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04 YZ250F Jetting issues

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The past year, my bike has been developing some serious backfire on decel. Deciding to work on my jetting, I purchased a JD kit for the bike. I went with the suggested:

185 main

Blue needle 5 groove down

1 3/4 turns out on fuel screw (also bought a zip-ty fuel screw)

To do all the work right, took entire carb out of bike and proceeded to do the above. Squirted myself good by turning throttle pulley and looking at the engine side of carb. Oops! I also installed a new cable for the hotstart as the cable was seized, not the plunger.

Once done and buttoned back up, the bike would not start at all. I got 2 revs that lasted about 1/2 second or so and some backfiring. Took it all apart, looked at things, installed a new plug. Have good spark as well, made sure all electrical connections were firmly seated. Still no fire.

Ok, start working backwards then. Took carb out, installed the stock MJ, a 180. This time I notice no fuel squirting out of carb output side. Hmmm I thought. Still no go. Reinstalled the stock needle, clip in 3rd position down. Fuel screw still the same. So in theory, I should be back at stock settings before I started all this work. STILL no starting.

Now I am thinking, go back to stock fuel screw? Carefully, with carb on bike, I removed fuel screw slowly so I do not loose o-ring and minuscule washer. Keep in mind, I left the petcock ON. Shouldn't gas have come out of the fuel screw opening without any fuel screw in? None did on my bike. When I put the stock main jet in, fuel did come out of the 17mm opening to access this, so that area is getting gas?

By the way, I did check the plug when back to stock setting to see what it looked like/smelled like. Looked nice and clean, no noticable residue.

Any ideas on this? Did some dirt get in the carb, blocking something? I know there is always residual gas in carb so turning the throttle pulley will result in few squirts. Would a new hot start affect this? I never ever used it, hence the cable seizing.

Last night, I took everything apart last night and thoroughly cleaned it with brake cleaner and compressed air. Put the JD kit back in with above mentioned settings. Got a 3 backfires this AM but no definitive engine start, turning over.

Note, the back ran fine prior to all this work so...you can see this is getting frustrating. Checked thumperfaq, but nothing geared towards non-starting carb issues.

Thanks all for the help. Will be checking back throughout the day to see what input everyone suggests.

mx813/mike

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you may need to rebuild it. My dad's wr 400 was starting to do the same thing so we are gonna rebuild it. it is a '01 but probably hasn't been ran as hard as a mx bike.

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The past year, my bike has been developing some serious backfire on decel. Deciding to work on my jetting, I purchased a JD kit for the bike. I went with the suggested:

185 main

Blue needle 5 groove down

1 3/4 turns out on fuel screw (also bought a zip-ty fuel screw)

To do all the work right, took entire carb out of bike and proceeded to do the above. Squirted myself good by turning throttle pulley and looking at the engine side of carb. Oops! I also installed a new cable for the hotstart as the cable was seized, not the plunger.

Once done and buttoned back up, the bike would not start at all. I got 2 revs that lasted about 1/2 second or so and some backfiring. Took it all apart, looked at things, installed a new plug. Have good spark as well, made sure all electrical connections were firmly seated. Still no fire.

Ok, start working backwards then. Took carb out, installed the stock MJ, a 180. This time I notice no fuel squirting out of carb output side. Hmmm I thought. Still no go. Reinstalled the stock needle, clip in 3rd position down. Fuel screw still the same. So in theory, I should be back at stock settings before I started all this work. STILL no starting.

Now I am thinking, go back to stock fuel screw? Carefully, with carb on bike, I removed fuel screw slowly so I do not loose o-ring and minuscule washer. Keep in mind, I left the petcock ON. Shouldn't gas have come out of the fuel screw opening without any fuel screw in? None did on my bike. When I put the stock main jet in, fuel did come out of the 17mm opening to access this, so that area is getting gas?

By the way, I did check the plug when back to stock setting to see what it looked like/smelled like. Looked nice and clean, no noticable residue.

Any ideas on this? Did some dirt get in the carb, blocking something? I know there is always residual gas in carb so turning the throttle pulley will result in few squirts. Would a new hot start affect this? I never ever used it, hence the cable seizing.

Last night, I took everything apart last night and thoroughly cleaned it with brake cleaner and compressed air. Put the JD kit back in with above mentioned settings. Got a 3 backfires this AM but no definitive engine start, turning over.

Note, the back ran fine prior to all this work so...you can see this is getting frustrating. Checked thumperfaq, but nothing geared towards non-starting carb issues.

Thanks all for the help. Will be checking back throughout the day to see what input everyone suggests.

mx813/mike

Check to make sure the hot start cable isnt adjusted to tight keeping it open.

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Check to make sure the hot start cable isnt adjusted to tight keeping it open.

hmmm...let me go check that. Will loosen it up until I get about 1/8" free play to ensure that it is firmly closed.

mx813

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tried that, got a few more back-fire pops and a few 1-2 second engine revs before quitting.

Fuel screw turned out to 3x now. This means???

mx813

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Tore into the carb today, took everything apart, cleaned thoroughly, float, accel. pump, all jets, etc...Carb is spotless.

STILL wouldn't start. Again, good spark and all, but things get interesting here. There is gas getting to the engine as evident by the pooling of gas in the intake manifold area. &%$#@!??? Hotstart cleaned again and verified working in case that might be the culprit. Nope.

After 4 hrs of working on it, taking tank off dozen of time, we said might as well try to compression start it. Boom, got some speed and it fires up.

However, there is a huge rattling sound coming from the exhaust side of the engine. Sounded like something was loose or what not. So now we are thinking, hmmm if exhaust valves are functioning, that will prevent the engine from starting? Is this so? So that is my underlying question. If valves are not functioning correctly, will this prevent the bike from starting? The force of the compression probably was enough to get it running. The bike would not start any other way, just compression start.

Tired of gas and working on the bike, I will be pulling the cam cover off and check my valve clearances tomorrow evening. There is something wrong so gonna dig into it tomorrow. What else should I be checking? I will also check the cam chain tensioner to see if that is working as chain could've skipped throwing timing off.

Input on this is much appreciated!

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.

Sounds like you are getting no gas to the motor or your timing is way off.

We are thinking mechanical somewhere in the cam/valve/timing....see my last previous post. Thanks for the pics. Carb is spottless, each jet cleaned out thoroughly.

mx813

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Thats the thing there...last time I rode it at playday at Eddieville, it was running fine 2 weeks ago, albeit the backfire on decel. Pressure washed it and started it to ensure any water in exhaust was dried out, etc... and put it away for a week. Then once I started working on jetting, this all started.

I did take exhaust header off to clean it up since it was getting pretty caked up but also made sure nothing went into the exhaust manifold.

Gonna check the valves tonight and go from there.

mx813

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did you remove the slide from the carburator when you cleaned it?

No, opened them up with the throttle pulley to clean passage way.

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well with the new info, 2 more things come to mind

Hot start plunger is stuck or adjusted open. You have to grease that thing!

Cleaned out, lubed and verified working and fully seated. Will order a new plunger as the O-ring around the middle appears to be worn to flush with the rest of the plunger. Also will order the plastic threaded housing that seats into the carb since current one is stripped bad. Why they made it out of plastic is beyond me:confused:

Fuel screw is missing or at least the O ring is not installed properly or in the right order. That will make some exhaust type of noise.

Shoud be O ring, washer, spring, screw.

Confirmed. Screw, spring, washer and O-ring.

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Just tried to check the valve bucket clearances to cam lobes. Feelers were too wide so I need to go back and get some narrower ones at NAPA that were sitting next to the ones I just bought. oh, well

At TDC, the buckets I can touch can spin but unable to get a true reading with my current clearance gauges. So there is some clearance on both intake and exhaust, how much, I will let you know tomorrow.e

Let's assume they are in spec, what would be next? Compression check? What is typical compression on for the bike? The manual did not specifiy.

mx813

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Here are 2 pics so you can verify that I am indeed at TDC. Little peace of mind for me.

dsc01855vk7.jpg

The black dots are from me last valve check. Made one on the chain as well to ensure everything was lined up just right.

dsc01860mf4.jpg

The TDC line looks off as I couldn't get the camera lined up right, but it is in line with marks on crank cover.

Thanks,

mx813

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On the crank, that is the only vertical mark present. There are stamped part numbers, a dyed blue dot on it and the mark that you see. Directly to the left of the vertical mark is the letter H.

Your right about accessing some of the buckets, that is why I said the ones I could reach.

Oil, use yamalube and changed 1 ride ago. Not sure what discoloration you are talking about. Engine parts? Pretty religious about oil. The oil is still clear from the last change.

Once I get the narrow feelers this afternoon, I will post my specs.

Ordering new hotstart plunger, spring, etc...today.

mx813

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Timing looks spot on. Even if the crank was 180 degrees the bike would still run, just it would run like crap. I made that mistake. Are you sure the float is moving freely when the bottom cap is installed? It sounds like it's not getting filled up. You can test this by taking your carb out and shaking it when its empty, you should hear the float clanking around. Now hook the fuel line up to it while it's dissasembled to fill it up with fuel, now shake it. If there is no clanking it's not that then.

Everything else you checked sounds spot on, it should start.

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When I get the new hotstart plunger and other parts, I will hook the carb up to my gas tank, but not installed in bike so I can verify the squirt, receiving gas visually.

Will look at the float as well tonight since I will be checking valves and bike is in pieces.

I did bump start it so it does run, but makes an awful sound that sounds mechanical in valves.

And your right. It should start, but the bike powers that be in the sky must be toying with me.

mx813

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did you change the spark plug?

last nite while working on an 02 wr250 with 18 mo old gas I drained the gas, cleaned the carb, put new gas in and kicked about 100+ times with an occassional start, run a second, backfire, pop die.

I changed the plug and it started 1st kick and ran fine!

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did you change the spark plug?

Yup. Bought 2 new ones in case 1 got fouled. Put a new one in after starting the jetting and they are still like new.

Once I compression started the bike, the plug I pulled last night was really black, slimey. And this only from 3-4 minutes of running on Sunday afternoon.

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Valve clearance update:

Measured valves and found the following -

Intake

Standard is 0.10 - 0.15 mm per manual

R - 0.221

M - 0.223

L - 0.223

Exhaust

Standard is 0.17 - 0.22 mm per manual

R - 0.221

L - 0.221

So based on these clearances, the exhaust are still in specs, albeit, barely.

Intake are not so I will adjust accordingly with new pads per manual table once I pull cams off and get installed numbers.

These current clearances in my opinion would not prevent the bike from starting. Maybe not open all the way to let appropriate mixture, but nonetheless, bike should start. Or am I totally wrong?

Later tonight, after I put stock jetting back in, will hook up carb. up outside of intake manifold and see what happens when gas is turned on and throttle twisted. See what gas is coming out.

Thanks again for your input! All the suggestions are helping narrow things down.

mx813

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