# Frustrated - ‘65 GTO engine stalling at >4000 rpm



## Roqetman (Aug 31, 2015)

Calling all Q-Jet/Carburator Experts. I am going out of my mind trying to solve this stalling problem. I have now invested in a new DUI electronic distributor and the stalling problem persists so... it must be fuel delivery problem. Is a 750 Qjet not able to keep up with fuel demands of a 400 engine with RAIII heads? The engine stalls/bogs >4000 rpm whenever I do a full throttle take-off and when I’m winding out first and second gears (TH400). It’s a little better if I peddle it but full pedal-to-the-metal and I have to let off and give it a second to catch up. After so many adjustments, I’m all out of bullets. I love the spreadbore design but I’m no longer sure this 52 year old carb has what it takes to make this newly rebuilt engine perform like I want it to. What’s a good replacement carb that can keep up with fuel requirements up to 5500 rpm? Or, is that too much to ask for?


----------



## PontiacJim (Dec 29, 2012)

Roqetman said:


> Calling all Q-Jet/Carburator Experts. I am going out of my mind trying to solve this stalling problem. I have now invested in a new DUI electronic distributor and the stalling problem persists so... it must be fuel delivery problem. Is a 750 Qjet not able to keep up with fuel demands of a 400 engine with RAIII heads? The engine stalls/bogs >4000 rpm whenever I do a full throttle take-off and when I’m winding out first and second gears (TH400). It’s a little better if I peddle it but full pedal-to-the-metal and I have to let off and give it a second to catch up. After so many adjustments, I’m all out of bullets. I love the spreadbore design but I’m no longer sure this 52 year old carb has what it takes to make this newly rebuilt engine perform like I want it to. What’s a good replacement carb that can keep up with fuel requirements up to 5500 rpm? Or, is that too much to ask for?



Another carb may not fix the problem anymore than a new distributor. You can try it, but the Q-jet should be able to keep up unless it is sick and/or a passage plugged. The fact that it pulls to 4K and then you have to let off the gas for the gas to fill up the carb bowl means fuel delivery. Where? That's what you have to find - carb itself or the fuel system.

Have you checked every inch of the fuel line looking for a "pinched" or bent line?

Replaced the fuel pump?

Fuel pump cam/eccentric incorrect or slipping so the pump arm is not being activated correctly?

Have you used a fuel pressure gauge to watch the actual fuel pressure at the carb?

Have you done any fuel pump flow testing?

All you steel line from the tank to fuel pump is 3/8" and the line from pump to carb is 3/8"?

All new 3/8" 50PSI fuel injection rubber lines at all connections, front and at the tank?

You pulled the fuel pick-up/sending unit out of the tank to inspect the sock on the end of the pick-up tube.

Have you even blown air through the 3/8" line to make sure there is no junk plugging the line? We had one member find junk lodge inside the line. Blew it out and no more issues.

Has the carb been rebuilt?

Float level too low.

Need a larger needle/seat from Cliff Ruggles?

Fuel filter junk or installed backwards or without the spring - if still using the factory style filter.


----------



## geeteeohguy (Feb 2, 2008)

Jim hit it dead on. I had the same issue with my '67 twice. (stock Q-jet) First time in the 80's it was a wadded up fuel strainer sock in the tank that would block fuel flow. I removed the sock. Second time in the early 2000's it was a cracked rubber fuel line from the tank sender to the steel line. Not enough to leak fuel, but enough for the pump to suck air and cause starvation under full throttle. A cheap and easy fix, after I found the issue. Your carb is running out of gas so it's more than likely a fuel delivery problem, not a carb problem.


----------



## BearGFR (Aug 25, 2008)

Like GeeTee and PJ said, If you're "running out of gas" at more than 2000 rpm and it happens "after" launch and not at the instant you hammer the throttle, it's not a carb problem. You have a fuel delivery problem. The carb is emptying the fuel bowl faster than the rest of your fuel system can fill it. There are many possible causes, such as weak fuel pump (just because it may be brand new doesn't mean it's good), clogged filter somewhere in the line, flattened fuel line, clogged/degraded sock on the pickup inside the tank, even a piece of loose debris in the tank that can temporarily restrict the pickup. 

Start with the inexpensive/easy to get to things first, and work towards the more difficult/more expensive ones.

Bear


----------



## ylwgto (Oct 27, 2009)

I’d check all rubber lines in your fuel system asap. The new gas really tears up old standard hosing…you may have micro cracking that only reveals itself under full throttle conditions.


----------



## lust4speed (Jul 5, 2019)

Very common problem with the stock fuel system in our A-bodies not keeping up with demands. Usually the engine pulls fine through first gear but somewhere in the upper ranges of second gear at full throttle the car falls flat on its face. If you still have the bronze filter inside the carb inlet, take it out and do another test run. If the engine dropped out at 4,000 RPM, then just removing the filter should give you at least another 500 RPM before things go south again. If you were right on the line, it might solve the problem. Those filter elements restrict quite a bit and going to an inline filter helps. Should be a steel cased filter about 2" in diameter and 3" long. Really every part of the fuel supply system needs improvement. The 5/16" line is marginal, the stock fuel pump is marginal, and so on down the list. You just have to address each area and improve it when necessary.

The float bowl on early Quadrajets is tiny, and the size dictates that a fair sized fuel supply is needed to keep the carb happy. I would still stay with the Quadrajet and concentrate on improving the fuel supply issue.


----------



## Roqetman (Aug 31, 2015)

Thanks for all the great advice! I’ve had this car since Aug 2015. In that time frame I have replaced the fuel pump with a new Holley mechanical fuel pump that has 7 psi of pressure at the engine. The gas tank has been flushed and coated with a ethanol compatible finish. The sock and pick up in the tank are new. I have ran it with no fuel filter at the carb, no improvement. The car has a 5/16” fuel line and 1/4” return line, the return line was not in use when I got the car. I made a brass “Y” connector so I could connect both the 5/16 and 1/4 lines together and connect them to the 3/8” pick up at the tank, I then made a second Y connector to connect the 2 fuel lines into one 3/8 line at the fuel pump. I thought this would be easy way to have a larger than 3/8“ fuel line (5/16 + 4/16 = 9/16” vs 6/16” or 3/8), still no improvement. (I stole this idea from Junior Johnson, back in the day he would run a 1” fuel line to hold more gas. The inspectors eventually caught him and had to rewrite the rule book again.) I have considered putting a new 3/8“ fuel line but I’m doubtful that will solve the stalling problem Unless… my 5/16 or 1/4 lines has a kink or blockage. I’ll check that next!


----------



## PontiacJim (Dec 29, 2012)

Roqetman said:


> Thanks for all the great advice! I’ve had this car since Aug 2015. In that time frame I have replaced the fuel pump with a new Holley mechanical fuel pump that has 7 psi of pressure at the engine. The gas tank has been flushed and coated with a ethanol compatible finish. The sock and pick up in the tank are new. I have ran it with no fuel filter at the carb, no improvement. The car has a 5/16” fuel line and 1/4” return line, the return line was not in use when I got the car. I made a brass “Y” connector so I could connect both the 5/16 and 1/4 lines together and connect them to the 3/8” pick up at the tank, I then made a second Y connector to connect the 2 fuel lines into one 3/8 line at the fuel pump. I thought this would be easy way to have a larger than 3/8“ fuel line (5/16 + 4/16 = 9/16” vs 6/16” or 3/8), still no improvement. (I stole this idea from Junior Johnson, back in the day he would run a 1” fuel line to hold more gas. The inspectors eventually caught him and had to rewrite the rule book again.) I have considered putting a new 3/8“ fuel line but I’m doubtful that will solve the stalling problem Unless… my 5/16 or 1/4 lines has a kink or blockage. I’ll check that next!


Well, you might have created a problem when you joined the pick-up line with the 1/4" return line.

Assume you purchased the sending unit having the 3/8" pick-up tube? Factory, and most I see available use the 5/16" pick-up tube, not 3/8". 3/8" was factory 1968 and up.

Next issue I see, and might be your problem, I get your Junior Johnson thinking, but did you extend the 1/4" return line alongside, and match, the 3/8" or 5/16" pick-up tube? If not, that is your problem right there. The 1/4" return line does not go to the bottom of the tank like the pick-up tube. It is much shorter and only returns the gas, so no need to go into the tank very far.

I snagged a photo of a 1965 GTO repop 5/16" fuel sending unit with 1/4" return line. Note how short it is. If this is you, you are sucking up air, not gas at the higher RPM's because there will be less resistance sucking air than gas.

Maybe the issue?


----------



## lust4speed (Jul 5, 2019)

Also worth noting is tubing is measured by outside diameter so that 5/16" tube is roughly 1/4" I.D and the 1/4" return line is roughly 3/16". The resistance inside the small return line means that it is adding almost nothing to overall flow.


----------



## Roqetman (Aug 31, 2015)

Thanks again for all the input! They say a picture is worth a thousand words so, I drew a diagram to show how I used two “Y” connectors to incorporate the 5/16 factory fuel line and the 1/4 return line in order to hopefully increase fuel flow to carb. Granted this still has not solved the fuel delivery issue >4000 rpm but I thought, if it worked it would be easier than installing a new 3/8 line. Another question, would using a fuel filter that has fittings for fuel to the carb and the return line to the tank be a good option since that is apparently how Pontiac designed It? Thanks again for helping me with this issue!


----------



## Roqetman (Aug 31, 2015)

Here is picture of the fuel filter with return line fitting. Would this help?


----------



## geeteeohguy (Feb 2, 2008)

I see no picture. The return line needs to run to the tank separately per the manufacturer's design in order to bleed off vapor pressure and allow the fuel to flow better and cooler. Much like venting a gas can. Tying the two together will cause issues and not help fueling issues. You need 3-5 PSI fuel pressure, and enough volume to fill a quart jar full of gas in a few seconds of cranking...like less than 25. Here's my original '67 vapor return set-up.


----------



## Roqetman (Aug 31, 2015)

Geeteeohguy, can you not see the diagram I drew or the fuel filter pic? Do you think using the return line with this type filter solve the problem with 5/16” fuel or do I need to install a new 3/8” fuel line?


----------



## geeteeohguy (Feb 2, 2008)

You need to find out why your fuel delivery is not up to par. You first need to perform a volume check. And go from there. I explained how the vapor return system works...it's a vapor return, not an additional fuel supply. As stated previously, the 1/4 return line does not go down where the pickup is in the tank. It's there to allow vapor and some fuel to return to the tank, not to provide additional fuel to the carb. If you are failing the volume check, it is a fuel pump issue that is likely. Both my GTO's are running the original small line and don't have any issues running out of fuel at even engine-grenading rpms. Find out what's wrong rather than trying to re-engineer the car.


----------



## Sick467 (Oct 29, 2019)

Roqetman said:


> Geeteeohguy, can you not see the diagram I drew or the fuel filter pic? ...


I see your fuel line drawing and it is better with your mods, but only slightly compared to the original 5/16" line. The two lines add some volume, but also add flow resistance. The added volume exceeds the resistance. It is my opinion that it only equates to a small improvement. Many Pontiacs have been beefed up and still do fine with the original smaller line...3/8" would be better, but maybe not necessary. I would look elsewhere for your problem so long as you have made sure your lines are clear and free flowing.

Make sure your fuel tank vent is working properly. I'm not familiar with the 65, but a partially clogged fuel tank vent (or gas cap) could cause fuel delivery problems that you describe. Especially if it is just partially blocked. Not blocked enough to cause normal driving problems, but blocked enough that you just can't get the vacuum in the tank to release under heavier fuel needs. Maybe a long shot, since this problem could be one or two of many things, but clearing the lines and checking the tank vent would be free and easy to do.

The fuel return line is a great idea, but helps mostly with vapor lock and reliving stress from the fuel pump. My build will retain this feature, but I don't think is has anything to do with your current problem.

My mechanical thoughts go with,

Clogged lines,
Weak fuel pump,
Clogged filter,
Collapsed/gummed up sender sock, &
clogged fuel tank vent.

Most of these things have been mentioned by better gear heads than I. Let us know what you have eliminated, we can go from there.


----------



## Baaad65 (Aug 29, 2019)

This is my setup for what it's worth, '65 with original tank and pickup and sock it looks like but not guaranteed, no return line just the filler neck vent, new 5/16" id steel braided fuel line to a Holley 110 gph mechanical pump, 5/16" id feed line with an Edelbrock 50 micron in line filter to a Quickfuel 850dp with the carb filters removed, all feeding a 461. I have 5-7 lbs of fuel pressure and on Sunday I shifted it at 5700 rpms through 4 gears until I hit 115 mph with no problems. I would look at the pump and filters like mentioned unless you do see a bad line I can't remember.


----------



## Roqetman (Aug 31, 2015)

Thanks again for all the input. To clarify my modifications to the fuel line, the return tube that goes into the tank is blocked off and is not used. The “Y” connector is to connect the 3/8 fuel pickup tube at the tank to the two lines along the frame and then another “Y” connector up front to connect those two lines into one 3/8 line the supply the fuel pump. I realize this may not have helped but just wanted you all to know that the return line going into the tank is blocked and not used. 

The Holley fuel pump is new and putting out 7 psi but have not yet checked the volume. 
Fuel filters are new and clean, one at the tank and one at the Qjet fuel inlet. Would a fuel filter that has the vent return line be a possible solution? 
The sock in the tank is new and clean. 
Fuel tank vent, I have not checked it but but the original gas cap states “not vented”. If there is a vent would it be in the filler neck or somewhere else on the tank?

How is the best way to clean out the fuel line, compressed air or do I need to somehow do a “roto rooter” treatment on it? I’m having another mechanic looking at it today and will pass this info to him. Thanks Again!


----------



## Baaad65 (Aug 29, 2019)

My vent comes off of the the filler neck, goes up through a valance piece with a small piece of rubber hose then hair pins down...that's from memory but I'm sure you can find a picture. Seems like I did replace the rubber and blow it out at one time when I was having your issues but mine was an inaccurate tachometer and I was hitting the rev limiter....you don't have one of those do you?


----------



## Roqetman (Aug 31, 2015)

Baaad65 said:


> My vent comes off of the the filler neck, goes up through a valance piece with a small piece of rubber hose then hair pins down...that's from memory but I'm sure you can find a picture. Seems like I did replace the rubber and blow it out at one time when I was having your issues but mine was an inaccurate tachometer and I was hitting the rev limiter....you don't have one of those do you?


No rev limiter. And as I recall there is a vent tube in the filler neck like you described. But if that’s true why would the gas cap state “not vented”?


----------



## O52 (Jan 27, 2019)

The cap is not vented since the tank is.


----------



## Sick467 (Oct 29, 2019)

Roqetman said:


> No rev limiter. And as I recall there is a vent tube in the filler neck like you described. But if that’s true why would the gas cap state “not vented”?


The vent tube that comes off the fill neck and into the trunck is your vent. The cap does not need to be vented since it is vented otherwise.


----------



## Roqetman (Aug 31, 2015)

O52 said:


> The cap is not vented since the tank is.


Ah-ha! Thanks for the clarification!


----------



## Sick467 (Oct 29, 2019)

Compressed air would be my go-to for blowing our the fuel lines and the tank vent...BUT start out with the air regulated down to about 20 psi. That should be all you need to prove that air can flow through easily. I would not go higher than 30 psi to be on the safe side...AND take your gas cap off before back blowing through the vent tube. You don't want to pressurize your tank. I would disconnect the fuel line at the tank and pump and then blow through. Do the same for the vent tube, disconnect at the fill neck and blow from there. You will have to blow into the tank to make sure the vent nipple at the neck is clear, but it's unlikely that there is a clog there and if so...the sock should catch it before it can get to the feed lines. Again, Don't forget to have the gas cap off. Aside from not wanting to pressurize the tank. YOu can hear better if air is moving.

Its a "feel" thing when blowing out the lines. If 20 psi of air seems to just go on through, I'd say that line is good. Bump it to 30 psi for fun if you want. IF the pressure seems to build and then "lets go", you have likely cleared a clog. You could bread-tie a rag over the end of the line, loosely like a sock, before blowing it out to see if you blow any boogers out.


----------



## geeteeohguy (Feb 2, 2008)

Roqetman said:


> Thanks again for all the input. To clarify my modifications to the fuel line, the return tube that goes into the tank is blocked off and is not used. The “Y” connector is to connect the 3/8 fuel pickup tube at the tank to the two lines along the frame and then another “Y” connector up front to connect those two lines into one 3/8 line the supply the fuel pump. I realize this may not have helped but just wanted you all to know that the return line going into the tank is blocked and not used.
> 
> The Holley fuel pump is new and putting out 7 psi* but have not yet checked the volume. *
> Fuel filters are new and clean, one at the tank and one at the Qjet fuel inlet. Would a fuel filter that has the vent return line be a possible solution?
> ...


Until you do this, you are chasing your tail. Why HAVEN'T you checked the volume, and why do you want to do all this other stuff instead? As a professional mechanic, I'm just curious.


----------



## Roqetman (Aug 31, 2015)

Geeteeohguy, Thanks for your interest. I’ll not a mechanic but I do like to tinker. The mechanic that installed the engine checked fuel pressure in his shop at 7 psi, this was one of the last things he did before he Retired! He did not check volume. So I took it to another old school mechanic and he checked the fuel pressure while driving and said it was 5.5 psi until the rpm’s got above 4000 and then it dropped to 3 psi. He said he didn’t have a good way to check volume. He put a new 3/8” fuel line and new fuel pump and now it runs like a scalded dog! You know, the way it should run. The fuel pump he took off was an Edelbrock not Holley. Thanks again for your input and interest!```


----------



## kszr (3 mo ago)

Roqetman said:


> Geeteeohguy, Thanks for your interest. I’ll not a mechanic but I do like to tinker. The mechanic that installed the engine checked fuel pressure in his shop at 7 psi, this was one of the last things he did before he Retired! He did not check volume. So I took it to another old school mechanic and he checked the fuel pressure while driving and said it was 5.5 psi until the rpm’s got above 4000 and then it dropped to 3 psi. He said he didn’t have a good way to check volume. He put a new 3/8” fuel line and new fuel pump and now it runs like a scalded dog! You know, the way it should run. The fuel pump he took off was an Edelbrock not Holley. Thanks again for your input and interest!```


Thanks for your posts. Had similar issues with our 66. New 3/8 line and sending unit fixed it!


----------

