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xr400 misfiring badly

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Hello,

I'm in need of some help. I recently traded for a 2000 xr400 that didn't run. I was told it ran fine last time it was ridden (of course). The gas smelled like varnish, so i emptied it, replaced it, and cleaned out the carb. the carb was caked, jets clogged.

I put it back together, and it fired up 3rd kick, seemed to idle ok ish, bogged when i revved it. I drove it away, and it fired ok until my rpms climbed, then it started misfiring badly, maybe more misfires than fires. 

I figured I would change the spark plug, see what that does. I noticed the two ends of the new spark plug were much closer together than the one i removed. the one i removed looked darkened, but not gummed up, necessarily. the two parts of the new one looked like they were almost touching. I'm not very knowledgeable about engines, so I just installed it. 

This time, the bike had a hard time starting, and only started with the choke on half way. after it started, i turned the choke on full to see what would happen, and it died immediately. I started it again. It would barely even idle, had to have the idle turned up to stay on. I tried to drive it, and it misfired much worse than before, at any rpm. I was surprised the thing even ran at all, it was so bad. 

Couple of extra bits of info: I live at 4500 feet elevation. the main jet is 145, don't know what the idle jet it (didn't see a number on it). I think everything is stock, but the tail pipe has its end bit removed, leaving a big hole in the back, and making it much louder than a stock xr400. 

Also, I found a small (1 cm or so) broken O ring on the floor that must have come from the bike, but I have no idea what it is for. 

Is it possible the old spark plug is fouled, and the new one is way too close together? 

Any ideas?

Any help is very much appreciated.

Thanks!

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Set your spark plug gap to .026". If you don't have a set of feeler gauges, buy one.

When you clean the carb, you can not omit pulling the fuel screw and clean that passage and verify its spring, washer and oring are accounted for and in good shape, especially when the other components like the jets were so bad. The bike will run poorly, if at all, if that passage is clogged up.

Is there an air filter installed? The stock unit is a foam filter over a stainless steel metal cage.

Is the rubber snorkle still in the top of the airbox or do you have a big hole there?

The baffle not being present in the exhaust is going to screw up the flow of the whole system and make it difficult, if not impossible to jet for and tune. I think it's useless to even try until you can remedy that with either finding a stock baffle for it or replacing the stock exhaust with an aftermarket one.

Edited by Trailryder42
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1 hour ago, Trailryder42 said:

Set your spark plug gap to .026". If you don't have a set of feeler gauges, buy one.

When you clean the carb, you can not omit pulling the fuel screw and clean that passage and verify its spring, washer and oring are accounted for and in good shape, especially when the other components like the jets were so bad. The bike will run poorly, if at all, if that passage is clogged up.

Is there an air filter installed? The stock unit is a foam filter over a stainless steel metal cage.

Is the rubber snorkle still in the top of the airbox or do you have a big hole there?

The baffle not being present in the exhaust is going to screw up the flow of the whole system and make it difficult, if not impossible to jet for and tune. I think it's useless to even try until you can remedy that with either finding a stock baffle for it or replacing the stock exhaust with an aftermarket one.

thank you. i will check all those things and report back

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22 hours ago, Trailryder42 said:

Set your spark plug gap to .026". If you don't have a set of feeler gauges, buy one.

When you clean the carb, you can not omit pulling the fuel screw and clean that passage and verify its spring, washer and oring are accounted for and in good shape, especially when the other components like the jets were so bad. The bike will run poorly, if at all, if that passage is clogged up.

Is there an air filter installed? The stock unit is a foam filter over a stainless steel metal cage.

Is the rubber snorkle still in the top of the airbox or do you have a big hole there?

The baffle not being present in the exhaust is going to screw up the flow of the whole system and make it difficult, if not impossible to jet for and tune. I think it's useless to even try until you can remedy that with either finding a stock baffle for it or replacing the stock exhaust with an aftermarket one.

i fixed the spark plug. now it starts easy, idles great, but when the rpms climb, it starts sputtering badly again.

the air box has a big rectangular hole in the top, no snorkel that i can see. 

EDIT i pulled the fuel screw from the bottom of the carb, found the washer, spring, o ring, cleaned the passage, will start bike up in the morning, see if it helped. fuel screw was two full turns out

what if my carb bowl oring is in bad shape? it is a little flattened looking

Edited by lanmind

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On 5/20/2020 at 8:59 PM, lanmind said:

Hello,

I'm in need of some help. I recently traded for a 2000 xr400 that didn't run. I was told it ran fine last time it was ridden (of course). The gas smelled like varnish, so i emptied it, replaced it, and cleaned out the carb. the carb was caked, jets clogged.

I put it back together, and it fired up 3rd kick, seemed to idle ok ish, bogged when i revved it. I drove it away, and it fired ok until my rpms climbed, then it started misfiring badly, maybe more misfires than fires. 

I figured I would change the spark plug, see what that does. I noticed the two ends of the new spark plug were much closer together than the one i removed. the one i removed looked darkened, but not gummed up, necessarily. the two parts of the new one looked like they were almost touching. I'm not very knowledgeable about engines, so I just installed it. 

This time, the bike had a hard time starting, and only started with the choke on half way. after it started, i turned the choke on full to see what would happen, and it died immediately. I started it again. It would barely even idle, had to have the idle turned up to stay on. I tried to drive it, and it misfired much worse than before, at any rpm. I was surprised the thing even ran at all, it was so bad. 

Couple of extra bits of info: I live at 4500 feet elevation. the main jet is 145, don't know what the idle jet it (didn't see a number on it). I think everything is stock, but the tail pipe has its end bit removed, leaving a big hole in the back, and making it much louder than a stock xr400. 

Also, I found a small (1 cm or so) broken O ring on the floor that must have come from the bike, but I have no idea what it is for. 

Is it possible the old spark plug is fouled, and the new one is way too close together? 

Any ideas?

Any help is very much appreciated.

Thanks!

Did you manage to find where that o-ring was missing from? That might be a good starting point 

 

*edit* sorry, my bad, I missed where you said you had accounted for the spring/washer/o-ring 

Edited by Twiggy0ne
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12 minutes ago, Twiggy0ne said:

Did you manage to find where that o-ring was missing from? That might be a good starting point 

I don't have a clue. none of the parts i disconnected were anywhere near that size. 

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On 5/20/2020 at 9:49 PM, Trailryder42 said:

Set your spark plug gap to .026". If you don't have a set of feeler gauges, buy one.

When you clean the carb, you can not omit pulling the fuel screw and clean that passage and verify its spring, washer and oring are accounted for and in good shape, especially when the other components like the jets were so bad. The bike will run poorly, if at all, if that passage is clogged up.

Is there an air filter installed? The stock unit is a foam filter over a stainless steel metal cage.

Is the rubber snorkle still in the top of the airbox or do you have a big hole there?

The baffle not being present in the exhaust is going to screw up the flow of the whole system and make it difficult, if not impossible to jet for and tune. I think it's useless to even try until you can remedy that with either finding a stock baffle for it or replacing the stock exhaust with an aftermarket one.

It worked!

two possibilities: either cleaning the fuel screw hole like you said was it, or I may not have seated the carb into the manifold tube properly the first time. When installing this time around, i noticed the carb went deeper into the manifold than the first time, and the two rubber knobs on the manifold tube rested neatly on either side of the carb knob. I'm pretty sure I didn't do this the first time, so it may have had a slight gap somewhere.

So the bike took off, and seems to be running perfectly.

Thank you so much for your help! You and all the people helping others in this forum are angels

Edited by lanmind
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New question: the bike now starts first kick every time, seems to run great, but the idle is not perfect. It doesn't seem to be able to do a comfortable low idle. If i keep the idle up a bit, it is fine. I also have a '97 that idles a lot better, so I can compare to see what it should be able to do. Another difference is that when i start the 2000, i have to take the choke off or it will die in 5 or so seconds. The 97 i can leave the choke on for much longer. I have tried turning the fuel screw, going from 1/2 turn to 2 1/2 turns out, and it doesn't really make a difference

Any suggestions for how to get it to idle better?

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Don't use the choke next time you start it from cold. See if it will start without it. If it does, the pilot jet may be too rich. You haven't stated what your jetting is.

You can still get the oring kit for the carb that comes with all the orings of it. It's not uncommon for the bowl and drain plug orings to flatten and need replaced from time to time.

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2 hours ago, Trailryder42 said:

Don't use the choke next time you start it from cold. See if it will start without it. If it does, the pilot jet may be too rich. You haven't stated what your jetting is.

You can still get the oring kit for the carb that comes with all the orings of it. It's not uncommon for the bowl and drain plug orings to flatten and need replaced from time to time.

i didnt see a number on my pilot jet. prob didnt look hard enough. think i'll leave it for a bit until i get a new spark arrester to see if that changes things, then i'll dive into the jetting. i'm waiting until i sell my other xr4 to buy all those bits.

thanks again

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