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Back firing through carb

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I bought a ttr 125. When I bought it, it didn't run. I've been trying to get it to fire up but I can't seem to figure it out.

The bike sounds like it wants to start. But it never stays running for more than a second. Even on starting fluid.

When I try to start it fire will come out of the carburetor (I have the air box unhooked so I can spray starting fluid into the carb) white smoke also comes out of the exhaust when I roll it over.

I have tried a new plug just to eliminate that and it doesn't help.

I was thinking it may be valves so I replaced the whole valve assembly and it hasn't helped any.

When I put it back together the timing mark on the crank I line up perfectly but the timing mark on the can looks a little off to me. But another tooth on the sprocket is too far. Is the back firing because the timing is a hair off?

If it is the problem how can I adjust it? Is my timing chain just stretched and wore out? The tensioner doesn't seem to get it any closer. Maybe its not off and just appears so? On the crank there is 3 lines. l-l l... I'm using the last one. Is this correct?

Also my valves are adjusted to .004 inches each. I got this number off line. Is this also correct?

What causes a 4 stroke to backfire through the carb? Also I will need to replace the fuel petcock on my gas tank because it constantly leaks. Could this have anything to do with it? It shouldn't right? Because the float stops gas from coming in

I'm trying to get it fixed by Christmas so I can give it as a gift so any help is greatly appreciated

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what year and model bike???

 

in general,

improper timing,,, FIND out what mark and set it to that...

flywheel key sheared if applicable...

improper valve lash...

air leak causing a lean condition...

valve spring/valve sticking/leaking...

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I assume, like the 4 strokes I'm familiar with working on, you need to be sure your crank marks are aligned when the engine is on the compression stroke. If its on the exhaust stroke when you make your valve adjustments, it's going to be wrong.

 

When setting the cam chain on the sprocket, with the crank marks and sprocket marks aligned properly, pull the chain tight on the side of the sprocket opposite of the tensioner and wrap the chain around the sprocket. That puts all slack in the chain on the tensioner side, to be taken up by it. If you wrap the chain onto the sprocket, leaving slack on the side away from the tensioner, then when you reinstall or release the tensioner, it will pull that slack out and move the sprocket off its alignment marks.

 

Definitely be sure the chain isn't so worn or stretched that tensioner can longer compensate.

 

NEVER rotate the crank with the tensioner released and a slack cam chain.

 

If you haven't had the carb off yet for a cleaning, you should. Make sure the jets are stopped up. Don't forget to remove the pilot screw and clean that passage and account for all parts. It's not going to run with those things being dirty.

 

If you want to make this bike a gift, give the gift of a shop manual with it.

Edited by Trailryder42

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Hey thank you guys for the replies. The bike is a 2004 yamaha ttr125.no I don't have a manual but I really need to get one. And I've cleaned the carb twice now. Taking all jets out and stuff.

How do I tell if it's on the compression or exhaust stroke?

If I had it backwards that could explain why fire is coming out of the carb?

I don't know much about 4 strokes

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