Ok, I’ll give the throttle tube marking a try. Not sure what the emulation tube o- ring is. Can you reference the diagram I posted? Should I try re-building the carb?

Your diagrams threw me for a second... Pretty sure 74 DT175 still has the "drain plug" in the side of the float bowl (actually the main jet holder)?

Looked at actual parts diagram and it does... I circled main jet holder and put a red dot next to critical o-ring #3.



O-ring seals float bowl to main jet... if seal leaks, fuel bypasses main jet and will run rich.

Most of the time carbs can be taken apart and carefully cleaned without buying a bunch of parts except that critical o-ring #3 on your carb. It's a good idea to take it all apart and clean it up and make sure all jets and slide needle are correct and original Mikuni parts.

Kits are all "aftermarket" and suck. I wouldn't wast money on one.

Also, make sure carb is original. With the $20 Chinese carbs on ebay a lot of people swap them and the cheap carbs range from "usable" to "junk".

Yes, the carb on my 175 is the one you show. It is a Mikuni but I haven’t ever taken it apart so no way to tell what’s inside. Is there an original rebuild kit available or do I buy parts individually? If individually, what parts do you recommend I buy? Also, when looking at the carb on the bike I noticed the overflow tube and air vent tube are not on the carb. How big of a difference would that make?

Tubes missing won't cause a running problem.

I'd buy parts from Yamaha. Your local dealer or I like RMATV-MC... link is usually on most pages here.

O-ring for sure. Maybe bowl gasket though if you're careful often can be reused. Doesn't hurt to have one on hand. Most everything else can be cleaned. Carb needs to be completely disassembled. My bet is still on that o-ring as a very likely culprit.

You don't seem to be having trouble with carb overflowing or starter circuit so probably don't need float valve or starter plunger.

Going to go through the carb and replace this o-ring first.

I ordered parts for the carb yesterday and just thought I’d take it out for a little ride today. Magically the roughness was gone. It ran great! Wound it up through all the gears and it revved and ran great. So good I thought I would take it for a ride down the road. I rode it for 2 or 3 miles and as I turned back down the road to my house the spare plug came shooting out of the top of the head. I tried to put a new plug in but the threads are stripped and the plug will not hold. I’m guessing a previous owner stripped it out and some how got that plug to grab. Is this repairable or do I begin my search for a new head? If I have to put a new head on might want to do the top end. Any suggestions on where to find a top end kit and piston?

heads are so cheap I'd probably just get a new used one rather than deal with fixing the old, and since the copper gasket can just be annealed it's a quick easy job that doesn't necessarily warrant a whole top end, like ten minutes to do

How likely is it that when I try to loosen the head nuts that the entire cylinder bolt turns and tries to come out of the case? Is there a way to keep that from happening? If it does happen, will I have to pull the whole cylinder?

I've only taken a few off but never had an issue with the cylinder bolt. From what I've read, seems more like an issue on the later engines. MarkT has a lot more experience though, and I'm sure he will advise well.

Thanks, hope to hear from Mark T.