Re: Yamaha 175\'s - need some tech help
You'll need a repair manual--you can still get them, I just got one for my '73 CT175.
Your description sounds like one doesn't idle, but will run if you keep giving it gas, right? If so, you need to adjust the 2 screws that relate to the idle. There is a standard procedure for that. It is in the repair manual--or I'll quote it if you want.
For both of them:
Of course, you replaced the spark plugs already....
Look for obstructions in the exhaust--mine was originally blocked with about a cup of water in the bend by the cylinder.
Definitely clean out the carb, get rid of all the old gasoline, put a new gas line filter in, get a new air filter, and check the points if you have them.
Check the reeds--they should lay flat against the reed block. If they don't, try turning them over; if they still aren't right, get replacements. Boyessen makes some.
And then if that doesn't work, you can pull the cylinder and check for carbon build-up in the ports, and on the head.
Those are the first things I'd try.