yz450f big bore kit


33 replies to this topic
  • kylerf150

Posted July 09, 2009 - 04:54 PM

#1

hey everybody i recently put a athena big bore kit in my yz450f and i cant get it to run at all.:banana: is there a way to check compression to see if everything is ok with the piston and ring or what do you suggest i do i am a stopping point i am getting sick of kicking at this thing.:worthy: any suggestions would be awsome!

  • haydug

Posted July 09, 2009 - 05:48 PM

#2

Compression gauge works fine. Does it have spark? Is fuel getting to the plug? Cams in time correctly?

  • kylerf150

Posted July 09, 2009 - 07:46 PM

#3

thanks for the fast reply. it is getting a spark.I have checked and double checked the timing but when i kick it just has a small pop and then nothing. I checked the compression and i was getting about 55 psi without the carb on.does that sound ok means the ring hasn't seated yet or am i wrong.would it start if i use a small squirt of starting fluid?:worthy: thanks again for the reply and any help is really appreciated!!!!

hey do you think changing my needle clip position would help.

  • 426 NOOB

Posted July 09, 2009 - 08:48 PM

#4

thanks for the fast reply. it is getting a spark.I have checked and double checked the timing but when i kick it just has a small pop and then nothing. I checked the compression and i was getting about 55 psi without the carb on.does that sound ok means the ring hasn't seated yet or am i wrong.would it start if i use a small squirt of starting fluid?:worthy: thanks again for the reply and any help is really appreciated!!!!

hey do you think changing my needle clip position would help.


To me the pop means its either lean (not enough fuel) or the timing is off. Since you say timing is correct then I would try playing with the a/f mixture screw and or maybe a pilot or main jet up. Try a/f mixture first and turning it out is richer. Also a compression gauge is sorta useless for an auto decompression bike. I would not worry about the reading its getting. A leakdown test is 100% more reliable.

  • kylerf150

Posted July 10, 2009 - 09:26 AM

#5

thanks 426 NOOB i will try turning out the a/f screw and maybe play with the jetting. its hard getting any advice here where i live the shops aren't very friendly and dont want to help you unless you give them a little of you hard earned cash.i will play with it and see what happens. what jet kit do you recommend me to buy a dynojet,jd jet kit ect. and have you ever used the zip ty fuel mixture screw are they worth the money. thanks for your reply i really do appreciate it!!

  • grayracer513

Posted July 10, 2009 - 09:46 AM

#6

To me the pop means its either lean (not enough fuel) or the timing is off.

You haven't learned this by now?

Excessive decel popping is an indication of either lean or very rich idle mixture, and/or an air leak in the exhaust system. Some decel popping is completely normal for a big YZF.

If you take an engine to the point that it never pops even a little, it will be too rich. It may appear to run "OK", but you'll often have trouble hot starting it, and your plug life won't be great.

If you have an engine that has a leaking exhaust causing a pop, you may never be able to jet your way out of it, and if you do, it won't run right at all.

http://www.thumperta...599#post3710599
http://www.thumperta...613#post3350613

  • kylerf150

Posted July 10, 2009 - 09:00 PM

#7

thanks grayracer513 i'll double check the exhaust tomorrow

  • kylerf150

Posted July 12, 2009 - 02:27 PM

#8

well here's an update. the exhaust is all tight and i can't find any leaks.I have played with the a/f screw and i still cant get the thing to start im thinking of maybe sending the carb into zipty for a accel pump mod. what do you think. Or should i check the valve clearances. right now im just getting a loud backfire about every 5 kicks. i have some feeler gauges but i would like to get some metric 45 degree ones to make it a little easier. what brand of feeler gauge do you guys have good luck with?

  • yoshgixxer

Posted July 12, 2009 - 06:33 PM

#9

i think gray read the post wrong..what he said is 100% correct but has nuthing to do with the question you were asking...he is a damn genious with thumpers so if he re-read it he might have a better answere for you!

you said:

.I have checked and double checked the timing but when i kick it just has a small pop and then nothing


so i take it as...you built the motor and now all you get is a " pop"?

let me ask you a question...did the bike run before you took it apart?

if it did run before the top end kit then id look into what you just re-built and less into carb jetting and leaky exhasut...the jetting cant be that far of that it wont even start just by using a big bore kit...it really sounds like cam timing...but maybe you damaged the coil? ?Bad plug? even if its new...maybe cracked the ceramic on the plug putting it in...

You said it did have spark...so you can also put some gas in a spray bottle and try squirting some into the carb with it open...then kick it...see if you get anything...also is the float stuck closed?

  • grayracer513

Posted July 12, 2009 - 09:21 PM

#10

You may want to check that you did not dislodge the flywheel key and mis-index the flywheel. Find true TDC using a screwdriver down throught he plug hole and compare that with indicated TDC on the flywheel.

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  • kylerf150

Posted July 13, 2009 - 01:33 PM

#11

hey guys
the reason why i rebuilt the motor is becuase the rod got seized to the crank 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.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. also how can i tell if the float valve is stuck

  • 426 NOOB

Posted July 13, 2009 - 07:00 PM

#12

hey guys
the reason why i rebuilt the motor is becuase the rod got seized to the crank 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.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. also how can i tell if the float valve is stuck


if the float is stuck you will have fuel running out the overflow tubes on to the ground. (Usually) Unless its stuck closed of course.

  • kylerf150

Posted July 13, 2009 - 09:41 PM

#13

yeah, no fuel pouring out. i think i will have to take it to the local shop and see what they think. and have them do a leak down test on it to see if everything is ok with the rings. the piston only has one compression ring on it so maybe the compression is leaking by i have no idea what is wrong with this thing! :worthy:

  • grayracer513

Posted July 14, 2009 - 08:08 AM

#14

Float needles can stick shut, too, although it's fairly rare. My CR500 used to do that regularly when I first got it. Open the bowl drain (3mm Allen head at the bottom of the bowl on the left side) by loosening the screw one turn or so. Fuel should run out one of the lines under the engine. If not, try rapping the side of the bowl smartly with a screwdriver handle.

Your problem is, if in fact your timing is correct, almost certainly in the carb, probably a lacquered pilot jet.

  • kylerf150

Posted July 14, 2009 - 09:28 AM

#15

ok gray thanks i'll look into it. i really appreciate you guyS helping me i have learned alot about how things work on this site it has been very useful.
THANKS GUYS!!!!

  • mjw1

Posted July 14, 2009 - 03:26 PM

#16

Have you look to see if your pilot jet is plugged it will gum up when the bike sits for a long time. That is what it sounds like to me. The only way to clean the pilot jet is to remove it and blow compressed air through it.

  • kylerf150

Posted July 14, 2009 - 05:08 PM

#17

mjw1 i have looked to see if any jets were plugged i can see through all holes with a flashlight.Thanks for your reply though.

  • grayracer513

Posted July 14, 2009 - 05:30 PM

#18

Did you check the cross holes in the pilot jet? The air jets in the mouth of the carb? Accelerator pump timing/squirt?

  • 642MX

Posted July 14, 2009 - 05:59 PM

#19

If your certain the timing is correct and your carb is perfectly clean, you might try to turn the pilot screw out about 3 or 4 turns and then kick it. I put big bore kits in both our bikes and the stock pilots where too small on both bikes.

If it runs with the screw 3-4 turns out, it means you need a larger pilot, so just turn out the screw for testing purposes.

Also, its not uncommon to foul a plug.... just because it pops (and shows some spark) doesn't mean it will start the bike.

  • kylerf150

Posted July 15, 2009 - 10:13 AM

#20

Did you check the cross holes in the pilot jet? The air jets in the mouth of the carb? Accelerator pump timing/squirt?


gray i did check the cross holes in the pilot jet and i removed all of the other jets nothing was plugged that i could see. for the accl. pump timing/squirt i havent really messed with it but it just misses the slide when i give it throttle

i will try to screw the pilot screw out to 3 or 4 turns.

thanks guys this is a great site!!!:worthy:





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