Help on Vibration/Pulsation Problem

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Specs: 1977 355 motor (1yo) TH350 transimission.

It started abruptly about 2 weeks ago and has become slightly worse.

The car feels like it's running on seven clinders.

It's more pronounced when the engine is under load and in a lower gear. It is most noticable when I have the car in 1st gear then coast in gear from 4000rpm.

I can hear the vibration in the exhaust tone--kind of a rapid burble.

The vibration goes away when I place the car in neutral at speed and coasting.

The car starts and idles smoothly.

I've noticed a drop-off in power.

The vibration is constant at 70mph.

There is no abnormal slippage from the transmission showing up on the tach.


I replaced the plugs, plug wires and distributor cap/rotor to no effect. All plugs looked normal. I set the timing, idle mixture, and secondary air valve. I made sure no plug wires were crossed. Changed transmission fluid and filter. No help/no change/no dice.

Any suggestions? I hate to take it to the shop!

 

[69autoXr]

Sounds to me like maybe a fuel supply issue. I would check into the operation of the pump, fuel pressure, and condition of the fuel lines and filters. Look into the fuel tank with a flashlight and see if there's any obvious contamination.

I had the same problem with mine, it would start and idle fine but as soon as any load was applied it would start to miss terribly. Turns out my brand new fuel filters were clogged with rust particles from the gas tank.

 

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It was the distributor--the pick-up coil fried and melted against the return spring assembly (which then stuck)--amazing it ran at all!

It must have been going bad for a long long time--the power is now tremendous (I have a 355 r/s motor) and the hard hot-start problem is gone now too.

I never new that a weak ignition would cause it to start hard when the engine was warm, but it does.

I installed a Pertronix Distributor and Accel 9000 high-temp ignition wires.