[Mgs] Stumbling / stalling ignition issue?

[David Ambrose]

On 4/17/2010 2:28 PM, Paul Root wrote:
> Hi all,
> 	I just got back from a trip to and from work. Less than 20 miles each way. At
> about 10 miles each way, I was getting some stumbling.
>
> 	The terrain varied, slight incline/decline or flat. Speed was constant,
> sometimes a lift off the throttle for traffic.
>
> 	The symptoms were 3000-3500 rpm, the tach would bounce down to  1500-2000,
> and then back up. A couple of times it dropped to 1000.
>
> 	It happened 3 times in a row on the trip home, within about 3-4 minutes, so I
> was ready and paying attention. The ignition light did not come on, ever.
>
> 	I decided after that to get off the highway and take surface roads the last 4
> or so miles. And I got another stall at 2000-2500 rpm. I downshifted to 3rd
> and the car restarted.
>
> 	At a stoplight, the car did stall. It took 20-30 seconds of cranking to
> start. Normally, the car starts within 10 seconds cold, and 2-3 seconds warm.
>
> 	I got home without further trouble.
>
> 	The car is a 1977 MGB, I have a pertronix distributor, that I put on new last
> year. A pertronix coil, that's 8-9 years old. Ignition wires are 8-9 years
> old. Dual SU (HIF) conversion.
> Alternator replaced to summers ago.
>
> 	I'm in the middle of my second tank of gas for the season, I did put in a can
> of heat. The oil is fresh, changed last weekend.   Oh, as a side note. I used
> NAPA high milage oil. It's made by Ashland, which is Valvoline. Anyway, it was
> rated SL/SJ. Not SM. Napa Gold filter.
>
> 	I recently put in a plumbing ball valve in place of the stock heater valve. I
> don't see any leaking there, but I'll check it again. I do have to pull it
> apart, I put the valve in backward for the cable to work.
>
>
> 	I'm going to go and do a fuel delivery test (1 imp qt per minute), and look
> things up on Paul Hunt's web site. Oh, and check the rotor and cap.
>
> 	What else?
>    
I think I've seen this before.....

The bouncing tachometer points to a problem in the low tension circuit. 
I would look for a loose or poor connection between the distributor and 
the coil, or between the coil and the rest of the car. Make sure the 
connections are clean and tight. This goes double for any crimped 
connectors in the wiring harness. Make sure the connector to the 
distributor is tight.

You may be able to diagnose the problem by connecting a test light to 
the distributor and wiggling wires.

Cheers,
Dave Ambrose