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popping xr400 with mikuni carb

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Hi guys, picked up a 2000 xr4 with a mikuni pumper. It has snorkel removed, kandn filter, E series can and stock headers. My main complaint is that the bike pops like crazy when I let off the throttle completely, and usually stutters when I go from 3/4 to 1/2 throttle. The bike also stutters a bit at wot. I am at 7300ft. The current jets are 125main and 12.5 pilot. The pumper screw is set all the way in, fuel screw 3 turns out. I don't know where the jet needle is at, but I'm guessing 2nd notch if it was left stock. I really can't believe that I'm running lean, but is that most likely causing the popping? I also have a bit of off idle bog when I crack the throttle, although it's not too bad (better than the stock carb for sure.) What are your thoughts?

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One other bit of info...the bike came with a white brothers e series can with 12 baffles, but NO solid plate at the end. It is my understanding that this muffler is designed so that air escapes through the baffles and NOT a big gaping hole in the pipe. Could this be worsening/causing my popping on decel? thanks

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I have the same muffler and with no end cap you are completely bypassing the disks in the back. This means you have virtually no back pressure. I also just recently put on a mikuni and it was shipped with a 12.5 pilot and 130 main. Needle was in center position. My bike would not even run with this. I am currently at 17.5 pilot, about 3 turns out, and the needle clip is at the very bottom. My bike is still a tad lean. I have a 135 main coming and I hope this will get rid of the leanness that I have at between 1/3 and 100% throttle. I would recommend that you put your spark arrestor back on your muffler. I think you will find that the bike will like that much better. Good luck!

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I agree, you're probably lean. Richening the jetting might work with that open ended can, but I would try first to find a cap for it and then the jetting you have will probably be ok, plus you'll quiet it down and get some low end torque back.

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Got some sheet metal today and made myself a solid plate for my E series. The bike does not pop nearly as much and the sound is a lot more tolerable for my neighbors. I just removed my carb, after peeking inside, my needle is on the 2nd clip. Do you think this still might be too rich? What are you all running for needle position? Again, I am at 7300ft and bike is gordoned with stock xr400 motor. Please help!

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If you go by the Mikuni HS40 tuning guide, it says stuttering when suddenly chopping the throttle from 3/4 to 1/2 means too rich on the needle.

When tuning, you dial in the pilot first, then the main jet, then do the needle if required.

If the homemade end cap is the best you're going to be able to do and it's still popping when letting off the throttle completely, go ahead and drop in the 15 pilot and retune the fuel screw. If the bike pops on decel with the fuel screw 3 turns out, it's saying the pilot is too small. See how the bike runs in the 0-1/4 throttle range and see if it's still popping.

Then drop in the 130 main and run the bike from 3/4 to full, see if it runs out better. During that same run, see if you can recreate the stutter when chopping from 3/4 to 1/2. How is acceleration from 1/2 to 3/4?, smooth and crisp, or not?

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I put in my 15 pilot and adjusted fuel screw 3 turns out. Actually, there was no difference from 2 turns out until the screw almost fell out. went for a test, and The bike still pops some on when chopping to zero throttle, no popping when with slight throttle. Much better with the homemade muffler end plate. Now, however, the bike takes about 5-10 kicks to start at any time and the off-idle bog is worse than the stock carb. I have played all day with the come-on screw, and nothing seems to help it. I took the auto decompress mechanism off my 440 and am used to finding tdc and kicking. Now, I don't know what to do with the auto decompress...I just kick and kick and kick and finally it will start. What is your starting proceedure with the mikuni and when to use choke? Also, what is the real way to adjust the fuel screw on the pilot system? Eddie's method is different from the one in the Mikuni manual which is different from Trailryders. Eddies makes the least sence to me as he says to see how many turns from rough to smooth idle. The mikuni manual says to see how many turns from rough idle to rough idle (lean to rich) and split it (mine wont stutter on the rich side.) TR, you told me to count turns from bottomed out to smooth idle and change pilot depending on the number of turns. I'm pretty confused, this jetting is kicking my rear.

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I couldn't get the slight popping on decel out of my Mikuni carb. Someone told me it was normal for the Mikuni on an XR400R and that only with the XRsOnly pipe was there less popping. It was a used carb and I wasn't sure if it was worn or this was normal.

There are lots of bits on the Mikuni carb. Main, pilot, needle, needle jet, air jet, pumper settings. I wonder if we are overlooking the air jet?

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Sorry to hear this is being such a pain.

The tuning guide actually says no more than 3.5 turns out before going to next size larger pilot. Beyond that, the taper of the screw has no effect, plus you risk the screw vibrating out and losing it.

Tuning the fuel screw, I've always preferred the rough idle to rough idle method, but I don't split the difference. I turn it back in slowly til the idle picks backup/smoothes out, which may not necessarily be the middle of the two rough idle points.

As it is now, When you lightly seat the fuel screw and start coming back out with it, how many turns out before idle gets better? Sometimes it can be really difficult, as in your case, to notice/hear a change in idle towards the rich end of the fuel screw. But the change to smoother idle coming off lightly seated is usually pretty easy to catch, so using that point, you can guesstimate how many turns out too rich would be, or where the mid point would be between the two rough idle points, if you you could detect them both. Make sense? Find that first smooth out point and go from there, backing out the screw a little at a time, checking throttle response with each small adjustment. See if you can make response better that way, as you're coming from too lean towards richer as you come out with it, instead of visa versa.

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Thanks TR,

You've been a great help. Well, the problem with starting seems to have been due to the fact that I did not tighten down the intake boot clamp enough when reinstalling the carb. I was wondering why the carb was coughing at me and sounding like a lean condition. The mikuni doesn't seem like it has enough intake or exhaust area to get a good clamp on. Anyway, it starts better now, although not as easy as my 440 with stock carb and autodecomp removed.

Now, regarding the fuel screw, the bike begins to get audibly lean at 2 turns out with the 15 pilot jet. Does this tell me enough to decide if that is the right pilot size jet? It seems to get best throttle response at 3 turns out. I just can't get over how gutless the stock 400 is compared to the 440, especially on the top end. It may be time to do some testing with needle and main jets, but I'm thinking overbore is in the near future.

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I know what you mean about the stock 400 seeming gutless compared to the 440.

I agree, the adapters they put on the Mikuni to interface with the XRs intake boots could have been longer. It can be a pain to mount and get an adequate seal if you're not careful.

3 turns out seems to be the target point for the fuel screw according to the tuning guide, so if that's where you have it for best throttle response off idle, go with that. Work on the main now and it's throttle range.

I didn't get my Mikuni til after my 440, so I'm unfamiliar with its performance on the 400. The procedure for jetting it is the same tho.

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