At my wits end with this buckin' xr400!

Long time reader but first time poster. I need advice here.  Been riding my 98 xr400 for about 8 years now and love it, except when it isn't running right.  Like now :thumbsdn: .  I am getting her ready for spring riding so i cleaned out the carb (clean, blow out, pressure test), poured in new clean gas, new plug and fired her up.  I adjusted the pilot jet and then the valves.  She starts up within 8 kicks cold and idles fine.  But when i open up the throttle she" pops" and "spits".  Hard acceleration cleans it up but then when i am just riding at a steady throttle opening she bucks and pops.  This happens in any gear at any speed.  I am basically out of ideas.  Any help would be greatly appreciated! :thumbsup:

Too lean on the pilot?

This may be a really silly question but did you check and verify the float level and that you have good constant fuel flow from the petcock?

 

Also...  The tank fuel screen in the old XRs deteriorates and keeps sending fine particles to the float bowl.  Take the carb off.  Clean it.  Works great.  Runs bad after a while.  Repeat.  Go insane.  Our XR250 drove us crazy with that issue.

 

Also make sure the carb is pushed all the way into the intake boot.

Edited by VortecCPI

You adjusted the pilot jet? Do you mean adjusted the fuel screw or changed the pilot jet?

 

If you didn't remove the fuel screw, its spring, washer and oring and clean out that passage when you cleaned the carb, you didn't do a complete job. All it takes for the bike to run like crap at idle and off idle throttle positions in an otherwise spotless carb, is a dirty/blocked fuel screw passage. Look at the orings condition closely too.

Have you checked that your decompression cable hasn't got moved out of place a bit or is maybe just barely holding tension on the little decompression lever on the head ... it just takes a very small amoint to give symptoms like you say are happening..

One more thing that drove me more insane than Im already. There are two small holes in front of carb,Even tho I cleaned them.I did not do good job.It did same thing.Get carb cleaner,with nozzle,blow the holes out with hi air pressure.While wet with carb cleaner,or maybe small wire off wire brush. Also check green ground wire on coil bolt.Also check spark plug cap,and where it connects to coil wire.If nothing fixes it.Replace pick up coil /pluser. Arter 7 years can go out at anytime.

As you can see, there are many possibilities.  

 

Eight kicks is no good.  It should start in one or two kicks when cold every time.  Did the bike sit a while to develop this problem?  How long?

 

Also, did you open up the exhaust or intake without changing jet sizes?  What size jets and needle clip position?

 

It's most likely a carb problem but how far out were the valves?  If you adjusted them a lot, you may not have actually been at TDC. 

everyone! Thanks for the feedback!! The bike has always been a 8 kick starter even when she is running great. 4 kicks with decomp lever in and throttle open then in 4 she fires up. When hot one or two kicks and on occasion she will start cold in two. But not usually. She sat for 6-7 months and ran like a top when i last rode her.not sure what size jet but it is the same one i have been using for years. Needle clip is the middle one i believe. Again. ..no change on it for years as well. Valves were not far out of adjustment. I am familiar with that procedure so i am confident i was at TDC. I cleaned the carb well an used pressurized air to blow it out. Reset the pilot to 1.5 turns out (98 xr) and then tuned the bike per the manual. I am sure it is something simple i have just overlooked. I will check the decomp lever (it has proper play but i have experienced it pulling open on hard left turns before), boots to carb and tank screen (didn't even know bout that one!). The float is set right and its getting plenty of fuel. Frustration is probably part of my problem. So im tsking it dlow and methodical. I'll let you all know my progress!

everyone! Thanks for the feedback!! The bike has always been a 8 kick starter even when she is running great. 4 kicks with decomp lever in and throttle open then in 4 she fires up. When hot one or two kicks and on occasion she will start cold in two. But not usually. She sat for 6-7 months and ran like a top when i last rode her.not sure what size jet but it is the same one i have been using for years. Needle clip is the middle one i believe. Again. ..no change on it for years as well. Valves were not far out of adjustment. I am familiar with that procedure so i am confident i was at TDC. I cleaned the carb well an used pressurized air to blow it out. Reset the pilot to 1.5 turns out (98 xr) and then tuned the bike per the manual. I am sure it is something simple i have just overlooked. I will check the decomp lever (it has proper play but i have experienced it pulling open on hard left turns before), boots to carb and tank screen (didn't even know bout that one!). The float is set right and its getting plenty of fuel. Frustration is probably part of my problem. So im tsking it dlow and methodical. I'll let you all know my progress!

Another thing to consider is the CDI.  Although uncommon a bad or faulty CDI can cause all sorts of bizarre issues.

Sorry for the dbl post by the way.  On my cell phone.

This may be a really silly question but did you check and verify the float level and that you have good constant fuel flow from the petcock?

 

Also...  The tank fuel screen in the old XRs deteriorates and keeps sending fine particles to the float bowl.  Take the carb off.  Clean it.  Works great.  Runs bad after a while.  Repeat.  Go insane.  Our XR250 drove us crazy with that issue.

 

Also make sure the carb is pushed all the way into the intake boot.

This. You must install an inline filter. I think you have some crud in the pilot circuit, the jet itself may be clean but the passageways are clogged. Clean the carb again, inline filter and new or perfectly clean pilot.

Just the fact that it needs 8 kicks,and the way you have to start it.Not Right. 10/4 on fuel filter inline.

Try replacing the jets in the carb.  also verify you do not have an air leak on the intake side, and what about checking the valves?

Ok! Bike is fixed and purring like a kitten. The main jet was set to max fuel. Took it back a notch and all is well. Nothing changed since i last rode but this fixed it. So "Murphys law" applies here. Thanks for all the great help and ideas everyone!! My bike is better tuned than ever before! Gonna ride like a banche on Saturday!

The main jet was set to max fuel. Took it back a notch and all is well. Nothing changed since i last rode but this fixed it. 

 

Am I the only one confused by this?

no I spent 30 minutes trying to find reference to it, I thought maybe it was the clip setting or something?

my XR4 is still hard to start but it been to cold to work on it  :cry:

Am I the only one confused by this?

 

No. I am too. I remember not too long ago, a post of a guy that thought you tuned the main jet by turning it in and out.

Ok! Bike is fixed and purring like a kitten. The main jet was set to max fuel. Took it back a notch and all is well. Nothing changed since i last rode but this fixed it. So "Murphys law" applies here. Thanks for all the great help and ideas everyone!! My bike is better tuned than ever before! Gonna ride like a banche on Saturday!

 

Are you speaking of the idle mixture screw at the front of the float bowl?  The main jet has no adjustment or setting.

Edited by VortecCPI

No. I am too. I remember not too long ago, a post of a guy that thought you tuned the main jet by turning it in and out.

Me three. Other things make me think  the OP is not "murican!

Edited by YHGEORGE

Ok, ok, ok!  YHGEORGE i am "murican" and my honarable discharge from uncle sams misguided children hanging on my workshop wall will more than prove that!  i guess my terminology regarding the "main jet" is incorrect. The c-clip on the large needle was reset to a higher slot as where it was set allowed a very rich mixture of fuel through the carb causing it to pop, buck, backfire and blow black smoke.  Once the c-clip was moved the bike fired up right away and is running really smoothly.  Does that make sense to all?  Again, i really appreciate the advice from all y'all.

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