The Stovebolt Site Forums The Parking Lot Big Bolts 51 gmc stalling out

In my 51 3 ton, I've got the 270 with the zenith carb. I just got the carb back from being rebuilt thinking that would solve the stalling out problem. It didn't solve it If I pour gas down the carb it starts and runs for a minute or so, so long as I keep pounding on the gas pedal. It just won't idle at all. I have an electronic fuel pump that is pushing gas into the fuel filter, but from there I'm not sure what is happening.
It's been doing this a long time. I just cleaned out the gas tank and put new gas in so the tank is not the problem, the pump shouldn't be the problem and the carb shouldn't be the problem. I'm not sure what to look for now. any help would be much appreciated!

Troy

Hy Red63, did you clean out/replace the fuel lines from the fuel tank to the carburetor and any shutoff valve that may be in the tank?

Thanks for getting back, 3B... Yea, I have replaced the lines from the carb to the tank if I remember correctly. I also cleaned the valve at the bottom of the tank as well, although that was a good while ago. I think I will try and replace the fuel filter once again. I did notice the fuel line where it connects to the carb fitting, that fitting is not exactly tight. Could that be a problem ? I will tighten that up, put the new filter on and see what happens and get back. I know it's getting gas, cause when I step on the pedal it revs up... It just won't keep up the idle, as it just bogs down and dies

I had a similar idle problem in a 66 gmc truck that was caused by a vacuum leak. After looking all around the carburetor and changing some fuel line parts (that were needed anyway), I found a hole where the hydrovac vacuum supply hose was touching the exhaust manifold in a hard to see spot.

I had the same vacuum problem,but it was the base of the carburetor. It needed primed to run,but with rough idle. Would not run without choke,either.

thanks a lot guys for getting back on this. Wetwilly, what did you do to resolve your problem with it ?

Hello Red,

I would start by plugging/ blocking off all the extrenal vaccume stuff I.E.. hydrovac / Two speed raerend supply hoses and see if motor will run without those things. If so start looking for a leak in one of those systems.

New fuel pump, also added electronic fuel pump, new lines, cleaned tank, rebuilt carb and it runs worse than before. Even used auxillary fuel tank and bypassed the in-truck tank. Every time I pour gas down throat it starts... I can rev it up and its getting gas in for a few seconds then falls off and dies. Repeat process and get same results every single time. At wits end now and know not what to do

So you're saying as long as you dump gas it will run?

Have you disconnected the fuel line at the carb and put the end of the line in a jar and turned the electric pump on? Will it fill the jar with a nice steady flow or pulse?

If you plug the line with your finger will it make pressure (5-6 psi)?

Mike B

If you are sure that lack of gas is responsible, there are only a few possibilities. You have probably done this, but the first step is to disconnect the fuel line at the carburetor and make sure gas gets that far. (An electric pump that turns on with the key can move a lot of gas in a short time, so make sure you can catch it all. The engine will have to be cranked to test a mechanical pump, which should produce a strong flow in spurts.) Gas gets from the fuel line to the jets through a bowl, and this supply is controlled by a needle valve that is opened by fuel pressure on the needle and is closed by pressure from an arm connected to a float that rises as gas fills the bowl. So the next step is to make sure that the needle valve is not plugged or stuck in the closed position and that the float is not stuck in the up position (which can happen during a rebuild) or is not set so low that it closes the needle valve without letting enough gas into the bowl. Hollow floats can also leak and fill with gas, but I have never had this happen to know if it leads to too much or too little fuel. Once gas fills the bowl to the proper level, it is hard to believe that all the power and idle jets could be plugged. At this point the engine might not run good, but it should at least cough and sputter. So if gas is getting to, but not through, the carburetor, the needle valve and the float are the next problem points to check out.

Thanks tons for gettin back guys.... I will do the fuel line in jar test and see what happens. How can I know if the needle valve or the float is the problem ?
If all else fails, look in that direction ?

JR and Mike have good suggestions.

Have you checked that the carb is adjusted properly? Is the idle mixture adjustment too lean? One and a half turns from seated is a good starting point.

Will it idle or try to idle with some choke?

Alex

Just a very little idle, if any.... I can't keep it running long enough to adjust the idle screw I guess I could just tighten it all the way down, then bring it back up a turn and a half ?

If it idles a little with some choke, then your idle mixture is too lean. That could be a lot of things but I would start with the mixture screw. Don't "tighten it down," lightly seated is enough, then back out a turn and a half.

Hope that helps. Let us know what happens.

Alex

Red,

Others will know better what might be done with your particular carburetor. I am sure that I haven't worked on one like it. In general, you need to get the bowl open to check the needle valve and float. Directions for setting the float level might be included with your rebuild kit instructions. Good luck. I understand the frustration. I have a 66 GMC truck that wouldn't start after changing the alternator wiring, and just by chance I tracked it to a loose wire in a multi-strand plug that had been jiggled while doing other work.

Your rebuild kit instructions might also advise a starting point for the screws that adjust the idle mixture. I haven't found this adjustment to be critical for fixing the type of problem you are describing because just a light throttle should keep the motor running.

Thanks JR and Alex.... Concerning the carb, I had it professionally rebuilt so I dont have the kit instructions. I can touch base with the rebuilder and see what he says. I will check these things out and get back soon. Thanks so much for all the input. I have been frustrated I've felt like giving up on this thing.

If the carb. was professionally rebuilt, then the base plate Should've been checked for "level". You may check it yourself with a straight edge.

Thanks so much for getting back on this Wetwilly... I will hopefully get to work on this tomorrow and can check that. I didn't know about the level on this thing being that important. Please forgive my ignorance

Well, I adjusted a screw in the carb, the one with the spring on it. I got the truck at least to idle a little better. As it was idling tho, it started running very rough, like a lost some cylinders My friend said I may have adjusted the mixture and got it incorrect, hence the rough idling... I ran into another issue tho, for some time the old starter has been dragging. It would eventually kick in tho. Know, it's totally stopped cranking and when I hit the foot starter all it does is bump, it won't turn over. Did the starter finally kill over you think ?

Yeah,it sounds like it may be time to rebuild that,too. Problems usually come in 3's,I've learned. hope you only have 2 to deal with! Good Luck!!

I was afraid that would be what I'd hear

Thanks for assistance on this.