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99 yz400f running issues

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I have a 99 yz400f that is having idle issues. It starts easy with choke on and runs ok with a little poping after reving it a little. When I turn the choke off it dies unless I feather the throttle, but when I do that there is a big loud poping like it's backfiring big time. I have pulled the carb multiple times and cleaned or changes any jets that needed replaced. Still have the same issue, I even tried bigger main jet ect.. now all I can think is something electrical or with the ignition. I am getting pretty frustrated but don't plan on giving up on this 400. Anyone with any suggestions or pointers? Any advise is appreciated. I do have a new ignition coil on its way because I thought that might be the issue. Thanks

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I would start out with a full breakdown and very through cleaning. You'd be amazed at the amount of crap that comes out of a 17 year old carb!

Remove every single jet and give it a good blast with a mild carb cleaner and compressed air, even a small bit of dirt in the wrong place can totally derail a carb.

Make sure you've got seals in all the right places and the pilot screw is set to stock setting.

 

I have a 98 400f and had very similar issues awhile back, turns out there was some gunk stuck in the pilot jet. (which is directly responsible for idle.) Once that was cleaned out, it runs like a champ now!

Don't both with the main jet when dealing with idle issues, main jet is only in play when dealing with 3/4 or more throttle.

If it's running somewhat smoothly off the choke, than your coil and rest of ignition system is probably fine.

With the behavior you've described, I'd be willing to bet you've got a clog somewhere in the pilot circuit, which is what delivers fuel at or under 1/4 throttle.

 

From there I'd move on to the plug, make sure you're running the correct plug (CR8E) and that it's not all fouled up with soot. If it is, replace it.

 

And lastly, check the TPS (throttle position sensor) that shouldn't prevent it from running, but if it's feeding the ECU a bunch of bogus numbers, it will cause the spark timing to move around unusually and lead to whacky behavior. (testing info for TPS is located in owners manual)

If you're not sure, just unplug it, it will run on a static map.

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I would start out with a full breakdown and very through cleaning. You'd be amazed at the amount of crap that comes out of a 17 year old carb!
Remove every single jet and give it a good blast with a mild carb cleaner and compressed air, even a small bit of dirt in the wrong place can totally derail a carb.
Make sure you've got seals in all the right places and the pilot screw is set to stock setting.
 
I have a 98 400f and had very similar issues awhile back, turns out there was some gunk stuck in the pilot jet. (which is directly responsible for idle.) Once that was cleaned out, it runs like a champ now!
Don't both with the main jet when dealing with idle issues, main jet is only in play when dealing with 3/4 or more throttle.
If it's running somewhat smoothly off the choke, than your coil and rest of ignition system is probably fine.
With the behavior you've described, I'd be willing to bet you've got a clog somewhere in the pilot circuit, which is what delivers fuel at or under 1/4 throttle.
 
From there I'd move on to the plug, make sure you're running the correct plug (CR8E) and that it's not all fouled up with soot. If it is, replace it.
 
And lastly, check the TPS (throttle position sensor) that shouldn't prevent it from running, but if it's feeding the ECU a bunch of bogus numbers, it will cause the spark timing to move around unusually and lead to whacky behavior. (testing info for TPS is located in owners manual)
If you're not sure, just unplug it, it will run on a static map.


It runs awesome now that I cleaned and put brand new jets in. I am just curious if I should put the jet needle back to stock position or leave it one down from middle. Will it hurt or damage it if I left It? Should I check the spark plug and decide from there? Thanks

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Awesome!

I would go with stock position if you went back to stock jets.

Just one position off from center shouldn't hurt however. I would ride it for 10+ minutes as is, and see how it runs, if it runs smooth all the way from idle to WOT, and the plug isn't all sooty or totally white, (too rich VS too lean) than you should be good to go!

The factory owners manual actually gives some pretty good tuning tips, so I'd pick up one of those if you can find it! (I've got a .PDF copy if you'd like that as well)

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Awesome!
I would go with stock position if you went back to stock jets.
Just one position off from center shouldn't hurt however. I would ride it for 10+ minutes as is, and see how it runs, if it runs smooth all the way from idle to WOT, and the plug isn't all sooty or totally white, (too rich VS too lean) than you should be good to go!
The factory owners manual actually gives some pretty good tuning tips, so I'd pick up one of those if you can find it! (I've got a .PDF copy if you'd like that as well)

Thank you. I just got a .PDF copy. Possibly going to go on a test ride after work hopefully everything is good to go! Your help is much appreciated.

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Awesome!
I would go with stock position if you went back to stock jets.
Just one position off from center shouldn't hurt however. I would ride it for 10+ minutes as is, and see how it runs, if it runs smooth all the way from idle to WOT, and the plug isn't all sooty or totally white, (too rich VS too lean) than you should be good to go!
The factory owners manual actually gives some pretty good tuning tips, so I'd pick up one of those if you can find it! (I've got a .PDF copy if you'd like that as well)


I started it up and it runs a lot better with very little to no poping. I can get it to run and idle. I had to fix an air leak as well. Now I'm just confused because I have a bog from idle. Is this common on the yz400f and is there anything I can do to get rid of the bog a little. Also I feel like my coolant is getting really hot, any ideas what that could be?

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Could the bog be a cause of putting in stock jets? Not being jetted right for my altitude? I feel like I had more power be for all this backfiring and not idling started, but I really haven't taken it on a ride yet because I want to get it close to right before I go ripping around!

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If you're getting bog coming off idle the pilot mixture is incorrect, or the accelerator pump is not working. I'd start out by backing out the pilot screw a half turn or so and then testing again. (this richens the mixture a bit) If it gets worse, go the other way a half turn (relative to where you started).

Does it run fine through the rest of the RPM range? Or is it only sudden power off idle that's giving you trouble?

Edited by Michael Cahill

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If you're getting bog coming off idle the pilot mixture is incorrect, or the accelerator pump is not working. I'd start out by backing out the pilot screw a half turn or so and then testing again. (this richens the mixture a bit) If it gets worse, go the other way a half turn (relative to where you started).
Does it run fine through the rest of the RPM range? Or is it only sudden power off idle that's giving you trouble?


I have only noticed off idle bog. I will be messing around with it later tonight for a little bit. Could the pilot passage still be dirty even though I cleaned it really good? I will let you know what I find out. Thanks again for your help and advise.

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So I started the bike and backed out the pilot screw didn't do much. I adjusted it the other way(in) a full turn from 1 1\2 out from slightly snug so that gives me 1/2 turn out from slightly snug little bog. Went for a rest ride and did well for a bit then I had to adjust a quarter turn out but didn't get rid of the bog. There is a bog when I wack on the throttle low end upper end seems to have no bog. The worst is from idle. Do you think I need a different pilot jet possibly?

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So you're only a half or quarter turn out on the pilot screw? (from lightly seated)
If so, I'd go at least a full turn out from slightly seated. Manual states "stock" is 1.5 turns out.

Does it idle fine on it's own? And does it run smoothly while slowly riding? (such as around the yard with 1/4 or less throttle)
Or is it super "boggy" even while just "putting around"?

If you're only noticing it directly off idle, and only when you suddenly jump on the throttle, I'd be suspicious of the accelerator pump. Which is what gives you a quick squirt of fuel when the throttle is suddenly opened.
If that's not working and then engine is running at a slower RPM, and the throttle is suddenly opened, there will be a temporary lean condition due to the sudden influx of air, but with little fuel.
AKA "bog".

 

Edited by Michael Cahill

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It's pretty boggy with 1/4 or less of the throttle. When I was riding I put it at one full turn out(worked the best for riding around. I'm going to check out my acc pump later. I replaced diaphgram, spring... ect.. it seems to idle ok it jumps down a little when I let idle by itself after throttle. I might have to clean my pilot port better. It is running so much better than it was at the beginning

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