YZ250F compression and carb

Ok my buddy gave me 2002 yz250f for a project. Started project by getting whole top end redone. Got new valves, and all that stuff. Perfect. Nope no compression. Tried all the techniques and all that TDC stuff. Nothing. Anyways now where I'm at is no compression to kick it over, yet we can bump the bike and it runs perfect(or so we thought) all day long. Don't die, don't bog, Anyways we thought it ran perfect when we loaded the bikes up we saw gas leaking fairly slow out a big bolt on bottom of carb. Any idea what this is? Also a good shake while gas line is on off some gas will leak.

And again the compression at the kicker is weak. Very seldom do we get any and just bump the bitch. I am ready to get this baby going right.

I been looking at cams and stuff would that be the way to go?

Please help me, I cannot take the sanity of the insanity of not being able to brapp!!! Thank you for all the help I receive.

The big bolt at the bottom of the carb is probably the bowl drain plug.  Are you sure it's leaking from the plug or is one of the vents leaking and dripping down and off that drain plug?  If you have fuel dripping out of the vents you need to look at the carb floats and float valve.  

 

Is the kick starter moving the piston?  I'd be surprised if there is zero compression felt at the kick starter but you can still get it to run by bump starting it.

Edited by Doc_d

The fuel dripping is probably coming from the drain plug, vent or the access hole for the jets(im about 99% sure that bike has an fcr carb). The access hole would be what you take off to change the main and pilot jets, its a hexagon with 6 triangleish indentations(sorry cant find a pic to copy). Wipe it off really good and see if you can watch the drops form. I would think the vent and drain would have hoses leading away from the carb so Im thinking someone didnt out the access cover back on right.

As far as the compression, which you have since it runs, I bet your kickstart assembly is in wrong somehow or one of the gears is damaged.

Being an '02, you have a manual decompression lever on the handlebars. I would look into that. Is the lever not releasing? Is it even hooked up?

You can also swap exhaust cams for the newer style with auto decompression built in. I recommend this mod anyways. Much easier to crank.

Edited by Foxracer117

Is that lever by left hand thumb?

So took carb apart clean everything no clogs

Put back together and gotba dirty filter. Probably from my dirty pantalons. Anyways still wouldn't kick over bumped it.

Ran bike up down street no leaks stop running it cause cops!!! Danm!!!

Ummm found idle but what was crazy during that process bike kicked over 7 times. Really crazy dude. Ran it ohh yeah said that.

So let it sit for a CIG break. Sat on top its been a hr and won't kick over. Had a shit ton of compression after that and still after this hr. Wtf can it be now one problem might be solved now the kick?

Is that lever by left hand thumb?

So took carb apart clean everything no clogs

Put back together and gotba dirty filter. Probably from my dirty pantalons. Anyways still wouldn't kick over bumped it.

Ran bike up down street no leaks stop running it cause cops!!! Danm!!!

Ummm found idle but what was crazy during that process bike kicked over 7 times. Really crazy dude. Ran it ohh yeah said that.

So let it sit for a CIG break. Sat on top its been a hr and won't kick over. Had a shit ton of compression after that and still after this hr. Wtf can it be now one problem might be solved now the kick?

Bike is a 03. Just ran the vin.

10th number is a 3 so that means 3 right?

Cleaned carb, made sure everything was cleaned. Like squeaky. Put carb back on tightend that bolt you said didn't kick over bumped it. Crazy bike built a lot of compression. Kicked over 7 times finding idle ran bike till cops came. Let bike sit wouldn't kick over. Been about 45 min got pissed and kicked over again like 7 times. Didn't run bike

Since I just found out it was a 03 is that the lever by my left thumb?

Your lack of compression could be several things. The decomp lever could be stuck. I've had bikes do that before. It could also be out of time. Read you service Manuel VERY carefully and follow every instruction on setting the timing. Don't trust YouTube! Also, your valve shims could be too big and the valves are sticking open. After you replaced the valves, did you 're shim them? I just replaced the valves in my 06 yz250f and the shims were way off. Also, what did you replace in the top end? Are you sure you put ALL the rings on the piston? Did you gone the cylinder, or is it new? And it could be in the kicker gears. And for the gas leaking. I've fixed more leaky carbs than I can count on my fingers. Find where the leak is coming from, and if it isn't a vent hose, silicone that sucker up with some good rcv silicone! If it is coming from a vent hose, it is probonly your floss letting in too much gas. Bend your float tab so that it the needle valve closes sooner. The leaking g with the gas off just means that the petcock is bad. They are like 20$ on eBay if it drives you mad enough to actually replace it. Most people just live with it. And if your feeling adventurous, use that left over silicone and see if you can fix the petcock! Hope this helps!

Bike when hot won't start looking up lever. Everyone says push that lever in and it should start. Never owned Yamaha or a 4 stroke at that. Had a 80 growing up finally for a big boy toy 20 years later.

Bike when hot won't start looking up lever. Everyone says push that lever in and it should start. Never owned Yamaha or a 4 stroke at that. Had a 80 growing up finally for a big boy toy 20 years later.

Bike when hot won't start looking up lever. Everyone says push that lever in and it should start. Never owned Yamaha or a 4 stroke at that. Had a 80 growing up finally for a big boy toy 20 years later.

Bike when hot won't start looking up lever. Everyone says push that lever in and it should start. Never owned Yamaha or a 4 stroke at that. Had a 80 growing up finally for a big boy toy 20 years later.

So it just doesn't start when it gets hot? Does it still have no compression? That decompression lever is not always just a straight up starter. Try to just pull it in a little bit. Just enough so you can feel it is working, but not pulling it all the way in

Theycallmesam9....it has compression it runs. But what it didn't have was compression when it first started but we started to mess with everything possible and now it kicked over. It didn't leak gas from bottom of carb and kicked over after running it for 30 min. What was crazy and made no sense was we ran it found the idle and it shut off 7 times all 7 times kicked over like I said it ran for 30 min smoked a half a cigarette wouldn't kick over hot. So I have that lever. Is that the hot lever? Looks like it and as for what my best friend who basically is our mini mechanic says it is kinda getting stuck. But again it sat for like 20 min I got angry it didn't start hot. Like 6 kicks in a row started. Wtf?

And as for compression. My buddy now has a bruise on his knee from a kickback on the kicker. Lol, I didn't do the top end build. The guys who did I known for years. They own a very respectable shop that I personally seen them work and perform miracles. So I'm danm near positive they did every single thing possible. They are really good at what they do. Old man that owns the place owned it for 30 years and worked at every single motorcycle shop in the north NJ area. Lol all the guys I had look at it when done he worked with. He's seen the old and young generation and still kicks ass. Lol

The head was rebuilt by a very reputable shop. All valves are new and I was told shimmed correctly . They also checked the timing twice. Before giving it back. And it was honed(hope I spelt that right).

Yes gas come out the vent tube.

the bike does not have a decompression lever. Just the hot start lever connected to the carb,which we haven't tried out once lol

Ok, so the lever by your. Left hand thumb is a hot start. It goes into the top of the carb and makes the ration leaner so the bike starts easier when it is hot. So push that lever in when going to start it when the bike is already warmed up. But once it starts running, tIe your thumb off it. If the lever is engaged once it starts it will make the bike run rough. The problem you are experiencing with it shutting off and leaking gas out of the vent tubes sounds like a float issue. If it runs perfect, then just shuts off for no reason, it sounds like it is running out of gas in the bowl. I reccomend that you take the carburator apart and check for correct action on the float. There are several YouTube videos on doing this if you are not familiar with it. Since it is an 03, it had what is called automatic decompression which means that when the bike isn't running something pops out of the exhaust cam On the compression stroke to make it easier to start. So it won't feel like it has much compression, but if that didn't work, you wouldn't be able to start it. If it feels like it had little to no compression, the I highly highly reccomend you chech the valve clearances. Even if that shop did shim the valves right, new valves have a break-in time and will have to be 're-shimmed after running the motor for a little bit. Just get a set of feelers gauges and chech them yourself. The valve cover is just 2 bolts and then it comes off. There are several videos of cheching valve clearances for the yz250f on YouTube the I know of.

What is a little bit? Maybe 3 hrs on bike. I'll let you know how it goes later. Thank you Sam. Thank you very much dude.

It was only running out when we bumped it and had to find the idle of the bike. Other then that haven't seen a leak or it hasn't bogged out but were gonna check what you said as well. We took carb apart yesterday and made sure everything was on proper. That seemed to help that. Now we wanna see if the bike will kick when hot cause you are a life saver and helped us realize that was the hot start. My friend who's our mini mechanic said it kinda gets stuck. Like I push it and its slow to return. Should I invest into a new one? They aren't that much. Please lmk and again thanks

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