Gas Won't Stay In Carb Why ?

[BAD2500HD]

I have a 1979 GMC K25 it has the 350 and is a auto the problem i am having is the gas won't stay in the carb , if it sits for just a day or two it will start up but much past that and you have to pull off the cleaner and spray in some gas to get it going cranking it and pumping the gas does nothing we have looked it over and can't seem to find a leak and it doesn't seem to be getting into the oil that i can tell or into the motor its just like it never had gas in it , it does have the duel tanks and i did put on a new fuel pump . once you get the truck running it seems to run great it has around 66,000 miles on it any clue as to what i need to do ? when person said the carb needs a rebuild another said i should put a check vavle in the line but i thought there was one in the pump ? also after sitting like i said for a few days the pump is dry too , any help would be great as i need this truck for the winter etc. ! Thanks

[Hooty]

I haven't owned a vehicle with a carbureter for close to 20 years but I did have a similar problem with an old motorcycle many years ago. The rubber gasket on the float bowl needle valve had deteriorated and would not seal on the seat anymore. Even though the fuel was gravity fed from the tank, it would still drain down the float bowls overnight. In your case it could be allowing the fuel to drain past the rubber seat, down to the fuel pump and back to the tank. Hence you would not see any leaks.

 

If you were to rebuild the carb, a new float bowl valve would be included in the kit. I can't remember if they are available separately.

 

Don't know if this is anywhere near the mark but it is my $.02 worth.

 

Cheers and good luck.

 

Hooty

[txab]

1 line goes to the carb from the pump. 1 one line comes from the tank. 1 line is a fuel return line to the tank. When your pump is pumping more fuel than the carb needs it is returned to the tank. The return line should be the smaller of the 2 input lines. If it has a factory valve underneath, it should be on the passenger side frame rail. There will probably be 4 rubber hoses on one side of the valve and 2 on the other. (2 from each tank. Again 1 will be fuel from the tank the other will fuel return. Since yours may have been changed from factory setup things may be different) If you bypass it for now, you can just buy some rubber fuel hose long enough to connect the metal lines from the tank you want to use to the metal lines on the other side of the valve. You can leave the valve in place for now. If you have an aftermarket valve it may be different.

 

I would definitely try this to see if this might be your problem. Your valve may be letting you draw from an empty tank.

 

Your valve may or may not look like this:

 

 

[builderpaul]

Stock Rochester Carb?

This takes me back in history. I had a similar problem.

Read the section on bottom plugs.

 

http://www.cartechbooks.com/cartech/contentfiles/10814.pdf

[REV353]

Having the same problem with my 65' Fairlane. I am waiting for a new fuel pump. I would not look towards the needle and seat, cars are different than motorcycles and snowmobiles like the above reply. Powersport items have the fuel tank above the carburetors, the pressure from the gas in there tanks force the gas to the carburetors, thus putting added pressure on there needle and seats. Your fuel pump is lower than your carburetor along with your gas tank. Anyway, my car pops off when I hit the key, then it dies, crank it for a bit to get the gas back up and it runs all day long. From what I have been told there is a check valve (one way) that is bad.

[Jsdirt]

I've got a '72 'Camino that did the same.

 

New fuel pump should solve that issue. EDIT: I see you've done that -- possible that the new pump is defective. See if you can try another one out to test.

[BAD2500HD]

got to craw under the truck this weekend for a min. and looked at the lines what a mess ! does anybody have any pics of what it should look like and what lines i need to unhook ? i'll be using just the pass. side tank ( right) they are all twisted and going every which way i can't really make heads or tails of whats what ! thanks

[txab]

I'll try to snap a pic tomorrow. It might help

[BAD2500HD]

Sorry guys its taken me so long to get back to you , i haven't had a chance yet to change the fuel line and disc. the second tank , had a bunch of stuff come up and had to put it on the back burner i hope to get at it this weekend if it don't rain both days (or snow !) .

[BAD2500HD]

Thanks ! it sure can't hurt ! i have never seen such a mess before no wonder i'm having trouble !

[txab]

Were you having this problem before you changed the fuel pump? So when you remove the line at the carb and crank it, you're not getting fuel? Or does it just take a while to finally start spitting fuel out?

[BAD2500HD]

Jsdirt , it did the samething with the old fuelpump thought the new one would fix the problem but didn't seem to. txab , yes samething with old pump it seems to be running back into the tank i guess when we took the old pump off there wasn't any gas in the pump or lines when we put the new one on we cranked it over and over but coulded get any gas up to the carb untill we dumped some down the carb the second time it ran and stayed running . Hooty , i thought of the floatbowl but not sure where the gas is running out to from the float ? the oil doesn't seem to smell like gas . builderpaul , yup its a rochester and i was never very good with carbs ! and it seems like there isn't really anybody around here that does work on them . my son just has no clue and doesn't even want to touch it ! and he works on cars all day but has no clue what is wrong with it like i said nobody works on them or knows how to tune them anymore , thanks for the input and any and all help on this is great . P.S. doesn't the fuelpump have a check valve in it to keep the gas in it or do they ? thanks again !

[txab]

I have a 1979 GMC K25 it has the 350 and is a auto the problem i am having is the gas won't stay in the carb , if it sits for just a day or two it will start up but much past that and you have to pull off the cleaner and spray in some gas to get it going cranking it and pumping the gas does nothing we have looked it over and can't seem to find a leak and it doesn't seem to be getting into the oil that i can tell or into the motor its just like it never had gas in it , it does have the duel tanks and i did put on a new fuel pump . once you get the truck running it seems to run great it has around 66,000 miles on it any clue as to what i need to do ? when person said the carb needs a rebuild another said i should put a check vavle in the line but i thought there was one in the pump ? also after sitting like i said for a few days the pump is dry too , any help would be great as i need this truck for the winter etc. ! Thanks

 

Ok. So even after sitting a few days it will start and run until the fuel in the bowl is gone. Correct?

 

What happens when you crank it for a while? Will it eventually get fuel back to the carb? Or do you just crank and crank and get nothing?

 

Have you looked into all your fuel lines and fittings from the tank to the fuel pump. Especially any rubber hoses that might have cracks that might allow a small amount of air into the line. If you're "losing your prime", a slight line leak on the suction side might allow the fuel to run back to the tank.

[BAD2500HD]

txab if it sits for just a couple of days it usely will start up again after that you can crank it for ever and it doesn't seem to pick up any gas thats one reason i thought it was the pump . i have gone over the lines and yes there is some rubber line on it but it all looked good i suppose it might be smart to just replace it all , i don't use the second tank ( on drivers side ) would it be ok to not worry about that line or should/would that have to be replaced to ? one other thing there is a couple of vac. lines that have been pulled off and pluged would that do anything ? i plan on trying to get them back to where they belong and have everything put back to right . thanks

[txab]

What kind of switching valve do you have for the fuel tanks? The electric type? If you're not ever going to use the the drivers side tank why not just connect the passenger tank direct, bypassing the valve. I believe the valve switches to the left (drivers) tank when power is applied. At least that is the way it on on my '79 & '80. I'm wondering if this might be where the problem is. Worth a try since you're only using 1 tank anyway, no need for a switching valve. Don't worry about the vacuum lines. If it runs OK without them, leave them alone for now. They probably are for the vacuum valve switches that control emission crap, the vacuum switches are probably bad anyway.

[BAD2500HD]

What kind of switching valve do you have for the fuel tanks? The electric type? If you're not ever going to use the the drivers side tank why not just connect the passenger tank direct, bypassing the valve. I believe the valve switches to the left (drivers) tank when power is applied. At least that is the way it on on my '79 & '80. I'm wondering if this might be where the problem is. Worth a try since you're only using 1 tank anyway, no need for a switching valve. Don't worry about the vacuum lines. If it runs OK without them, leave them alone for now. They probably are for the vacuum valve switches that control emission crap, the vacuum switches are probably bad anyway.

I would say thats what it is since there is a toggle switch so it must be electric , to be honest i haven't really checked out the valve etc. on the tanks is there any certin way it has to be done by bypassing the valve ? i kind of wondered if maybe something like that was what was giving me trouble it might be worth it to take/unhook it and try it that way its not a factory tank it something somebody had made and put in there i believe it did new come with the two tanks, i plan on someday buying the factory one and putting it in. also the fuel pump on this truck has three lines does that have anything to do with the two tanks ? thanks