I recently purchased a 1980 Yamaha DT 125g enduro. Replaced the battery, carb, air filter, etc and thought I had everything in place. The bike will start relatively well and sounds okay. The last few times I tried to start it the bike will idle fine with the choke on and sometimes off, but anytime I turn the throttle the bike will seemingly take off in rpm and match what I put the throttle at without going back down. I have to turn the bike off to stop this. Once it even took off on its own to 7000 rpm without me touching the throttle. Has anyone had a similar problem? I'm really confused what the problem could be and any help would be appreciated. 

Wondering why you replaced the carb as generally speaking just a thorough clean of the jets will have it good as new. Anyway, back to your problem, is the cable possibly frayed & hanging on, not flicking back to closed in the slide. Without motor running does the throttle feel smooth to open & does it close by itself with a feeling of the slide hitting the idle speed screw. Maybe back of the speed screw just so you can feel the slide hit right down in which case the motor should stop when closed. If none of that is the problem then off to air leaks & crank seals but unlikely. Good motor those!!.

I blasted out the oem carb and replaced the needle valve gasket along with adjusting float height but couldn’t get the carb to stop from dumping fuel out of the overflow. The previous owner had another carb he never put in (not oem) and that stopped the fuel leak problem but now this problem started. It seemed like the throttle itself was snapping back into place fine when it wasn’t running but when I bought the bike my friend noticed it was pretty sensitive and revving up quickly. I might buy a new throttle body to see if that fixes the problem. Do you know where I should check for air leaks?

Hard to guess where to start but think i'd be looking at the original correct carb to see why its leaking & try that one before worrying about air leaks.

I just looked at the bike again today after letting it sit last night and it seems like there is fluid or something coming from the crankcase, probably near the magneto. This is probably the gasket right? 

The plot thickens. No, don't worry about the gasket but take the mag cover off for a start. It's possible the crank seal has been forced out by a belly full of fuel in the crankcase from a faulty float valve or something like that. Or if its been sitting for years it can-- if faulty--let the oil tank oil slowly empty into the crankcase with a similar result. I'm hoping its not the seal behind the mag but think what i'd do is--with ignition OFF', push it around the yard with the plug out just to try & get rid of excess fuel. Could loosen the exhaust at the cylinder too to let any out if its got into the pipe bend. Then try starting it. See how much fuel is in the cover but might be lucky & just found its way past the crank seal over time so it'll still run. If none of those ideas work you might need to pull the mag off to eyeball the seal. RH seal is also a possibility but will smoke more than you can imagine but unlikely to start if that seal is out of the case. I'm forgetting what the original problem was, so if it ran once it should run again but hopefully wont rev uncontrollably.

Very common problem on that model.  Remove the magneto and replace the crank seal with proper dual lip one from a 77-79 IT175 part number 93103-25125-00 
Can be bought online or from your Yamaha dealer.  You will be chasing your tail until you replace that seal.

Thanks for the help RT325, Im definitely going to look at the crank seal portion. I'll try out what you said. It was still running and even idling really well for a while. It would either just die when I gave it throttle or start to reach high rpm and stick there. I ordered oil gaskets and a new throttle body just to see if that would help. 

@MarkT Are the ones from 77-79 better than the 80? Thanks for the reccomendation.

If you don't have one, buy a good flywheel puller like the Motion Pro 08-0026  You will need it to replace the seal. There is a gasket on the plastic side cover but it's there to keep water out more than keeping anything in.

Crank seals on your engine need to hold both pressure and vacuum which is best accomplished with a proper dual lip "SW" type seal.  Yamaha used a single lip seal on the magneto side of your 1980 DT175. 

The 77-79 IT175 had the "race" version of your engine and came with a proper dual lip seal on the magneto side, direct replacement for the seal in your bike but a better design. 

Your symptoms are classic air leak and the leak from the magneto area confirms what I've seen on my own bikes...  I think I currently have 4 DT175's?  Three of them are 1980/81 models.

 


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