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Help! 2006 yz250f wont start

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hi I have a 2006 yz250f and im ready to bash my head on the wall until I black out

 

over the winter I took the motor out, installed a wiseco 13.5:1 wiseco high compression piston, a new wiseco cam chain.

I also had 5 brand new titanium valves, valve springs, guides and seats installed at Machine racing. when I rebuilt it I honed the cylinder for a new mating surface and assured all torque specs were followed and rings were on the right way.

 

now that its back together it will not start... ive taken the carb apart multiple times, cleaned it out and have triple checked EVERYTHING..

 

The timmg is exactly aligned with the T.D.C mark on the flywheel and the marks on the cams,  all the shims are perfectly in spec so no valves are staying open.. or at the wrong time or anything!

 

im getting crazy amount of spark with my new NGK iridium spark plug so the wirering must be fine?

 

the plug is getting fuel, when I kick it, and kick it.. and kick it.. then pop the spark plug out its wet.

 

there is no blockage in the air flow/ exhaust passages anywhere.

 

any old gas in the tank and carb have been flushed and I have fresh 94 octane premium fuel with added octane boost for 7 extra digits ending up at 102 octane for the high compression.. and I have also tried the 110 octane racing fuel my local shell offers.. both don't work

 

I did not mess with the throttle positioning sensor or anything that way that could cause anything..

 

any ideas on why my bike wont start would be appreciated.. it wont even show a sign! barely the odd back fire or sputter after 30-40 kick or long stretches of attempted bum starting

 

 

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I assume it ran before you rebuilt it?  Have you done a leakdown test?

 I read a post a while back where the bike got a spark with the plug connector/coil was laying on the engine but it wouldn't run.  Turned out the coil had a small crack that leaked spark when it was down in the hole.  You might be able to wrap some rubber around it to insulate it.  

I would also check the wiring harness connector to the coil, see if it's wet or corroded.  

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yes it ran before I did the rebuild! used to start first kick.. i didn't do a leakdown test because i had just assumed being my head just got back from machine racing with brand new valves and springs there shouldn't be an issue? plus you can hear and feel the compression..

 

when i had the wireing harness off i inspected and cleaned the hole thing.. it didn't have any noticeable cracks, tears or corrosion anywhere from the coil, cdi or anything.. and i cleaned all the conections with electrical contact cleaner followed by a bit of dielectric greese.

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I noticed when I did my last rebuild, I didn't get the idle adjustment back to the same position. 

Too wide or closed and it will not start reliably.  I got mine in the ball park by holding it up to the light and opening the idle adjustment until I could see some light under the slide.  There is a spec for it, about 1/64 to 1/32 should get you close.

 

Also, high octane fuel is notorious for bad starting.  The thing that makes it good running in a hc engine is slow burning, which is really bad for starting, especially in cold weather.

 

Last, if the spark plug is gas fowled, it will not start.  I'd put a new plug in it.

good luck

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If you had the flywheel off, verify TDC (top dead center) by probing down the spark plug hole to the piston.  If the marks don't agree with the piston, you've dislodged the Woodruff key on the flywheel and mis-indexed it. 

 

The carb set out off the bike all the time you worked on it, and the pilot jets are notorious for closing up when this happens.  Open the carb up and double check the pilot jet.  A #40 jet has a metering orifice only .40mm in diameter (.016").  If a film of dried gas no more than .001" thick should form in that jet, it will effectively be a #35, and way too small to make the bike want to run. 

 

Read:

 

http://www.thumpertalk.com/topic/681801-bike-wont-start-dont-know-what-to-do/#entry6879695

 

Also check to see if any varnish has formed around the end of the fuel screw, and that the O-ring on the screw is in good shape.  If you had the slide out, be sure you have it in with the release plate on the engine side right side up ("square" edge down).

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If you had the flywheel off, verify TDC (top dead center) by probing down the spark plug hole to the piston.  If the marks don't agree with the piston, you've dislodged the Woodruff key on the flywheel and mis-indexed it. 

 

The carb set out off the bike all the time you worked on it, and the pilot jets are notorious for closing up when this happens.  Open the carb up and double check the pilot jet.  A #40 jet has a metering orifice only .40mm in diameter (.016").  If a film of dried gas no more than .001" thick should form in that jet, it will effectively be a #35, and way too small to make the bike want to run. 

 

Read:

 

http://www.thumpertalk.com/topic/681801-bike-wont-start-dont-know-what-to-do/#entry6879695

 

Also check to see if any varnish has formed around the end of the fuel screw, and that the O-ring on the screw is in good shape.  If you had the slide out, be sure you have it in with the release plate on the engine side right side up ("square" edge down).

Thanks for the advice! this is very usefull ;) I will defenetly look in to this! but why is the plug wet if its a carb issue? and why wouldn't the bike show a sign of starting when I directly put a bit of gas in the cylinder directly??

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"..........but why is the plug wet if its a carb issue? and why wouldn't the bike show a sign of starting when I directly put a bit of gas in the cylinder directly??

 

When you put the carb back on, you may not have put the cables back in the same relative position, and you could be accidently applying throttle / accelerator pump is wetting the plug;

or the float could be too high, or the high octane fuel is preventing a start.  Put lower octane fuel in for the first start and work from there after its running and adjusted.

I think you may see some posts from guys going high compression and actually having to go leaner on jetting. 

Did you get the fuel screw back in with the same number of turns out or did you just go to 2 turns expecting to have to adjust it?

 

Pull the plug and put a new on in.  Then for the next try, use starting fluid into the airbox, and have the seat off.  Just spray some into the airbox filter area and kick away.  Some bikes are finiky on the first start after a rebuild.  Once you get it fired up and running, be ready to adjust throttle position, idle screw,  and choke. 
 

Edited by ronbuell
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... the high octane fuel is preventing a start.  Put lower octane fuel in for the first start

This is not the problem.  Bad gas could very well be your problem, but high octane fuels are no less easily ignited than any other gasolines.  Octane number is not a measure of how easily the fuel burns.

 

Four strokes, especially high performance ones, are very fussy about the mixture on start up.  With the choke on, your carb delivers fuel from the starter jet and the pilot jet, assuming the throttle is closed.  If the pilot is clogged, it won't start, but the fuel from the starter jet can wet the plug.  When you prime the engine with raw gas, consider how much goes down the hatch when you work the accelerator pump, and that while the bike pro0bably does want one or two good squirts to get it going, four may flood it and require some air from a tipped open throttle or the hot start to clear.  A teaspoon's worth amounts to a gross flood, and will drown the thing.

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Thanks for all the help and sugestions guys! I went ahead and replaced the pilot jet wich was smaller then it should of been! but I finally figured out that it was the flywheel! for some reason the keyway was off by about 1/8 and an inch! im not really sure how the flywheel went on if it wasn't in the keyway properly but once I corrected it my timming was a tooth off so I corrected the timming to the new and proper flywheel position and she started right up!

 

BUT after about 5 minutes of running and a couple easy lasp of the yard the bike got really lound with ticking noise in the top end.. after I got the new valve job, to reshim to spec I was using the thinnest shims my hotcms kit has wich I found odd since it should be the opposite? because as you wear when you do your ajustments you put smaller pucks as the gap closes.. could it be that the valves have somehow seated and now I have too shim thicker? because that's what it sound like!

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Tilted shim, maybe.  If you don't insert the shim in the retainer squarely, it can get stuck in the pocket and not settle all the way in, only to snap down the rest of the way later on.  One possibility.  Check your valve clearance.

 

Valve clearance closes up as the valve OR the seat wears.  If you have fairly serious wear on the seat, a lot of extra metal will have to be cut out of them to get them back to where they are concentric and squared up right.  That will mean you need a smaller shim.  

 

What size did you end up needing?

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Tilted shim, maybe.  If you don't insert the shim in the retainer squarely, it can get stuck in the pocket and not settle all the way in, only to snap down the rest of the way later on.  One possibility.  Check your valve clearance.

 

Valve clearance closes up as the valve OR the seat wears.  If you have fairly serious wear on the seat, a lot of extra metal will have to be cut out of them to get them back to where they are concentric and squared up right.  That will mean you need a smaller shim.  

 

What size did you end up needing?

I just checked my valve clearences and there all still in spec... I had to use 1.20 on the exaust and I belive it was 1.35 on the intakes. im running 94 fuel with octane boost is it possible im getting pre ignition? the knocking started after I cracked the throttle a little bit for the second time..? the bike starts second kick but imidiate start to knock at idle. the cam chain is tight and the cams are fine.

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