:

thanks again for all of your help i just went back to work for 2 weeks so when i get back home i will try unplugging tps. thanks again guys :banana:

  • mxmaddman

Posted July 17, 2009 - 09:33 PM

#24

i was getting about 55 psi without the carb on

You should be closer to 80 psi.
Make sure the shims are correct in the buckets. If you mixed them up its not going to run. Either the timings off or your shims aren't in correct.

  • mjw1

Posted July 18, 2009 - 09:55 AM

#25

Check to see if one of the shims moved to the side of the valve if it is not centered it will hold the valve open a little that happened with my brothers 450.

  • Jeepentj

Posted August 23, 2009 - 03:50 PM

#26

Did you end up identifying the problem? What was it?

  • kylerf150

Posted August 24, 2009 - 01:04 PM

#27

sorry guys i have not been able to touch the bike the past month i have been working non stop right now im only getting about 4 days at home and im on the road again. when it slows down i will start working on my little problem sitting in the garage again. i will post on here when i start working on it again sorry guys.:ride:

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  • kylerf150

Posted April 09, 2010 - 06:24 PM

#28

Well eight months later i have started to work on my little problem.I took the head and cylinder all back off to make sure every thing was ok there and it all looked good to me but that doesnt say much.The ring end gap on my new piston and cylinder is right at .012 and according to my oem manual it says the max is .022 so i think that my gap is ok. Im thinking of puttin the cams on the head with the head off of the motor and to see if the shims are all in the right place is that the right way to see if they are all working correctly i know i never messed with them but you never know what happens when your not around.i really want to get this thing going, summer is coming up fast.

  • Powderhound

Posted April 10, 2010 - 05:22 AM

#29

I put a big bore in a crf 450 that I used to have. The first time that I started it I had to kick until I was blue in the face and then kick some more. The bike would not start worth a crap. I remember after a while I got it to start and then after going out and seating the rings the bike started OK after that. I couldn't get mine to run very well until I leaned out the jetting. I don't remember off the top of my head how much I went because that was a few years ago. The bigger cyl adds displacement which creates a larger vac signal through the carb causing the engine to draw more fuel through the existing jets.

  • Powderhound

Posted April 10, 2010 - 05:43 AM

#30

Well eight months later i have started to work on my little problem.I took the head and cylinder all back off to make sure every thing was ok there and it all looked good to me but that doesnt say much.The ring end gap on my new piston and cylinder is right at .012 and according to my oem manual it says the max is .022 so i think that my gap is ok. Im thinking of puttin the cams on the head with the head off of the motor and to see if the shims are all in the right place is that the right way to see if they are all working correctly i know i never messed with them but you never know what happens when your not around.i really want to get this thing going, summer is coming up fast.



I don't think I would trust that you have the proper valve clearances unless the head is torqued down on the cyl. I'm guessing the rings just need to seat and the low compression is what's making it nasty to start. Does the compression ring have a proper orientation (if it has markings on it, like an X then that goes up). Did you measure ring end gap at both the top and bottom of the pistons travel in the cyl? New parts can be bad and new cyl can be out of round.

  • NJDirtdog1

Posted April 10, 2010 - 05:59 AM

#31

Yeah i would go for checking the cam timing if anything, because if it is off it wont start. But if you figure all that out check your clearances, i would recommend buying the motion pro tappet feelers. I dont know what year your yz is but i have an 03 and the intake side clearance cold is .10mm- .15mm and the exhaust is .20mm- .25mm and i know they have feelers that are perfect for that clearance range, they are small and prebent very easy to use. Oh and i would not recommend a dyno jet kit. The guys on here no more than me but i would say go with JD kit if anything i here good reviews. Good Luck.

  • kylerf150

Posted April 10, 2010 - 06:29 PM

#32

thanks guys im going to have the local dealer adjust the clearances and go from there i hope that they are the problem and i can get this thing running. if i cant get this thing going in the next couple of weeks ill be going back to the stock setup.

  • teamtoxic

Posted April 11, 2010 - 06:34 PM

#33

I was hearing similar "pops" the other day while trying to start my bike after a valve adjustment. Turns out I had installed my exhaust cam just one tooth off its correct timing position. After fixing the problem, it was one kick to fire. Double check your work.

  • thaoe_thumper

Posted August 08, 2016 - 02:12 AM

#34

Ok I just bought an 06 YZ450FI live in Lake Tahoe so elev. does play with tuning. That being said, The guy i got bike  from said has new top end so i thought great right!           WRONG. I get the bike home (by the way guy somehow had bike running i got there to buy it go figure) and went to start it and same kida problem as your having, i would kick the shit out of it only to get a back fire every third or fourth kick.

So i went through everything i wont even begin to bore with the details, just listen to me and re-check you your cam timing. find yourself a pic of same  bike at TDC                       "top dead center" with the valve cover off it   (hopefully the pic is taken from the rt. side of the motor) and see if your intake and exhaust  cam lobes are in approx. the same position. like with my bike according to the pic i found online  my exhaust was at the 3 O'clock ish position and my intake was at 9 O'clock ish @ TDC. Mine was not like the pic come to find out that my Exhaust cam gear slipped a couple of degrees on the cam shaft. So after a new set of Hot Cams My bike started on the 1st kick.







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