Been through everything, What am I missing?

[garyd1961]

Originally Posted by AASmedic

Yep, checked all the fuel lines, no leaks

If the lines are old they may not leak but still suck air at higher speeds. I had this happen years ago on my 69 442, the rubber line coming from the tank had a crack. It would run perfect at slow speeds but if I got into it it would just die.

[AASmedic]

Originally Posted by davepl

I must say it does indeed sound like a fuel starvation (delivery) issue from the symptioms.

I'd say fill the float bowls with fuel so it'll run a few seconds, then disconnect the fuel line at the carb and put it into a clear bottle so you can see what's happening. Fire the motor so it'll run for a few seconds using the residual fuel in the carb.

You -should- get a strong and steady flow of gasoline into the glass jug. If it's anything but an impressive flow of fuel, I'd imagine you've confirmed it.

Goes without saying, but gasoline is evil and dangerous. Be paranoid and safe around it.

(BTW, if its q-jet, don't forget the filter and screen in the inlet of the carb itself)

Originally Posted by KJSR

Those RPM's are not that high. Does it start at 2500 and continues to max rpm (over 3k)?

You may want to run the truck in the driveway at night and see if you have spark jumping from anywhere at higher RPM's. That is after its warm and is missing.

Originally Posted by KJSR

Oh yeah, I didn't see.....HEI or points? You mentioned a "pick up" in the distributor so I'm guessing electronic. I would also consider the ignition module if its an HEI. The coil can cause this as well especially since its doing it after its warm.

Originally Posted by Already Gone

I just cured a similar problem with my truck. I too changed the fuel pump and everything else is new. My truck kinda died at fast idle then sputtered while driving and even backfired sometime. A friend told me to try running Premium Gas in the truck. I have been gassing my truck at the same place for 10 years always running regular unleaded but was at my wits end so filled it up on Saturday with premium and it seems to have cured the problem the last 2 days driving around city and highway.

Originally Posted by 68 P.O.S.

Clogged fuel sock on sending unit in the tank? What carb do you have? What is your timing set at?

Originally Posted by custom10nut

Did you check the filter on the pickup line in the tank?

Originally Posted by rsgt

Check the crimps and connections that power the HEI. Where do you power it from?

Originally Posted by whtgmc68

I had nearly the same problem with my truck when it still had the 350 that was in it when I bought it. I replaced the carb, fuel pump and filters. Turned out there was so much gunk in the fuel tank that I just replaced it since it was about the same price to have the orignal boiled out and properly cleaned.

Never had another problem and when the engine finally got a proper flow of fuel, it ran so well that I almost hated to replace it with the 383.

Originally Posted by garyd1961

If the lines are old they may not leak but still suck air at higher speeds. I had this happen years ago on my 69 442, the rubber line coming from the tank had a crack. It would run perfect at slow speeds but if I got into it it would just die.

OK Guys, I have read all of the comments and have some feedback.
I have an HEI which is powered directly from the fuse block (replaced the harness in the rebuild) with all new components installed. Timing is set to 12 degrees BTDC and starts instantly and runs well. Tanks were cleaned during rebuild as well. I do need to re-inspect the rubber lines that connect the tank to the frame rail line and then to the pump.

Here's something that may be of significance though. I do not have a vented fuel cap nor do I have the vapor canister system. It was removed at some point before I owned the truck. Logic would tell me that if all of the air/vapor has been sucked out of the tank by the fuel pump and no air to replace it then the rubber lines collapsing would make sense as well as the fuel starvation issue. Thoughts on this theory?

[57taskforce]

Correct me if I’m wrong but I don’t think the 3/4-1ton trucks had the vapor canister in 72 only the 1/2 ton. My K20 doesn’t have it. The tank has 72 markings (indicating to me that it’s original) on it and it has no provisions for the vapor line or the return line. That said you definitely need a vented cap with out the evap setup.

[Steeveedee]

Originally Posted by 57taskforce

Correct me if I’m wrong but I don’t think the 3/4-1ton trucks had the vapor canister in 72 only the 1/2 ton. My K20 doesn’t have it. The tank has 72 markings (indicating to me that it’s original) on it and it has no provisions for the vapor line or the return line. That said you definitely need a vented cap with out the evap setup.

Yup. This is likely the culprit. It may bleed in enough that the vacuum in the tank isn't bad enough at low speeds, but once the engine gets revved up, the fuel pump can't overcome it. This is easy to check, if the OP wants to put a vacuum gauge in the fuel supply line. Any number of inches of vacuum means that the supply is restricted.

[AASmedic]

Originally Posted by 57taskforce

Correct me if I’m wrong but I don’t think the 3/4-1ton trucks had the vapor canister in 72 only the 1/2 ton. My K20 doesn’t have it. The tank has 72 markings (indicating to me that it’s original) on it and it has no provisions for the vapor line or the return line. That said you definitely need a vented cap with out the evap setup.

Not sure if that is true or not. I have seen so many different set ups and read so many conflicting things that I couldn't tell you for sure if it had one or not. Some books say yes, others are vague while some say not.

[AASmedic]

Originally Posted by Steeveedee

Yup. This is likely the culprit. It may bleed in enough that the vacuum in the tank isn't bad enough at low speeds, but once the engine gets revved up, the fuel pump can't overcome it. This is easy to check, if the OP wants to put a vacuum gauge in the fuel supply line. Any number of inches of vacuum means that the supply is restricted.

I'm thinking a quick test will be in the works tomorrow. Thanks for the input guys, i will update findings as they are available.

[frank69]

I had similar problem with 71 longhorn with duel tanks, bypassed switching valve and just used seat tank and problem went away.

[Dirt's72]

Did you talk to Orval? Gotta ask the man the myth first off then ask these folk up in here. Good luck and see you at the show on Sunday.

[geezer#99]

I see you have a fuel tank switching valve.
Have you tried bypassing it and running just from the main tank?

I’ve seen air leakage from those valves before!

[ZEKE68]

Had a very similar problem on a 55 chevy I had once. Turned out the ballast resister had gone bad and was sending 12 volts to the distributor. These trucks have a resister wire instead of a ballast I believe, so it's likely not the same problem. But as stated ignition problems can seem like fuel problems.

[AASmedic]

Originally Posted by ZEKE68

Had a very similar problem on a 55 chevy I had once. Turned out the ballast resister had gone bad and was sending 12 volts to the distributor. These trucks have a resister wire instead of a ballast I believe, so it's likely not the same problem. But as stated ignition problems can seem like fuel problems.

Not an issue here. I have an HEI ignition.

[AASmedic]

UPDATE!!!!

Thanks for all of the input, suggestions and comments. Good news, I found the culprit. As some of you mentioned, the rubber hose that was installed from the tank to the cross-over valve was bad. When I rebuilt the transmission and installed the saddle tanks, I replaced the valve and hoses. I didn't realize that the hose was cheap CHINA made stuff. And, as predicted by some of you, the rubber became soft and collapsed with the higher fuel demand. It took a bit of trial and error but I finally can drive my truck again without fear of it leaving me on the side of the road. I connected the rigid lines together with a small chunk of fuel line and will now replace all of the "hose" with rigid line as well to prevent future issues.
Again, I want to thank all of you for looking at the thread and offering your suggestions.

-Kevin

[SIXTY9]

GREAT glad you found what was wrong.