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About to mod a KDX 175, any tips?

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I just bought a 1980 KDX175. I took it out this week end to establish a baseline for the engine suspension etc. befor I start to modify it. After it was good and warmed up, I took the stock silencer / spark arrester out. I just did it temporarily to see how it ran, thinking if it ran better I would drill it out and put the hap back on the exhaust pipe and rejet. What I noticed was more low end grunt. So my idea is to drill out the silencer and put the cap back in the pipe, cut a larger hole in the air box, then increase the main jet size. Anyone have any experince with this? I am running super unleaded pump gas with Golden Spectro at 24:1.

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todays oils are so good the first thing you should do is change your ratio. Fully synthetic 2stroke oil is completely safe to use @ 40:1. You would be hard pressed to find anyone who will agree to any exhaust hacking. Seat of your pants impression of more low end grunt is probably wrong, as a free flowing exhaust usually benefits top end with a LOSS of low end. Do yourself a favor, pull off the jug and smooth out the ports, put a fresh set of rings,REPACK your silencer. That may or may not require jetting changes. Cutting airbox is ok idea, just stay away from water. The key would be to spend effort getting the suspension sorted and that will make you faster than any engine mod

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Thanks for the info. I have changed the fork oil to a thicker oil, 10w. It helped a bit, as it looked like the original oil. I am planning to go with heavier springs or doing a fork swap from a bike with thicker tubes, like a KX maybe. The rear suspension is in pretty good shape but it looks like the linkage pivot arm bearing is worn out.

As for the ports, the engine was just resleeved with an iron sleeve by a guy who has been building race bikes for 40 years, along with new crankcase bearings, new crank, rod, piston and rings. KDX 175's dont have packing in the silencers. It is just a long tube with holes drilled in it and the end welded with a plug in it. I'm not sure if it actually has more "low end" with it out, but it definately had a quicker response to the throttle at the very low end. When I was creaping in 1st or 2nd over technical terrain, it responed a lot quicker when I snapped the throttle to pull the front wheel up. I did a similar mod to an old DT 125 and it had more power through out the entire rpm range. More air in, more exhaust out. Just wondering if anyone had ever done it to a KDX 175.

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I will guarantee that there was pacing in there at one time, for your info there have been many studies that have proven a freshly packed silencer will make more HP than worn packing or oil saturated, which by the way if you continue to run 24 to 1 you'll have. Seriously, I would save my money, your working on 30 year old technology and unless your just working on it as a project, your wasting your money.

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If your handy with welding you could cut the expansion chamber open and remove the internal baffling. The old enduro bikes, PEs, KDXs etc had a mesh type baffling against the inner walls of the pipe. Some even had a matting of sorts to quiet them down. I know the PEs have it and late model KDXs also. I know that DG made a pipe for it back in the day and Answer used to make a silencer for them also. You might be able to dig them up on the various vintage sites or the popular auction sites.

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Thanks for the input. The studies you referred to were correct. A fresh clean silencer will get more horse power, becaue a clogged one will restrict the flow of exhaust. If you remove the packing then there is nothing to clog up. However, on the KDX 175 you don't have to do that because they never came with it. I know you gauranteed it, but they dont have it, never did. As far as cutting out the wire mesh, I had an old Yamaha that I did this on, (pics on the site somewhere) and re-welded the pipe back together. It worked nice. But on the KDX you dont have to cut the pipe at all, there is one bolt and the entire piece comes out. You can look down into the hollow pipe. That is why it would be such an easy (and cheap) mod. I am based in reality, I know it will not make a huge difference, I just noticed a little bit quicker throttle responce on the low end.

I know before you all go on about a 30 yr old bike and all that. I have been riding for 36 years, and have had the new disco styled bikes (2004) kx 250 and they were fun and fast. But there is something to be said for having a simple trails oriented 2 stroke that hauls butt, with no radiators to break, no water pump to seize, no power valve to get stuck, and an engine that can be rebuilt in the garage in half a day for $200. So, if there is any one else out there that loves vintage old school bikes just for the sake of having a vintage bike and likes to make a few minor changes to tune the bikes, I'd like to hear from you.

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I have quite a few old bikes (71 Maico 250,73 Maico 400, 74 Honda CR250, 77 RM 125, 82 RM 250) but I ride my 2 KX's and my KTM. I like the old bikes and have no intention of getting rid of any of them, the 71 Maico is my second bike I ever owned. I do take the RM's out for a rides sometimes. One day I may try some vintage racing but I don't really care to race just have fun. The old bike are just plane neat but I sure like the newer stuff to ride.

Good luck with your bike and have fun with it.

Ed

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I don't have many hop-up pointers for your 175, but I will agree that it's nice to have a simple vintage bike with long travel suspension. I just picked up an '87 KDX 200 which I'm in the process of reassemblng. The previous owner sucked a mouse nest through the motor when trying to start it three springs ago. The bike is bone stock, with the exception of an old Answer silencer. Oh, and it picked up an incorrect KX left fork leg at some point in the last 24 years. It's a fun, simple project. Maybe not as simple as it could be, since I'm having some trouble tracking down parts for the linkage push rod. It has spherical bearings and special bolts with grease fittings that nobody makes aftermarket replacements for. Here's hoping the Babbit's OEM order comes up with some parts. Otherwise, I might have to get creative.

About the only hop-up hints I can offer would be opening up the air box with some holes drilled in the cover, DG exhaust if you can find one, maybe have some one smooth the ports a bit when the top end is off, rejet, make sure the suspension (and linkage!) is in top condition, set your sag, and twist the throttle futher!

Oh, and some pictures would be nice... :bonk:

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dfeckel the old full floater Suzuki's used spherical bears in that setup that should be close to the same size, you might check them out.

Ed

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Im having the same issues as dfeckel with my 84. Let me know what you come up with.

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What is the OD and ID and width of them and I will check my old stuff. I have one floater frame in my storage shed.

Ed

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I believe you can still get a universal muffler just need to know the size of pipe of the stinger end of the pipe. but it will be alot louder, pic would be good I owned one and put alot of time on it. I do agree with you on taking it out I just took out my 76 yz 250 out on the ice and my friend brought out his 76 rm250 and the race was on.

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What is the OD and ID and width of them and I will check my old stuff. I have one floater frame in my storage shed.

Ed

Thanks for the offer on checking out the full floater. I don't have the measurements handy, but I'll post them up when I get them. I'm hoping that I'll be able to get what I need OEM, but I haven't heard from babitts, yet.

Worst case scenario, I might just fab up a steel push rod that will let me use off-the-shelf needle bearings and seals of the appropriate width. If anyone knows a reason why that wouldn't be a good idea, please chime in.

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its very possible a air cooled kdx200 cylinder fits.

call Ferdette Racing.He has been racing Kdx since they were 175.He knows more about that bike than any human ever could

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