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03 YZ450 _ New Top End - Can't Start

By Kevin_S,
in YZ400F/426F/450F/450FX

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Kevin_S

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I just replaced the top end myself. Carb. Took slide out and cleaned. Wouldn't start.

Double Check cam Timing - Good.

Spark - can hear clicks. See spark in dark, seems weak but compared to other bike, ok

Spark Plug comes out wet after a kicking spree.

Carb off - check slide - Black side of Slide upside down. Noticed seal warped in several spots. Corrected slide to be rightside up.

All back together, no start.

Plug wet.

New Plug.

No Start.

Take Coil off an test primary coil - low on ohm meter. Take to shop with mechanic with good rep, he says the book is wrong. .05 is ok even though the book say .08-.1.

Secondary coil ohm test no discrepency with book.

Is that seal really important enough to not allow it to start if it is warped but looks like it will seal?

Or is this beast just really hard to start after a rebuild. I did use a fair amount of oil on the top end.. I wonder if that could be it.

Any help would be appreciated.

Kevin

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Frostbite

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When you say you replaced the top end, what exactly did you do? Head off, new rings, barrel.....

If you have spark and tried a new plug, and have compression and your cam timing is correct I'd look at the carb. A few turns of the idle knob and my WR won't start. I've run into a similiar problem and ended up backing the idle knob off until it completely clears the throttle mechanism and then try to start, turn in 1 turn, try again, turn in one turn until it started. It may be something more complicated but eliminate the simple stuff first.

Another thing. I just bored my engine and installed a new plug. I ran it for 4 hours and a few days later when I tried to start it there was no way. It seemed electrical so I pulled the plug to check for spark. Same as you, it seemed weak but I stuck another brand new plug in the cap and the spark looked the same in the dark. I tore the bike apart until I ran out of options and then stuck the 2nd new plug in and it fired first kick. Plugs can be funny, your new one may be defective.

I think the rubber O ring on the throttle slide plate is more of a vibration damper than a sealing gasket, I don't think that would cause the engine not to fire.

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Kevin_S

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I replaced the piston / rings / pin and had the cylinder inspected / honed at the local yama shop.

I tried 2 new plugs.

I am also leaning towards the carb thing. I will try the idle screw thing to see if that helps.

What do you think about the oil residue on the top of the cylinder? Could that be an issue. It is just the layer of oil that the rings pushed up during the install.

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Frostbite

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What do you think about the oil residue on the top of the cylinder? Could that be an issue. It is just the layer of oil that the rings pushed up during the install.

I loaded mine with oil during the rebuild and it started 2nd kick. It burned blue for a few minutes and then cleared up. Unless there is a puddle of oil on top of the piston or in the valve cutouts I don't think it could vaporize with the air fuel mixture fast enough to foul the plug while attempting to start. Once running, the heat and flow frees up the oil and allows it to mix and burn off.

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Run-Run131

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Sorry this is my first post... but I just changed the top end on my '04 this past weekend, and I had a no-start issue that took me about a day to finally figure out...maybe I can help? Does the bike seem to kick over a little too easily (compared to before)...lack of compression?

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Jetsprint2

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Sorry this is my first post... but I just changed the top end on my '04 this past weekend, and I had a no-start issue that took me about a day to finally figure out...maybe I can help? Does the bike seem to kick over a little too easily (compared to before)...lack of compression?

How much wear did you find in your 04', I am curious because I have the same bike. Do race/trail ride how hard are you on your bike. Did it blow up or seize? My bike has low hours but I like to hear about other 04's that have more time on them than mine.

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Frostbite

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Sorry this is my first post... but I just changed the top end on my '04 this past weekend, and I had a no-start issue that took me about a day to finally figure out...maybe I can help? Does the bike seem to kick over a little too easily (compared to before)...lack of compression?

What caused your problem?

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Run-Run131

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I've had the bike for about a year...bought it new in March '04. I ride about three times a week...once a week at Racetown395 (pretty hard riding) and about twice a week in the desert behind my property (mostly cruising). Just figured it was about time for some of the bigger preventative maintenance items. Everything looked pretty good...cylinder looked great...slight (normal) wear on piston skirt, valves were still within spec. limits (although a little toward the tight end). It's been a great bike. The only real issue I've had is the kick-axle stopper breaking...twice! First time, bike was still under warranty (dealer fix). But, it broke again two weeks later...not much of a fix! Did the work myself after that, and it's been fine since. Again, I think it's a great bike...just trying to keep it running strong.

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Run-Run131

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The middle intake valve shim slid out of its groove and slipped toward the side of the bucket. I actually found this tip on an older post here in TT. I suppose it may have slid out when I was cleaning the head up a bit. Anyway, I checked all of the valves, and it was the only one that looked off-center. I snapped it back into place, put the cams, etc. back together, and the bike fired up in three kicks! Before this, I assumed it was timing... I reset the timing about three different ways...12 pins, 13 pins, 14 pins...nothing worked! I've never been one for chat rooms, etc, but ThumperTalk has made a believer out of me!

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Run-Run131

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Correction...the top end article with Gary Jones is in Feb. '05 issue of Dirt Bike. I wasn't trying to imply this is the reason for the no-start problem that began this thread; I was just trying to support your post with yet another source that recommends not to hone these cylinders. Sorry if this led the topic astray or caused any confusion.

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Kevin_S

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Thanks for the help. I am going to try all of the suggestions; starting with the valve issue as I did move the head around quite abit. While there I will check the air screw on the carb to make sure the oring is seated properly. If I still have trouble, then I will try the idle screw thing. And, if none of that works, I will re-think everything another 30 frigin times until it starts.

The honing process was very light, just enough to break the glaze, and it was done by my local yamaha shop mechanic who has a good rep. He also did it to my neighbors YZ250F at the same time and his bike runs like a champ.

Kevin

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Kevin_S

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?

Run-Run 131 - Problem Repeated. Thanks, Problem solved.

Exactly as Run-Run 131. The middle intake valve spacer was not in its correct place. As a result, the middle intake valve was constantly open which created a compression issue.

After spending a couple of hours checking it out. The bike is now running strong. New Motor, Carb cleaned several times, life is good.

Thanks again for the help and solution.

Kevin

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