Hey guys. I have a 1980 DT175. For some reason the throttle slide won’t go down all the way and the throttle cable is maxed to its length. What should I do to get more length from my throttle cable?

Thanks! 

What you're describing is not at all a normal problem if you're using original parts and everything is in good condition and assembled properly. 

More info and pictures or video would help...  when did this start? Are you using original parts?  How did you determine the slide isn't going all the way down?  How much is slide open?  Do you have the cable routed correctly?  Is the slide installed correctly with guide pin aligned with slot?  Is the slide needle bent?  Is the slide needle dropping into the jet?

Did it have enough free play originaly for the lower nipple to get out of the slide & hold it partialy up--shouldn't happen really.
Is this a new problem with original cable--or a new cable.
Is cable in the carb top adjuster or got out of adjuster--i guess it's an angled adjuster on those so maybe crimpted to not get out.
Outer cable popped out at the junction box--although maybe retained in on those models.
Cables twisted conecting in junction box--not sure if possibe & takes three to get fitted crossed, on a twin cylinder & round junction box [yours might be a flat-ish junction box].--& everything Mark says also.
Not been screwing screws & have the stop screw [idle speed screw] wound right in.

When I got to the point of reassembling the carb slide on my 73 MX250 after it sat for 35+ years, I did so before I added the carb to the motor so I could see what was going on before it was all covered up on the final assembly. It was not closing all the way, so after much much scrutiny, and still not able to understand why the original cable was now shorter for some reason. The only place I could "adjust" was the stop that the twist throttle had. It has a large plastic stop that could be reduced to give more cable length by allowing the  twist grip to turn in the return position a bit farther.When I got to the point of reassembling the carb slide on my 73 MX250 after it sat for 35+ years, I did so before I added the carb to the motor so I could see what was going on before it was all covered up on the final assembly. It was not closing all the way, so after much much scrutiny, and still not able to understand why the original cable was now shorter for some reason. The only place I could "adjust" was the stop that the twist throttle had. It has a large plastic stop that could be reduced to give more cable length by allowing the  twist grip to turn in the return position a bit farther.

 



With the white portion of the grip, where the cable attaches to I, being a carpenter, used a very sharp wood chisel to remove some of the plastic tat acts as a stop with in the housing by trying each time after removing a bit and doing so until the throttle closed all the way.

I just went through a situation like this. Bought the bike in 2015 and stored it until last summer. I bought a few new items for it at that time and one was a throttle cable. Upper and lower. I think it was KSI brand and it was made in Japan. May still have the package it came in. Half the summer I was trying to figure out what was wrong with the bike as it would rev to the heavens after starting. Reading everything that I could find on this site I finally came to the conclusion to take the carb off the bike with the cable still attached and low and behold the slide is around 3/4 open. I must have read a dozen times to make sure the slide is closing. Had a mental image in my head that I had absolutely done that. Thought about it all winter, kind of nagging me. When I did see that slide up there I almost hit the ceiling with absolute JOY! I must give a lot of credit to RT325. He would stress that in a number of posts I read. 61 years old and must be getting a little daft! Anyhoo, I ordered a new upper and lower cable from KDI.rerpros and the DT1E is on the road again. Im reliving that moment I saw that slide up there! Pure JOY!
 

Honey! I shrunk the cable.

The pattern cable that came with my RT2 actually had an instruction to remove about 10mm of the outer cover before fitting to some models although, unhelpfully, it didn't say which models!  . Anyway, it transpired that this mod had to be carried out for the RT2 and so I got my local bike shop to do it for me.