Fuel starvation issue

RPO LS5

RPO LS5

Member
Joined
Dec 25, 2008
Messages
24
Location
PA
Hi Folks, I'm in need of a little help, and this should be the right place to ask I'm sure.

While cruising at 60 to 70 mph and hitting the gas, my 72 sb falls flat on it's face. A quick history, fresh complete rebuilt 350 (8000 miles), carb. fresh rebuild with a new float and accel pump two weeks old. Brand new fuel pump and rubber lines with new fuel filter at the carb. The old fuel pump failed, thought that this was the problem but it ran poorly today after the new pump was installed. It seems that heat might be part of the problem? In the morning it ran fine later in the day the stumble happened. The sock in the tank looks clean, and I have a vented locking gas cap. The motor is just not running the same?

Any help would be appreciated,

RPO LS5
 
NEW fuel pump...mechanical I presume....the suction hose from the frame rail has been referred to by others as having to be molded, just a normal hose will collapse when warm from the suction forces and the U bend it makes....

:drink:
 
RPO LS5 said:
Hi Folks, I'm in need of a little help, and this should be the right place to ask I'm sure.

While cruising at 60 to 70 mph and hitting the gas, my 72 sb falls flat on it's face. A quick history, fresh complete rebuilt 350 (8000 miles), carb. fresh rebuild with a new float and accel pump two weeks old. Brand new fuel pump and rubber lines with new fuel filter at the carb. The old fuel pump failed, thought that this was the problem but it ran poorly today after the new pump was installed. It seems that heat might be part of the problem? In the morning it ran fine later in the day the stumble happened. The sock in the tank looks clean, and I have a vented locking gas cap. The motor is just not running the same?

Any help would be appreciated,

RPO LS5
Click to expand...

talk to lars but it sounds like it may be a secondary carb problem.
 
mrvette said:
NEW fuel pump...mechanical I presume....the suction hose from the frame rail has been referred to by others as having to be molded, just a normal hose will collapse when warm from the suction forces and the U bend it makes....

:drink:
Click to expand...
Thanks, Yes the pump is mechancal, I will order the S molded fuel line on Monday! The fuel line I have on now is rather long, in order not to make a kink.

DARROW 74 said:
RPO LS5 said:
RPO LS5
Click to expand...

talk to lars but it sounds like it may be a secondary carb problem.
Click to expand...

Not so sure, after another test it feels a little sluggish going throught the gears now also? I think it's carb. related too, but the carb. was recently rebuilt by a reliable rebuilder. This problem started this season, I drove the car last year the 8000 miles without trouble. The trouble started while on a 10 hour (600 mile)drive last weekend. After 8 hours the fuel pump failed but I was able to struggle with it the rest of the way, then changed the pump. On the way home the engine did "cough" a couple times, but again I made the distance. Finally yesterday I took the car out again, in the morning it was fine but later in the day the temps went up and the problem reoccured. A few weeks ago the carb. was flooding terribly onto the manifold, my rebuilder replaced the float and the rubber seals on the accelorator pump claiming the gas ruined the rubber seals? It's not flooding but now "starving"? I did put a new set of plugs.

Any more suggestions?
 
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PERK, BTDT, especially with a long hose on that input....

IMO, stick a electric pump in back and toss the mechanical aside for now, or maybe at least as a test trick.....prove the point so to speak.....
 
RPO LS5 said:
The carb. was recently rebuilt by a reliable rebuilder.
Click to expand...

That just means it could be screwed up worse than an amateur cold ever get it...

A fuel pump won't cause the issues you're seeing, but several other things, and combinations, could.

Your symptoms are indicative of a lean condition. Assuming you have verified that you have at least 12-16 degrees initial timing, 36 degrees total timing, and a correctly functioning vacuum advance, the following items will cause the symptoms you describe:

  • Low float
  • Incorrect float
  • Needle clip installed backwards on float arm, restricting needle opening
  • Secondary airvalve spring windup too loose (should be 3/4 turn)
  • Low APT/power piston setting
  • Stop pin cut off of power piston
  • Idle air bleed holes intentionally plugged in float bowl
  • Power piston spring too soft
  • Primary rods too big
  • Jetting too lean
  • Incorrect secondary rods
  • Non ethanol-compatible accel pump
...to mention a few. If timing is correct, the carb needs a teardown and a thorough visual inspection by somebody who knows what they're looking at. Make sure the carb is jetted to spec and that setup parameters are right.

Lars
 
lars said:
RPO LS5 said:
The carb. was recently rebuilt by a reliable rebuilder.
Click to expand...

That just means it could be screwed up worse than an amateur cold ever get it...
  • Secondary airvalve spring windup too loose (should be 3/4 turn)
  • Low APT/power piston setting
  • Stop pin cut off of power piston
  • Idle air bleed holes intentionally plugged in float bowl
  • Power piston spring too soft
  • Primary rods too big
  • Jetting too lean
  • Incorrect secondary rods
  • Non ethanol-compatible accel pump
...to mention a few. If timing is correct, the carb needs a teardown and a thorough visual inspection by somebody who knows what they're looking at. Make sure the carb is jetted to spec and that setup parameters are right.

Lars
Click to expand...

I suppose that I wouldn't be having carb. problems if he was as reliable as I was told, so I guess I should take that back.

To be honest, most of the items on your list are above my head, but I will copy and print them out and have the items looked at by another.

Thanks Lars!
 
Why don't you just swap the carb out with another one to see if the problem persists. If it goes away, the carb is at fault, and one or more of the items on my list are likely the culprit. If not, you have other issues. You can borrow a Q-Jet off any Chevy truck or from a Corvette buddy, and the swap will take 7 minutes.
Lars
 
lars said:
Why don't you just swap the carb out with another one to see if the problem persists. If it goes away, the carb is at fault, and one or more of the items on my list are likely the culprit. If not, you have other issues. You can borrow a Q-Jet off any Chevy truck or from a Corvette buddy, and the swap will take 7 minutes.
Lars
Click to expand...

Good idea I'll try to get my hands on one this week. I'm still going to change out the fuel line to the molded type, it might be part of the problem. Today I didn't have much time to spend on it but I did find a pretty good exhaust leak at the exhaust manifold to pipe connection. (pointed right at the fuel pump area). Weeks ago I removed the heat riser for a better fit on the pipes under the car. Put it back on, no more leak. I mentioned this because it seems like the starvation problems get worse the hotter the car gets. Hope I can figure out the problem this week I have two events coming up this weekend and damn I wanted to show off the car!

Thanks again!
 
A little follow up. Last night I ran the car out to the carb. rebuilder, yes the same guy. Let him take the car out for a test drive, when he returned he said the same "starvation" issue occured while he got on the gas. I did put in a new set of points and the car runs much faster but still falls on it's face. He inspected the carb., and put in a new filter. I'm now thinking that I have another bad pump, along with the collapsing fuel line. Parts are ordered I'll keep you posted.
 
Pusher pump in the rear.....I had a car drive me crazy long time ago, the only cure was the pump in the rear....thank you Carter.....

:shocking::yahoo:
 
New GM pump and molded line.................. no change.

Mrvette I hear you on the rear electric pump, but the car didn't have any trouble with the mechanical pump last year?

Guess it's time to get to work on Lars list!

Again, these items were fine last year?
 
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RPO LS5 said:
New GM pump and molded line.................. no change.

Mrvette I hear you on the rear electric pump, but the car didn't have any trouble with the mechanical pump last year?

Guess it's time to get to work on Lars list!

Again, these items were fine last year?
Click to expand...

Sincerely, best of luck, as I think about that nitemare I had years ago...it WAS fine one day, then the troubles started, electric pump cured it.....I never looked back....damned if I know WHY, just WAS.....

:bestwishes:
 
Still stumped................tore the carb down , again found nothing. Falls flat on it's face at the top end, in any gear. Really frustrated, anymore suggestions?
 
Check your secondary airvalve spring windup and verify your choke pulloff is functioning and hooked up. Secondary spring windup should be 3/4 turn. If it still falls on its face, try 7/8 turn. The choke pulloff should hold the airvalve firmly closed at idle.
Lars
 
lars said:
Check your secondary airvalve spring windup and verify your choke pulloff is functioning and hooked up. Secondary spring windup should be 3/4 turn. If it still falls on its face, try 7/8 turn. The choke pulloff should hold the airvalve firmly closed at idle.
Lars
Click to expand...

Checked the secondary spring wind up, and tried it at both 3/4 and 7/8. It ran better at 3/4, but still fell flat at around 4200 rpm in second 3500 in 3rd. Today, I also replaced the main shaft in the distributor in order to repair the tach gear (unrelated). I'm thinking I should check fuel pressure after the pump? I have a party to go to this weekend, man I wanted to take the car!
 
Yes, you need to check fuel pressure at the carb inlet. Install a "T" fitting at the carb inlet and run a long rubber hose to a gauge that you can tape to the outside of your windshield right in front of the driver's seat. Observe fuel pressure when the problem occurs. At no trime should the pressure drop below 3 psi.
Lars
 
Turns out to be the sleeve or sock on the sender after all. Yesterday I rigged up a 2 1/2 gal gas can to a 10' piece of fuel line. I then connected it to the fuel pump inlet, car ran fine. I relocatd the rubber line to the rear on the metal line, car ran fine. Drained all the fuel from the tank removed the sender and inspected the sleeve. On the outside it looks like new, but on the inside hard as a rock, like cement almost covering the inlet tube. That just goes to show that a visual inspection through the filler neck is not a sufficiant way to check the sock! Now I have a very well tuned motor with perfectly functioning fuel pump and a brand new sender w/sleeve.

Thanks Lars and Mrvette for your help.:thumbs:

Funny thing several of my buddies told me "filters" all along but it "looked" just fine.

It does seem to be sucking down the fuel pretty quickly now?

Oh, the Mrs. came home sick from work yesterday and we never did make the party!

RPO LS5
 
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