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Discussion Starter #1
Hey folks. I found a 1968 CD175 (also CA175) in a garage. She'd been sitting for 21 years. I cleaned and rebuilt the carb (single carb), cleaned the air filter, replaced the spark plugs, replaced the battery, put fresh gas in, started her up. She ran fine for a half a mile to a mile, then started cutting off - bogged down. She'd start again with the choke on, but was hesitant to run. I took the gas out, took the petcock apart, reassembled it clean, put a fuel filter inline, and a magnetic catcher for any rust particles (the tank looks pretty good), blew any loose crap out of the tank, replaced the gas (straining), and re-cleaned the carb. She started right up, ran for a mile, and cut off again. Same thing, would fire, but not run, with choke.

It's not crap in the carb, or a gunked up line. Any other thoughts? Any help would be greatly appreciated. I'm trying to get away without taking her to a shop. I'm handy, just out of ideas.

Thanks.
 

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It really doesn't take much rust to clog a carb up. There is probably a lot more rust in the tank than you realize. Most of the tank can't be seen thru the filler neck. If the rust is not flaking off then I'd try the Yamaha Tank Rust Cleaning kit. It's in two parts. One is acid to get the rust out and the second is a light coating to keep whats left sealed in.
 

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I had the same problem with mine I own a 1967 cd 175,you need to clean your gas tank because the rust in there is blends with the gas if youl notice as you clean the carb there some brown sticky substance thats a mix of rust and gas residue,if it cant be clean then you will have replace the gas tank
 

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Discussion Starter #4
Figured it out - fuel cap vent clogged

Thanks for the responses, especially the ones still coming in! I sold this bike over a year and a half ago!

Summary - a faulty fuel vent causes a vacuum to form in the tank, thereby depriving the carb and engine of fuel.

Explanation:
The tank was essentially clean, and I'm a whiz at cleaning carbs (I did it more than once). The problem was that the fuel cap vent was clogged. When fuel would get pulled out of the tank for the engine, it would create a vacuum condition in the tank, and eventually the fuel would stop flowing because of that. My first test to establish that this was the case was while I was riding, it would start to stutter or shut off, and I pulled the fuel cap off. It would either stay running or start right up again. So I started to ride and while I was riding, I would loosen the cap or pull it off every minute or so - the bike stayed running. So I loosely put the cap on, so the rubber gasket didn't make a seal, and left it. The bike stayed running. So, I went home, took the cap off, removed the rubber gasket, and soaked the fuel cap in carb cleaner, then blowing compressed air through the little pinhole vent. This worked well enough to get me riding. The replacement cap was going to take a week or two, and I wanted to ride right then. It worked beautifully, and when the replacement cap came, I saw that it was different than the one on my bike.

All The Gear, All The Time
(Even if you're riding a hopped-up moped!)
 

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Try to eliminate the variables. If the bike starts cold,runs for a mile,and then cuts out, it could be heat related as the carbureation demands change as the bike heats up as my Cb360 does. I had a Honda CB 100that would run fine and than cut out, then run fine again until it warmed up. It turned out that a mounting screw for the points shorted out the points when the arched spring that closed the points expanded with heat and contacted that screw.
What you should do is start the bike cold and when it cuts out,wait a half hour and see if it runs OK again. If it does,you can confirm that the problem has something to do with operating temps. You can try carb settings, or you may have to look at electrical issues as sometimes the coils can get weak when the bike warms up, or a marginal electrical connection spazzes out when it overheats. That means checking the spark at the time of failure. A good troubleshooter never assumes that just because something worked ten minutes before that it is working now. It takes time,but try to eliminate just one variable at a time.
 

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Discussion Starter #6
See previous post. I fixed the problem, and sold the bike almost two years ago. Incidentally, stopping the bike, waiting for an hour and then retrying was misleading in this case, because the air pressure had a chance to slowly equalize in this case.

Thanks
 
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