Been having some fuel related problems recently that I need to solve. Made a thread a while ago but never had any closure on it. Here I am again, in need of help.

Vehicle:

'68 Corvette Convertible
327/350
4 speed Muncie

Aftermarket:

-Holley 70gph mechanical fuel pump (had an electric fuel pump somewhere, could not locate, but pulled the fuse for it). Has no return line to the gas tank, not sure if I need it?

-BG Road Demon 650cfm, vacuum secondaries, electric choke. Screws turned 1 full turn out, on all 4. Idles around 750rpm.

-Weiand Stealth Air-Strike Intake Manifold.

-Summit HEI distributor, Vacuum advance hooked up to a ported/timed source on carb. Timed engine to 34* around 2500rpm (revved the engine, with vacuum plugged, until it stopped advancing and then timed to 34*.)

Stock throttle cable which doesn't fully open the carb, so I can't do WOT..

---Initial Problem---

Car would idle and run fine. I could drive it up and down the streets near my house without a problem. I could rip through the gears, and rpm range, with no problem. Seemed to be perfect. But then when I went on a cruise...I would encounter problems.

Example 1: Would be cruising around 65mph at around 3000 rpm. I would start to go up a slow ascending, long hill...and halfway up (it isn't steep, just long), the car would start to act fuel starved. It would start to sputter and buck, causing me to get off the gas, out of gear, and flutter the throttle until a steady "rev" could be achieved...then put back in gear, and try to take it easy (55mph) up the rest of the way. Sometimes it would be worse.. Point is, I don't have that problem at any other time it seems, but with those variables. It doesn't appear to be correlated to how much fuel is in the tank..which was my first guess.

Possible diagnosis:

-Incorrect tuning of the carb?
-Fuel pump losing pressure or not working properly?
-Fuel pickup line in the gas tank gone bad, or not mounted correctly (allowing it to float or whatever to the top of the gas tank?)
-???

Example 2:

A month after the above mentioned, consistent problem, I was cruising again, on the highway. No hills or big inclines.. Decided to step on the gas a little while in 4th gear. I was cruising at 65mph again, 3000rpm, and took it to about 3500rpm before it started sputtering and wanting to die. I quickly backed down and was able to maintain 65mph with no problem, for the rest of the trip. I could also rev all the way to 6000rpm in the lower gears, from a stop or whatever.

Diagnosis:

-Incorrect tuning of the carb?
-Insufficient engine vacuum?
-Insufficient fuel pressure?

---Newly Arisen Problem---

Example 1: Drove the car to a local car show, about 15 miles from me. Drove great, no problems, was able to rev it as much as I wanted while on the road. Cruised at about 65mph, but didn't take it beyond that. Drove back, and no problems until I got about 3 miles from home. Then for no reason it seemed, it started sputtering and not wanting to maintain speed. No hills and was only trying to maintain speed. Dropped my speed down as well as tried fluttering the gas pedal. Didn't help. It continued to sputter until it got around 30mph and I was able to limp it home. Didn't sound as healthy or like all of the cylinders were firing either. Gas tank was around 1/4. I filled it up at the gas station next to me with Premium.

Diagnosis:

-Fuel leak somewhere?
-Fuel pickup line in gas tank?
-Vapor lock? Car was sitting outside the whole time, but temperature wasn't that bad. Engine never got over 180*

Example 2:

Drove the car this morning to church. Again, NO problems the entire time. Went up and down hills at cruising speed, was able to accelerate whenever I wanted...no issues. Drove back, again, no issues.

Then I drove it this evening around 5PM. Was going to a Corvette Club meeting across town, same distance that the previous car show was. Barely made it a mile on the highway before the car started sputtering and bucking at cruise. No hills, and was not trying to push it past 3K rpm in 4th. I tried fluttering the gas pedal to no avail. Had to drop speed down to about 30 mph, before I could maintain some sort of speed. Would try to flutter it back up to speed but it would choke and sputter. Then tried aggressively fluttering the gas pedal (pushing it farther to the floor, and more rapidly) and the engine would sputter and hesitate, and then catch and accelerate moderately well. But when I had to back off the gas due to a gear change or speed limit, the car would go back to acting the way it was. When I pulled over the car and checked the fuel bowls, the front primary one had no fuel in it, the back one was where it should be. Limped it back home and parked it.

Diagnosis:

-Fuel leak, made worse by pressure?
-Something clogged in carb?
-Vapor lock?

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I know this is a lot of info, with wild guesses on my part, lack of complete information and description, but any help you all could provide would be awesome. I'm out of ideas..

Thanks,
Wes

 

sounds like I problem I had ages ago. I kept thinking it was fuel. Instead it was my coil....when it got hot, it would act erratic....of course as soon as I opened the hood it would cool off, and then run fine...so it was hard to find.
Try changing your coil. It's cheap and easy.

 

You're talking about an external coil, right? I have the HEI setup so it is on the distributor. It is also brand new..

 

There must be a blockage on the primary side of the fuel feed. Or the vent tube is blocked on top of carb. I would start with that, check filter at primary bowl. If you can make it happen after that, try removing air filter completely and try driving like that. Has anything changed like gas tank or fuel lines?

 

Sounds like a fuel delivery problem to me. Either a stopped fuel filter, a suspect pump or restrictions in the line. I would FIND and remove that electric fuel pump, and check the filter.

 

Other than the little mesh screens in the fuel inlet lines at the carb, I'm unaware of any fuel filters on the car.

When I had a gas pressure gauge hooked up, it showed between 5-7 psi at idle and possibly free-revving.

Don't think the vent tube at the top of the carb is blocked..

 

Is it possible that there is something jamming the needle and seat on primary side? Holding it closed or stopping it from opening? That empty primary bowl you mentioned is what I would consider to be your problem. Stick with it we will figure it out.

 

First, I would remove the inoperable electric fuel pump. I don't think that's your problem, but things like that just bother me. Now on to your problem. You mentioned that you had a Holley mechanical pump. That means you have either replaced the flexible rubber hose (S hose) that goes from the steel line to the intake side of the fuel pump, or you reused your original.
I had the exact same problem with my '72 that you are having. I could drive around town with no problem. Short trips were OK along with mild cruising. But once things were really warmed up (hot) and I got into it, it was like someone was turning off the gas. I'd have to pull over and sit for 5 or 10 minutes just to make it home at a limp. I replaced my fuel pump and thought that would take care of the problem, but the same thing happened. I crawled under the car while waiting at the side of the road one day and noticed right away that the S hose (which I hadn't replaced) was very soft and kinked. I replaced the hose the next day with a piece of rubber fuel line, but that also kinked when it got warm. I didn't want to wait for the preformed S hose from Eckler's, so I made a replacement with some 90 degree bends of steel line and some braided stainless line. I have never had the problem again. Replace that hose with the correct S hose or make yourself a good replacement. Wish I had the ability to post pictures so I could show you what I did in place of the stock hose. I hope something this simple solves your problem. Good luck!
Duane

 

If you have a rubber s at the pump this can be your problem. Also I missed the part that you had electric as well, that has to be removed, or bypassed.It should be located by the gas tank.

 

i think you have some good advice here, i strongly reccomend finding that electric pump and remove it depending on the pump it may be a big restriction.

 

What kind of gasket/plate do you have under the carb? Most carbs need an insulator plate under the carb to keep the engine heat out of it...or you get the fuel hot enough to bubble/boil and get the kind of behavior you are describing. Just another idea to mull over...

Another possibility is that the fuel float is set too low and is starving the system when you get it up to a given flow rate. Your last 'incident' did not indicate any slope to the roadway, so I don't know if that is a possibility or not.

 

As far as the throttle cable not being able to reach WOT, I had that years ago. In my case, it turned out that someone stepped too hard on the gas pedal, and bent the arm. Easy fix was to reach down, and pull the pedal upward to bend it back where the pedal would allow full travel before hitting the floor. Not sure if this is what's limiting your throttle travel, but its an easy check/fix.

 

This may sound really stupid, but is the vent in your gas cap clear ?? If you're not getting enough air in the tank, it'll starve the motor.
Cheapest to check.

Good luck
John

 

What spark plugs are you using? What are they gapped at? Sounds like they are loading up to me.

 

All your symptoms seem to point fuel starvation problem.

Do a flow test on your system. Disconnect fuel line. Place it in a container capture the fuel. Start the car to see how much flow you get. Run the car for about 15 seconds to determine your flow rate.

 

Sounds like a classic case of a blocked vent or you used a non-vented gas cap on a tank that requires vented. Easy check: Remove gas cap and go for a drive (do it with 1/2 tank or less so you don't splash gas all over).

 

I have seen that hose at the pump do some weird stuff as well..also the gas cap..and vapor lock without a insulator under the carb..it could be the old fuel pump also..the little gears in there may not allow enough flow thru them if not powered..everyone has put up some good stuff here..now you just have to eliminate the ones that are not the problem..have you considered also the HEI pickup..they do some weird stuff also when they get hot..even a new one..you may have more than one problem ..good luck

 

Not sure, but I can check.

I never thought about that hose...I'll definitely check that out. You may be on to something there...

I've looked by the gas tank, with the electric pump doing its thing and making noise, and I still couldn't find it...it may be inside the tank..

However, I had driven the vette to Myrtle Beach, then to the mountains of NC, and back to Columbia, SC with that electric pump still there, but un-powered. Had no problems. I don't think these recent problems have to do with it, but I can be wrong..

See the above comment.

I have the gasket that came with the carb. Brand new. Don't think this carb needs an insulator plate, though I know what you are talking about.

Fuel float is supposed to be just under the sight glass at idle, correct? That's what I read online on numerous sights. However, I have mine at the midway mark, of the glass..

Hmmm, will check that out. It's been bothering me that I couldn't seem to get it right..

Honestly, I don't think my gas cap has a vent. But I thought that the gas tank itself had a vent tube? I've been using this same gas cap since I got the car. And like I mentioned earlier, I've driven across the state, to another state, and back with no issues...

Mmmm, can't remember off the top of my head, but I think they were Autolite or ACDelcos that ended in 47S.

I'll try that later...don't have the time this week to do that. Thanks.

That's an easy test..will try it. But it could drive perfectly fine with it on one day/trip...and the next it could be causing me fits. It's (problem) inconsistent...

Haven't considered the HEI pickup..

 

I'm not sure about the 68, but later cars have a "sock" on the fuel pick up in the tank. Those can get clogged and colapse, especially when they're over 40 yrs old. Could that be contributing to the issue? It was on my 81....

 

i had a similar problem on my trip from NY to TX. it was my HEI. this is what i did to make it to the store...


The wires going into the dizzy were bent.. it would make contact and run awesome, and lose contact and dog out.. while its running, you could wiggle them and see if anything weird happens...