1998 XR400R bogs and runs rough past 1/4 throttle

[travisantonissen]

Hi.

I've had my 1998 XR400R for about a year now, and it's been running perfect. One day I was riding it hard, and it felt like something changed all of a sudden. Very fast, it was not gradual. 

Now the bike runs really rough pretty much past 1/4 throttle and completely dies at WOT. To add, the engine and exhaust note deepened significantly. 

I've done a carb rebuild, replacing the air idle mixture screw, pilot jet, main jet, and needle, and cleaned everything thoroughly, but I still have the same problem.

Are there any other causes for the issue I am describing? I'm thinking to replace the air filter next, spark plugs, and do valve adjustment.

Bike is all stock, no mods. Like I said, it was as if something snapped, broke, or changed all of a sudden. 

Thanks

Edited April 24, 2018 by travisantonissen

[Romy]

Maybe part of the exhaust baffle came loose, blocking the outlet? Exhaust note change would make me start with checking the muffler, it has happened. 

[S.O.A.N.Z]

Carb ACV ok ?

[Trailryder42]

2 hours ago, S.O.A.N.Z said:

Carb ACV ok ?

Carb ACV?? What is ACV?

I'd check to make sure the choke lever bracket isn't loose. The choke flipping on to full or half choke by itself in rough terrain because of a loose bracket has happened more than once.

When you had the carb off, did you happen to notice if the choke plate was all there?

 

[S.O.A.N.Z]

18 minutes ago, Trailryder42 said:

Carb ACV?? What is ACV?

Are you for real - type in google carburetor acv - wow amazing

[travisantonissen]

The air cut off valve (ACV, just googled it :] ) looked like it was all in tact. 

I tightened the screw for the choke lever, and the plate looked all legitimate. 

I'll start checking the exhaust. Thank you!

[Trailryder42]

19 hours ago, S.O.A.N.Z said:

Are you for real - type in google carburetor acv - wow amazing

I hate abbreviations. I always say Air Cutoff Valve. Reading ACV didn't click.

[S.O.A.N.Z]

1 hour ago, Trailryder42 said:

I hate abbreviations. I always say Air Cutoff Valve. Reading ACV didn't click.

All good - i dont click sometimes as well 

[countrykiller76]

My xr did that too. Tighten the boot that goes from air filter to engine. There's a leak

[Rockwraith]

Check your cam timing! If you were running it hard and your cam chain tensioner has seen better days and is loose, you may have skipped a tooth on the cam. Do a compression test. If you have gone through the carb and everything else looks good check the cam timing and the condition of your chain and tensioner. Common issue on the XR4. That is where I would go next. 

[travisantonissen]

Well, I checked the muffler, exhaust, I checked the boots that connect to the carb. Everything looks good. looks like I'm off to check the cam timing.

[GregWB]

Have you checked and re-checked the fuel? We spent days and numerous times pulling things apart looking for the same problem on XR400. We were convinced the fuel was ok, until we went and purchased some more!

[dirtjumpordie]

My XR did that.  Although it was after a rebuild.  The wire going into the spark plug cap was not pushed all the way into the cap.  So it had weak spark.  It would run fine at low load and low RPM.  It ran like shit at higher loads.  And at WOT it would cut out and backfire and misfire and stuff.  

Its an easy thing to check.  Otherwise check that resistor in the spark plug cap.  

[travisantonissen]

16 hours ago, dirtjumpordie said:

My XR did that.  Although it was after a rebuild.  The wire going into the spark plug cap was not pushed all the way into the cap.  So it had weak spark.  It would run fine at low load and low RPM.  It ran like shit at higher loads.  And at WOT it would cut out and backfire and misfire and stuff.  

Its an easy thing to check.  Otherwise check that resistor in the spark plug cap.  

I hope it's this! Bike has been sitting...

[geezer99]

Funny thing, this just happened to me.  The bike ran perfect one day, and the next it would not run without a lot of choke, or above 1/3 throttle.  It sure acted like too lean a mixture.  I was in the process of removing the carburetor to see what was up when I noticed the carburetor insulator, the rubber/aluminum thing between the head and the carb was coming apart.  As someone else pointed out above:  air leak!  Massive, almost completely separated.

So I'm trying to glue it back together with a very tough urethane flexible caulking compound.  The stuff dries to a very strong rubbery compound, but the real test will be to see how well it adheres to the rubber and aluminum.  I'll post back on results.

tom

[Casper50]

4 hours ago, geezer99 said:

Funny thing, this just happened to me.  The bike ran perfect one day, and the next it would not run without a lot of choke, or above 1/3 throttle.  It sure acted like too lean a mixture.  I was in the process of removing the carburetor to see what was up when I noticed the carburetor insulator, the rubber/aluminum thing between the head and the carb was coming apart.  As someone else pointed out above:  air leak!  Massive, almost completely separated.

So I'm trying to glue it back together with a very tough urethane flexible caulking compound.  The stuff dries to a very strong rubbery compound, but the real test will be to see how well it adheres to the rubber and aluminum.  I'll post back on results.

tom

Just out of curiosity, why not just buy a new one for about $40 instead of going through the hassle of trying to glue it back together and having a higher risk of failure again? Not only will a small hole lean your bike out but all of the unfiltered air that it's sucking in possibly causing damage. Don't get me wrong, I'm all for saving a dollar, but this has the potential of going the other direction to the tune of way more than $40.

[geezer99]

in responce to casper:

that's plan B.  Because the surfaces are quit broad, greatly suitable for adhesive.  Partzilla said they were not available.  I haven't looked at ebay yet.

meanwhile, home on the ranch:

I gave the urethane sealant a couple of days to dry, and I couldn't tear it apart with my bare hands, so I reinstalled it, and it's working great.  I'll report back in 6 months or so.  Meanwhile, if anyone wonders, the stuff I used to repair it is:  Sika, Sikaflex Construction Sealant.  Data sheet here:  http://usa.sika.com/dms/getredirect.get/us01.webdms.sika.com/951

please note that I'm not advocating for this product.  It was simple something I had on hand, and seemed to have a chance of working.  I don't race, don't ride dozens of miles into the desert by myself, or do anything else that would put me in danger if the bike dies.  I'm retired, on a budget, and like to experiment.  Most of the folks on this site probably don't want to "half-ass" stuff, but some of us do like to be independent, and are willing to experiment.

tom