- Joined
- Jul 21, 2012
- Messages
- 3,240
- Reaction score
- 16
- Location
- South Florida
- My Car
- '71 Mustang Mach 1 M-code "Soylent Green"
'69 Plymouth Valiant 100
'68 Plymouth Satellite
Afternoon, all:
I've been fighting a bizarre, completely intermittent rough idle issue on the '78 Mark V, which runs the Ford 400 with Duraspark II. It turns out the rotor centering tab was completely sheared off on the inside - something that could apply to just about any engine with a mechanical distributor (including Ford's 302/351W/351C).
This is what I found inside the distributor - little pieces of the centering tab strewn about. The Duraspark distributor is the same as you'd find in a '71-73 with a 351C, just with a magnetic pickup unit inside it instead of the cam-driven points and condenser, and a wide step-up plate to fit the big cap. In theory, this could happen to a points/condenser car too:
This is (was) the centering tab inside the Standard-branded plastic rotor:
The bad Standard rotor at left, and a replacement at right - which, quite honestly, looks like a really cheap part.
This all seems simple upon reading the solution, but the odd part was trying to diagnose the problem. The engine would idle and run perfectly one moment, then it would begin hesitating out of nowhere - with NO backfire. Fuel delivery was fine, and attempts to force the engine to run in this situation caused one of the catalytic converters to glow red hot from a rich, unburnt mixture of fuel and air.
With the fuel system confirmed as not being a factor, this left the ignition system. The thing is, there's not much to fail intermittently in the ignition system - most of these parts (coil, Duraspark ignition module, magnetic pickup) either work at 100% or fail 100%; though Duraspark is known for heat failure - but the idle problem would crop up either cold or hot, and it could return to a good idle with no explanation at any time.
Of course, the problem was that the rotor was gradually rocking itself back on its shaft, thus retarding timing at will - but short of a malfunctioning vacuum advance with retard built in it (which this car did not have), it didn't make much sense.
My only concern at this point - now that I look at the photo of the dizzy - is that the shaft slot appears to be very sharp - and could have been the cause of the problem. I should note that the distributor cap itself showed no evidence of the rotor tip ever having come in direct contact with it.
Stress fracture from the injection molding process? Who knows? That rotor couldn't have had more than 1,500 miles on it - I remember replacing it about 4 years ago, and the Mark V hasn't seen over 30 miles over the last two years.
-Kurt
I've been fighting a bizarre, completely intermittent rough idle issue on the '78 Mark V, which runs the Ford 400 with Duraspark II. It turns out the rotor centering tab was completely sheared off on the inside - something that could apply to just about any engine with a mechanical distributor (including Ford's 302/351W/351C).
This is what I found inside the distributor - little pieces of the centering tab strewn about. The Duraspark distributor is the same as you'd find in a '71-73 with a 351C, just with a magnetic pickup unit inside it instead of the cam-driven points and condenser, and a wide step-up plate to fit the big cap. In theory, this could happen to a points/condenser car too:
This is (was) the centering tab inside the Standard-branded plastic rotor:
The bad Standard rotor at left, and a replacement at right - which, quite honestly, looks like a really cheap part.
This all seems simple upon reading the solution, but the odd part was trying to diagnose the problem. The engine would idle and run perfectly one moment, then it would begin hesitating out of nowhere - with NO backfire. Fuel delivery was fine, and attempts to force the engine to run in this situation caused one of the catalytic converters to glow red hot from a rich, unburnt mixture of fuel and air.
With the fuel system confirmed as not being a factor, this left the ignition system. The thing is, there's not much to fail intermittently in the ignition system - most of these parts (coil, Duraspark ignition module, magnetic pickup) either work at 100% or fail 100%; though Duraspark is known for heat failure - but the idle problem would crop up either cold or hot, and it could return to a good idle with no explanation at any time.
Of course, the problem was that the rotor was gradually rocking itself back on its shaft, thus retarding timing at will - but short of a malfunctioning vacuum advance with retard built in it (which this car did not have), it didn't make much sense.
My only concern at this point - now that I look at the photo of the dizzy - is that the shaft slot appears to be very sharp - and could have been the cause of the problem. I should note that the distributor cap itself showed no evidence of the rotor tip ever having come in direct contact with it.
Stress fracture from the injection molding process? Who knows? That rotor couldn't have had more than 1,500 miles on it - I remember replacing it about 4 years ago, and the Mark V hasn't seen over 30 miles over the last two years.
-Kurt
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