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Discussion Starter · #1 ·
Hey guys so I purchased a 78 Yamaha xs750 late June which was running fine and hasn't ran since then. I did some work which included a carb rebuild and now I get it to try to turn and almost hear it turn with the engine almost coming to life. I do believe it's the carb since I have done all tests for other problems (compression, spark, and starter) but I don't know what to do with the carb since I think I have it all perfect. The carb for reference is a Mikuni mk ii bs34 if I'm not mistaken. I have recently ordered new jets that the manual recommended a 145 main jet and a 42.5 pilot jet, not recommend by the manual due to the size not being on jetsrus but I'm using their recommendation for my specific bike. Any idea on how to get the carb to working order again? Thanks guys.
 

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Ghost in the machine
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Is the bowl getting fuel? You can check by opening the drain screw. If the bowl is filling with fuel, my suggestion is to remove to carb and do a complete cleaning again, making sure there was nothing done wrong in the reassembly. Another thing to watch for (I recently had this problem) is that some rebuild kits can contain parts which are not to spec with the originals. CAREFULLY compare all the main jet parts (especially the jet guide) with the original that you replaced. I recently had an issue with a main jet guide from (carb specific) rebuild kit that was barely noticeably different from the original, that caused me to pull my hair out.
 

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Discussion Starter · #3 ·
Hey man, thanks for the reply. Yes they are getting fuel. I actually had a leak in one I had to fix, float needed to be better cleaned, and actually while I was checking the leak I did a whole nother clean. I let it soak with as much as I could remove off the carb for about two days. I looked in the manual of the bike and saw the main jets I have, oem equivalent 130, is not what the manual said I needed. It says I need 145's which seems high but I ordered new genuine Mikuni jets with the correct sizes. Another question; my pilot screws stick out about .75 of an inch. Pilot screws scare me as I don't know what a soft seat is and don't want to hurt it. How far should they sit?
 

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Ghost in the machine
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First off, don't be scared of a "soft seat". You will be able to tell when it hits bottom, just don't try to "tighten" it to the bottom. then back it out to the OEM specs.

As far as specific jet sizes, I can't speak to that specific model. I find that when in doubt, I check a site like BikeBandit, look at the diagrams for the carb and use the sizes that are shown for OEM.

Lastly, I must tell you that I only consider soaking as a single step in the proper cleaning of a carb. All ports should be cleared (I use compressed air), and very small, barely visible, holes (like the one that feeds the idle) are best cleared with a very fine wire, like a small guitar string.
 
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