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Yamaha MX175

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Yamaha MX175

Postby ap72 » Mon Feb 14, 2011 4:54 pm

Does anyone have any info on modifying this dirt bike? Its a 2-stroke and I'd rather follow something tried and proved rather than experiment since I'm new to 2-strokes.

Its a 82 model I'm fixing for my brother (it was his as a kid) and I would like to make it just a little nicer, I'm open to anything from milling to carb modifying to porting, etc, just give me some direction please!
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Re: Yamaha MX175

Postby levisnteeshirt » Mon Feb 14, 2011 10:22 pm

follow this site

http://www.macdizzy.com/

i used to have a friend with one of those ,, fun bike
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Re: Yamaha MX175

Postby Unkl Ian » Mon Feb 14, 2011 11:30 pm

This one was posted here recently: http://www.bimotion.se/
Just because you never studied the Laws of Physics,
doesn't mean they won't try to kick your ass.
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Re: Yamaha MX175

Postby JES » Tue Feb 15, 2011 12:54 am

Not trying to upset anyone. But that bike and engine combo has very limited potential. It would need a special expantion chamber, highly modifyed ports and reed mods, a much larger carb/free flow air box etc. That bike was ment to be a play/beginner bike when new. Not an MX bike as the name suggest. If all this were done, the suspension would still be inadequate. IMO, just rebuild like stock. Clean-up on the ports is O.K. but don't change the hight or shape. Make sure to clean the silencer at the end of the pipe, as this likes to get clogged, and really restrict the power. All this said, it is still a fun bike to ride, just not a fast one. John
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Re: Yamaha MX175

Postby ap72 » Tue Feb 15, 2011 10:02 am

I actually don't want this to be too fast, so that's good, I just want to improve the low end and throttle response, make it more enjoyable to ride.
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Re: Yamaha MX175

Postby af2 » Tue Feb 15, 2011 11:04 am

ap72 wrote:I actually don't want this to be too fast, so that's good, I just want to improve the low end and throttle response, make it more enjoyable to ride.


The very first thing I would do IF THE ROD IS TIGHT would be to cut the squish down to .025 and be 50% of the bore circumference. You might have to also cut the top of the barrel to get there. I have found on a trail type bike 175lbs is pretty much max compression.

Without touching anything else you will be suprized how it acts.
Have fun!
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Re: Yamaha MX175

Postby ap72 » Tue Feb 15, 2011 11:09 am

af2 wrote:
ap72 wrote:I actually don't want this to be too fast, so that's good, I just want to improve the low end and throttle response, make it more enjoyable to ride.


The very first thing I would do IF THE ROD IS TIGHT would be to cut the squish down to .025 and be 50% of the bore circumference. You might have to also cut the top of the barrel to get there. I have found on a trail type bike 175lbs is pretty much max compression.

Without touching anything else you will be suprized how it acts.
Have fun!


That's something I can easily do, thanks for the tip! is .025" too tight though? I get nervous whenever I go past .035".
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Re: Yamaha MX175

Postby af2 » Tue Feb 15, 2011 11:16 am

If the rod bearings are good you will be fine. I have not had 1 give any witness marks at .025.
Another thing I do is polish the chamber. It makes it nice to clean down the road on a freshen up.
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Re: Yamaha MX175

Postby williamsmotowerx » Tue Feb 15, 2011 11:47 am

.025" is way way to tight for a roller/needle bearing crank. I would put it at .050"
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Re: Yamaha MX175

Postby gasman5.0 » Tue Feb 15, 2011 12:15 pm

JES wrote:Not trying to upset anyone. But that bike and engine combo has very limited potential. It would need a special expantion chamber, highly modifyed ports and reed mods, a much larger carb/free flow air box etc. That bike was ment to be a play/beginner bike when new. Not an MX bike as the name suggest. If all this were done, the suspension would still be inadequate. IMO, just rebuild like stock. Clean-up on the ports is O.K. but don't change the hight or shape. Make sure to clean the silencer at the end of the pipe, as this likes to get clogged, and really restrict the power. All this said, it is still a fun bike to ride, just not a fast one. John


Well said... If your just wanting to play with the little thing for fun then go for it. I've done many of older bikes like this for just the pure fun & enjoyment of it with real seat of the pants improvements :D , But it will never run 1/20th as good as new 2 stroke 250cc or 4 stroke 450cc motorcross bike.
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Re: Yamaha MX175

Postby af2 » Tue Feb 15, 2011 12:23 pm

ap72, Here is a web page you need to read. Chapter 2 explains everything I have posted.

http://issuu.com/jeanlulu/docs/a-graham-bell-two-stroke-engine-performance
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Re: Yamaha MX175

Postby Dirty Imp » Tue Feb 15, 2011 8:11 pm

I owned one of these as a kid.. like was said, if the rod big end is good, bring quench to .025-.030. Put in a set of carbon fiber reeds, or try and find a rage cage for the bike, an fmf silencer tends to help a touch, and port jug only on the leading edges of the chambers, bring them to a razor sharp edge and take any casting out, polish the shortturn radius's as much as possible, and port the exhaust outlet to match the copper crush gasket, bringing it to a gentle taper down in the port, dont open up the csa on the exh port any more than stock. Rejet accordingly, 2 strokes live and die by the tuneup. Do all this and that bike will have you tumbling on your ass when the power hits. It will shred like satan got to it, i promise. Been there, done that with almost every old yamaha 2stroke ever built...
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Re: Yamaha MX175

Postby af2 » Tue Feb 15, 2011 8:53 pm

Dirty Imp wrote: Rejet accordingly, 2 strokes live and die by the tuneup.


Although I totally disagree with the knife edge ( I still have the SF110 to prove it) The jetting WILL have to be UPPED.
They do run the best right before they pop!!!
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Re: Yamaha MX175

Postby Dirty Imp » Tue Feb 15, 2011 9:40 pm

Just posting what worked well for me.... but that was YEARS ago... but my lil 175 wud run with most 250s around my neck of the woods.... in the desert.
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Re: Yamaha MX175

Postby levisnteeshirt » Tue Feb 15, 2011 9:45 pm

i had an old 77 Honda MR 175 ,,, we did a few things to it ,,, it didn't even have reeds and we picked it up ,,,, first bike i ever bought, ( 1980 ) ,, 150 bux in pieces ,, cosmetics in great shape , just not running ,, got it running in 2 weeks ,,, ahh the days ,,,
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