• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Is it possible to drain a carb dry under acceleration?

moparedtn

moparedtn

I got your Staff Member riiiight heeeere...
FBBO Gold Member
Local time
3:49 AM
Joined
May 14, 2011
Messages
17,241
Reaction score
32,432
Location
On the Ridge, TN
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?
 
Hi Ed, Im not as near as smart on this stuff as a lot of the pros on here, but I am betting the ignition is good or 1st and 2nd would show the same problems at the upper RPMs.
The fuel supply set up you have going is the same basic set up used on the factory 6pack cars and Hemis from back in the day.
3/8 supply and pickup , carter hemi pump, Im betting you also have the 3/8 filter.
Could the pump be weak even though its newer ? Possible but prob not if it shows good pressure and steady readings.
I though back when you went for that 650 you were on the right track. Ive seen more over carbed engines than under.
You may be running out of fuel after that long through all the gears pull. Are you at max for your rods and jets on that tune kit ?
I got to ask if your positive the float level setting is good and your not starting off with a low level in the bowls.
 
All the carter style carbs carry the same amount of fuel. To be sure run it up til it stumbles. Click it off and pull over. pull the top of the carb off and have a look if its empty or not. I think you will find plenty of fuel.
 
Ed, just get the 800 cfm avs or a big Holley/clone. If you like flooring it to 6k in all gears which it sounds like you do then its time to let the engine breathe!

It sounds more like not enough fuel. A fuel pressure gauge should give more info on the fuel system. If it goes to zero wide open under load you are not getting enough to keep them full.
 
1967coronet said:
Hi Ed, Im not as near as smart on this stuff as a lot of the pros on here, but I am betting the ignition is good or 1st and 2nd would show the same problems at the upper RPMs.
The fuel supply set up you have going is the same basic set up used on the factory 6pack cars and Hemis from back in the day.
3/8 supply and pickup , carter hemi pump, Im betting you also have the 3/8 filter.
Could the pump be weak even though its newer ? Possible but prob not if it shows good pressure and steady readings.
I though back when you went for that 650 you were on the right track. Ive seen more over carbed engines than under.
You may be running out of fuel after that long through all the gears pull. Are you at max for your rods and jets on that tune kit ?
I got to ask if your positive the float level setting is good and your not starting off with a low level in the bowls.
Click to expand...
Yeah, I made a point of checking float levels when I first got the AVS.
I've got it jetted up to what the 800cfm AVS has in it stock now, too, although there's always a bigger jet size to pop in there, I suppose.
One thing on these AVS carbs I've noticed over the Holleys I've had in the past - there is a very pronounced feel you get in the gas pedal when you go for the secondaries - you can feel the additional spring tension the secondaries have and the engine lets you know in no uncertain terms that the flapper on the secondaries is opening. Reminds me a lot of the old Quadrajet carbs in that regard.
No hesitation or stumble, just a very pronounced sensation both in foot and in response.

So....IF (and I say IF) I'm actually managing to drain the bowls, is it possible that the "direct feeding" of the fuel straight from inlet to bottom of the bowls/into the jets at that point cannot keep up with engine demand?
I don't even know if I'm saying this right...

Oh yeah, I agree, the ignition at this point seems pretty dang close. The throttle response right off idle is insane at this point to boot - instant and snappy as heck. Very happy with that as well.
 
Last edited:
hangn0ut said:
All the carter style carbs carry the same amount of fuel. To be sure run it up til it stumbles. Click it off and pull over. pull the top of the carb off and have a look if its empty or not. I think you will find plenty of fuel.
Click to expand...
Ok, taking your theory in hand here - if that's the case, what's happening? The jetting is still too small on the secondaries and it's actually pulling so much fuel through them that it sucks them temporarily "dry"?
 
I drained my 6BBL once at the drag strip - only about 100ft to the finish line and engine starts coughing - fuel pump push rod had worn out, and was about 3/8" shorter than it should be.....so yes it is possible to drain your carbs. :D
I won the race BTW....by 0.001 to one of NZ's best DYO racers.
 
SlinktRR said:
Ed, just get the 800 cfm avs or a big Holley/clone. If you like flooring it to 6k in all gears which it sounds like you do then its time to let the engine breathe!

It sounds more like not enough fuel. A fuel pressure gauge should give more info on the fuel system. If it goes to zero wide open under load you are not getting enough to keep them full.
Click to expand...
Eh, could be?
These Carter pumps are rated 120GPH @ 6.5PSI, a pretty stout volume for a stock motor I'd reckon - in fact, I hesitated fetching one because I've read this is actually too much pump for a mild street engine, especially with no means of returning excess fuel to the tank.
Knock wood though so far - it hasn't flooded out the carb/"run over" the fuel inlets yet.
Interest part, at least to me, is that these pumps have only 1/4" inlets and outlets, the smallest part of the whole fuel system as far as that goes.
 
kiwigtx said:
I drained my 6BBL once at the drag strip - only about 100ft to the finish line and engine starts coughing - fuel pump push rod had worn out, and was about 3/8" shorter than it should be.....so yes it is possible to drain your carbs. :D
I won the race BTW....by 0.001 to one of NZ's best DYO racers.
Click to expand...
WTG! :thumbsup:
Ah yes, been there done that with the pushrod. This one's a brand new hardened one from Comp Cams that Mancini recommended with the Carter pump. My old pushrod was quite worn.
 
I am FAR from an expert, and I was going to say, it seems highly unlikely to me that your fuel demand could exceed the fuel pumps ability to supply the fuel you need, but as I thought about it, it dawned on me that the fuel supply (regardless of the pumps ability in GPH) is limited by the float bowl inlet pintle valve. Maybe you ARE outrunning your supply. Think of X amount of water pressure pushing water through a 1/2" pipe and another through a 1" pipe. Which do you think will fill a gallon quicker?

Solution: Would it be possible to drill out the supply port while not exceeding the diameter of the pintle valve so that it still seals and shuts off or restricts fuel flow at normal demand?
 
A few things. First all Carter/Edelbrock carbs have the same size bowls. All are generally jetted very close from the factory. I race 900hp motors on these carbs. I believe your condition is do to lack of fuel. First raise the float level. Set it at 5/16" instead of the recomended 7/16". Make sure to check that it has maximum float drop. Try running it with the fuel cap off, could have a tank venting issue. If you still have an issue perform a fuel delivery test into a gallon can. 1/3 gallon in 30 seconds should be enough for 500hp. Less than that look to your delivery system.
Doug
 
My car is doing the same thing !! I'm doing a complete fuel system up grade from, fuel line to fuel pump Going -8 or -10 Also do you have a fuel Gage on your system you should be at 6lbs-psi with out dropping, mine is dropping down to 3lbs-psi under acceleration.
 
Last edited:
What Doug said...no need to mess with the factory needle/seats or go crazy with the jetting. The 650 is plenty for a street/strip set-up.
 
Here,try this. Drive normally up to about 70 mph or so. Now downshift and punch it. Now you should be able to go well past that point where it stumbles if it is a fuel problem as you are at full throttle for a much shorter period of time.
 
A round back alternator with electronic ignition can give you problems...just saying.
 
Ranger16 said:
I am FAR from an expert, and I was going to say, it seems highly unlikely to me that your fuel demand could exceed the fuel pumps ability to supply the fuel you need, but as I thought about it, it dawned on me that the fuel supply (regardless of the pumps ability in GPH) is limited by the float bowl inlet pintle valve. Maybe you ARE outrunning your supply. Think of X amount of water pressure pushing water through a 1/2" pipe and another through a 1" pipe. Which do you think will fill a gallon quicker?

Solution: Would it be possible to drill out the supply port while not exceeding the diameter of the pintle valve so that it still seals and shuts off or restricts fuel flow at normal demand?
Click to expand...
That's the base question I'm asking, right there.
Is it possible to actually starve the thing because of things like the restrictions in the carb itself?
 
dvw said:
A few things. First all Carter/Edelbrock carbs have the same size bowls. All are generally jetted very close from the factory. I race 900hp motors on these carbs. I believe your condition is do to lack of fuel. First raise the float level. Set it at 5/16" instead of the recomended 7/16". Make sure to check that it has maximum float drop. Try running it with the fuel cap off, could have a tank venting issue. If you still have an issue perform a fuel delivery test into a gallon can. 1/3 gallon in 30 seconds should be enough for 500hp. Less than that look to your delivery system.
Doug
Click to expand...
1. Ok, raising floats was something I wondered if I could do. Check, will try.
2. Fuel cap is already vented and new, even though the tank vents are also in place (these are supposed to have non-vented, remember). Tank is also brand new.
3. The delivery test is intriguing. With this 120GPH pump and 3/8 lines, I should about drown, eh?
Thanks!
 
moparcrazy said:
My car is doing the same thing !! I'm doing a complete fuel system up grade from, fuel line to fuel pump Going -8 or -10 Also do you have a fuel Gage on your system you should be at 6lbs-psi with out dropping, mine is dropping down to 3lbs-psi under acceleration.
Click to expand...
Fuel line does have a little outlet for a gauge. I could slap one on, I suppose. Guess I need to figure out how to get it positioned so I can see it from behind the wheel.
What's an acceptable minimum pressure under full boogy?
 
You must log in or register to reply here.

Similar threads

67 Coronet Conv
Fuel system upgrade and problems
Replies
17
Views
1K
Aron Gleason
Aron Gleason
1969VAGTX
Fuel Pump Saga Continues - I Am At a Loss!
Replies
15
Views
1K
Bee1971
Bee1971
66 Sat
Confusion on Fuel Line I.D. and O.D
Replies
9
Views
1K
Geoff 2
G
C
TQ fuel leak at fuel supply line
2
Replies
24
Views
1K
moparedtn
moparedtn
katmachine62
Holley Carb 2300 2BBL Fuel Bowl slowly draining internally!?
2 3
Replies
53
Views
2K
katmachine62
katmachine62
Auto Transport Service
Back
Top