Hesitated at highway speed - Vintage Mustang Forums
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post #1 of 6 (permalink) Old 09-13-2019, 03:29 PM Thread Starter
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Hesitated at highway speed

So I have found some posts on similar issues but not exactly what mine does. 289 2 barrel carb... car drives fine for about 20 min at highway speed. Then starts to hestitate badly and eventually stall altogether. I have to sit on the side of the road for 3-4 min and then it starts back up and will drive again for another 5 min or so and then repeat the whole process. Other similar posts in here seem to say it happens when they get to highway speed. It in my case it only happens after having been going that speed for a while. It accelerated hard when I step on it so fuel doesn’t seem to be an issue. New ignition coil, plugs, and wires. Has an electronic ignition that wasn’t done by me so maybe that? Timing has been an ongoing source of confusion for me. When I time it to factory specs the car runs very poorly... really lumpy. The more I advanced it the better it runs. At about 25 degrees it runs super smooth but i know that’s way too advanced. Any help is greatly appreciated.
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post #2 of 6 (permalink) Old 09-13-2019, 11:11 PM
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The next time it does it, run to the back and loosen the fuel cap. If you get a big "whoosh" of vacuum you'll know the vent in the cap is plugged....

Other potential issues... whatever electronic (solid state) ignition is installed....
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post #3 of 6 (permalink) Old 09-14-2019, 12:48 AM
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When it fails, pull the coil wire and verify spark. If you have it there pull a plug wire and verify spark. Actually had a cracked rotor that would open up and kill spark out of the cap when hot. Spark from coil but not to the plugs. Cool down and it would restart.
If you have spark and no start then you have isolated it to a fuel delivery issue. If that is the problem then +1 on a bad fuel cap as that is a prime candidate for your symptoms. Loosen and drive to see if that is it, but I would try to recreate the problem and check spark first. Good detective work...
On your timing issue, sounds like your marks might be incorrect.
Verify tdc by checking to see #1 piston all the way up with a screwdriver in the plug hole and see if your pointer really is on tdc. You might have a timing cover mismatched to the balancer. You are checking the base timing with the vacuum hose unhooked, right?

Last edited by Matter; 09-14-2019 at 12:58 AM.
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post #4 of 6 (permalink) Old 09-14-2019, 02:49 AM
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Originally Posted by Woodchuck View Post
The next time it does it, run to the back and loosen the fuel cap. If you get a big "whoosh" of vacuum you'll know the vent in the cap is plugged....

Other potential issues... whatever electronic (solid state) ignition is installed....
this is a possibility, more likely however i think you might have a vapor lock issue. check where the fuel line runs. i fit is within 1/2 of the exhause system, then you need to move it, or install an electric pump in the back near the tank.

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post #5 of 6 (permalink) Old 09-14-2019, 08:52 AM
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Also check that your choke is disengaging. I have a 2V 289 that likes to stay choked for a while, the spring housing may need adjusting. This will make my car sputter at highway speeds until I’m warmed up, because it’s not allowing enough air in. Changes in ambient temperature require that to be adjusted so the choke just closes when cold. Also, if your heater hose is not running adjacent to the spring housing, it may not be warming that up as fast as intended. The steel lines that go to the exhaust manifold also help control this. Once you stop and wait, you might be heat soaking enough to finally open it up. Just another idea.
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post #6 of 6 (permalink) Old 09-15-2019, 11:13 AM Thread Starter
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I’m going to take a serious look at all the suggestions. Obviously the challenge being I have to drive it at highway speed for quite a while before I can replicate the problem and then I am diagnosing along the side of the highway... I did notice today that the file line running out of the pump and up to the fuel filter was touching the block... could that be vaporizing the fuel when the block gets hot? I don’t even know if such a thing is possible...
as far as timing I took out the spark plug from cylinder 1 and it feels like it is tdc at the timing mark. Assuming I am doing it right. And yes I checked timing with the vacuum advance disconnected. Question... where should the vacuum advance attach to the engine. Mine goes into a spot below the choke between the carb and the manifold. I was told by a friend that I should connect it to the outlet at the back of the engine that feeds the rest of the accessories. Oddly very different vacuum pressure. Basically none at idle from the carb and around 20 at the other spot.
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