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Hi was hoping to get some help here. My 68 GTO convertible has its original 400 WT 4 spd and motor was rebuilt long ago only has maybe 3,000 miles on it. Recently when I drive it hard (foot to floor) it literally loses all power. I floor it and it lays wheels and revs high and fine through first, second, but as soon as I get to decent speed through third the engine almost completely cuts. Literally feels like fuel gets completely cut off. Engine doesn’t completely stall but almost. It just drops down to idle and almost stalls. Background on the car is that in past couple years I’ve had the original Rochester 4bbl rebuilt and also an HEI ignition installed. Engine had no issues after both of these procedures. It literally just appeared last week. I do suspect a vacuum leak because the hideaway headlights stopped going up on their own so thought I should mention that. Not sure if issues are related or not. Any help would be greatly appreciated.
 

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Well first clamp off the hoses to the hideaway headlights and retest drive......

if that does not change it could be a few different things, fuel may not be meeting demand, are the secondary throttle plates opening? A paper clip or marker on the throttle linkage to letyou know if you cannot tell.....if it bogs and stalls they may be opening but not delivering fuel just air....so you bog and stall at that demand....

this car has low mileage and today’s gas can gum up rather fast even with a new carb.

I would put a bottle of Redline S1Complete fuel system cleaner in a full tank of gas and drive a bit, let that work through. Don’t let the counter man sell you something else saying it is just as good, it ain’t most stuff is snake oil and Redline has more Techron in it than a bottle of over the counter Techron.....

HEI and spark can die under high demand as well, but you may want to check vac and fuel first given it has been running ok...secondary jets may be all gummed up from sitting
 

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Sounds like fuel starvation.
Things to check:

Strainer inside gas tank
Fuel pressure
Filters ( filter in carb as well)
I had a float that was sticking in my carb. Turns out it had the wrong float.When I would get on it, the float would stick in the up postition shutting off the fuel.
The float looked like it was OK, but the arms were just long enough that it would occaionally touch the end of the fuel chamber, and when, you guessed it, under accleration, the fuel would slosh to the rear, lift the float and it would hang there, shutting off the fuel. Drove me nuts for a while. I put in another float ( verified correct) and it has been good to go
 

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Discussion Starter #7
Thanks for all the responses. So I got the Redline S1 and put it in the tank and took it for a good 20-25 mile drive. I'm sure it has to work its way through the system so I don't want to jump to any conclusions yet, so we'll see. But I really tested out the car hard and what I noticed is that the issue happens in any gear somewhere between 2600 and 3400 rpm. I can make the issue happen consistently, so it definitely isn't sporadic or random. Seems to happen in the 2600 range when going up hills and 3000-3400 range on flats. It cuts out hard and immediate. I mean hard enough that a passenger would jerk forward and definitely notice something was wrong. I'm wondering if a vacuum issue would cause such clear cut off in power? A vacuum advance?

Anyway fuel is relatively new, although I do drive the care very little so at beginning of summer the gas was probably a year old. I've put probably about 20 gallons in it this summer though.
 

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remove your distributor cap and rotor and take a look at your weights and springs, ar3 they loose or jammed?...work them with your hand do they turn out and spring back?...also look at your rotor and cap for any issues.

it could be there, but what you are describing, up hill and above 3000 is when the engine is demanding more more,....this is when power valves and accelerator pumps and secondaries come on..

a weak coil, a bad ground to the HEI, a failing module, a weak voltage from the alternator can all play here. It still might be a fuel issue like LATech said, how is your fuel filter?.....

Something is failing when more power is demanded......a spark plug wire could be shorting and on increased demand it arcs to a ground, will it stall not under load?

like when you rev it up to 3000 Rpm in Park? ......
 

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I am leaning towards a fuel issue. Your description of quits pulling hard enough to jerk you forward is indicative of the fuel bowl running dry and there is 0 fuel in the bowl until you let off the pedal and wide open throttle and the fuel refills the bowl again.

You can note in 3rd gear what road speed you are at the moment it begins to quit, then shift into 4th to drop RPM's and give it about 30 seconds and see if the bowl fills, then maintaining the same road speed/RPM as you were in 3rd gear and the problem occurred, push in the clutch and bring the revs up to the same RPM's you noted the problem, then drop down from 4th to 3rd and hit the gas hard and see if it will pull to say 4,000 RPM's or dies. If it pulls past the problem RPM's and dies higher up the scale, then a gas delivery issue and your fuel bowl is running dry.

Fuel filter first. Then test. This may be a trial and error thing, but start there and report back. Do not begin replacing a bunch of things at once or make a lot of adjustments as this will not help to narrow it down.
 

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Sounds "fuelish" to me too. Check (and maybe go ahead and replace) all the fuel filters including the one in the carb inlet and also the one inside the tank - the 'sock' on the end of the pickup tube. That one's a pain becuase you have to drop the tank and remove the inlet/float assembly to get at it. How old is your fuel pump? It might be running out of gas (literally) at RPM and not able to keep up with demand. If you do drop the tank, make sure you check it thoroughly for any debris/garbage that may have somehow gotten inside.

I had a similar problem with my '69 that I chased for more than a year. It didn't do it "all the time", but occasionally it would run like a banshee until just after the 2-3 shift, then act like it had been shut off. The problem turned out to be a small piece of debris in the tank that now and then would manage to get sucked into the tank outlet and almost completely block it off. (I've modified my tank and added a rear sump.)
137880


Bear
 

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I had the exact same problem. Replaced the sending unit in the tank, all fuel hoses, then added an electric fuel pump and fuel pressure gauge so I could delete the mechanical pump. Problem solved.
 

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I had the exact same problem. Replaced the sending unit in the tank, all fuel hoses, then added an electric fuel pump and fuel pressure gauge so I could delete the mechanical pump. Problem solved.
All well and good, but that never isolated the actual cause of the problem. Not everyone wants to go to that extreme and replace everything, nor go to an electric pump.
 

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Yup...mine turned out to simply be a hose on top of the gas tank out of sight of course...it had gone brittle and had a little slit that was able to pull gas under normal use but there was this one hill on my way to work that would stall it right there, every morning...for months, before I decided to pull out the tank for lack of any other solution...after that I'd blast up that hill as if I were trying to flatten it!
 

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Yup...mine turned out to simply be a hose on top of the gas tank out of sight of course...it had gone brittle and had a little slit that was able to pull gas under normal use but there was this one hill on my way to work that would stall it right there, every morning...for months, before I decided to pull out the tank for lack of any other solution...after that I'd blast up that hill as if I were trying to flatten it!
There is a "sock", or filter, on the end of the pick-up tube in the tank. These can collapse, get plugged up with debris, or break apart. Most miss this as the problem. You could also find that the tank vent is plugged, or even a wrong gas cap was at some time installed.

Trying to isolate the problem can take time and you have to go step by step. Some want to keep their cars as original as possible, so stock is what they want. One member found some junk clogging the steel line running along the frame rail. Pressurizing some air into the line with it disconnected at the tank produced the clog, blew it free, and no more problems - probably a chunk of deteriorated rubber gas line as the ethanol laced fuel will soften, deteriorate, and even cause rubber fuel lines to seperate and shed.
 

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Sounds like fuel. There are two pieces of rubber hose on the suction side of the fuel pump, one at the pump and the other on top of the tank. I had a car that randomly stalled, like vapor lock. I decided to replace the fuel pump, and found that the 2" of hose at the fuel pump was seeping/weeping gas. An occasional drop on the ground, but not much. The rubber in the hose has gone soft, and pretty much dissolved! With engine running, the outside of the hose would dry out, and the pump would start sucking air. So, periodic replacement of the rubber hoses is a good idea.
 

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Another thought: new car reviews of 67 GTO's noted that the (new!) cars went flat at about the top of second gear. But only if the fuel level was below 1/4 tank! Under strong acceleration, the fuel runs to the back of the tank, and away from the fuel pickup. There's enough gas in the carb to get to about 60 MPH. Don't know if this situation applies to 68's, but you might want to test with a full tank of gas an see if the problem goes away.

Holley has some new magic fuel pickup device that replaces the sock on the end of the pickup, and will draw fuel if any part of the mat touches fuel. Sounds like a good idea, but they want about $100 for it. I haven't tried it yet.
 

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Another thought: new car reviews of 67 GTO's noted that the (new!) cars went flat at about the top of second gear. But only if the fuel level was below 1/4 tank! Under strong acceleration, the fuel runs to the back of the tank, and away from the fuel pickup. There's enough gas in the carb to get to about 60 MPH. Don't know if this situation applies to 68's, but you might want to test with a full tank of gas an see if the problem goes away.
Which is exactly the reason I added a rear sump to my tank and relocated the pickup point.

Holley has some new magic fuel pickup device that replaces the sock on the end of the pickup, and will draw fuel if any part of the mat touches fuel. Sounds like a good idea, but they want about $100 for it. I haven't tried it yet.
I was considering that, but the last time I researched doing it there was some reason I abandoned the idea. The product is called Hydramat. None of the ones I found were a "sock replacement". They all had some sort of different fitting where they attached.

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I mentioned this before, but I had a clogged exhaust where a baffle had come loose and blocked it on single exhaust. The car would idle fine but wouldn't accelerate. Probably not your problem as you have dual exhaust, but worth a mention.
 

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Vacuum leak ,is my first guess ,but that Quardajet probably a biggy , I have 68 Convertible GTO also , I went with a 800 theromquad don't get one ,no low end ,my next time I'm going with 800 or 830 Holley most likely because I'm foot too floor also and some type of torqer intake .I sold mine and bought another 68 Convertible GTO turquoise and pearl interior, frame off resto again .
 

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Another thought: new car reviews of 67 GTO's noted that the (new!) cars went flat at about the top of second gear. But only if the fuel level was below 1/4 tank! Under strong acceleration, the fuel runs to the back of the tank, and away from the fuel pickup. There's enough gas in the carb to get to about 60 MPH. Don't know if this situation applies to 68's, but you might want to test with a full tank of gas an see if the problem goes away.

Holley has some new magic fuel pickup device that replaces the sock on the end of the pickup, and will draw fuel if any part of the mat touches fuel. Sounds like a good idea, but they want about $100 for it. I haven't tried it yet.
No it doesn't apply to 68 GTO Convertible , I ran mine empty or damn close constantly .
 

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