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KX 125 won't start after top end rebuild

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So I'm sure this question has been asked several times before, but I'm new here and I'm stumped. Just trying to see if anyone had any good suggestions to look at.

So I just recently rebuilt the top end. New piston, rings, gaskets, and jug. Replaced all electrical, so it's getting spark plus I checked and it's definitely sparking. I've cleaned the carb. Checked all jets and they're fine. Checked reeds they look brand new and in perfect shape. I even torqued the bolts down to specs 18 ft lbs. Good compression.. right around 120 psi. New flywheel and the woodruff key is still intact... but I did notice that the gasket kit I order came with 2 different gaskets for the bottom part of jug. They were the same gasket but slightly different... so I don't know if that makes a difference.. so if anyone has any ideas or suggestions feel free to shoot them my way. Thanks

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I was planning to try that tomorrow.. but I'm positive I didn't do the rebuild wrong. I even took the top end apart earlier to check and make sure I put the piston on correctly and I did. Arrow facing toward exhaust. I do have a slight thought that fuel may be not getting to the cylinder to ignite. Took the spark plug out and it's dry. And so was the cylinder when I took it apart. But I can't figure out what the issue would be with the carb. I checked everything but everything looks good.

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My bike had that problem too try and put carb cleaner with your gas it's a special bottle and it goes in your gas but put the gas straight into your engine where your spark plug goes then your bike will pull the gas it needs and your bike should be running great but bike should start on first kick with the gas in the spark plug bay

My bike had that problem too try and put carb cleaner with your gas it's a special bottle and it goes in your gas but put the gas straight into your engine where your spark plug goes then your bike will pull the gas it needs and your bike should be running great but bike should start on first kick with the gas in the spark plug bay

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My bike had that problem too try and put carb cleaner with your gas it's a special bottle and it goes in your gas but put the gas straight into your engine where your spark plug goes then your bike will pull the gas it needs and your bike should be running great but bike should start on first kick with the gas in the spark plug bay

My bike had that problem too try and put carb cleaner with your gas it's a special bottle and it goes in your gas but put the gas straight into your engine where your spark plug goes then your bike will pull the gas it needs and your bike should be running great but bike should start on first kick with the gas in the spark plug bay

Sorry, but your advice isnt accurate at all.

1. Never put unnecessary additives in your fuel. Not necessary.

2. Your advice would only be a band aid for the problem because its assuming the bike will "fix itself".

Seeing as though its fuel delivery issue, trace it back from your petcock to your carb. Is the fuel line kinked or clogged? Take off your float bowl to see if its got fuel in it. Then go from there and let me know what you find.

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The petcock and fuel line is good. I already checked that so it's getting fuel to the carb. I don't think it's getting in the bowl though.. I did notice one of the wires that goes to the fuel cut valve was a little bare. I'm going to fix that tomorrow and see what happens.. I was reading the manual and it stated to check the cut valve with a 12v battery and when the valve actuates to measure how far the rod sticks out. So idk.. I've never messed with a carb that has a fuel cut valve but it seems like to me it could be the issue. Correct me if I'm wrong.

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The fuel cut valve is part of the powerjet assembly. Its kind of confusing the way it is in themanual (mine anyway).To test it you need a 12 volt battery. Remove powerjet valve from the carbConnect 12 volt battery to the powerjet electrical connector Measure valve rod length (from the edge of the power jet valve body, minus the threads, to the tip of valve rod).correct length is 21.3-21.7 mm or 0.839-0.854 in.disconnect battery and measure again. correct length is 19.4-19.6 mm or 0.764-0.772 in.

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Could also very well be your power valve. When you took off your cylinder, did you happen to bend the vertical actuating rod when disconnecting the assembly?

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The actuating rod is fine. I made sure I didn't damage it when taking it apart. This isn't my first rebuild but first encounter with this problem. Would the power jet valve possibly be the cause of the issue though? And yea it says the same thing in my manual. Doesn't really seem confusing.. I'm just trying to figure out if that could be the issue as to why I'm not getting fuel delivery.. because everything else in the carb checks out ok. And it makes sense to me because it looks like it's directly ties in to the fuel line.

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120 PSI sounds like it's on the low end for a new top end.

 

 Lean the bike on it's side and see if gas comes out the carbs overflow

 

Try a couple drops of pre mix gas down the spark plug hole.

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My bike had that problem too try and put carb cleaner with your gas it's a special bottle and it goes in your gas but put the gas straight into your engine where your spark plug goes then your bike will pull the gas it needs and your bike should be running great but bike should start on first kick with the gas in the spark plug bay

My bike had that problem too try and put carb cleaner with your gas it's a special bottle and it goes in your gas but put the gas straight into your engine where your spark plug goes then your bike will pull the gas it needs and your bike should be running great but bike should start on first kick with the gas in the spark plug bay

Bad advice ignore this

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Yea it is on the low side.. but not much, it should still crank. I got a used cylinder off someone. The one that was on it was completely trashed.. I bought the bike off a guy in this condition. The more I work on the more I get pissed because this kid did so much dumb shit to the bike and so many parts were missing.. good thing I only gave $300 buck for the bike. But just found out the bowl is getting fuel to it but I don't think it's making it to the combustion chamber. The spark plug doesn't get wet with gas. So I think I am going to try pouring some premix through the hole. But I'm still positive that it's this fuel cut valve. Was reading the manual and it states that it will keep bike from starting if the valve stays closed. But I have no idea where to get one. Can't find it on eBay without having to buy a whole carb. Not trying to do that when this is a perfectly good carb. Just that one part.

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Oh and yea I wasn't going to do what he mentioned. I don't use fuel additives on anything. It's a waist of money. Like mentioned earlier it's just a band aid. But can't knock the guy he's only trying to help.

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When you say fuel cut valve do you mean the power jet? That should not affect the bike running at idle at all. All it does is opens or closes a auxiliary jet at full throttle to allow more or less fuel to the engine. If the bike doesn't start and it's a fuel issue it's going to be one of three things normally, the float isn't allowing enough fuel to by pass it, the pilot circuit is clogged or the reeds are junk.

 

And the compression should be between 160 -190 on a fresh top end. you're more then just a little low on that count. Try another compression test and look up the proper way to do it. If done right and the top end is fresh and you still get 120psi you've got an issue with your rebuild, far beyond not getting any fuel. 

Edited by moose_338

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The manual calls it fuel cut valve... so yea sure maybe it's the power jet... but it still says in the manual that if it's not functioning that it won't get fuel... pilot circuit is good. Checked it over and over. Reeds are brand new. Maybe the float.. but checked it several times and cleaned it. But like I said the cylinder is a used cylinder. It came off a running bike. So I'll do another compression check... but if the compression is low it's the cylinder... because I didn't have it replated or anything. Just put it on. But right now I'm trying to figure out why I'm not getting fuel. I'll worry about the compression once I figure this out.

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Think I found the problem. The kid put the wrong throttle cable on. It's entirely too short and won't allow the slide to close enough allowing way too much air come through the carb. Wish I would have noticed that a while ago. I been racking my brain. Plus compression is good. Checked it again and it's at 170..

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