Topic: Gas Flow Problem

[Honda_Dan]

There appears to be a problem with the Gas flow into the Carbs on my '76 CB750F. Before I go any further, I am a mechanical newbie, so pardon the level of my questions. I read through most of the archives on carburetor and fuel system problems and haven't found a good answer.

Here is the problem: The bike starts easily cold or after warmed up. It idles perfectly at about 1700 RPMs after warmed up.  After a little bit the bike just dies. I realized that when I drain the flow bowls after it dies, there is very little gas in them. Also, when I pop the gas gap, there is a sucking sound.

I tried running the bike for a while with the tank open, and it seemed to run for longer. I did this twice, and one time gas started flowing out of the left most carb release valve. The second time, it seemed llike gas may have flowed out of the carbs into the air filters (Bike has pod filters). 

I  took off the gas tank and pulled off the petcock. I assume the tubes/straws with what looks similar to a small condom wrapped around them is a fuel filter. It had ruptured in the top and there was a minor amount of rusty looking dust around it.

I also inspected the air intake/vent on top of the tank right under the latch. I poked a cotter pin and then sprayed a touch of carb cleaner in there.


What else should I do to fix this, or perhaps even confirm the issue. I could take off and clean the fuel lines, but from the outside they appear in pretty good shape.


Thanks for help guys. I have found the archive and this forum to be a fabulous resource.



--Dan from New Hampshire--

[Gordon]

Sounds like you have a couple of problems.  The first one you already figured out was the gas cap vent.  If it's blocked, you'll build up a vacuum in the tank which will keep fuel from flowing to the carbs.  The second problem seems to be stuck float valves in one or more of the carbs.  Try tapping the offending carbs with a screwdriver handle to see if it stops the flow.  This could easily be caused by rust and other debris from the tank getting into the carbs through the torn filter in the petcock.  You need to thoroughly clean out the carbs and install an inline fuel filter between the tank and the carbs.  Good luck!

Gordon 

[Honda_Dan]

Thanks Gordon,

I had a couple of quick clarifications: Should I install and inline filter and get rid of the one on top of the petcock, or should I just leave that one as is. Do I put the line filter just between the petcock and where the fuel line branches into two, or is there some other technique.

Also, what is the best way to deal with the air flow issue from the tank. I think the way my bike works is that there is smalll whole right in front of the gas cap. I sprayed some carb cleaner in there, but have no idea how else to clean it.


-Dan-

[Gordon]

If it's not doing any good, then there's not much reason for it to be there.  If you have a "T" connection in your fuel lines, it's easiest to fit one filter before the T.  There's less chance of developing air locks, too. 

As for the gas cap vent, you may have to take apart the inside of the cap to be able to clean it out well.

Gordon

[Harry]

I  took off the gas tank and pulled off the petcock. I assume the tubes/straws with what looks similar to a small condom wrapped around them is a fuel filter. It had ruptured in the top and there was a minor amount of rusty looking dust around it.

Gee, I dunno, what does a small condom look like 

I am not familiar with your bike, but on my 550 tank the hole in front of the gas cap is to take the overflow should you overfill your gas tank. Don't ask how I know. The gas cap vent is a hole in your gas cap, you may need to poke about until you figure out how it works! I suggest you fix this problem before proceeding to the next one...

[Honda_Dan]

I have kind of fixed the problem. The gas flow issue is solved when I run it on Reserve.  I'm going to pull apart the petcock and see what is limiting gas flow.

Drove the bike about 50 miles and and more less ran like a champ, except that it leaked a bit of gas out the carbs onto the pod filters. Any thoughts on what is causing this?

Now onto the headlight....

Thanks for all your help!!


Dan

[Ibsen]

If you have an inline filter already, the problem could be that it is partially clogged. Or as already mentioned, the filter in the petcock.
And when you turn the petcock to reserve, or open the gas cap, the gravity will increase a little bit, just enough to get some fuel through the filters. I had this problem with the inline filter on my bike early this spring. I replaced the inline filter and that solved the problem.
But as alredy suggested, also check the vent hole in the gas cap.

[Honda_Dan]

I do not have an inline filter, but plan on installing one as it looks like there was rust in the stock filter. Does anyone know a good place to get an inline filter and also a replacement for the stock one on top of the petcock.

Dan