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Evening All,

My first thread on here having trawled through looking for answers earlier today - no luck, so time to sign up and try properly!

I need some help identifying a probable cause for my brother's XR400's misfire. He bought it a year or more ago as a seized up 2004 model, which was in good condition (apart from the seized lump and the seized swing arm mount). He bought the parts and put it back together as a 440cc and when he first fired it up, it ran well apparently. It has hardly been used since, but over the past few trips out, it has deteriorated and now wont rev out at all. It starts well (for an XR) and is happy to tick over, but as soon as you turn the throttle any more than a fraction it splutters as if hitting a rev limiter.

I did a few hours on it on Friday and you just have to keep it working on it's plentiful torque, as any throttle just renders it useless. He has a few other bikes in his shed, so doesn't worry about fixing this one, but when I'm over there visiting him, it could be a perfect trail tool I reckon.

I'm no mechanic but he is, so any input from you guys as to what this may be would be appreciated. My first thoughts were plug cap or carburation, but some experienced thoughts would be useful.

It will rev much better in neutral than it will in gear by the way - but hold the throttle open in neutral and you can hear it struggling then too. When riding it, it prefers to just be upshifted at around 2K rpm

I read about the butterfly flap in the carb breaking up on these and dropping into the engine causing horrible messes - I don't know if that was why this one seized or not.

Any help much appreciated.


Mike
 

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Hi Mike

Sounds like its your carburetor and it has a blocked main jet, time to strip it down and blow your jets through with compressed air.
 

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The Angry Pasty Muncher
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6,170 Posts
Evening All,

My first thread on here having trawled through looking for answers earlier today - no luck, so time to sign up and try properly!

I need some help identifying a probable cause for my brother's XR400's misfire. He bought it a year or more ago as a seized up 2004 model, which was in good condition (apart from the seized lump and the seized swing arm mount). He bought the parts and put it back together as a 440cc and when he first fired it up, it ran well apparently. It has hardly been used since, but over the past few trips out, it has deteriorated and now wont rev out at all. It starts well (for an XR) and is happy to tick over, but as soon as you turn the throttle any more than a fraction it splutters as if hitting a rev limiter.

I did a few hours on it on Friday and you just have to keep it working on it's plentiful torque, as any throttle just renders it useless. He has a few other bikes in his shed, so doesn't worry about fixing this one, but when I'm over there visiting him, it could be a perfect trail tool I reckon.

I'm no mechanic but he is, so any input from you guys as to what this may be would be appreciated. My first thoughts were plug cap or carburation, but some experienced thoughts would be useful.

It will rev much better in neutral than it will in gear by the way - but hold the throttle open in neutral and you can hear it struggling then too. When riding it, it prefers to just be upshifted at around 2K rpm

I read about the butterfly flap in the carb breaking up on these and dropping into the engine causing horrible messes - I don't know if that was why this one seized or not.

Any help much appreciated.


Mike
Bad news about fitting the 440kit the motors can't can't cope with them for long ( my previous posts will show that) every bike i know of thats had a 440 put on it has let go quite badly. Check your valves that will make it start easier. Jetting, mine was changed by previous owner when th ekit was fitted, with the kit i think mine went up to a 179. Is it spluttering due to fuel starvation or is a mis fire?? mine had a big flat spot which would sometimes seemed like a mis fire. mine is all back as standard now and is far better than the 440 to ride, although the 440 was a fun fast ride but over-revving and metal fatigue sorted that out normally 440 kits get put on as a cheap fix when the motor's seize.
If your friend is a mechanic he will have no problems sorting an xr as there nothing to them. You don't need to remove the carb to check main jet just undo the 17mm nut in bottom of carb and then unscrew the jet with a 6mm spanner. Also if it hasn't been used for a while drain the carb and use fresh fuel
 

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Red Blooded Biker
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Check the seal between the inlet manifold & the cylinder head...if the seal has deteriorated like i've seen before, they leak air past the seal & weakens the mixture. This makes the carb quite difficult to set up the air/mixture screw.
 

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If it was running ok and now it isn't...it is unlikely to be caused by wrong jetting.

Check the simple things first, before the 'Doctor Dooms' have you ripping your engine apart.
First, are you sure you are turning the choke fully off? It has 3 positions:

Down- fully off
Up 1- half choke
Up 2- full choke

Also: Check the airfilter is clean, properly fitted, and nothing has got past it into the intake. Sometimes on old negletted machines I have seen the airfilter foam degraded so bad, it was breaking up and going down into the intake!! If the airfilter looks dirty or poorly maintained, I would suspect your main jet could be blocked.
Like david247 says..check for blocked jets anyway, and contamination in the float bowl cap, it is an easy job.
Also change the spark plug regardless, it may be breaking down under load.

It's true a 440 kit may need to have the Carb rejetted to suit... but it is usually just fine tuning that is needed, and not enough to cause this kind of drastic poor running.
 
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