Mikuni VM34 Jetting Advice


XS400 Member
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Miami, FL
Hi to all members here on the forum. I would like to share some thoughts on my experience dealing with aftermarket carbs. I got my 1980 XS400 for $200 from a widow. The gas tank was perforated and the bike had been sitting on a garage for 6 years. I tried to keep the original carbs but one of them had the pilot jet stuck and was stripped inside so I could never get it out. The other carb had one of the pieces where the floats are mounted broken, plus both diaphragms had holes. So I decided to go aftermarket. After that I first tried some PWK38's in Amazon at $29 each and I found that these worked good. But the idle screw was a pain to work on. I returned them and bought two Mikunis VM34's, left and right. These come with a 35 pilot jet and a 260 main. I tried them with this original setting and got them to idle good. The problem was that the bike would not go over 4,000 rpm and could not go above 40-42 MPH. I changed to 42.5 pilot and 160 mains. The bike run better and idle good too, but although the RPM's went over 4,000, I still felt that the bike could do better. I then changed the mains to 135 and now the bike runs great. I can feel the power and I feel like she is free now with no restrains. My current combination is 135 main and 42.5 pilot although I think that the 35 pilot would do good too. I did not move the jet needle nor made any other changes to the carbs. I wanted to share this because I tried to get information on these carbs and could not get real input from people that actually owns them and have them working on a XS400. I hope this could help our fellow members out there that are dealing or deciding on carbs. I also have a 2013 Triumph Thruxton but for some reason the XS400 is my favorite to ride around, although not for speeds of 55 and above because of the vibration. Just 3 weeks ago I also bought a 1982 XJ650 Seca completely disassembled. Now I'm in the process of assembling it and getting her to run. The XS400 has the original exhausts and I put these filters on the Mikunis. https://www.amazon.com/gp/product/B00SQNJXME/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1. I attached here some photos of the XS.... and the Thruxton.


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Thanks for your input. You may want to use a much better filter as those are nothing more than a screen. A set of black foam uni's would filter dirt and dust much better. Just make sure to oil them first.
I have a set that fit 54mm carbs. They are in great shape and barely used. $28 shipped
Thank you. I don’t see how that will fit on mine. I already bought a pair of pods. I will let u know if those don’t work.
Great to here!! The longer the pod and foam backs are even better!! But I am glad to hear you have it sorted now.