The number refers to the size of the opening in the tip of the jet in hundredths of a millimeter, so a 100 MJ is, in theory, 1mm in diameter. Main jets are numbered and usually are in a range of about 90 to 140, with stock motors usually being in the 98-110 range. There are, for our purposes, only four jets and one adjuster screw in play in an SI carb:ġ) The Main Jet (MJ) - The little jet at the end which mainly impacts Wide Open Throttle (WOT) and which people just getting started with jetting seem to think is the only jet that matters. Next, let's make sure we're all using the same terminology. I finally got around to producing a cleaner version and wanted to share it here, along with some basic guidance on how to jet an SI carb, because this comes up all the time.Īs always, feedback is appreciated, because while I'm not bad at this, there are always others who are better. Official discussion thread for the wiki page: Wiki Guide: SI Jetting Chart + How to Tune an SI CarbĪ while back over in another thread, I threw together a quick n' dirty hand drawn jetting chart. Updates to this article are being made over on the Wiki page: Wiki Guide: SI Jetting Chart + How to Tune an SI Carb, so look there for the latest & greatest version! Tl DR: Don't use this thread for scenario-specific questions start your own thread. ![]() It benefits everyone, because it will allow you (and the rest of us) to keep the conversation cogent and focused on your problem, meaning better guidance, plus provide you with a log of your tuning steps and process. If you have questions regarding your own setup or carb tuning, please start your own thread! Not recommended - except for the really impossible to find jets - and then get your self a set of jet files.īut the old scab XR runs like a scalded dog now and cost Nothing but time and patience !Please Note: This thread is for discussing the content of the SI Carb Tuning Wiki Page. 6mm = very small hole - had to use a torch tip cleaning file & strands of wire for measuring size ![]() Now drilling out pilot jets is a much more involved process !!Īs they are only. 160 mm or something like that - it will be obvious when you measure the drill that fits in your 155 I was working on an old scab xr and didn't want to spend any $$$ġ60 is. ![]() If you have a digital vernier caliper you can measure the drill and get the one that is the right sizeīut jets are cheap so buying a 160 is only going to cost $5. I have drilled jets with good luck - just file off the original # or scratch it out so you know it has been modified If you have a set or access to a set like that then give it - what can you hurt the 155 is not right ![]() I do know that Mikuni, Dynajet, and Kehin all have different sizes for the same number: mostly because I'm not sure how they measure the jet size. but I'm not sure thats what you want to do.
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