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120 custom pipe dimensions

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Ok, still getting everything back together but before I even ride it I know this stock pipe will be holding me back on main jetting. Plus it hangs off the back like crazy now with the trials seat. Current setup is 58mm BBR piston in TB jug with ShiftUp Spartan head, Kitaco step 1 cam and 200X carb (15p/95m @6500ft asl). Stock pipe is one piece and has ID of 7/8” or about 22mm with no step. I intend on keeping the stock pipe since it hasn’t been gutted and a spark arrestor is required for OHV sticker but want to look at making a custom low pipe like the XR75 for the most performance. Seems like I remember 1 1/8” or 28mm being a size but I’d love some info on diameter of header and where to put step. 

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Well the snow melted enough to take it around the block a few times on 19/16 wheels. I definitely think it needs a 17 up front with the leading axle XR forks, feels a bit like a chopper with the soft stock pogo stick shocks. Might need to go a bit longer on the rear shocks but I’ll wait to see what wheel combo I come up with. As far as the pipe the bike feels like it’s really holding back up top. The inner diameter of the stock exhaust is 22mm at the header and actually gets smaller before it goes into the muffler ( I had a junk stock one I cut open to find out). The exhaust valve on the Spartan head is 23mm. I found somewhere that a good port to pipe ratio was 1.1 to 1.25 so I need about 25-28mm diameter pipe. Also runnning the 200X carb so I could go bigger there also. Any info or recommendation would be great! 

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Edited by FightingRed

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I picked up a broken FMF pipe on the cheap and hacked it up to make my own shorty muffler with spark arrestor intact. Repacked with fiberglass and new rivets. I’m using a stock XL100 head pipe that I modified and piece of steel 7/8 handlebar for a midpipe. The header is only slightly smaller inside diameter than the FMF but thicker walls which means quieter pipe and less likely to break again. Should have it welded up this weekend and then I’ll wrap it since I won’t be running a heat shield.

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We have a CRF125FB with a Yoshimura exhaust system. The external diameter of the head pipe starts at 25.5mm, with a single step up in diameter around 50% of the length of pipe. I believe the stock exhaust valve for that bike is 20mm, so your ratio of 1.1 to 1.2:1 pipe diameter sounds right on the money. The factory pipe was only 18mm external diameter and boy that bike was anemic until I swapped on the Yoshimura.

Edited by MarkfromNC

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They all seem pretty choked down for noise and emissions. Forgot to post a pic of the mocked up pipe before welding and wrapping. What do you guys think?

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