Has anyone had this issue with their 4.2? Common?

 

What fuel? How recent has timing and tune been checked. 4.2s usually do not ping unless out of adjustment.

 

Its not pinging but over-running/dieseling. Once you shut off the ignition the motor will keep running unless you put it in gear and step on the brake before shutting it off. Wondering if its common with the 4.2 Jags

 

Dieseling is not common. Mistune, dirt in fuel system, plugs, settings are out of wack. Idle could be set wrong. Dieseling is worse than pinging, as it loads cylinders on shut down. Timing off for fuel used. If using 87 or 89, not enough for this older car. Ethanol could be a problem, use 93 Octane. If this solves problem, reset timing.

 

Do you have the stock '70 vacuum retard unit? If so, try capping the vacuum system, and advancing the idle advance to 10 degrees BTDC...(Stock is 5 with the retard) .Not difficult...Those retard units "suck" IMHO, and were more aggravation than they were worth...Just used to meet the "new" emission requirements.
Edward

 

How about 91? 93 is pretty rare here in California.

 

Whats that look like?

 

It looks pretty much like the standard unit...Only the numbers on the unit will tell you....See if you can see the distributor number. If I remember right , it's a different # than earlier years...At any rate, if the vacuum is hooked up, cap it at both ends...Start the engine and let it warm up. Then check the idle advance...Set it to 10 degrees, (you can even go to 12), and see if that cures your problem.
Edward

 

The diaphragms in the unit get old and need replacing. Can be bought and DIY. 91R will be fine.

 

If it's the vacuum retard unit, I'm pretty sure that it is permanently sealed.....
It's a terrible dizzy, and works ten times better with just the centrifigal advance.
Edward

 

Hello Edward,
The Retard Unit can be taken apart by carefully levering up the crimped down edge. However, for what they're worth, I rather replace the device with a new unit. This device is used for the purpose of emission control during idling and when the accelerator is backed off rapidly. Its more of a vacuum switch, rather than aiding in the mapping of the Advance Curve. When at idle, or in a rapid, foot off the accelerator pedal situation, the vacuum is at its greatest and therefore, the Retard Device is at its full stroke. The engine will be slightly more responsive with it disabled, but there exist a greater potential for back firing in the mufflers.

Regards,

Brent

 

Brent
I can only go by my own experience with the retard unit...Not wanting to spend $400 on a NOS retard unit , for a dizzy that I wasn't sure of, I just disconnected the unit...In addition, I removed the secondary throttle plates and rods...The difference in performance has been spectacular....Before I would put the retard unit back together, I would buy an older 4.2 dizzy , with a vacuum advance....
I have had no backfiring as a result of the changes.
Where I live, there are no emission requirements for cars of this age, so going back to what was a poor system to begin with, would not make any sense.
Edward

 

Hello Edward,
If its not working, we only reinstate if the client insists and we don't pay anything like $400.00 for replacements. The backfiring issue is a potential one and seems to happen more with V12s. Its not a big backfire, more of a rumble during rapid deceleration.

Regards,

Brent