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XR 400 tune-up done, but now help needed to dial it in.

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Last week, I picked up a '01 XR400 and after absorbing all the best known methods here on TT -- I tore into it today to give it a once over.  My objective was to check the "gordon mods" and to perform the usual maintenance.  I did verify that the exhaust port, aftermarket can and desnorkling was done.  

 

I feel good about all of the steps I've performed, but I'm having some hot running issues that I'd like to see if I can get some pointers.  But first, I'll give the set-up.  When I picked up the bike, it ran well - started OK (I wanted it to be better). 

 

Here's a list of what I did:

  • Replaced filter with Uni filter (oiled it up first)
  • Adjusted the valves.  I think the right term is they were "tight" as the feeler gauge would not go into the gap.  Adjusted them to 0.004 intake and 0.005 exhaust.
  • Pulled the carb, did a minor cleaning (very little varnish present in the bowl) - replaced the existing jets (148/55) with (160/60).
  • Replaced Air/Fuel screw with Kouba FS-3 adjustable screw (turned out the suggested 1 1/4 turns per Kouba's instructions (the screw I took out was at 1 7/8)
  • Replaced spark plug (existing was fouled rich) with Denso IX24 plug
  • Changed oil / filter and cleaned screen.
  • Installed K&N fuel filter.
  • Used new fuel (premium - no ethanol)

When I was done, it cold started fine, ran fine.  Took it for a spin in the snow.  

 

Here is where I think I screwed things up.  I started to follow Kouba's instructions on dialing in the air/fuel.  This was done by bringing down the idle speed and then going -in / out on the air/fuel screw to find the middle between just about stalling on both ends.  In hindsight, I think I did this too soon (engine not yet at the right temp).

 

Here's my problem and what steps I took:

  • It was running fine at 1/2 choke (normal idle speed, etc) - then when moved to choke off, it would die
  • Here I adjusted the idle screw to increase idle.
  • Now 1/2 choke was still fast idle, but would still die when choke was "off" 
  • Increased idle screw more, but I noticed it would not "hot start" without 1/2 choke. 
  • At this point, I increased the idle speed screw even more and it would hold idle at choke off (albiet too slow and rough) 

This is my first dirt bike and I'm a total novice with regards to carburation, etc.  

 

Suggestions?  Thanks in advance.

 

 

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What is your elevation? I'm guessing it's pretty cold there if you're riding in snow. Humidity is a factor too.

I prefer to set my fuel screw with the idle speed set at where I am ultimately going to want it, not like you've done. You'll have better throttle response off idle and easier starting if you'll run a little higher idle speed. The trade off is that you'll have less engine braking when letting off the throttle when riding, but depending on what terrain you ride and your experience and speed, less engine braking can be a good thing. The bike will keep its momentum like a 2 stroke bike does when letting off the throttle.

I tune my idle speed to give me the specific amount of engine braking I want at the time and for the terrain I'm riding.

2 turns out on the fuel screw is the sweet spot you're aiming for when jetting/tuning the pilot circuit for best idle and throttle response, with 1.5 - 3 turns out being the acceptable range for a specific pilot jet before needing to change it.

Assuming the Kouba fuel screw taper is the same as the stock one, we around here always suggest starting your fuel screw tuning at 2 turns out and fine tune from there for best idle and throttle response off idle, on the rich side. That means, when starting at two turns out, with the bike at the proper warmed up temp, come out slowly with the screw until the idle begins to drop, stop there and begin to turn it back in slowly and stop at the point where the idle speed increases again. That's very close to best idle "on the rich side".

If you were to do the opposite, turn the screw IN and then back to best idle, that would be best idle on the "lean side". The fuel screw meters fuel. OUT is more fuel/ richer and IN is less fuel/ leaner.

When you replaced the fuel screw with the Kouba part, did it come with a small washer and o ring too? When you pulled the stock screw out, you should have had a spring, small washer and o ring come out with it. If not, you need to account for those parts. They are either still in the bore or weren't there to begin with. The new Kouba part also needs those. The spring goes on the screw first, then washer, then o ring, then you slide the whole assembly into the bore. Screw it in til it "lightly" seats, then back out 2 turns.

Be sure you're using the choke lever correctly. All the way down is choke off. All the way up is full choke. If you have that backwards in your thinking, you'll never get it to run right.

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What is your elevation? I'm guessing it's pretty cold there if you're riding in snow. Humidity is a factor too.

 

When you replaced the fuel screw with the Kouba part, did it come with a small washer and o ring too? When you pulled the stock screw out, you should have had a spring, small washer and o ring come out with it. If not, you need to account for those parts. They are either still in the bore or weren't there to begin with. The new Kouba part also needs those. The spring goes on the screw first, then washer, then o ring, then you slide the whole assembly into the bore. Screw it in til it "lightly" seats, then back out 2 turns.

Be sure you're using the choke lever correctly. All the way down is choke off. All the way up is full choke. If you have that backwards in your thinking, you'll never get it to run right.

 

Well - turns out I was going the wrong way with the choke - DOH!  Once I corrected for this mistake, all is well in the world.  I was able to tune in the Kouba part and adjust to the mid-point between the lean turn-in "fall off" and the rich turn-out "fall off".  

 

Good info on the idle speed and I wanted to answer about the Kouba part assy.  When I took out the stock air/fuel part, the spring and washer came out with it (btw - kouba doesn't provide these parts and need to reuse OEM).  I could see the O-ring, but it didn't come out on it's own.  Since I didn't have a replacement and to risk buggering it up using a pick to take it out, I just left it there and reassembled the Kouba part with the sequence of new screw, spring, washer, and by default, the o-ring already in the carb.  The shape of the Kouba part is a little different as it's more tapered at the part where the needle meets the main body.

 

My elevation at the house is 650 ft and it was below freezing yesterday and a little above freezing today.  Work is being done in a heated garage, so around 45F with the door slightly open.  My riding will be mostly around 1000-2000 ft.  I'll keep the 160/60 jet combo in the stock carb for now and see how it behaves.  

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I'll bet that jetting works great. And you are FAR from the first who has got the choke position wrong. Very common mistake.

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