brodysZ28

This issue has me stumped.

Motor:
383, 330hp 385 ftlbs
Barry Grant 750 mechanical secondaries
72 primary, 96 secondary jets
0.110" needle and seat assemblies
110gph brand new jegs fuel pump
brand new inline holley fuel filter
3/8 lines
under 3500 rpm its real rich
over 4000 its 12.9:1 afr

Issue:
Under WOT the fuel bowls both primary and secondary are being drained. I have observed (wideband) and felt (stumble) this happen after rowing a few gears. On the dyno we observed the bowls being drained through the sight glasses.

Causes?
Replaced fuel pump and filter, no change
Fuel pump eccentric measured at 0.350" (seems good) I can't find a spec but read comp is around 0.340
Fuel lines kinked? Inspected fuel lines and saw no signs of issues
Everything in the carb seems to be working properly, the floats are working good, needle and seats weren't plugged.

What other tests or inspections can I run to track this down???

Thanks.

sofakingdom

iTrader: (1)

Replace the "sock" filter thing in the tank and rod the pickup tube out with a coat hanger or the like, and lacquer thinner. Maybe even rifle brushes to the extent you can get em through there.

No "tests" or "inspections" that I'm aware of.

Dyno Don

iTrader: (7)

Let's see if I can explain this so it is easy to understand.

Optimal fuel pressure should be 4-4.5 PSI, if it is greater that will cause you to lower the needle and seat to stop over flowing, thereby pushing the float too low and causing condition you are experiencing.

ie. not enough fuel in the bowls to sustain WOT.

Assuming you have already checked for adequate flow on the 3/8th's line.

HTH

tealman92

Is your float level set too high?

brodysZ28

So there is an in-tank filter? I haven't touched anything with the tank and I have had the car since 2005. That is a good recommendation though.

Dyno Don - I am not following. So when the float comes down the needle should come down with it. At that point the fuel pressure is forcing the needle down as well. The more fuel pressure, the more force is trying to open the needle and seat. Wouldn't that cause the bowls to be overflowing through the vents?

BTW I might be looking into your headers and y-pipe. I will pm you.

brodysZ28

It is set high. Just over the high mark. This way they weren't draining immediately.

Dyno Don

iTrader: (7)

If you have too much fuel pressure, you have to screw the needle and seat down to stop over flowing, that pushes the float down so there is less fuel in the bowl at WOT.

So what I am saying is.... Check your fuel pressure.

brodysZ28

I think I see what you are saying. The bowl would displace the fuel, I can get a guage and rig something up. But shouldn't the needle and seat be able to flow MORE than enough to keep up with the jets? If the needle and seat is open, shouldn't the bowl only increase in level, not decrease as I am seeing?

Dyno Don

iTrader: (7)

The fuel in the bowl pushes up on the float to shut the needle and seat, if the pressure is too high you have to lower the float to shut it off, lowering the amount of fuel in the bowl.

It also prevents the needle and seat from opening fully.

The best way to check this is, remove the bowl, turn it upside down and see how close the float is to the bottom of the bowl. If it is near a quarter of and inch, it is too low.
But you can't raise it if the fuel pressure is too high (5-6 PSI) or it will over flow. So, bottom line is you have to get more fuel in the bowl. The pressure from the float shuts the needle.



I'm not saying this is your problem but check it to make sure it is not.

But first check the fuel pressure.

The smartest auto engineer I ever met was Bill (Grumpy) Jenkins he said, and I quote, "Get every variable under control" if you want things to work right.

brodysZ28

Yup, I am following you now. I pulled the needle and seats out already so I will have to get it all back together and re-set the floats to where they were and then pop the bowl off and see how low the float actually is. The pump I got is rated for 6.5 - 8 psi, so that is up there. I had a pump once that only put out 3 psi as it wore out and it stumbled all over the place above 4,000. But you really shouldn't need hardly any pressure to fill those bowls.

brodysZ28

I pm'd you on those headers too Dyno Don. No rush, get back to me whenever on those.

sofakingdom

iTrader: (1)

Carbs don't work off of fuel PRESSURE; they work off of fuel VOLUME.

If your carb's fuel bowls can be kept full by the fuel system's delivery rate at .000001 psi, then .000001 psi is all you need. (won't happen in actual reality of course; it's the concept that counts)

When the fuel bowls empty during a run, it's not because you don't have enough PRESSURE; it's because you don't have enough FLOW VOLUME. Granted, you can measure something about volume by looking at pressure, and make certain deductions; but, turning up the fuel PRESSURE will NOT fix a fuel VOLUME (flow, delivery) problem. You can have so much fuel pressure at idle that the needle & seat can't close properly, but STILL run out of fuel after a few seconds of high-RPM WOT (high fuel flow demand).

To fix that, you have to find the restriction in the system, and open it up.

Odds are, that restriction, is in the tank. Almost always is. If the car has sat around for more than acoupla years at any time during its life, the fuel has probably turned to tar in all the parts in the tank.

http://www.rockauto.com/catalog/more...nid=573&jpid=7

Strainer... goes on the end of the pickup and kinda lays in the bottom of the tank

Drop your tank, replace the "sock", and clean up the whole length of the pickup tube. Don't bother messing with "pressure".

Night rider327

iTrader: (13)

I got to agree with sofa. It's a volume/flow prob not a pressure prob. The holley and BG carbs are made to work with 6-7 psi so thats not the issue.

You are NOT getting enough fuel supply to your engine. Your 110 gph pump is plenty for a mild 383 so the prob is somewhere from tank, line, fittings, etc.

I have seen this prob before when people use hardware store 45* and 90* brass fittings in their fuel pumps, carbs, filters, etc.

Also see it from junk fuel filters that couldn't flow enough

And seen it many times from clogged up fuel socks in tank like sofa pointed out.

Test your fuel flow with and with out the tank and stock lines in the picture...

Take fuel line off carb, put it in a gallon jug, dis-arm your ignition system and crank over the engine for 10 seconds.. See how much fuel is in your jug.

Take off and plug the supply line going to pump, hook up a rubber fuel line to pump and stick it in a 5 gallon can full of gas. "prime" the new line for a few seconds.. Now crank engine over again for 10 seconds and see how much fuel is in your jug.

10 seconds on a 110 gph pump should give you a 1/4 gallon of fuel

FRMULA88

You don't need more than 7 psi for a CARB. Do you have a pressure regulator and a fuel pressure gage ?
You want to set fuel pressure at 1500-2000 RPM During WOT fuel pressure should not drop lower than 5 PSI.
if your pressure is dropping with the car idling or at WOT it drops below 5 psi or even zeroes you are losing pressure.
Fuel delivery is the likely problem so check the pump, supply line, filter, & tank.

A healthy 383 -8AN (1/2) is better at supplying more fuel to the pump.

if you are using the stock lines on your third gen that is more likely to be your problem
The stock line & pick up tube is 3/8 O.D. but the I.D. is a scrawny 5/16. this is like sucking gas thru a straw.
OK for a 180 TBI engine but will starve a real engine !


I have the reverse issue right now... I have -10AN (5/8) line so fuel delivery to the pump is not the problem, the POS pump is !

brodysZ28

Thinking about the system as a whole, all I need is flow. Now flow and pressure are directly related, so more pressure, more flow. Being a brand new pump and having the same issues, I'm confident that's not it. I ordered the fuel pump screen (sock). Hopefully that will do it.

To test that theory though ya I could pull fuel from a 5 gallon jug for x second and do the same from the tank while cranking. But would I see a difference? I mean at 5000 rpm for 5 seconds the bowls only decreased half an inch. Now if there was no restrictions I don't know how much the needle will be of the seat in the carb during those conditions. Hard to say I guess. Easy enough to test though... Maybe I'll try it.

brodysZ28

Most guys don't change the stock fuel lines do they? I mean the needle and seat stock is 0.110. That's a lot smaller than 3/8 or 5/16.

Dyno Don

iTrader: (7)

You are spot on, it doesn't take a 5/8th's line to feed two .110 needle and seats.

Most go over kill in that department.

FRMULA88

Increased fuel pressure is generally misconceived as increased volume, in fact it is the exact opposite.

http://www.quickfueltechnology.com/t...-fuel-pressure

CARB & EFI NOT THE SAME principle.

A CARB wants a high VOLUME of fuel at low pressure to keep the bowls filled with fuel at WIDE OPEN THROTTLE.

Raising pressure past 7 PSI on a carb does nothing...

http://www.quickfueltechnology.com/t...ents-fuel-flow


The float moves according to volume in the fuel bowl and as the bowl fills up it has to shut off the inlet to prevent flooding (think of the tank on a toilet). If you have your fuel pressure set at 9 p.s.i. then it will require 9 p.s.i. from the float to shut off the inlet (needle & seat). Floats in a carburetor, no matter what brand, are simply not designed or capable of operating at this pressure. Ideally fuel pressure needs to be between 5-7 p.s.i. Setting your float at a pressure of 6.5 p.s.i. is usually a good start and will give you plenty of float travel.

Put simply, more float control leads to a more stable fuel system. A carburetor that can sense and react to variations in fuel level will always be first to the finish line; combine this with a pump and regulator setup that can get the fuel forward and you have a winning fuel system.

FRMULA88

Your fuel pump should be up to the task you lines should be fine with 3/8 -6 at that power level.

but only 330 HP out of a 383?! seems very low (is this crank or wheel HP)

I recommend using port regulator and set pressure at 6.5 - 7 psi

A fuel gage on the regulator is useful tool, especially if you can see it from the driver's compartment.

I would

Good Luck !

Dyno Don

iTrader: (7)

According to this theory 110 GHP pump would support a 1350 HP combo.

I ran a BB 427 with 2 1050 dominators (= 4 .110 needle and seats) thru a 1/2" line 110 GPH pump 4.5-5 PSI making 760 HP 1/4 MI. 8.90/152 MPH and never missed a beat.

I grant you having plenty is the objective, but some are over kill.

brodysZ28

When it's cheap and easy to have a little overkill, I do it as long there are no down sides... like the 110gph vs 80gph. Going to the extent of changing the fuel lines is another story though. So I ordered the sock. I am going to need a gasket of some sort, correct? Is this what I need?

brodysZ28

I was disappointed... the only thing we could come up with is the 1 and 5/8 hooker headers were killing me. afr was good, timing was good. Maybe a little too much scatter though. The motor sounded pretty clean. If the afr guage or 02 sensor was that far off and I was really rich I would have thought we would have noticed something...

sofakingdom

iTrader: (1)

How much tar and crusties did you push out of the pickup with your rifle brushes?

brodysZ28

I haven't pulled it out yet. I will take some good pictures when I do though.

crazynights

iTrader: (3)

Same thing happened to me with my 383. Did everything you did and finally tank was the suspect. Changed everything like Sofa suggested and even took out the intank fuel pump. Works fine now.

FRMULA88

Get enough fuel to that carb and it should wake up...

brodysZ28

Really, that is good to hear. I was seeing something about in tank pumps on early cars... does my 83 originally L69 have an in tank pump as well as the obvious mechanical one on the motor? So you replaced the strainer, cleaned the tubes, and did you replace the gasket/nut that I posted above? Anything else I will need?

brodysZ28

Not on a dyno pull I don't believe because it has the fuel in the bowls that lasts enough for the pull. Its only an extended period of WOT that the fuel supply has an effect on. I've got another thread going for the dyno... looks like the valve springs might be on their way out.

sofakingdom

iTrader: (1)

AFAIK none of the 83 L69s came with the electric pump. (mine didn't) AFAIK they were added at the dealership to address warranty complaints. Later years were somewhat different maybe. But who knows, you might have one in your car, only way to know for sure is to look. Won't be hard to spot while curing the crusties and the funk.

FRMULA88

You have engine & drivetrain load but you have no g-force when strapped on an chassis dyno.

The fuel pump has to have enough pressure to overcome acceleration
(g force) in order to keep pushing the fuel to the carb bowls.

Restrictive fittings & undersized supply line make matters worse under acceleration.

FRMULA88

in this an aftermarket IN TANK pump. or an externally mounted IN-LINE PUMP ?

if it's in-tank pump and you are re-using the OEM fuel line & in-tank pickup line this is your bottle neck. The line is small and probably dirty. for a healthy 383.

if this is an external in line pump and new 3/8 line to the carb how are you feeding the pump?

You must gravity feed an inline pump so the gas tank must be sumped to install a feed line, you cannot suck fuel from the top of the tank. electric pumps push fuel not pull... if you are not gravity feeding the pump this will overwork / overheat the pump.

brodysZ28

It is an in-line mechanical cam driven pump. 3/8" line all the way from the tank to the carb. I am going to pull the pickup out and clean it out/replace the screen.

FRMULA88

ok, you have an engine mounted cam driven pump...

Depending on what brand you have it could simply be the pump is no good for the application..

You need to confirm volume at 7 PSI & 6000 RPM (under load).
Free flow ratings don't mean anything, they are an advertising trick.

with a mechanical pump you also have to contend with vapor lock (unless you are running a return line)

I had a similar problem with my pump.. so ditching this POS and getting electric in tank

FRMULA88

http://www.robbmcperformance.com/ima...vy550HP_45.jpg

http://www.robbmcperformance.com/ima...vy550HP_FF.jpg

your Jeg's pump over 4000 RPM is probably only flowing 60 GPH (at best) at 4.5 PSI

at 7 PSI it' is probably less than that.

This is why Free flow ratings are useless in the real world.

FRMULA88

For example, the Aeromotive A1000 pump on a carbureted NA engine, set to 7 psi running on 13.5 volts flows 791 lbs/hr, feeding 1582 hp @ .5 BSFC. On the same engine with EFI, regulated to 43.5 psi on 13.5 volts, the volume drops to 614 lbs/hr, feeding 1228 hp @ .5 BSFC. Add in some boost–6 psi, an 8:1 boost reference regulator, intercooled, pressure regulated to 91 psi on 13.5 volts and the flow craters to 370 lbs/hr, feeding 616 hp @ .6 BSFC. This demonstrates a flow reduction of 53%.

More pressure requires more fuel pump volume.