2004 YZ450F will not start

12 replies to this topic
  • aligshow

Posted August 28, 2012 - 08:27 AM


I have a 2004 yz450f and i cann't get it to kick start at all. installed a new je piston about a month ago start four different time in garage and ran great.
1) compression.
je piston 13.5 to 1 only a month old
the valve clearance is good .20 to .25 for intake and .10 for exhaust
timing is prefect the "I" is lined up below and the marks on the cams line up with [color=darkgreen][size=1]top[/size][/color] of the heads the other marks are around 12 o'clock.
new coil and spark plug. did a spark gap test .misfired around .30
did an ohms test and every thing is with spec
cdi test good and sparks at the right time.
3) Gas
i only vp [color=darkgreen][size=1]fuel[/size][/color]
carb is clean
have good vac from the engine

any ideas? please help

  • grayracer513

Posted August 28, 2012 - 08:45 AM


On timing, the marks "around 12:00 o'clock" are meaningless from a practical standpoint. They can be handy as an assembly aid, but can't be used to set timing. The first thing is to verify that the flywheel is indexed where it belongs. Pull the plug and probe for the top of the piston with a tool such as a Phillips screwdriver, that can't be dropped into the cylinder. If the "I" on the flywheel agrees with the piston about where TDC is, proceed.

The engine should be rotated forward to TDC, and at that point, the intake cam should have an "E" at ~9:00 o'clock, a punch mark near 12, and, importantly, an "I", the actual timing mark, should be lined up with the gasket surface on the top of the head. The exhaust will have two punch marks, one near 12, and another at near 9:00 o'clock, again, lined up with the head surface. The marks almost never line up perfectly, but the question is whether moving them one way or other will make them line up better or worse.

The most common cause of hard starting apart from the user just doing it wrong is the carb. It is possible for the inside of it to look clean while the pilot jet is wholly or partially clogged. A drop of dry varnish only a couple thousandths thick can cut the jet size in half, and you won't see it. Read:


Check the fuel flow into the carb, and verify the float level. Also check the starter jet. Then, too, understand that the bike usually will like to be primed before cold starting, so after verifying that the accelerator pump works, try twisting the throttle from 1-3 times prior to a cold start (never when hot). None of my 450's would cold start unless this was done.

One last thing to check in the carb is the condition and position of the throttle vacuum release plate on the engine side of the carb. It can be installed upside down, and looks like it should be to most people, but it's supposed to be in with the "square" side that has a hole near the edge down. Check the plate for cracks or missing corners, too.

  • aligshow

Posted August 31, 2012 - 01:35 PM


thank you for the reply
installed all new jets in the carb. -all stock jetting
replaced the cdi, stator, coil, and spark plug.- all stock parts
and it still will not start!!

any idea before i take it the dealership and it over charged!!!

  • grayracer513

Posted August 31, 2012 - 02:09 PM


You do not mention whether you addressed the issues raised in the first two paragraphs.

  • aligshow

Posted August 31, 2012 - 03:32 PM


sorry! i did double check the timing and it was spot on. the "I" and TDC line up.and the cam are all in the right spots!

  • aligshow

Posted September 26, 2012 - 09:02 AM


I replace all five vavles and had the seats cut . head is like brand new. ran a compression test and i have no compression at all. any ideas?

could it be the new piston ? maybe a ring folded over ?

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  • grayracer513

Posted September 26, 2012 - 02:05 PM


Valve clearances or cam timing would be most likely. Clearances will change with new valves and seats.

Another common mistake is to have an adjustment shim jammed in a tilted position in one of the valve spring retainers instead of sitting down where it belongs.

  • aligshow

Posted September 27, 2012 - 07:41 PM


What are the symptoms of leaking valves or bad seat . Would cause the bike not to start, because there is not enough compression?

  • grayracer513

Posted September 28, 2012 - 06:57 AM


If a valve doesn't seat, compression will be from low to non-existent, depending on how big the leak at the seat is. The engine depends on there being at least a little compression to start and run, and remember that in the auto decompression bikes, the compression ratio when starting has already been cut from the normal operating level of 12.5:1 down to about 7 or 8:1 by the decompression system. Much more of a leak and they just can't get interested in running.

If you have access to a leak down tester, that will pinpoint the exact source of the leak for you, but you will often be able to hear a distinct hissing sound during the compression stroke while cranking from either the exhaust pipe or through the open carburetor when a valve leaks.

Pull the cam cover back off and take a look, if you haven't already. May be as simple as the tilted shim I mentioned. If noting's wrong there, get a leak down test done on it.

  • aligshow

Posted October 19, 2012 - 08:08 PM


So I replaced the all the valve seats with pro circuit copper seat . All new titanium valves values and high rev spring . Basic the whole head is brand new. Every part on the top end is new. I did a compression test and only got 30 psi . I did get the bike to start a few times, but it was a lot of work . Any ideas on why I'm not getting more compression. It hold the 30 psi in the cylinder for a long time Valve clearance is good .

  • aligshow

Posted October 20, 2012 - 01:55 PM


Its running. Thank you for all the help.

  • grayracer513

Posted October 21, 2012 - 07:49 AM


I know you already got it running, but a standard compression test is not really very valid when the engine is equipped with automatic decompression. Even if it did not have such a device on board, compression tests are easily influenced by several typical things, among them the point in the engine's cycle at which you start, the speed at which you crank the engine, and whether the throttle is open or not. The highest readings will be had by testing with a good fast kick through the complete compression upstroke with the throttle opened. Testing like that with a YZ450 should probably produce something on the order of 100-120 pounds, but that's just a guess.

Leak down testing is much more definitive.

Glad to hear that it's running.

  • Kawabuggy

Posted October 21, 2012 - 09:32 AM


As mentioned above, if you are only getting about 30PSI on compression stroke then something is seriously amiss. Did you replace the rings on the piston as well? Put them on in the right order?

Here is what I would do:
1. Find TDC on the COMPRESSION stroke. Remember, the piston is at TDC two times-once for compression, and once for exhaust. Be sure you are on compression stroke.
2. Put the trans in first gear then lock the rear tire in position so that it cannot rotate. You can do this by stepping on the back brake during the test. You must do this because when you apply compressed air to the cylinder, the air will want to push the piston back down in the cylinder & open up the valves. Lock the piston at TDC.
3. Remove spark plug.
4. Remove the crank case oil fill/check plug.
5. Now using a rubber tipped air nozzle, apply compressed air into the spark plug hole. Have a friend with you so that as you are applying the air to the cylinder, and holding the rear brake, he can check the exhaust for air coming out, and check the crankcase fill plug for air coming out, and also remove the air filter and check for air coming back through the carb/injection system. You know those incense sticks they sell at some gas stations? Get one of those and hold it in front of the exhaust tip, carb/injector, and crankcase oil dipstick/fill plug and the smoke will show you if any air is moving. You could also use matches after blowing them out but the incense stick works better.

Now, once you find where the air leak is, here is what you do next:
1. If air coming out of crankcase-your rings/piston/cylinder are NOT sealing and you will have to remove the head to figure out where the problem is.
2. If air coming out of carb/injector then your cam timing is off, your valves are out of adjustment or not sealing.
3. If air coming out of exhaust then your cam timing is off, your valves are out of adjustment or not sealing.
Lastly, if your head gasket is not sealing, and this is a water cooled engine, remove the radiator cap and watch for small bubbles to start appearing in the coolant. If you see the bubbles while you are applying air to the cylinder, then the head gasket/sealing rings are not sealing. Remember, it may take quite a bit of air into the cylinder before you start seeing bubbles so just keep the air going in the cylinder until you can identify where the leak is.

With only 30PSI on a compression gauge, you have a pretty big leak and it should be easy to isolate. Further, if you have a friend that has a "smoke machine" such as we use when working on cars (I'm a mechanic) you can substitute smoke for compressed air into the cylinder and it will quickly show you where the leak is UNLESS it is leaking into the coolant at which point you will need compressed air to see the bubbles. Or, if you suspect the head gasket is leaking, put a pressure tester on the radiator and apply pressure and watch the gauge. If the cooling system won't hold pressure, the head gasket may be leaking and allowing the coolant to fill the cylinder. Please post back and let us know what you find.

One other thing, before you do all of this, put the bike on the ground, put it in second gear, let the clutch out and push the bike like you are bump starting it. You are not really trying to bump start it you are just checking to see if the compression really is that low. If it is low, the bike will move easily without the rear tire locking up. If the rear tire locks up and just drags, try a different compression gauge to see if the first one is giving you a false reading.

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