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XR250 jetting - a little lean ?

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Did the first test today of my newly recommissioned 1991 XR250. I dispensed with the airbox and it breathes through a Uni filter connected directly to the carb.

It feels a little lean, and seems to want a little choke to start even when warm

Jets are 128 & 42 with needle J75 clip in the middle position. Would I be advised to richen a little and would that be both jets ?

 

 

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Edited by FFMan

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Needing choke to start when cold is normal. After the bike is warmed up, flip the choke to half, then full choke positions while it's running to see if it runs better or worse. If it runs the same or better, pilot could use bumping up.

Get out and ride the bike at 3/4 to full throttle to see if the main is too lean.

If you have problems with the bike stalling for no reason while riding, ditch that problematic stubby gas cap vent.

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> After the bike is warmed up, flip the choke to half, then full choke positions while it's running to see if it runs better or worse.

i did this by accident the other day. i forgot to take the half choke off once it was warm. it would not rev, died dead if i opened the throttle wide

 

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Mikuni's website has a tutorial on tuning, the main jet tuning exercise can also be applied to your carb, it has worked for me.

"The Roll-Off technique is the quickest and is almost as accurate as the Roll-On method. First, one gets the engine warm on the way to a safe roadway. If there is room, use fourth gear as this allows more time to assess the result.

Now, get the engine rpm high enough that it is on the cam and in its power band. This may need to be as high as 4000 rpm with some cam choices. Apply full throttle. Let the engine accelerate for a couple of seconds until it has settled in and is pulling hard. Quickly roll the throttle off to about the 7/8ths position. When you do this, the mixture richens slightly for a second or so.
If the engine gains power as you roll the throttle off, then the main jet is too small and you need to fit a larger one.
If the engine staggers slightly or has a hard hesitation, then the main jet is too large and you need to fit a smaller one.
"

http://www.mikuni.com/fs-tuning_guide.html

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thanks for the tip & link

i've never heard of this test, but the too lean scenario is exactly what I get on my kawasaki road bike. perhaps as i fitted a K&N i need a larger jet in those 4 carbs ?

i'll test the xr250 this weekend

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4 hours ago, coasty1 said:
  • how many turns are you out on the pilot jet ?

I hope you mean the fuel screw (pilot screw), as the jet itself always gets seated and snugged down. It is not adjustable.

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You will probably find the bike will be VERY difficult to get jetted properly without the airbox.
We've bought quite a few poorly running bikes with missing airboxes, we buy the parts off eBay and it fixes the problem.
Same with multi carb streetbikes.
Good luck tho

Edited by arthur6

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interesting point arthur6 - will see how i get on but bear advice in mind

 

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