# Still can't figure out the problem....starting to piss me off



## xconcepts (Jan 4, 2009)

Ok, so as some of you might remember from previous posts, I'm having fuel issues while accelerating. Once it gets to about 2-2500rpms she will bog down like she's not getting any fuel. I either have to shift to another gear or let off the gas completely and slowly ease back on it. Hell started to do it on an incline after a light yesterday and I didn't think I would make it up. Started when I had a 4 barrel qjet on that did need rebuilt, but I decided to go to a tripower and the carbs were build originally by a member on here that was recommended. New fuel filter and pump(from autozone), and new fuel tank filter were added

At first the issue went away but after a few months of maybe every other weekend driving her it started to creep back into doing it in 4th gear then 3rd gear then 2nd. So after asking around figured it was a points issue and not getting good spark so I went to a Pertronix unit inside the distro and again went away for a little then came back but only in 3rd. Also had the carbs rebuilt as this time as I was told they were leaking. Then the Pertronix unit went bad after about a year and I went back to points for the moment. I replaced all the rubber fuel lines, blew air through the steel lines and cleaned the tank out. 

Still had the issue but I needed to replace the rear main seal and once we pulled it decided to get it rebuilt. After rebuild I added an MSD box. Seemed to help but still issues in 3rd. So just recently had to have the motor rebuilt again due to a bad bearing, which was covered under warranty, I added a ready to run MSD distro and now I'm back to square 1 with having issues in 2nd, 3rd and 4th......

I'm at a loss at what is wrong and what I should do next. It was mentioned to go to a electronic fuel pump, but not sure why I should need to do that being a stock 389 for the most part besides the MSD units. I've read the fuel pumps from autozone, etc, are not correct and allow too much fuel or something. Could that be the issue?


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## Indecision (Oct 24, 2010)

I'm speculating and far from an expert, but I've heard about the parts store pumps being WAY high from the factory. Not the same type of car, but a VW mechanic I knew told me he had one come to him straight from napa putting out 12psi, which is 4x what it should be. So it just overpowered the needle valve/float, and it would piss fuel out the vent tube and venturi nozzle. 

In your case, with the engine in higher RPM, it's possible that it's making more pressure than the carb can take and doing something similar, though obviously not as extreme. 

If you have a fuel pressure test gauge, I'd start there... even if it's not the problem you can at least eliminate it as a problem.


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## chuckha62 (Apr 5, 2010)

It sounds to me like the "sock" filter on the fuel pick-up is collapsing under high pump volumes. These things tend to partially clog over time and will collapse under high demand. Short of taking it out and replacing the pick up you can blow air pressure back through the line to re-inflate it. This will also partially clear it, but it does tend to put debris back into your fuel.

Just one thought...


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## geeteeohguy (Feb 2, 2008)

He said he replaced the sock. That was my first thought too, having BTDT. I would eliminate the sock completely, as I did on my own '67 back in 1988. No problems since.


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## xconcepts (Jan 4, 2009)

Yeah sock is only about a year old


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## chuckha62 (Apr 5, 2010)

Looks like my third grade education failed me again... Reading comprehension, ugh!

It's possible that the pump filter could still be collapsing. I'd do what Geetee did and eliminate it entirely.


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## jmt455 (Mar 26, 2012)

I'm also inclined to think this is a fuel delivery issue.

Is your fuel tank vented? If the tank vent is too small or plugged, it MIGHT cause this kind of aggravation. If you have a vented gas cap, make certain that the vent is working properly.

If the venting checks out OK, I would put a fuel pressure gauge on it and verify that the pressure is correct (not too high OR too low). I have had new fuel pumps recently that put out over 7psi. That could be enough to blow the needle valve off the seat, as mentioned earlier. I am now adding fuel pressure regulators to my mechanical fuel delivery systems.


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## pjw1967 (Mar 10, 2014)

I'm not any help with your problem, but I thought I'd capture your post nbr 455 before it became 456.


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## xconcepts (Jan 4, 2009)

jmt455 said:


> I'm also inclined to think this is a fuel delivery issue.
> 
> Is your fuel tank vented? If the tank vent is too small or plugged, it MIGHT cause this kind of aggravation. If you have a vented gas cap, make certain that the vent is working properly.
> 
> If the venting checks out OK, I would put a fuel pressure gauge on it and verify that the pressure is correct (not too high OR too low). I have had new fuel pumps recently that put out over 7psi. That could be enough to blow the needle valve off the seat, as mentioned earlier. I am now adding fuel pressure regulators to my mechanical fuel delivery systems.


Ok, so I don't believe I have a vented cap so where would it be vented? I've had the tank dropped and never noticed another vent.


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## jmt455 (Mar 26, 2012)

xconcepts said:


> Ok, so I don't believe I have a vented cap so where would it be vented? I've had the tank dropped and never noticed another vent.


There should be a vent tube in the filler neck, near the top of the filler neck tube. 
It should be connected to this vent tube assembly:


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## 666bbl (Apr 13, 2014)

I have repro and OEM tanks. The OEM has a vent in the filler neck, the repro does not. Being a 65 with the fill behind the lic plate I'd prefer the OEM with the vented neck vs running a vented cap. Back when many of the GM 'used cars' were on the road I'd often see a car splash fuel out the back as they accelerated forward. Chevelles from our era were notorious for that. If you have a new tank and a no-vent cap this may be your issue. Don't shoot the messenger here, but that's an awfully simple fix considering the angst you have now. Just as a test you could tape a rolled up clean rag into and around the filler and take it for a spin. If it is a vent issue you'd probably hear the tank collapse and go back into shape, something like a big oil can. If you have aggressive exhaust you may never hear it. Worth a try as the test would be free.


Quick edit: If it was too much pressure you'd surely smell it and see evidence of raw fuel on the engine.


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## pjw1967 (Mar 10, 2014)

Here are a couple of photos of the installed tank vent. Both the old tank and the news stainless tank had the vent tube at the filler neck.


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## 70_GTO_JUDGE (Dec 8, 2010)

xconcepts said:


> Ok, so as some of you might remember from previous posts, I'm having fuel issues while accelerating. Once it gets to about 2-2500rpms she will bog down like she's not getting any fuel. I either have to shift to another gear or let off the gas completely and slowly ease back on it. Hell started to do it on an incline after a light yesterday and I didn't think I would make it up. Started when I had a 4 barrel qjet on that did need rebuilt, but I decided to go to a tripower and the carbs were build originally by a member on here that was recommended. New fuel filter and pump(from autozone), and new fuel tank filter were added
> 
> At first the issue went away but after a few months of maybe every other weekend driving her it started to creep back into doing it in 4th gear then 3rd gear then 2nd. So after asking around figured it was a points issue and not getting good spark so I went to a Pertronix unit inside the distro and again went away for a little then came back but only in 3rd. Also had the carbs rebuilt as this time as I was told they were leaking. Then the Pertronix unit went bad after about a year and I went back to points for the moment. I replaced all the rubber fuel lines, blew air through the steel lines and cleaned the tank out.
> 
> ...


I had a truck that would do that....sounds like something in the fuel tank, like rust


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## xconcepts (Jan 4, 2009)

70_GTO_JUDGE said:


> I had a truck that would do that....sounds like something in the fuel tank, like rust


I've cleaned out the tank, no rust. I have the original tank to my knowledge, if not it's still 30+ years old. I'll have to check the vent, see if it's bad. Thanks for the pics

Sent from AutoGuide.com Free App


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## xconcepts (Jan 4, 2009)

So I do have the tank vent and it is connected. So looks like I'm leaning towards a electric fuel pump or the stock pump flooded the floats 

Sent from AutoGuide.com Free App


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## chuckha62 (Apr 5, 2010)

Okay, Just to be clear... From what you describe, it's sounding like you're fuel STARVED not flooded, under load. I can't even begin to imagine the condition that must exist in order for you to have too much fuel (given your current set-up) while under load above 2600 RPM. The excess fuel pressure that was mentioned is (in my opinion) a red herring. That is only going to be an issue at idle, NOT under load and especially at the RPM that you mentioned. 

You have a restriction someplace. Whether the sock has collapsed again, or you have a plugged filter, or you have a pump that is failing under load. Excess pressure is not the issue given what you have described. put a pressure gauge in it that you can see while driving and what what is happening. That's the only way you're going to know what's going on.


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## Indecision (Oct 24, 2010)

chuckha62 said:


> The excess fuel pressure that was mentioned is (in my opinion) a red herring. That is only going to be an issue at idle, NOT under load and especially at the RPM that you mentioned.


It may or may not be HIS problem... but I personally had the EXACT same problem on my wife's bug. The fuel pressure was so high, the needle was overpowered and fuel would spit out the venturi nozzle and vent until the car died. Once I got the fuel pressure fixed, it ran like a champ. So however unlikely it may be to you, I can assure you that it is in fact possible.

And for the 5min it takes to hook up a fuel pressure gauge and actually SEE whether it's a problem, even just for the sake of eliminating it as a problem, I think it's absolutely stupid to say it's not worth looking in to... especially KNOWING the pump was replaced with an autozone pump that are known to have non-standard pressures out of the box.


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## chuckha62 (Apr 5, 2010)

Indecision said:


> It may or may not be HIS problem... but I personally had the EXACT same problem on my wife's bug. The fuel pressure was so high, the needle was overpowered and fuel would spit out the venturi nozzle and vent until the car died. Once I got the fuel pressure fixed, it ran like a champ. So however unlikely it may be to you, I can assure you that it is in fact possible.
> 
> And for the 5min it takes to hook up a fuel pressure gauge and actually SEE whether it's a problem, even just for the sake of eliminating it as a problem, I think it's absolutely stupid to say it's not worth looking in to... especially KNOWING the pump was replaced with an autozone pump that are known to have non-standard pressures out of the box.


Believe me... and read my previous posts and you'll see excess fuel pressure is my hot button issue. I am well aware of what it can do. On this car, under the conditions he's describing it is HIGHLY unlikely to be the source of his problem. But as I stated as well, a fuel pressure gauge, visible from the driver's seat, will give him a wealth of information.


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## topfuel67 (Dec 23, 2008)

I had this same problem in a 65 I bought cheap years ago because of this hard to find issue. The guy thought it needed a new motor and I planned on putting a new motor in anyways. When I put the new motor in it still had issues revving above 2500 like it was starving for fuel. It turned out being a wiring issue. The positive for the coil was coming from 12v off the heater blower instead of from the battery/alternator/starter junction. The coil needs a strong gauge wire.


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## chuckha62 (Apr 5, 2010)

topfuel67 said:


> I had this same problem in a 65 I bought cheap years ago because of this hard to find issue. The guy thought it needed a new motor and I planned on putting a new motor in anyways. When I put the new motor in it still had issues revving above 2500 like it was starving for fuel. It turned out being a wiring issue. The positive for the coil was coming from 12v off the heater blower instead of from the battery/alternator/starter junction. The coil needs a strong gauge wire.


That's a good point. The coil also needs to be adequately grounded.


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## xconcepts (Jan 4, 2009)

Ok, so I wanted to bump this for an update.

First off greetings from Lajes Field, Azores. The GTO was trucked to my dad in Texas and has been there for about 2 months. I got an update from him today and he is NOT having the bogging down problems that I was having in Arizona at all. He can get on it with no issues, grant it he hasn't punched it too much since it was just rebuilt again. But I didn't have to either and still had the problem just before I shipped her out.

The issue he is having is every time he hits a light or stop sign she stalls out. I never had this issue except a couple years ago when the carbs were messed up. The only difference is the weather and premium in Texas is 93 and premium in AZ is 91. He did say there is a small fuel leak at the brass fitting where the fuel comes into the carbs, which he is getting fixed.


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