Neighbor has a 1971 CT1C 175 It won't start, won't even try to start. I'm stumped. He was riding it yesterday and it began to lug in 3rd, 4th and 5th he said. He'd twist the throttle and the rpm's would drop like he was climbing a hill, then it shut off.
He had been running it with the incorrect air filter and a tank full of rust. Yesterday we put in the correct air filter, cleaned the intake boot, cleaned the fuel tap and blew through it with compressed air, put on a clean tank, cleaned and rebuilt the carb. Fully disassembled the carb (it was full of rust), cleaned all the parts in my ultrasonic cleaner scrubbed with a tooth brush using a degreaser and sprayed each passage with brake clean to make sure everything was clear before assembly. Still wouldn't start. It has spark and compression, but seems like it's not getting fuel. Needle is on the second groove just as the one we replaced was set (slide is in correctly with the grove aligned with the pin and it moves with a twist of the throttle as it should), the pilot jet is the same size as we replaced and we used the original main jet since the one in the rebuild kit wasn't the same size. There is a filter inline on the fuel line and there is some restriction when blowing through it, but gas was going into the bowl. He's to order a new filter. He replaced the spark plug, had an NGK B8ES, he picked up a BR8ES, don't know if that'd be any issue at this point. The spark plug he'd been running is dry and a redish brown on the end. We pulled the carb again and split it to make sure the float wasn't stuck at the top not allowing the bowl to fill. The float's fine with no fluid inside it. Did notice the holes in the bolt the main jet is in wasn't aligned with the hole in the bowl, put the original washer back on the bolt and the holes lined up. New O-ring on the setter needle jet down into the bowl to the main jet. Even pulled the head to make sure there was no top end damage, there was none.
Help!

BR8ES is a resistor plug. Not sure if that will be a problem. When buying plugs online some vendors will substitute resistor plugs.

Will it fire with starting fluid?

He put some gas oil mix in the cylinder and said it still wouldn't fire, wouldn't even try. We've not tried starter fluid yet.

My mistake, we did replace both the main and pilot jets with what was in the rebuild kit. The one we didn't replace was the jet the spring pin sits in to operate the float, it was labeled a different size than what was originally in the carb. The kits pilot jet had no markings on it, the main was the same size as what we replaced.

I did notice last night the intake manifold has a crack in it. The crack is where one of the bolts goes through to the attach the carb to the jug.

Using "rebuild kit" carb kit jets and slide needle has been recommended against many times here.  Best practice is to use genuine Mikuni jets and slide needle.  Even if the kit parts have the same "size" marked on them, they are not the same calibration as genuine jets and slide needle.  

The one "kit" part you can generally use is the float needle and seat. The float needle and seat is a SET.  The "spring pin" (float needle) you mentioned and the brass seat the float needle fits into must be kept as a matched set.  From what you wrote and the problem you're having, there is a chance that if someone used a new kit float needle (what you call a "spring pin") in the old brass seat.  Mixing old and new float needle and seat parts can result in flooding or starving for fuel.  As long as the hole in the new kit float needle seat is as big or bigger than the original, you should be okay replacing as a set.  The matched float needle/seat is the one "calibrated" part in the rebuild kit that is likely okay to use.

(Also, just FYI, the holes in the main jet holder don't need to align with anything.)

I'd install and verify original Mikuni pilot jet, main jet, slide needle jet, and slide needle.  Then make sure float needle and seat are a matched set.

Note that spark plug can spark outside the engine but fail under compression.  I'd install a new plug gapped to no more than 0.024".  Even better is to test the plug in a running bike (one of your bikes?) to verify it's good.  I've come across NGK plugs that are bad right out of the box.  

Hope you get it running soon!

Could it have broken off the flywheel key. Quite a common problem if the flywheel nut isn't tight enough. It will still spark but timing could be too far out to run. Just my guess for the night.

Yes good comment about the plug. I was to ride out with a friend and the bike just wouldn't start, put a fresh plug in and boom off we went, albeit 30 minutes and a bit sweaty later. I like to carry a fresh plug. B8ES work well for me (and I believe the R plug has no 'real' difference in this situation). Check for spark before kicking, if plug is wet fuel is getting in. Personally I would remove the fuel filter until the bike runs and use a clear line so you can see. Usually starting on choke with no to very little throttle is a good option. Others advice is way better than mine!

I’d always leave the filter in place. When buying, get extra. Those are way cheaper than a rebuild.
Well, at least where I am.. I hope there’s another month of riding left.. right before the freeze up.

Okay, here's where we're at now...
Carb back off and opened up. Put back in the original slide needle (E clip on second grove), pilot jet, main jet, float needle and it's seat after blowing each through with Brake Clean and running a very small denture brush through every orifice. All these pieces had spent an hour in my ultrasonic cleaner last Saturday, they're clean. The old fuel filter is still on and I know it should be replaced, we have a clear piece of hose from the fuel filter to the carb and we can see that gas flows to the carb. The float needles seat from the kit was smaller than what had been in the carb 2.0 vs, 2.5 so both originals are back in..
Put silicone in the cracks in the intake manifold. The cracks were only to the outside of the bolts holding the carb on, so shouldn't have been able to leak air.
New, correct, NGK B8ES spark plug gapped at .024" is in, I put it in my DT2 first to make sure it was good, my bike started first kick with it in, so no spark plug issue.
Cold compression at 112psi,
Still wouldn't start, squirted some starter fluid into the cylinder through the spark plug hold, nope still wouldn't pop.
Pulled flywheel cover off, removed the flywheel, the key's in the crankshaft and the grove in the flywheel was over the key.
Excessive oil in the left side and could see the left side crank seal was out. Got the seal back in place, cleaned up all the oil, ran a piece of 500 grit sandpaper between the points. Put everything back together and she still wouldn't pop.
Thought what the heck, and tried to jump start it, it fired! Ran it around the neighborhood a few times and decided to take it out for a run. Was running well, made it about three miles and it stopped again. Had him switch it to reserve just to make sure we didn't have a clogged petcock, started and ran almost a minute and it stopped again.
Went back and got his truck to bring the bike back home. Said he was done with it for today.
Unless someone has any better ideas, I believe a new fuel filter and left side crank seal are next. I don't know if the seal has popped out of place again, I won't know until he's ready to start working on it again.

That does sound like progress.

Keep going, and don't forget to update us! 

Great progress, in my opinion you may as well put new points in too since they are cheap and could have been fouled by oil once or twice. Compression gauges are notoriously inaccurate on small displacement, but that is getting a little bit on the low end, if it is accurate, but as long as it'll idle, not contributing too much to the running issue. I'd also pull the petcock to make sure it's not clogged with rust, they like to do that.