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Top end rebuild = no compression... race in 4 days!!


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Just finished up my 07 250f top end, went to kick it, and there's no compression, and fuel pours out of the drains. I know I'm going to have to tear it back down, but in the meantime I'm hoping it may be something simple that I overlooked.... any help?

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I would verify your valve clearances. NOT that your valves have moved, but sometimes, the shim under the lifter bucket can get upset and make a valve be tight.

If this is the case, you will see that a valve has no clearance. You will have to pull the camshaft(s) to correct the problem.

For your flooding carb, yeah, that's likely a gummed up needle & seat. Before you remove the carb (which is a pain on alum framed YZFs)- Try tapping lightly on the side with the plastic handle of a screwdriver. Or, you can spray some carbon cleaner outboard stuff down thru the fuel inlet. Sometimes, if you fill it with fresh gas and shut off the tap and then wait a day, it will get better by itself.

Hope this helps.

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I'm going to tear back into it, I hope it's something simple like a shim that had moved or maybe I set the timing just slightly off.

The bike ran beautifully 2 weeks before I tore it down, I doubt the carb gummed up in that short of time. Could it just be that because there is no compression, the fuel is not getting drawn into the cylinder and therefore is filling up in the bowl and running out the vents?

Also - I did use quite a bit of that stickly moly lube on the cams, but I was careful not ot get it onto the decomp system, and I did oil the decomp system with motor oil, per the manual.

The only shim changes I made were one size smaller in all the intakes, as clearances had tightened up somewhat. The valves were right in the middle of spec after the rebuild.

I am really confused on this one... this is not my first top end rebuild, but the first on this particular bike.

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Okay, quick update. I took it back down, and found the middle intake cam shim was displaced. I placed it into it's recess, and made sure it was good. Replaced everything - still no real compression.

This is what the cam sprockets and flywheel timing mark look like at TDC:

yztop.jpg

yztop2.jpg

Now I'm wondering about something.... is it possible that I have everything lined up at TDC on the intake stroke, and not the exhaust? Should I bring the timing mark on the flywheel to where it should be when the cam sprockets are 180 degrees from where there should be, and then move them to the proper place? It's very possible it got switched around when I did the rebuild, especially since I replaced the cam chain. Would this cause the symptoms I'm experiencing?

Oh - the decomp system is not stuck, and is working just fine.

Any ideas? I'm struggling here guys, and getting panicked. ­čśĆ

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Okay, quick update. I took it back down, and found the middle intake cam shim was displaced. I placed it into it's recess, and made sure it was good. Replaced everything - still no real compression.

This is what the cam sprockets and flywheel timing mark look like at TDC:

yztop.jpg

yztop2.jpg

Now I'm wondering about something.... is it possible that I have everything lined up at TDC on the intake stroke, and not the exhaust? Should I bring the timing mark on the flywheel to where it should be when the cam sprockets are 180 degrees from where there should be, and then move them to the proper place? It's very possible it got switched around when I did the rebuild, especially since I replaced the cam chain. Would this cause the symptoms I'm experiencing?

Oh - the decomp system is not stuck, and is working just fine.

Any ideas? I'm struggling here guys, and getting panicked. ­čśĆ

These bikes fire on the compression and exhaust stroke, so no you shouldn't have to advance everything one rotation.

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The carb may have gotten a piece of dirt stuck on the needle or seat; spraying carb cleaner in the fuel inlet should clear it as suggested above.

If cam timing is correct, and the valves are closing all the way, then the valves are not mating to the valve seats correctly. Is there any compression what so ever, or does it feel like the spark plug has been pulled out?

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Try this- hold the auto decompression lobe out and kick it slowly. You should have compression like never before.

If still no compression, then get someone to listen while you kick- have them listen in the air box and then listen in the exhaust and also listen if the air seems to be huffing out the crankcase.

There is only 4 places for the air to go- exh valves, int valves, past the rings, or thru the exhaust gasket.

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Okay, compression is back. ­čśĆ The middle intake valve shim had gotten displaced, but I kept looking for answers because it didn't feel like it had compression with a wrench on the crankshaft... I didn't realize how much mechanical advantage there is when turning the crank from there. Anyways all is good on that end, as compression is definetely back.

However it would only run for a brief few seconds and then it died - something is definetely up with the carb. As soon as I turn the fuel on, it starts to run out of the drain. Something must have gotten lodged in there.

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Completely cleaned the carb. No more overflowing, but stil no starting either.

I'm really really getting frustrated. I have never had this much trouble with a bike in my life.

I guess I'm going to check timing..... again.

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Alright, that's not it. Cleaned the carb, put it back together, still nothing.

Plug is wet, and I'm getting spark.

Took anoter look at the top end, and the timing is perfect. The indexing marks on the cam sprockets are perfectly in line with the head surface, and the rotor mark is spot on.

Here's the only thing left I can think may have something to do with it:

When I first checked it before stripping it down, the ex valves were right in the middle of spec, but the intakes had tightened to .09mm. Manual calls for .1mm - .15mm. The OEM shims were 185, 189, and 191 read from left to right. Replacements are only available in 05 and 00 numbers, so I installed 180, 185, and 185 from left to right. Like and idiot, I just went by the chart in the manual and never re-checked clearances. Well, I just did, and they're all just slightly too big, at .166mm, except for the middle valve, which seems as though it may still be too tight, but I am also having a hell of a time getting a feeler under the lobe without it dragging on the cam holder, so that may be part of it.

Is this enough to prevent it from starting?

If it is, what do I do at this point? The dealer only wanted to trade shims, so my OEM ones are gone, and they were too big anyhow. The only shims I have been able to find were the 00 and 05 ending numbers as I mentioned before....

What the hell now?

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Well, color me confused as hell but happy.

On a tip from another buddy, I turned the fuel screw from 4.5 turns out (that's how it's been since last year and has always started and ran beautifully) to 2 turns out and went to town.

I kicked it about 10 times, throttle closed, choke on. It popped a few times, leading me to think I was onto something. I kicked it a few more times with the choke closed, but nothing.

In a fit of desperation, I kicked it with the hot start lever opened, and viola! It roared to life, held idle, and took throttle. No weird noises, and i seems to be running great. I varied the idle and let it run for about 4 minutes, then shut it down so I can let it cool and double check shit like coolant level, etc.

I don't have any clue *** happened, but I'm just happy it started. Maybe all that kicing I've been doing flooded the cyl?

Hopefully it starts again on Sunday for the races....

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What happened is the bike was flooded........ You had a wet plug which was the first indication it was flooded. Basically all your kicking and adjusting before had cause the bike to become flooded, so with the choke off and the hot start lever open you gave it some nice fresh air and leaned the mixture so it could actually premote ignition....Now your second issue.....

Now that the bike is running, you need to address jetting, Pilot was too lean if you had to have the fuel screw out 4-1/2 turns. screw should be between 1 and 3 turns, anything more or less you need to fatten it or lean it. With the fuel screw out 4-1/2 turns you run the risk of the screw backing out and falling out of the carb....

If the bike runs best with the screw out 4-1/2 turns, pull the pilot and put the next step up pilot in and adjust the screw to 1-3/4 turns out to start then adjust to fine tune...

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Yeah, I hear ya on the flooding, but wouldn't all that kicking I've been doing cleared it out at some point?

As for the fuel screw, the kid I bought it from actually said something about having a special spring or some other setup that allowed it to be that far out without being in danger of falling out. I can't remember exactly, but you're right for sure - 4.5 turns out on the fuel screw means the pilot is too lean.

I'm just wondering why the previous owner had it set up this way, everything ese he had done to it was pretty spot-on mechanically.

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