'66 Having trouble at high speeds

JohnHunt · 07-25-2009, 05:02 PM

Hey there, I have a straight six that has been having some rather odd issues of late. My car has been coughing and dying on me when I give her some hard gas on the highway. This is a new issue, and it has never happened before. The coughing will go away for a bit before coming back stronger. I would pull off the road and it would sound as though she is out of gas and dies. When I wait for a about 5 mins, I can start her back up but can't drive far without it doing the same thing.

I figured it was something to do with a vacuum leak - turned out the vac hose from the carb to the distributor vac advance was completely oxidized. Naturally, I replaced it and she ran great for about 2 days before the same issue started again. The metal tube from the vac advance was loose and I noticed it wasn't crimping properly. I flayed out the end of the metal tube to give it a better setting and the car ran ran great again for about 2 days.

I am really now at a loss as to what could be the problem.
I can run her just fine if I don't push it hard around city streets but, if I take it out on a country road for a few miles (run it up to about 50 - 70 mph), it coughs like its striving for gas again and I have to ease way up on it to keep it alive if it can. I thought it could be a temp issue (due to it being very hot days this summer), but Im surely not ruling out carb adjustments and vacuum leaks.

Any help or ideas will be greatly appreciated.

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Mesomorphman · 07-25-2009, 05:12 PM

My car is doing the EXACT same thing. I tested the fuel pump output by putting the hose from the carburetor into a glass and cranking the engine for 5 seconds. I got about 1 ounce of gas. I was told by another member that a cup of gas is more appropriate. So I'm going to attempt to replace my fuel pump today. I'll let you know how that works out. Any help would be appreciated, though, as I am far from an expert.

67mustang302 · 07-25-2009, 05:25 PM

Bad fuel pump, bad fuel filter, trash in the carb, plugs, wires etc.

JohnHunt · 07-25-2009, 08:45 PM

It is definitely not the fuel filters (for I have more than one), I cleaned the carb just last week and inspected it yesterday - no junk or buildup, plugs are clean, and the wires are fresh with solid connections. The fuel pump however could be checked out... I'll get to that shortly and let you know.

JohnHunt · 08-04-2009, 05:35 PM

The fuel pump is not the problem. Had it taken to a shop and checked out. I changed the vacuum tube running from the carb to the vacuum adv. just in case there was a leak somewhere in there - but was met with no success. I'm pretty stumped right about now.

blue66mustang · 08-05-2009, 09:11 AM

check for junk in the fuel tank that may be cloging the pickup. It may get picked up and starve the pump and fall off after a few min of sitting.

fakesnakes · 08-05-2009, 09:45 AM

Blue is exactly right. A mechanical fuel pump works harder at higher engine rpm. Junk (rust, dirt, etc.) in the fuel tank gets sucked up against the pick up sock as suction increases and time passes. When you first start the car, it runs fine. Then as the crap in the bottom of the tank gets sucked up against the sock, performance starts to fall off as the engine starves for fuel. Once you shut it off, all the crap falls to the bottom again and the whole cycle starts again. Clean out or replace your tank if everything else seems fine.

JohnHunt · 11-05-2009, 05:22 PM

So I pulled off the fuel tank and opened it up. I flushed it out and filtered the gas that was in it. Everything was totally clean - even the pickup. I cleaned it anyways for good measure and put the tank back in. The only thing I changed out was a 5" fuel hose coming from the sending unit to the metal fuel lines leading to the engine. The old fuel hose was in pretty bad shape, so I figured maybe when the fuel pump got going and created a stronger vacuum in the line, it was causing that hose to collapse thus starving the engine.
It ran great again for a few days, but I just recently made a long trip and noticed that at around 65 mph, she started giving me small bumps - or just the speed wasn't smooth. About an hour later, I notice it getting slightly worse until the point of a full blown cough and backfire pop. So I dropped the speed down to about 55; which lessened the stuttering and made it home.
Any ideas?

67mustang302 · 11-05-2009, 06:32 PM

Check everything in the ignition system. It could be something as simple as a bad electrical connection to the coil.