Yours does sound a little weak possibly...  air filter on that model is already very restrictive so make sure foam element is not old and that it's clean and lightly oiled.  Don't use that modern "sticky" air filter oil..  Twin Air oil works great.  (foam can look great but won't flow air properly as it ages)  This sounds most likely as you seemed to think it might be rich.

Other thing I'd suggest is replacing ignition side crank seal with the 77-79 IT175 version if you see any signs of oil under the ignition cover or it acts lean at idle. 

Make sure plug is gapped to no more than 0.024" 

Well Tinkicker you are mistaken about me. I am still in my youth and this is the first motorcycle I've ever owned. The only reason I think this motorcycle should be faster is because of people who grew up on these bikes telling me their stories every time I come in for parts. And also the specs telling me the top speed should be 70mph. I may keep this old motorcycle around to just go around town or tear through the woods. Might find a newer motorcycle that has a bit more power to it.

For MarkT, the air filter is new but I will try the twin air oil. I thought it was running rich so I played with that for a few days and didn't notice a difference. Just got a brand new plug for it but didn't think to check the gap, I'll give it a look see sometime over the weekend.

A few other things I have tried, I checked to see if the gearing had been changed at some point but it appears to be stock. Tried cleaning the exhaust but that had no effect. Adjusted the throttle cable which was needed anyway and it gained me 5mph. May try an engine rebuild next winter but lets see how this goes first.

A little bit of background for the motorcycle that may be important.  So, the story is that my grandfather found the motorcycle on a rock pile after it had been sitting there for a few years, he repaired it having very little mechanical or electrical knowledge. It was then only used for taking into the woods for hunting. After about 3 years of that it got put in a barn and forgot about till about 2008.  It then got passed around to multiple people in the family and they all had enough know how to keep the motorcycle in good condition. Then from 2012-2020 it sat in the back of my brothers garage. In 2020 I bought it from him for a single dollar and then started fixing everything I could on it. So long story short, I don't know how well this bike has been maintained but I am certain that it wasn't handled with much care. I don't know if any of that is important but thought it might be.

Well I'm an old guy, but I distinctly remember riding my CT-1 in 1971 and later, with a buddy seat on the rack, and my girlfriend who weighed about 130 pounds (about 30 of those in her chest) up and down PCH doing 60.

I also distinctly remember doing hill climbs and beating 360s, and being able to pull wheelies in 3rd gear.

It's all in the technique. Wyatt's bike just has something going on that we don't know about yet... I would keep working on it.

If I wasn't 68 now, I would try to pull wheelies again and do some power slides, but I would like to last a little bit longer 

Agreed, I don't want us over stress this old engine either. I tend to not go higher than 6000 RPM with it in any gear. The way it's geared now, it's doing 60 mph at 6,000 RPM, and that's just where I stop. If I laid down on the tank it would probably do 70, but why?

Mine ran very similar to what you are experiencing, basically it felt as if it was being strangled and didn’t want to rev out. 
Mon the straight it would struggle to hold 50mph and you couldn’t do a wheelie if your life depended on it. 

it then broke down on me that made me start to ask advice and look into it; today I cleaned the points and set it up correctly and what a difference, it revs all the way, goes straight past 50mph and the front comes up if you want it to. 

I thought I remembered how a 175 rode and when I rode this I thought I was wrong, nope, it brought a great big smile to my face again today. Just give it a once over mechanically and you will have the bike that you thought you bought. They are great! 

worth checking the timing if you don't know the history but I think the CDI is less likely to go out of adjustment compared to the early points models.
might also be worth popping off the head to take a look in there... my DT 175 felt slow but a rode fine and didn't make any too loud of noises but this is what the Piston looked like...
 

I’m fifty something and never had a DT back in the day. I enjoy riding, done a few track days, had a F800Gs that I used to take off roading and camping in the mountains, tours into Europe etc. There are some “badges” that I feel obliged to get. Knee down was one, which was achieved at a knee down school (never got anywhere near achieving that anywhere else, but I’ve got scuffed knee sliders that I can wear with pride!). Taking the GS into the desert… done. And a wheely on my DT… now also done. All of six inches clutching it up in first….   but that’s ok and I have the badge!
 

To provide an update on this… I didn’t think the DT was as nippy as it should be. So I changed the rear tyre for standard size and in doing so realised the rear sprocket was 49T. So i also fitted new c&s with standard ratios.
 Definitely a bit nippier, and pulls top gear. It even accelerates in top now from 45 to 55+. Feels a bit more lively… overall a good result.
 

Holy thread resurection batman.

Someone at work made a casual comment today about something else entirely, but it made a lightbulb flicker in my head re performance not being as good as remembered in the past.  I immediately remembered this thread.

Apart from my weighing in a 200lbs instead of 140lbs in my youth which is a big performance killer, the other elephant in the room may be the piss poor modern fuels we are saddled with today.
I have heard of problems with other machinery using modern fuels and having combustion chamber shapes not designed for it. Esp with offset sparkplugs.
Also ethanol has a higher latent heat of evaporation than gasoline, so it may be that the ethanol is not vapourising as readily as old time gasoline did , esp in the cooler crankcase conditions and therefore having a cooling effect in the combustion chamber as it takes heat out the process to fully vapourise.

Food for thought?

I think they're pretty restricted in the airbox & carb including exhaust. First thought for lack of performance [not performing as it used to] is 'exhaust'.