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I have a 68 Mustang straight 6 1 BBL carb, 3 speed manual.

Noticed the other day that when driving at constant speed that the car hesitates (like I am letting off and pressing down on the gas pedal). It accelerates just fine.

No RPM gauge in car. When ever it is a constant speed, through city streets doing 35 or on the highway doing 60.

So, being an old car - where are the most likely problems areas - fuel pump? Coil? improper point gap? vacuum?

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Have you checked your spark plugs yet? –  Tim Aug 29 '12 at 14:55
    
We put new ones in last year, but good point, need to check the gap. This issue just started to occur in the last month. –  boyd4715 Aug 29 '12 at 15:01
    
I had something very similar happen and it turned out one of my spark plugs had a hairline crack in it. –  Tim Aug 29 '12 at 18:09
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2 Answers

My guess is fuel, try running a can of fuel system cleaner through it. A good spray of carb cleaner wouldn't hurt either. I'm guessing you've got a gunked-up jet in the carb that isn't giving a good spray pattern. It could also be a sticking needle valve that's keeping your float bowl from working properly, but I'd expect you'd have a problem when trying to accelerate too if that were the case.

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The carb was rebuilt in the spring, so I don't think that would be it. I don't drive it a lot, maybe once a week, so there could be some gumming of the needle value. –  boyd4715 Sep 4 '12 at 19:10
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Points would be the first thing I would check. It sounds like the ignition is breaking down i.e. not enough spark getting to the plugs. The next thing to check is the wires. If you have access to an ignition scope or a Kv meter it can make the diagnosis easier.

Pulling the plugs and inspecting them can give you more clues see my answer here for help on diagnosing based on the plugs

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