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Discussion Starter · #1 ·
I now have my '70 CL350 running (timing/valves/cam chain tensioner are all adjusted). At idle, it's not working great, but I'll save that for another thread (likely the carbs I think). At speed it starts off well though and pulls nicely at all rpm, with only occassional hesitations here and there.

My problem is that it bogs down and stalls out after about 3 miles of riding (different than the poor idling I have since it won't restart when it does this). I'm convinced it's a fuel flow problem because the petcock seems to flow a bit erratically at different times when I've taken it off the bike. I'm eventually going to take off the petcock to check it (supposedly it was rebuilt last year, but you know how that goes), but I really suspect I have a tank venting problem that's the major issue. At idle I think the fuel flow might be sufficient to feed the carbs, but at speed I wonder if it's running out of fuel because of poor venting....

Why do I think the tank cap is screwed up? Because EVERY time I start to open the cap I get a huge air noise (like you get when opening a sealed gas can). It does this after even a few minutes of just sitting or idling. Does this sound like a fuel tank cap venting problem to you? If so, how can I check/test/fix the cap? The two holes into it don't seem messed up, and the hold in the middle of the bottom of the cap looks open. Now there is a bit of Kreem on it from where the last owner did the tank, so I wonder if he clogged something. It also looks like there's a spring in there, so maybe it's messed up somehow?

Thoughts? Thanks guys!
 

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Discussion Starter · #3 ·
I took a closer look at the cap. The side tabs move and the larger hole in the middle of the bottom of the cap is clear, but down inside the hole in the middle it looks like there's supposed to be a second open hole of some sort. That one appeared to be covered in Kreem. Aha! I scratched around that hole with a nail and then blew in some air through it and the tiny hole just inside the cap liner.

POP!

I think I might have fixed it.

If not, I see a new cap is only $9.
 

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Discussion Starter · #4 ·
Guess I didn't get it. There's still a lot of pressure when I start to unscrew the cap. Oh well. Guess I'll get a new cap and go from there ;)
 

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My tank has been relined at a radiator shop, new OEM petcock last summer, new lines and filters..

I've been having a fuel starving problem too. Run her hard for about a mile...bogs down and dies. Open the cap, revs hard and i can take off again. (I can even see the fuel refill in the filters under the petcock when I open the cap)

So I decided to tear the cap apart to clean it. Questions about the internal part of the cap I have in the pic...

A: Is the (?)washer or metal disc indicated in yellow supposed to move and flap around (like mine does)

B: The cap looking part with the tiny hole (in red) looks like two halves. Is that supposed to be able to come apart? After cleaning the hole with guitar strings, I blew compressed air in the hole and it split the cap. I tried carefully separating it using a flat blade, but it wouldnt budge more than the air split it. I didnt want to force it any further until I found out more. Does that whole thing come apart for service?

Also, I've reading around here that having fuel filters may cause vapor lock. Especially if more than 1 filter per line is used.
I have 1 per line installed just under the petcock-about an inch or two. And they are the clear ones with the copper stone(?) cone shaped ones. I wanted the extra security of capturing stuff, since that tank had been relined and all. Should I remove them?
 

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Hondatwins to the rescue again. I just got my CB450 back together and was able to ride it around the neighborhood a bit. But after a mile or two it would kind of choke out and die. Then after it sat for a minute I could start it again but it would die out again a short time later. I was getting ready to take the carbs off and see what the heck was happening. As I was getting ready to drain the tank I could hear some gurgling noises as I opened the petcock. I disconnected one of the lines from the petcock and began draining the bike but after a moment the flow slowed to almost nothing. It was at that point that I realized that maybe the problem was that the tank was not venting. I remembered then that when I cleaned and coated the inside of the tank with POR15 I also painted the inner face of the gas cap. A quick look on the forum showed me that I had painted over the vent. I cleaned it out with a sharp screwdriver and found that the fuel flowed smoothly again. I ran the bike for a bit and it was fine though a road test tomorrow will be needed to be sure. Just wanted to share that in case some other dope did the same thing I did. Thanks Hondatwins.
 

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Also, I've reading around here that having fuel filters may cause vapor lock. Especially if more than 1 filter per line is used.
I have 1 per line installed just under the petcock-about an inch or two. And they are the clear ones with the copper stone(?) cone shaped ones. I wanted the extra security of capturing stuff, since that tank had been relined and all. Should I remove them?
The sintered bronze filters are very restrictive. Filters don't cause vapor lock, but the sintered ones reduce flow significantly in a gravity feed fuel system. Stick to filters with metal screens or paper elements. I favor the slightly larger filters with folded paper...

Motor vehicle Auto part Vehicle Fuel line Engine

Vapor lock is simply the fuel starting to boil in the lines. This is from heat. The gas vapor is high enough pressure to keep the fuel from flowing.... Hot days, hot engine, fuel lines behind engine, that's enough heat especially at slow speeds. If the bike was sitting in the sun, the fuel can be very hot to start with....It isn't all that common, but it does happen.
 
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