The Stovebolt Site Forums The Shop Area Engine Shop I'm stumped...carb?, fuel pump?, other?.... any ideas???

Hi guys, here's the story. I've had the truck about 8 years- '50 3600 with a '62 235 motor. Not long after I purchased the truck, I stopped to fill up and the truck wouldn't restart. Opened the hood and found gas pouring out the throat of the carb when the starter was running. I ordered a new carb ($70 plus core charge), put it on and all was well.

About 2 years ago I had driven the truck about 5 miles to the dump. After dumping the trash it wouldn't start. I have a clear fuel filter just before the carb, and it was bone dry. I also had a weak battery, so the engine turned over pretty slowly. I got a jump start which made the engine turn over faster and pumped fuel, so finally got it started. I figured the mechanical fuel pump must be weak so I replaced it as well as the battery. Again, all was well....

Last Saturday I drove to town to fill up- about 12 miles. After filling up, about a mile from the gas station (where the speed limit goes from 35 to 55, so I was accelerating) it started acting intermittently as if it were starved for fuel. It would run fine for a minute or two, then almost die. These episodes got more frequent, but I kept going to try to get home, About 4 miles from home it died for good. I checked the clear filter and it was dry. I took the fuel line loose from the pump to the tank to blow thru it (suspecting trash in the tank had clogged the pick-up) but plenty of fuel flowed thru, so the pump is getting fuel to it. I figured maybe the 2 year old fuel pump had failed, or maybe the cam that drives it had finally worn, so I called my wife and she brought the pick-up and a tow strap and towed me home. I took off the fuel pump and it definitely acted like it was pumping. I went ahead and took it apart anyway, but it looked ok inside- no evident trash. I reinstalled the pump and the truck started right up and the clear filter filled with gas.

On Thursday I drove the truck to work (about 15 miles) with no problems. Driving home, I got to the same place on the road where I had problems last time (55 mph, so again I was accelerating) and the truck did the same thing- acted like it was fuel starved. This time I only made it about 1/4 mile before it quit. Sure enough- dry fuel filter.

It's something how many people will stop to offer help when an old truck breaks down. I had plenty of help with the following: I took the fuel hose loose between the pump and the filter and ran the starter. No fuel was being pumped. Ran the starter for 30 seconds or so and NO fuel was ever pumped out of the fuel hose. Then I took the hose loose between the tank and pump to check the flow- plenty of supply. I did this several times, taking the hoses apart at different locations. No fuel was pumped thru the pump. So... the problem has to be the fuel pump. I called Napa and Advance Auto - nobody had a fuel pump in stock. Sooo.... AAA to the rescue. At least the flatbed truck was a Chevy, too. Took the truck home and put him in the barn.

Yesterday I opened the hood to try to find the problem and noticed the fuel filter was about 1/2 full!!!!. ???? How'd that get there? The filter was dry yesterday and the fuel pump hadn't run since. Hit the starter and he fired up, so the pump must be working again??? After running a minute or two I saw that a fair amount of gas was leaking from the carburetor, but it was leaking around the area of the throttle lever, back on the cab side of the carb,, not out of the top of the carb like the first one had. Something must be wrong with the carb, I went to town yesterday and talked to a guy at the auto parts place. He said if the carb is leaking around the throttle shaft it can't be rebuilt. The price for a new rebuilt carb now is $225!!!! and would have to be ordered.

I ended up buying an electric fuel pump and another filter. I installed the new filter just after the hose leaves the tank, and then the new electric pump just after the new filter. I bypassed the mechanical filter all together. I then hooked everything back up, started the motor. It ran fine and no apparent leaks. Next I went for a 5 mile drive (staying close to home). I stopped several times and checked everything- fuel filter up by the carb stays 1/2 to 3/4 full, and no leaks around or from the carb.

Any ideas???? Do I still need a new carb? What would cause it to leak (a lot) intermittently? How'd the fuel filter get fuel in it after sitting overnight? How is it that the mechanical pump worked sometimes but not others? Do I have 2 separate problems (fuel pump and carb) or is one part causing the other to malfunction. I have ordered a new carb, as I don't like the idea of gas running down on a hot motor. I have everything running again with the sometimes leaking carb, but I'd like to know what the problem is (was?) before heading out on the road again. Any ideas or suggestions greatly appreciated!!!! Other than replace the carb and hope for the best I'm not sure what to do next.

Have had the same problem with my 1960 235 in a 52 3100. After some frustrating time under the truck at the side of the road. And replacing the fuel pump I finally found the culprit. The needle in the carb that the float controls would stick. Some times closed..no fuel in the bowel and some times open...fuel leaking on to a hot manifold. Not the best of conditions. I took some 600 grit paper and polished the needle on the three sides that rub on the side of the seat. That solved my problem. Don't touch the tip. Don't touch the tip. That will make it leak all the time. Old tank, old gas, new alcohol fuels and a new kind of sticky varnish will add to our issues. Good luck with yours.

Thanks for the reply, Steve. But how would that explain that the fuel pump not pump fuel with the supply hose not connected to the carb? Red Herring??? I hate it when things don't make sense to me, and this doesn't.

If the needle and seat are closed. No fuel will move from the pump. Over time this will over come the check valve in the pump and so not be seen in the filter. I have a clear filter in front of the pump. You can see the fuel slosh up and down as the pump works. I have another just in front of the carb. You can almost see the fuel move in it. Good news, sort of the big junk is stuck in the filter before the pump and the fine stuff is stuck in the filter by the carb. Makes me feel like I am helping things any way. :)

The float valve is the most likely the main problem. I have seen the problem several times before. stuck closed, no fuel into the carb, open carb floods. By the way, if the throttle butterfly is closed and the carb is flooding, gas will leak out the throttle shaft in any carb. Doesn't mean anything is wrong with it.
But you could have a secondary problem. The tank requires a vented cap. Without it a vacuum will form in the tank as gas is used up. This happens very quickly when the tank is full but takes much longer when the tank is low on fuel. When the vacuum gets higher than the pump spring can push, no more pumping. Even a vented cap can become blocked.
How did fuel magically appear in the filter? In either case, when the fuel pump can't move fuel the diaphragm bottoms out against the spring. even with the engine not turning the spring is pushing on the diaphragm. If the stuck valve opens due to temp change or vibration, or the vacuum in the tank is relieved, the diaphragm will then be pushed up by the spring pumping some gas.

To add a wee bit to Jim Sears response:

If the tank vent is not open, not only can a vacuum develop in the tank, but differences in ambient temperature can cause a pressure in the tank sufficient to cause carburetor flooding.

The lack of a functioning tank vent can cause some of the weirdest intermittent problems imaginable.

Jon.

Have you taken the gas tank out and checked inside it, also check the fuel pickup in the tank?

Thanks, guys!!! I'll check all these scenarios out and see what I can find. I thought about the tank vent, even took the gas cap off while waiting on the tow truck- it still didn't pump fuel. Would it take a while to equalize pressure (vacuum) before it would pump?

Also- now that I have the electric pump, is it any problem to just leave the old mechanical pump attached to the block pumping air, or should I remove it and get a cover plate? I tried to use both the pumps in tandem but the elec. pump wouldn't
pump through the mechanical pump if the engine wasn't running.


Edit: thinking about the vacuum issue- when I took the fuel hose loose between the tank and pump, while sitting on the side of the road, there was plenty of fuel flow to the pump via gravity. The pump just wouldn't move it. Could the mechanical pump get an air lock or something?

Removing and cleaning the tank has been on my "to do" list for 8 years. It's on the top of the list now...... soon as I run this tank of fuel out.

Just experienced this problem yesterday on the first long trip on the new rebuild with a brand new fuel pump and a new Rochester carb from Carb Exchange. Ran fine at 50 to 55 going down the mountain until I opened her up a little near the bottom of the hill. Started cutting out. Got to my engine builder where he had a timing light and set the timing. Took it out on the freeway up to 65-70 and it started cutting out till I dropped it back down to around 55. Speedo does not work so I'm guessing speed. Took the frontage road towards home at around 50 and it ran fine. Loaded ten bags of salts at Home Depot and headed home about 30 miles away. Was fine at around 50 till I had a line of cars behind me and speeded up. Started cutting out so would periodically pull over to let them pass. Runs fine at 50. This new rebuilt carb must have a float filling problem. That's where I'm gonna look first I think.

Anzan,
Your problem seems to me that it is jetted too lean, my Rochester B does the same thing, fresh rebuild, I had to go to a 61 jet for it to run well. I'm at sealevel so it is worse for us guys. To diagnose lean jet/power valve take your truck out and get into heavy load and wait for it to cut out/stumble then pull the choke out a teany bit, if it starts to pull harder you are too lean on your tune. BTW, I might add, the single Rochester B seems to be a little under carb'd for a 235 (56 blue flame high lift cam engine 140hp). When I had my later model mono jet on it, it had a ton more power and better drivability too.
Steve

there would be no reason to have an electric fuel pump, you are bandaiding some other problem, sounds a lot like you have a partially plugged suction, filter or something. The mechanical pump can get a very small piece of rust caught in one of the valves and cause it to not work, I fought rust in the tank for about 2 years, found a NOS tank for $75 quite nearly the best $75 I have ever spent.

Thank you for all this information. It will help me resolve a problem I have.

This may be a red herring, but I will tell you about a problem I had with an old Volvo SW. I would make a right turn and it would stall. But being fuel injected with an electric pump it would suddenly start again. The fuel pickup line inside the tank was rusty - mainly from previous owners keeping the tank empty. So when I turned right, the rust hole in the pickup line would draw air. After a few minutes the fuel would cover the hole again and away I went.

I would check your pickup line inside the tank for holes or obstructions that could be a problem when the fuel moves around in the tank. I don't know what kind of strainer is on the pickup, but I also experienced a problem with a customer's car that had water in the tank. The water would block the intake screen. I doubt that is an issue but it might get the creative problem solving juices flowing.

All that being said, most experienced techs will tell you it is almost always the most obvious thing e.g. the float needle.