1968 GMC 250 motor won't stay running

[68gmcdude]

I have a '68 GMC with a 250 motor and 3 on the tree. I was driving the truck when it started surging. It would run good for 10 seconds then feel like it was starving for fuel for 10 seconds but it kept running. This went on for about 10 minutes or so. Whenever I accelerated hard it seemed fine with no missing or backfiring. After about a 1/2 hour of driving the truck just shut off. I coasted into a parking lot and eventually got it restarted. It ran for about a minute then quit running again. It did this 4 or 5 times until now it only runs for 5-10 seconds and quits. I ended up having it towed home. It's like it's starving for fuel. Could it be the fuel pump? Thanks!

[HO455]

Yes it could be but I would check some things first. Verify how much fuel is in the tank. Nobody likes to change a fuel pump just to find out the guage is wrong and the tank is empty. If you have at least half a tank try disconnecting the fuel supply line to the pump. Fuel should run out freely. BE SAFE when doing this! If your flow from the tank is slow or not at all your pickup in the tank is clogged. If you have a good supply of fuel then run a line from the pump outlet to a bucket and have someone crank the engine over. You should have good pulses of fuel into the bucket. Once again BE SAFE! No sources of ignition. Low volume from the pump would indicate a a bad pump. If this all seems fine then you have a plugged filter in the carburetor fuel inlet or a stuck float valve in the float bowl.

[Coley]

I had this happen in the past as well....ironically on my 250 6 cyl/'66 chevy.
Turns out it was a fuel problem....particularly the filter kept plugging up.
It would run for a while until debris built up on the filter surface...thus eventually starving the engine and shutting it down.
When/while the vehicle shut off ...then all the debris would loosen enough and pull back.
This would then allow enough fuel flow on a restart to allow the engine to 'go'.
At least until the debris/dirt/etc built up enough again in the filter to shut the engine down again.
Check and replace the filter if you get a chance....very cheap maintenance event and get a spare filter when you put the new one in.


my two bits.
Coley

[68gmcdude]

Thanks for the suggestions. I will check it out over the next couple of days... I do have over a half tank of gas...

[MARKDTN]

I had something similar to this on a '71 Eldorado. It had gunk in the fuel pickup lines that would get caught in the bends. I took some stiff wire and worked it out. Replaced the sock. It was OK after that.

[68gmcdude]

So today I removed the fuel line from the fuel pump and there was no fuel coming through it. I then removed the fuel filter thinking that was the problem. It was definitely filled with junk but when I removed it from the line not a drop of fuel flowed through the line. I'm assuming at this point that the tank is full of rust, sediment or whatever and it is clogging the line leaving the tank. Looking down the filler neck it looks like there is a bunch of rust. Shining a flashlight and looking into the tank it looks like there is a lot of junk at the bottom. Should I just pull the tank and install a new one? Here are some pictures...

[68gmcdude]

I just got a quote to clean and seal the tank for $300-$350. Seems kind of pricey to me. Looks like new tanks are anywhere from $150-$200 so a new tank seems like a no brainer. Where is the best place to get a tank? LMC? Classic Industries? Local auto parts store?

[697498chevy]

I recently replaced the tank and sending unit on my 1969 C 10 . I'm thinking the tank's $150 and the sending unit's $50 . Also a $20 oversize charge for shipping the tank . Got it from LMC . Had to enlarge the tank mounting holes to get the fuel neck centered in the cab hole . There's 2 tanks , mine's the un-vented one . Hope this helps .

[HO455]

Originally Posted by 68gmcdude

So today I removed the fuel line from the fuel pump and there was no fuel coming through it. I then removed the fuel filter thinking that was the problem. It was definitely filled with junk but when I removed it from the line not a drop of fuel flowed through the line. I'm assuming at this point that the tank is full of rust, sediment or whatever and it is clogging the line leaving the tank. Looking down the filler neck it looks like there is a bunch of rust. Shining a flashlight and looking into the tank it looks like there is a lot of junk at the bottom. Should I just pull the tank and install a new one? Here are some pictures...

EEEEEWWWWW! I would replace the fuel line too. It won't be easy to clean the old one completely.

[Steeveedee]

Originally Posted by 68gmcdude

So today I removed the fuel line from the fuel pump and there was no fuel coming through it. I then removed the fuel filter thinking that was the problem. It was definitely filled with junk but when I removed it from the line not a drop of fuel flowed through the line. I'm assuming at this point that the tank is full of rust, sediment or whatever and it is clogging the line leaving the tank. Looking down the filler neck it looks like there is a bunch of rust. Shining a flashlight and looking into the tank it looks like there is a lot of junk at the bottom. Should I just pull the tank and install a new one? Here are some pictures...

Whatever else you do, get rid of that inline filter. They are known to unscrew and leak fuel where you really don't want it. Personally, I'd recommend taking the tank out and dumping the fuel into another container to see what you have in the tank. Maybe pump the fuel out first. I grew up so poor that I couldn't even pay attention, and had to work everything out as cheaply as possible. I wouldn't pay to clean the tank, or buy a new one until I had an answer for what the problem actually is.

[custom10nut]

Does your truck sit idle for extended periods? The gunk in your pictures looks like sediment from separated fuel. The Ethanol fuels tend to separate when they sit for more than a month or so.
I had the same problem in my truck after I took it out of storage after five years.
I ran the tank down to about 1/8, and parked it across an incline with the filler on the low side. I then sloshed it back and forth to get any sediment to the filler side and siphoned the gas out. I was suprized how much crap came out.
Then I and got one of the clear filters, like you have, with extra inserts, and ran the rest of the gunk out. I put Sea Foam in a couple times and replaced the filter element 4 or 5 times until the gunk stopped showing in the filter.

[68gmcdude]

Here's an update... The entire fuel system from the tank to the fuel pump had all kinds of sediment, rust, etc. The tank was rusty inside so I installed a new tank, sending unit, lines, in-line filter, carb filter and fuel pump. Truck stays running now however when driving, it seems to bog down at a steady speed. When accelerating there does not seem to be an issue. There is also a backfire every so often when I am idling and give it gas. Could the float have a stuck valve? Is that easy to fix? I have never rebuilt a carburetor.

[HO455]

I would bet your float bowl has more than its fair share of debris in the bottom. Those glass filters are good at catching gravel and fire but little else. The passages in you carb are drilled to exact sizes and a .010 piece of rust in the wrong place will mess up the mixture. A tear down and good cleaning is in order.

[68gmcdude]

UPDATE: I took the carb apart, cleaned and am now starting to put together but I have a couple of questions... I have attached pics for reference.

1) The one pic shows the original needle seat that was in the carb (left) vs. the new one that came with the kit. They are very different. When I put the new seat in, it seems to not be correct as it comes down too far and interferes with the filter and spring (see pic). The new needle is not long enough to seat in the bottom of the old seat with the hanging clip on it. Can I cut off the part of the new seat that hangs down and interferes with the filter?

2) When I took the carb apart there was no needle in the original seat. Shouldn't there have been?

3) I was watching a video on how to reassemble the Monojet and it said there should be a spring in between the float bowl top vent screw and the plastic fitting it attaches to. Mine did not have one. Is it necessary? If so, what kind of spring is it and where can I get one?

Thanks as always!!!

[Steeveedee]

Originally Posted by 68gmcdude

UPDATE: I took the carb apart, cleaned and am now starting to put together but I have a couple of questions... I have attached pics for reference.

1) The one pic shows the original needle seat that was in the carb (left) vs. the new one that came with the kit. They are very different. When I put the new seat in, it seems to not be correct as it comes down too far and interferes with the filter and spring (see pic). The new needle is not long enough to seat in the bottom of the old seat with the hanging clip on it. Can I cut off the part of the new seat that hangs down and interferes with the filter?

2) When I took the carb apart there was no needle in the original seat. Shouldn't there have been?

3) I was watching a video on how to reassemble the Monojet and it said there should be a spring in between the float bowl top vent screw and the plastic fitting it attaches to. Mine did not have one. Is it necessary? If so, what kind of spring is it and where can I get one?

Thanks as always!!!

That's the wrong seat. If it were me, I'd return the kit and go to Cliff Ruggles for a kit. http://cliffshighperformance.com/

I realize that he's advertising as a Q-Jet guy, but I'll bet he can steer you straight. When I bought a kit for my truck (Q-Jet) when I gave him the part number he was surprised that it was the original carburetor -which I knew- it's been in the family for 44 years! He didn't look the part number up inn a catalog either, when he identified it as original.

That bowl vent spring is a lever type.