Motorcycle Repair: 1978 Yamaha DT 175 Sticky Throttle, yamaha dt 175, throttle return spring

Question
QUESTION: I have a 1978 Yamaha DT 175 that has some issues. I am the second owner and it only has about 4000 miles on it. Today I was riding it down a long dirt straightaway with the throttle pegged and it got stuck wide open. I shut it down, removed the twist grip throttle and examined the cable which appeared to be good. Fired it up again, still stuck at full throttle. Next I removed the cap to the mixing chamber and twisted the throttle to observe the cable action. Everything appeared to be normal. I replaced the cap being sure to match groove in slide to little bearing embedded inside of mixing chamber, screwed it back together good and tight, fired it back up, still stuck at full throttle. Finally, I adjusted the cable adjuster near the carburetor by screwing it towards the carb, fired it up, and it seemed to be unstuck. I also decided to adjust the idle screw to lower the idle as well. While revving it in neutral, it seemed to be a little sticky, but serviceable. However, during the subsequent test ride, the throttle would get stuck after hard blips, and would eventually drop down given enough time. Also, bike seems to smoke excessively when throttle is wide open, blueish color. Bike seems to top out at 60 MPH, top speed is supposed to be 71. Although the tachometer pegs out at over 10000 RPM at full throttle, motor doesn't sound like it's achieving full power at wide open throttle. Motor also makes a ticking, clacking sound (valves? cam chain?), most noticeable at idle. Any suggestions are greatly appreciated. Thanks,
Chris

ANSWER: Start by removing the carb and disassembling and cleaning all the parts. Be careful not to get carb cleaner on rubber parts as that will damage them. Use a fine strand of copper wire to clean the jets. Check that there's adequate tension on the throttle return spring.

It's likely your throttle cable is rusted out so I would at the very least inject some WD40 and some cable lube. They sell a cable lube attachment which you can use to force the lube into the cable. You should lube all cable once a year.

Once your are running with clean carbs check the spark plug color to give you an idea of the mixture. You want a medium tan color.

The DT175 is a two-stroke machine so you do not have any valves to worry about. Make sure you are running on the appropriate 2-stroke mix or if your bike has a separate tank for the 2-stroke oil, make sure its got oil and that its actually using it.


---------- FOLLOW-UP ----------

QUESTION: Thanks Jim for your quick response and excellent suggestions: I cleaned the carb, lubed the cable, checked-and cleaned- the spark plug (pretty black and fouled), and leaned out the air screw one turn and-presto! no more sticky throttle, and motor screams out at full throttle with much more power. Tachometer used to top out at 10000 RPM, now needle swings way past the 10000 point- almost going completely around the dial-when I peg the throttle. Oil pump is definitely working, however I'm still seeing a bit more smoke than I'd like to at high RPM, and sometimes there is a whitish tinge to it. The transmission oil seems to be staying at a constant level, but I haven't tried adjusting oil pump output yet. The metallic clacking sound I hear-most notable at low idle-is a little unnerving, any idea what that could indicate? Is that a normal sound for this bike to make? I'd also like to know at what RPM this bike should idle at. It seems a bit rough under 2000 RPM, and the bike tends to die if I lower the idle under 1500. Thanks again for taking the time to help me out,
Chris

Answer
The clanking sound may be some play in the bottom end. Probably nothing to worry about on a 30 year old bike.

I don't know the recommended idle speed since I don't have the service manual but on an old bike which may have been treated harshly I would expect idle to run on the high side.

You will find that if you lean out the idle mixture it will need more RPM to idle when warming up. You need to find that balance of idle mixture, idle RPM and plug color which keeps the plugs from getting fouled but provides a rich enough mixture to let it idle stable when warming up.