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XR4 jetting detail - fine tune pilot and screw

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I finished grinding welds, drilling 3 holes in SA. no snorkel, with stock carb (52, 142, stock clip position, front mixture screw two turns out) New plug. I started bike. It was hard to start and gave a popping backfire at idle and wouldn't idle well. Turning the mixture screw out didn't help much. I put in a 55 pilot jet.

Started better, but needed choke for a long time. Didn't idle well. After it was warm, the back fire was less. As I turned the idle adjustment in and the mixture screw out a little at a time the RPM got better with every turn out of the mixture screw. I kept dropping the RPM with the idle thumb screw and turning the mixture screw out as long as the revs increased. I repeated this fine tuning until I maxed out the mixture screw (it fell out).

Do I need to go up one on the pilot? Will the main have any influence on the idle? Is this the right way to get the mixture screw and pilot fine tuned? Any advice is welcome.

I'm at 3000 feet, 65 degrees indoors where I'm working on it. We ride at 4000 and up.

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I'm sure others will chime in, but I'll tell you what I learned from hours of reading up on this exact subject... from what you are saying it sounds like you are still lean at idle - go up on pilot jet. I suggest you try a 58 or 60.

The correct procedure is to start with the fuel screw about 3 turns out and start turning it in until the idle rpms are maximized, then turn it out 1/4 turn from there and leave it.

The needle should be in the third (middle) position.

The main jet does not have any effect until you are open over half throttle. But, I would say you will be very lean on your main jet. Try a 160 or 162.

I am at sea level and currently running 60 pilot and 160 main jet. My fuel screw does not affect idle rpms (until 1/2 turn out, and then it reduces rpms) so I need to go down another size. I am going to try a 58 pilot this weekend and see if I can get the fuel screw to have the effect I described above. Oh, and I am running a Uni filter and FMF PowerCore IV-Q pipe.

I hope this helps you.

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here's a starting point:

0000 ft > 160 / 60

1600 ft > 158 / 58

3200 ft > 155 / 55

5000 ft > 152 / 55 (54 act., turn pilot in more)

6500 ft > 150 / 52 (53 act., turn pilot out more)

9800 ft > 148 / 50

adding one of these also help adjust the pilot screw:


see the order form (the xr4 carb is the same as the klr650)

i use mine all the time for adjusting for elevation changes.

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