03 yz450f won't kick start, hesitates on throttle


23 replies to this topic
  • misterwaterfallin

Posted May 23, 2011 - 11:35 AM

#1

Update: New pilot jet, valves all set within tolerance, same problems

The bike is a 2003 450f
Full DRD exhaust
drilled airbox
Boyesen quick shot
Stock everything else engine wise

It won't kick start to save its life, and will only start with a jump. It revs and idles ok, but when I try to ride it it has a bad hesitation even on constant throttle. Kind of jerky and pops a bit. I checked timing, replaced the plug, cleaned the carb, new fuel/oil for good measure.
I checked the valves and the clearances on the intake side are very tight, but in my experience its usually a bad sign if they are loose and not too tight.

Any pointers on where to go from here? Im going to try replacing the pilot jet in the carb so see if its clogged or something. I don't want to pull the head if I don't have to but would like to ckeck for bent valves just in case.

Edited by misterwaterfallin, May 29, 2011 - 02:28 PM.


  • BlueMagiC

Posted May 23, 2011 - 02:34 PM

#2

How tight is "very tight" ?

  • misterwaterfallin

Posted May 23, 2011 - 03:06 PM

#3

How tight is "very tight" ?


spec is .10mm, I couldnt slip a .05mm (smallest I had) through. That didnt really make sense to me as bent valves usually stick open so I don't think that's the problem, a valve that is.

  • grayracer513

Posted May 23, 2011 - 03:09 PM

#4

I checked the valves and the clearances on the intake side are very tight, but in my experience its usually a bad sign if they are loose and not too tight.

Your experience is about to go down a new path. They get tighter with normal wear, not looser.

The first thing you need to do is to get the intake valves (and the exhausts for that matter) properly clearanced. If it takes more than two shim sizes smaller to get them right, you need new valves.

  • misterwaterfallin

Posted May 23, 2011 - 03:24 PM

#5

Your experience is about to go down a new path. They get tighter with normal wear, not looser.

The first thing you need to do is to get the intake valves (and the exhausts for that matter) properly clearanced. If it takes more than two shim sizes smaller to get them right, you need new valves.


Thank you for that, that's what I was looking for.
Sense I have been wrong up until this point I might as well ask, would the symptoms point towards having valves out of spec? I ask because I just want to get this thing running right so if I think I need new jets I might as well just get them ordered now rather than later

  • grayracer513

Posted May 23, 2011 - 04:04 PM

#6

Thank you for that, that's what I was looking for.
Sense I have been wrong up until this point I might as well ask, would the symptoms point towards having valves out of spec? I ask because I just want to get this thing running right so if I think I need new jets I might as well just get them ordered now rather than later


This symptom does:

It won't kick start to save its life, and will only start with a jump.


Until you get the valve clearance taken care of, there's no point in trying to address the rest of it. There may well be other problems, but the valves being tight can cause a lot of things, and overlap with other problems, making them hard to sort out.

  • misterwaterfallin

Posted May 29, 2011 - 02:29 PM

#7

Alright it looks like I am back where I started
Put in a new Pilot Jet
Set intake/exhaust valves

It still wont kick, and still hesitates on throttle. I am at a loss, I think it's running lean but that wouldnt make it not kick start would it?

  • grayracer513

Posted May 29, 2011 - 02:41 PM

#8

I think it's running lean but that wouldnt make it not kick start would it?

Assuming the engine is mechanically sound, having the correct idle mixture is critical to good starting.

Get it running and see if you can dial the mixture in better:

http://www.thumperfa.../jetting.htm#PJ

http://www.thumperta...ad.php?t=327405

Other things to check are the vacuum release plate on the slide (square edge down, no cracks or chips) and be sure the O-ring is on the pilot screw.

  • misterwaterfallin

Posted May 29, 2011 - 06:52 PM

#9

Assuming the engine is mechanically sound, having the correct idle mixture is critical to good starting.

Get it running and see if you can dial the mixture in better:

http://www.thumperfa.../jetting.htm#PJ

http://www.thumperta...ad.php?t=327405

Other things to check are the vacuum release plate on the slide (square edge down, no cracks or chips) and be sure the O-ring is on the pilot screw.


i double checked the o ring on the pilot screw after buying new one. where is the vacuum release?

  • grayracer513

Posted May 29, 2011 - 10:40 PM

#10

On the carb slide.

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  • misterwaterfallin

Posted May 30, 2011 - 12:34 PM

#11

On the carb slide.


just checked that

one point my dad brought up when I talked to him was spark timing. Is there any way to adjust that? I set the cam timing in relation to TDC, but what about spark?

  • grayracer513

Posted May 30, 2011 - 06:39 PM

#12

Spark timing can't be adjusted, which means it can only be wrong if there's a fault of some kind.

Check TDC at the piston against the mark on the flywheel to be sure the flywheel is indexed right.

  • misterwaterfallin

Posted June 03, 2011 - 06:50 AM

#13

Went up a pilot size to 45 and still won't kick start
Is there anyway to verify timing ie the cams in relation to the compression or power stroke? I know they are timed at TDC but there are 2 TDC's and only spark on one of them correct?

  • grayracer513

Posted June 03, 2011 - 08:27 AM

#14

Is there anyway to verify timing ie the cams in relation to the compression or power stroke? I know they are timed at TDC but there are 2 TDC's and only spark on one of them correct?

Think about that if the bike runs at all...

And no, that's incorrect. There is a spark on every revolution of the crank. Installing the cams is what makes one stroke the compression and the other exhaust.

  • misterwaterfallin

Posted June 03, 2011 - 08:47 AM

#15

OK that's what I thought. So I am still dumbfounded. Up another pilot size then possibly?

  • grayracer513

Posted June 03, 2011 - 08:57 AM

#16

Have you checked the accelerator pump squirt and timing?

When starting, have you tried the trick of cracking the throttle slightly open?

Have you verified that your hot start closes?

  • misterwaterfallin

Posted June 03, 2011 - 09:06 AM

#17

Have you checked the accelerator pump squirt and timing?

When starting, have you tried the trick of cracking the throttle slightly open?

Have you verified that your hot start closes?


verified the acc pump. I have tried to start with the throttle slightly open with no luck. How do I verify the hot start?

  • grayracer513

Posted June 03, 2011 - 09:50 AM

#18

How do I verify the hot start?


If it appears to work, cold start the bike, and while the choke is on, operate the hot start. It should work a bit like a thumb throttle, with a definite change in rpm between open and closed.

  • misterwaterfallin

Posted June 04, 2011 - 12:22 PM

#19

If it appears to work, cold start the bike, and while the choke is on, operate the hot start. It should work a bit like a thumb throttle, with a definite change in rpm between open and closed.


the rpm's raised when I pulled it in so I believe its working. It runs better with no hesitation and less popping with the new needle, but still wont kick over. I have no clue what the problem is

  • yz450fcranker

Posted June 05, 2011 - 08:49 AM

#20

you might have a bad engine or a clogged carburetor main jets maybe or an electrical problem start with cleaning the carb to near new check the plug cap get a good look inside the engine for flying pieces of metal





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