1970 yamaha u5 keeps running out of fuel tank is full

May want to check for restrictions in the fuel line. Like the fuel filter or may have trash in the carberator/injectors. The reason it starts after sitting, is the fuel is seeping through the line very slow. Thus. Reaching the carbs/injectors.

I hope you used a fuel filter in between the gas tank and the carburetor as rust from the gas tank will block the carburetor ! and i think you replaced the spark plug with a new one ? cleaned the points ( if it uses them instead of CDI ) A good way to get this bike to run is to use something better than fuel ... like paint thinners which has a higher alcohol rating than petrol , this is only used to start the bike and make it run for a half minute ! most bikes have a choke lever at the carburetor or on the left handlebar check to see if its working ok plus clean air filter and battery is charged fully, if you have a large hill nearby clutch starting the bike can some times force it to run longer .

When you have to go full throttle does it accelerate like a normal one when given full throttle, or does it just barely keep running? Is there a carburetor or is it fuel injection? Try some fuel system cleaner. But you say it's intermittent, correct? If so then it's a real mystery.

Just a question. Do you run your car on empty. If so that could be your problem with an empty tank you are sucking all the dirt through the filter and making it plug up. It is better to keep your tank above half full and does not cost any more than to run on empty. All mechanics will tell you to always keep your tank above half full.

Hello
Your running out of gas.
the fuel filter needs replaced.
your fuel filter is clogged and isn't letting a full flow of gasoline through.
change your fuel filter.

Also check to make sure your gas tank vent isn't clogged.
a clogged gas tank vent with pull a vacuum and try to **** your tank in, but it can't, so your fuel flow stops if it can't breathe.

That should take care of your problem.
have a great weekend
Big Al

Hello,

It sounds like you have bad gas in your fuel system. If your bike sat for a while, you will need to drain, and clean, the carb bowl(s) (turn off the fuel valve before doing so), check the fuel filter for any blockage (replace if necessary), and then add Sta-bil to your fuel tank. This will stabilize the remaining fuel in your tank. With today's fuel having Ethanol added to it, the fuel gunk's up quicker.

Turn the fuel valve back on and start your bike - run a full tank through the fuel system. After a full tank is used, get a bottle of SeaFoam and add the recommended amount to your fuel tank when filling up. The SeaFoam will clean your fuel system and your bike will run better. Use Sta-bil when your bike is not going to be used for a while and use SeaFoam every other full tank - if you are not going to use a Premium fuel with no Ethanol added.

Hope this helps.

If you find this information useful, please vote on this post. Thank you.

Regards,

Oz

Hello,

I know that quite a few of the V-Star's have an issue with fuel that has Ethanol additives. What happens when the bike sits, even for as little as two weeks, the fuel start to "gunk" up in the carbs. If you had any trash in your tank, you could of clogged the fuel filter.

Something you can try, without a lot of diagnostic testing, is to add SeaFoam to the gas tank. Use the recommend amount of SeaFoam for the amount of gas it takes to fill it up (I believe it's about 4 gallons or a bit more). Also add 1/4 ounce of carb cleaner (B-12 works great).

Before doing any of the above, I recommend that you check the fuel filter, fuel lines, fuel on/off valve and remove the carb bowls to check for any trash or gunk.

Now you can try starting it again, but put a trickle charger on the battery to help keep the battery up while starting. Start out with the Choke at full and try to start it for a few minutes. If it starts, let it idle for 10 - 15 minutes to allow the additives time to run through and clean the fuel system. Make sure not to run the battery completely down.

If it still won't start, pull a fuel line off a carb and try to start it. If you are getting fuel through the line, it's, more than likely, not the fuel system and could be electrical. Although I've never heard of an electrical issue when running any bike out of gas.

One other thing... try starting it with the fuel on/off valve in the RES (Reserve) position.

This may sound silly, but check the Run Switch to make sure that it didn't get knocked out of place - even slightly.

I've been racking my brain since your question appeared over 3 hours ago. I hope the information that I have given you, will give you enough of a starting point that you can figure out the problem.

If you find this information useful, please vote. Thank you.

Regards,

Oz

NOTE: I have done this, but I do NOT recommend it. Do this at your own risk. Get a spray can of starting fluid, find your air cleaner and give it a quick shot. Back away from your bike and try starting it. It will either start, or pop and backfire. If it didn't start, try one more quick shot, again backing away from the bike while trying to start it. Once the bike starts, and you have added the above chemicals, you shouldn't have any trouble starting your bike again. One more thing, if you use low octane fuel it will have Ethanol in it... use the SeaFoam every other full tank. Only use the B-12 once. If your bike is going to sit for an extended period of time, get a bottle of Sta-bil and add the recommended amount to the gas tank. Run it for a bit, shut it off, and shut off the fuel valve.

If the bike sat for quite a while, and you didn't drain the tank and carbs, your gas probably gummed up the carbs, jets, needles, etc. If you had them cleaned... this wouldn't be the problem although it could still be a fuel problem. Have you checked the fuel filter? I have found that using a gas stabilizer when the bike is not in use, keeps this from happening.

Before I ever take my bike into the shop, after winter sitting, I always run a mixture of Sea Foam (get it at Wal-Mart) with a full tank of gas for the 1st run of the season. This will take care of most fuel issues.

Hope this helps!

This sounds like water in the gas. Condensation in the gas tank makes it's way into the float bowl and after enough accumulates the bike dies because water just doesn't burn like gas. Drain the carbs and then let them fill up again. The good thing is that the cost to eliminate this as a problem is zero.

Your fuel tank breather is blocked. As the fuel is sucked out of the tank a partial vacuum is built up and no more fuel can come out.
Check the breather holes in the filler cap or the breather pipe if it has one.