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Is it possible to drain a carb dry under acceleration?

Ok, my '68 GTX. Basically stock 440 w/mild Comp Cams, headers w/3" duals, stock intake with Edelbrock 650cfm AVS jetted "fat", Carter "hemi" fuel pump. 4 speed, 3.55 gears.
Has been converted to Mopar electronic ignition, all in at 36 degrees advance - vacuum advance plugged off, the car hates it.
Entire fuel system, including tank and 3/8" lines, is new. Hell, most everything is new.
Engine checks out at 130psi on compression test for all cylinders, not so much as 5 psi difference between cylinders.
Y'all may recall my various threads bringing this beast back from the dead....if not, lots of background to be found in those threads.

Current situation is that she has really crisp throttle response off the line and pulls like a freight train up through 1st, 2nd and into 3rd gears, pulling to 5500RPM before shifts.
Once into 3rd and hard into it, she pulls fine until she gets over 4000RPM, then she noses totally over as if it has literally run out of gas.
I'll get off the throttle and give it a second and it comes right back, doesn't even stop running - it literally runs out of "breath" (I'm assuming fuel).
A few seconds later, everything returns to normal once I've stopped thrashing on it.
Yes....I know the carb is undersized....sigh.

Question: I know the fuel pump and the rest of the fuel system is plenty big/stout enough for this setup, may even be overkill - and that I'm likely totally emptying the fuel bowls on the carb when it noses over like that - but is it literally possible that the fuel pump isn't able to supply enough fuel "straight up" to meet demand when it noses over?
In other words, shouldn't the carburetor just take all the fuel pump can deliver "straight shot" when under that demand, regardless if the bowls are empty, and keep on trucking?
OR are the fuel inlets not capable of allowing enough fuel straight through to meet that high demand?

This may well just be how the carburetor demonstrates that it isn't big enough to handle the demand, but I know it's jetted big enough for the situation and the fuel delivery system is big enough surely?

No not at all. The size of the carb with it's jetting will only allow a certain volume of fuel through the orfices which will never exceed the amount of fuel in the bowls can deliver regardless of engine vacuum. If the carb is too small it may not be able to supply enough fuel for the engine at high rpm's, or if the deliver fuel volume to the carb is not to spec you may not be able to keep the bowls full, but the engineering of the carb is such that the bowls will always have enough fuel to supply the various circuits of the carb even at wide open throttle. Perhaps you should look into a larger carb, better volume of fuel delivery that's constant, or, undersized/collapsed/ partially plugged fuel lines or restricted pick-up sock back at the sending unit. Also check the push rod length to determine if your getting full stroke of the pump lever or a deffective pump.
 
moparedtn-After reading all this,Im still thinking needle&essay on the floats. It's been a long time for me but I don't remember a bigger seat assy. for them as for the holleys. I still think ur draining the bowls. Or your pickup sock is collapsing?

Please see Doug's post #12. No way a basically stock motor is draining the bowls. I, too, have used these carburetors for over 50 years on street cars and race cars. If the problem is indeed fuel, the problem will be found in the delivery system.
 
Please see Doug's post #12. No way a basically stock motor is draining the bowls. I, too, have used these carburetors for over 50 years on street cars and race cars. If the problem is indeed fuel, the problem will be found in the delivery system.
Thanks, Dave, but it's quite apparent the thing is totally "running out of gas" when it noses over - I have to wait on it to get more back in the carb before it will take off again.
The jetting, as Doug and everyone else says, are not the issue here. I know this, especially since I've got it jetted up almost exactly like the factory 440 carbs were (and that corresponds to the factory jetting on the bigger 750cfm AVS they sell now, by the way).
That means that, for whatever reason, the bowls are going temporarily "dry", since that's the last place the gas is before entering the engine, right?

No doubt something is going on with it not getting enough fuel delivered to the carb under high demand. Totally agree there.

Beats me what, though. With a new 110gph pump, a new pushrod, all new 3/8" fuel lines, new tank and new pickup, I'm stumped.
No external leaks to report, either. Been over it and over it.
Tank venting can't be the issue, either. It has the factory vents on it (verified clear) as well as a vented gas cap, even though the car didn't come with one originally.

I just replaced the small piece of rubber fuel line again that goes from the end of the hard line along the passenger side frame rail underhood to the pump last night. No reason, other than it felt a little "soft", even though it's as new as the rest of the system. Sort of a busy work thing, really, doubt it will make any difference whatsoever.

Am I right in assuming that an undersized, but properly functioning, carburetors' behavior at high RPM would simply be to limit how fast the engine is allowed to rev? As in "that's all the flow I got, that's as fast as the engine is going to spin"?

Could this totally nosing over, yet not totally dying, situation really just boil down to not enough carb/cfm?
 
Am I right in assuming that an undersized, but properly functioning, carburetors' behavior at high RPM would simply be to limit how fast the engine is allowed to rev? As in "that's all the flow I got, that's as fast as the engine is going to spin"?

Could this totally nosing over, yet not totally dying, situation really just boil down to not enough carb/cfm?

NO! If you had a carb problem, it would shows it head EVERY time you hit that point in the RPM and load.

You replaced one hose, I would replace all the hoses and clamps from Fuel Pump to Tank, (Use Fuel Injection clamps and hoses, not the standard screw clamps and cheap hose). I would also pull the sending unit to check the sock, but that is me.
 
NO! If you had a carb problem, it would shows it head EVERY time you hit that point in the RPM and load.

You replaced one hose, I would replace all the hoses and clamps from Fuel Pump to Tank, (Use Fuel Injection clamps and hoses, not the standard screw clamps and cheap hose). I would also pull the sending unit to check the sock, but that is me.
As I've posted ad nauseum, my friend, they all HAVE been replaced - and yes, I've learned that there are different fuel lines at the old parts store these days.
One thing I've learned since getting back into the hobby in these last years now is that todays' fuel eats the "old school" rubber fuel lines from the inside out.

I recall going into the local rendition of the "red golf shirts" auto parts store, quite tiffed at them for selling me a gas line that failed after less than a year.
Fella behind the counter says "oh, you want the good fuel line?"
I about smacked him back into the storeroom...

Anyways, back to your post, for which I am grateful. :)
All the factory hard lines are brand new and the remaining short pieces of fuel line (hard line to sending unit in rear; front of hard line - which I don't understand why they ended it where they did; it's asinine - to the pump in front)
have been replaced with the "good stuff", along with those nice f.i. clamps that don't chew into said rubber.
I located the fuel filter between the pump and the carb and have it mounted down on the frame rail for what I thought to be easy access as well.

I took her out for a nice drive to town to the store yesterday, with my wife as co-pilot. She behaved admirably, probably the best it has ever driven really since it's resurrection (the GTX, not the wife :) ).
It was a hot day (90F or so) but the car ran 180-185 on highway and city.
I gave it a few romps and banged a few gears and she never missed a beat, but I haven't done another total wring out in all gears pass yet.

Finally, I do agree I'm going to wind up pulling the damn tank again to pull the sending unit, no doubt.
I sure wish I could find a better quality one. The Mopar ones used to be out there for the fetching, but I can't find any of those now.
Anyone know where I could find one?
 
Good question! Wish I had a simple answer. Generally speaking a pump produces flow, and pressure is the result of restriction(s). Different pumps are designed to produce flow at different pressures (pump curves). So basically you need to know the demand of the engine at high rpm (flow) but also to understand where significant pressure drops or restrictions exist, either on the suction side or pressure side of the pump. Without flow or pressure gauges there might be a way of calculating where to focus, but you might still have to resort to some trial-and-error... (translation: start with cheapest/easiest-to-access components then proceed to spend more and more money).

This is an interesting article which might be helpful; it also includes some engine fuel demand related calculations. Good luck!

http://www.superchevy.com/how-to/en...ou-need-to-know-about-aftermarket-fuel-pumps/
 
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I'm by no means a pro racer or super duper mechanic, but I had the very same problem on several of my Mopars . Fuel starve and blamed it on the carb (s) , then fuel pumps , then lines . what I discovered, under very hard acceleration the fuel was moving so far back in the tank that it was away from the gauge pick up and hence fuel starve. never read all the replies so maybe you tried a full take of fuel already or maybe somebody already told you the same thing . Good luck with it .
 
I'm by no means a pro racer or super duper mechanic, but I had the very same problem on several of my Mopars . Fuel starve and blamed it on the carb (s) , then fuel pumps , then lines . what I discovered, under very hard acceleration the fuel was moving so far back in the tank that it was away from the gauge pick up and hence fuel starve. never read all the replies so maybe you tried a full take of fuel already or maybe somebody already told you the same thing . Good luck with it .
Thank you.
Yes, the thought has been stuck in the back of my mind that the repro sending unit may, in fact, not be sitting low enough in the tank - or even that the make-up of the "sock" on it isn't very good at scavenging fuel, for that matter.

That's why I was asking if anyone knew where I could find a Mopar-manufactured pickup.
I wish I had kept my original now....oh well.
 
Good question! Wish I had a simple answer. Generally speaking a pump produces flow, and pressure is the result of restriction(s). Different pumps are designed to produce flow at different pressures (pump curves). So basically you need to know the demand of the engine at high rpm (flow) but also to understand where significant pressure drops or restrictions exist, either on the suction side or pressure side of the pump. Without flow or pressure gauges there might be a way of calculating where to focus, but you might still have to resort to some trial-and-error... (translation: start with cheapest/easiest-to-access components then proceed to spend more and more money).

This is an interesting article which might be helpful; it also includes some engine fuel demand related calculations. Good luck!

http://www.superchevy.com/how-to/en...ou-need-to-know-about-aftermarket-fuel-pumps/

Thanks. Interesting article for sure, but if you've actually read up on my situation, you already know I'm way past the "start with cheapest/easiest-to-access components" part.
It has ALL been replaced.
Perhaps it's easier for me to state what is known on my particular car:
1. The engine in question is made up of a stock mid-70's 440 bottom end (meaning low compression) with a fresh set of 906 heads.
Nothing earth-shattering in the cam department, either - a Comp cams unit that's only slightly more lumpy than a stock 440 magnum one.
In other words, this thing isn't even approaching 400hp.
It has next to zero miles on it, pretty much. Very good oil pressure, etc.

2. ALL fuel system components, front to rear, have been replaced with new.
3/8" line all the way, front to back, including repro sending unit.
Tank new.
Pump is a HOSS of a mechanical one, a reproduction (from Mancini) of the famous "hemi fuel pump" Carter rated at over 110gph - WAY more than needed here.
Fuel pump pushrod is also new, a Comp Cams unit with bronze ends to resist wear.

3. The carburetor, although small at 650cfm, is new and has been "jetted up" similarly to its' 750cfm brother - which happens to be the same jetting as the factory Carter AVS was back then, pretty much.
To say she runs a tad rich would be an understatement. :)
Not to the "black smoke under acceleration" rich, but plenty fat.
Float levels have been checked and re-checked; so have all the ports and such.
The car actually likes this carb very much. Throttle response is CRISP, the way I like it and she pulls hard all the way up to my self-imposed redline in the mid-5's.

4. When the car exhibits the problem, it literally goes from pulling HARD (no stumbles, no slacking off, no gradual falling away of torque at all) to "flat on its' face" - as if someone shut the fuel off all at once.
There's no advance warning of when this is going to occur - it doesn't hiccup once before it does it. There's no slacking off of torque, no stumbling, no fading away of all that great 440 torque.

For a hodgepodge of parts that the former owner (and yes, now me) slapped together, it actually runs quite well otherwise.
I don't know that the car has ever had less than a quarter tank in it since I've redone everything - the repro sending unit lies to me on the fuel gauge, so I always think there's less in it than there really is.

The more I go through this, the more I'm led to think "sending unit".
 
It has ALL been replaced.
Ok, you had stated that earlier and I overlooked it. Guess I was thinking along the lines of some earlier posts, like 747 iirc who suggested drilling out what looked like a perfectly good fuel pump because in fact it was acting as a restriction.

More to the point, here is a simple and cheap way of measuring fuel pressure on the suction and pressure sides of the pump to confirm your hypothesis of fuel starvation at the pickup.

http://www.mityvac.com/pages/info_service_fp.asp

Without a gauge of pressure or flow we are still guessing. I realize you are asking for help to make the best educated guess but I was suggesting measuring (or calculating) before continuing to guess. Sounds like you have some great ideas and others have made many great suggestions that will lead you to the solution.
 
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My car was doing the exact same thing, it would only do it if I was hard on the gas for long pulls. I had been running the same exact fuel delivery system reliably for 2 years so plumbing size and sending unit issues were out, checked all lines then strapped a fuel pressure gauge to the hood and bingo. 7 psi at idle then a sustained 5 psi but keep pushing threw the gears and running the rpm's up and it noses over as the gauge nearly zero's out..... not enough volume. Pitched the imported Holley, bought a Carter, drilled and tapped out the ports from 1/4" to 3/8" NPT to be less restrictive, no more problems.
This was the post about using a gauge then drilling.
 
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Here's something to check, as it happened to me and I was like WTF?!?! Back at the fuel tank, check the rubber hose that connects the hard line to the fuel sending unit. Is it as straight as you can get it, with no extra amount of hose that can kink while under pressure?

I had a similar issue with my new 440 install and new hemi fuel pump. Car would start fine and idle, but when going for a short drive she'd die and be a bitch to restart. I did everything up front like you did. Went back to the tank/sending unit to see what that looked like, and noticed that short rubber fuel line was folding over on itself causing a kink. I used an X-acto hobby hand saw (to avoid sparks) to cut off a small piece of the hard fuel line to get rid of the odd bend in the rubber line and the car's been fine ever since.
 
Ok, you had stated that earlier and I overlooked it. Guess I was thinking along the lines of some earlier posts, like 747 iirc who suggested drilling out what looked like a perfectly good fuel pump because in fact it was acting as a restriction.

More to the point, here is a simple and cheap way of measuring fuel pressure on the suction and pressure sides of the pump to confirm your hypothesis of fuel starvation at the pickup.

http://www.mityvac.com/pages/info_service_fp.asp

Without a gauge of pressure or flow we are still guessing. I realize you are asking for help to make the best educated guess but I was suggesting measuring (or calculating) before continuing to guess. Sounds like you have some great ideas and others have made many great suggestions that will lead you to the solution.
Thanks much. :)
I'm already halfway there, since I have a pressure gauge on the fuel rail up by the carb.
 
This was the post about using a gauge then drilling.
He's got a point - seems silly that a "high performance" fuel pump like the Carter "hemi" one would have 1/4"NPT inlets and outlets on them when the fuel line itself is 3/8".
On the other hand, it's rated as it sits at 110-120gph with those little orifices...
 
Here's something to check, as it happened to me and I was like WTF?!?! Back at the fuel tank, check the rubber hose that connects the hard line to the fuel sending unit. Is it as straight as you can get it, with no extra amount of hose that can kink while under pressure?

I had a similar issue with my new 440 install and new hemi fuel pump. Car would start fine and idle, but when going for a short drive she'd die and be a bitch to restart. I did everything up front like you did. Went back to the tank/sending unit to see what that looked like, and noticed that short rubber fuel line was folding over on itself causing a kink. I used an X-acto hobby hand saw (to avoid sparks) to cut off a small piece of the hard fuel line to get rid of the odd bend in the rubber line and the car's been fine ever since.
Another excellent suggestion, thanks. :)
Yes, when I replaced all the hard fuel lines, that one was a biyotch to wrestle in there amongst the shock and exhaust and suspension - a very crowded area, especially with 3" exhaust piping.
I managed finally and actually made a point to have the end of the line looking at the tank to be pointed directly at the outlet of the tank - for just the reason you state.
I've replaced that little piece of rubber line as well since then, too.
It's amazing to me what today's gas does to the old school rubber lines and how quickly it does it.
There's a piece of "fuel injection" rubber line there now.
 
As I read this, if memory serves me at all. You have done just about everything out there. However, I think you still run through a fuel filter. I wonder if that is the restriction that might cause starvation. I had an old Ramcharger that ran great except when running up a long hill with a load, it would start to exhibit the same thing you experienced. Turned out that the fuel filter was partially plugged. The stupid thing was steel with a 1/4" return on the side. With the new crappy fuel, they don't seem to last as long as they used to. What I did was buy a new "steely" filter and poked a hole with an awl, strait through the center and then put a real (see through) 2 port, plastic filter after it. Not only are the plastic see through less than half the price but I could see that I had fuel. The point is it would be nice if you could install a pressure gauge after the pump before the filter and also after the filter to see if it is at least part of the problem. This might be a bunch of "hogwash" but wouldn't it be funny if was the whole problem all along! Good luck, you have me on the edge of my seat!
 
That would be goofy for sure, multimopes.
The fuel filter is as new as the rest of it all and is indeed a metal one (3/8" in/out).
No return line, though - I removed all of the return line stuff when I first got the car and capped off that outlet on the tank.

We've had the car out for short trips to town this last couple weekends and it's behaving a bit better, thankfully.

It's still shocking to me that the engine sounds & feels so much different now than it did before I did the head swap, though - and I don't think it should have made that much of a difference, since the cam is the same and the bottom end didn't get touched at all.
But hey, what do I know?
It has lost that "almost missing" thing it had with the stuck valve/bent pushrod, of course, but it's just.....different.
I don't know if it's a good different or a bad different, though.
I guess only time will tell on that one.
 
Tank is Vented, Yes? Check your Gas Cap...
 
Again, not to sound like an A-Hole, but have you checked it. With the issues you are having, something is not working as it suppose too.. Just because a part is new or says vented, does not mean it is or is operating the way it should..
 
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