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YZ400F Compression & starting problem

By Sebastian Pavich,
in YZ400F/426F/450F/450FX

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Sebastian Pavich

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I have 1 1999 yz400f and i took my carb to a shop to have them rebuild it so my bike would be at tip top condition. well when i got it back my bike wouldnt start. i adjusted the idle knob a thousand times even at all the way up and the bike still wouldnt idle, nor start. after kicking several times, the kick almost be came super easy. i wouldnt have to use the decompression lever at all, i could feel it build up compression at top dead center then kinda like leak it out if i kept presser on the kickstart lever. i rip apart the top end and the depression system that pushes the exhuast valve down. it was returning to proper postion and the valves werent gap and it look like the piston and rings where in really good condition ( i think the rings are good). Idk y i have this compression leak and y my bike wouldnt start does any one have an explantion????

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Apprentice

Sebastian Pavich

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it has a good plug, valve on spec, timing dead on, i guess the compression loss was just the bike saying f**k u to me, but what do i do about the carb? i adjusted the idle and air screw still wont start but if the throttle is held down a bit it starts, sometimes.... and honestly the reason i even messed with the carb is because it was backfiring from dirt being in the polit jet, i didnt want to mess with it any more already rebuilt the whole front end and just was done so i took it to a shop to let them adjust and clean it out. and got it back like this....

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kiwigrant

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i saw the same thing happen as my mechanic was taking the carb on and off to work on it. The bike went from running a little rough to not at all. zero compresssion it seemed like. I could tell there was no way the bike was going to start as i knew the bike and that condition was way off. He said the timing got thown off somehow in the process and took care off it but it stumped him at the time for sure

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  • 2 weeks later...
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slyfox83c

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Do you have the factory jets in this carb? I fought the same thing this spring but with out the compression issue. I had to go down in size on pilot jet and main jet. Now mine will start on the first or second kick. when they rebuilt it did they take the whole thing apart. Because if there is any thing in the idol circute on that carb it will cause what you are fighting. Most placed will just pull the bowl and pull the jets and spray carb cleaner through them and blow them out and call it rebuilt. You have to remove the lower section off the carb and run thin wire throught the idol circuts and spray them out. My carb was rebuilt 3 times. then i did it my self and found one of the idol circute ports plugged up. If you want to know what size jets i put in mine let me know and i will post them.. Gray Racer didnt agree what what jet sizes i used but it worked for my bike. I got the kit through JD Jetting. Seems like from the factory they put to big of a pilot jet in them and that was a common problem. One other thing i had to do with mine to make it easy starting is when it is cold i twist the throttle 4-5 times then kick it. With in 2-3 kicks it will fire up.. Hope this helps.. I just had alot of the same issues with my 1999 yz400fl1 and now they are fixed....

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Sebastian Pavich

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im not sure what exactly what they did. honestly they could have done any thing. the paper says new diapham gasket float bowl gasket and some other gasket and then it just says cleaning solutions and labor haha. they are an actual yamaha dealer but they are ****ing pricks. doesnt make sense how i can bring them a carb that is dirt but my bike will run with it dirt and now it wont run when its clean. i believe they left the stock jets in there. and as far as the carb cleaner and wire deal the manufacturer that made that carb told me not to use carb clean to try to use like purple power and mean green, that the carb cleaner will destory the gaskets, and the manuel told me not to use wire but compressed air i know every one uses wire.

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slyfox83c

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Using compressed air is the best but the idol passagases are so small that if one is pluged air and a degreaser will not work.. Yes you will have to take all your gaskets off before you use any carb cleaner... To do it rite there is a 1 gallon can of carb cleaner that you can buy and let it soak over night. You have to compleatly dissasemble your carb before you do this.. Your carb has 3 gaskets and 2 o rings. The upper gaskest where your top plate and a middle gasket where your idol circute bolts to the main body then the bowl gasket.

You should check the o ring inbetween you gate valve. This is what moves up and down when you twist the throttle and lets air into the motor. (Different people call it differant things.) That o ring can crack and break and cause a lean condition. If you want to check your idol circute just remove your bowl off your carb and the section that your bowl bolts to is your idol circute. The allen head bolts that hold it on can be tricky to get out. But if you have a small set of metric allen wrenches you can get if off without to much trouble.. Just make sure you clean the dirt out of the allen holes before you start. they like to plug up with dirt and if you round them off its going to be double tough to get them out. As Long as you clean them out before you start you will be fine. This is a good place to check as well. If they look like they havent ever been out then you know your dealer has not compleatly cleaned and rebuilt your carb. Once you get this off take the gasket off and take a thin wire and run through all the ports and passagages. Spray them out with carb cleaner and blow them out. Once you have this section cleaned you can reassemble the carb and should just be a matter of adjusting your idol.

What kind of Plug are you running in this bike?

I put a irmidium plug in mine and made a world of differance!

Let me know what you find? I fought the almost the identical symptoms that your fighting with mine and ive got mine starting within 2 kicks. If you just got this bike make sure you have the starting sequance down and she will fire...

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Grand Master

grayracer513

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Yes you will have to take all your gaskets off before you use any carb cleaner... To do it rite there is a 1 gallon can of carb cleaner that you can buy and let it soak over night. You have to compleatly dissasemble your carb before you do this.. Your carb has 3 gaskets and 2 o rings. The upper gaskest where your top plate and a middle gasket where your idol circute bolts to the main body then the bowl gasket.
I do not recommend this.

For one, the gasket between the upper float bowl casting and the main carb body is not serviceable; nobody sells it, so if you screw it up taking it apart, reassembling it, or soaking the carb in an aggressive cleaner, you're hosed.

Secondly, the TPS should not ever be soaked, which means it has to be removed (and the manual specifically says you should not do so unless it's to be replaced), which means you will at a minimum need to readjust it as you reinstall the carb.

Read:

https://www.thumpertalk.com/forum/showthread.php?p=8021726#post8021726

On the other hand, the older FCR's do sometimes develop leaks or blockages that can only be addressed by opening that joint, and in these cases, one runs a certain obvious risk. At some point, the old carbs just get to a point where they refuse to be fixed anymore, and the best bet then is a replacement with a later unit from one of the 450's.

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slyfox83c

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I do not recommend this.

For one, the gasket between the upper float bowl casting and the main carb body is not serviceable; nobody sells it, so if you screw it up taking it apart, reassembling it, or soaking the carb in an aggressive cleaner, you're hosed.

Secondly, the TPS should not ever be soaked, which means it has to be removed (and the manual specifically says you should not do so unless it's to be replaced), which means you will at a minimum need to readjust it as you reinstall the carb.

Read:

https://www.thumpertalk.com/forum/showthread.php?p=8021726#post8021726

On the other hand, the older FCR's do sometimes develop leaks or blockages that can only be addressed by opening that joint, and in these cases, one runs a certain obvious risk. At some point, the old carbs just get to a point where they refuse to be fixed anymore, and the best bet then is a replacement with a later unit from one of the 450's.

As for the Tps is not a big deal to remove and reset.. Ive remover mine a few times when i was going through the carb. Just mark One line on the Tps and a line on your carb to line it back up with and it will be set at the correct position.

Just be real careful and you will be fine.. Try running a small wire through the idol passages before you try the carb cleaner. I WILL NOT SAY TO DO THIS IN A BOOK, BUT SOME TIMES YOU HAVE TO DO WHAT WORKS TO GET IT FIXED Gray Racer.

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slyfox83c

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Heres what i had to do with my carb.

This bike is a little different than the others that people have. I have to twist the throttle 4 times then kick it through the compression stroke 3 times then twist the throttle 3 times and normaly on the 2nd kick it fires rite up and it idols.

I have done a ton of work on this bike in the last week and ruined 2 pairs of boots trying to kick start this bike.

I have adjusted the valves and rebuilt the carb and went completely went over the electrical system on this bike.. the only real changes i have made to it is adjusted the valves, and i went from a 175 pilot jet to a 170 pilot jet with the blue needle set on the 3rd position. I also put a IRIDIUM CR8EIX spark plug in it as well.

Now it runs like a dream.

Im running stock Jets other than the Pilot. The factory put to big of a jet in it and it has caused a lot of people a lot of problems. Im sure it was fine when it was new but when they start getting some wear on the motor it has caused problems. I just went out today and ran mine for a few min.. Its 48 degrees here today and it hasnnt been started in over a month. Twisted the throttle 5 times, pulled the choke, on the 4th kick it fired rite up!

Hope this helps you. I know how aggrivating it is...

Chad

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Grand Master

grayracer513

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Yes. It will operate in a default mode without it. The function of the TPS on the carbed bikes is to modify the ignition timing map for part throttle conditions in order to make them run a little cleaner. The CDI maps the advance based on RPM by using the tach signal from the trigger coil, and trims that for load by using the TPS. Without the TPS, it assumes a full throttle operating condition full time and sets the timing based on speed only. A lot of riders can't really tell much difference with it unplugged.

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