Guys the 540 in my ragtop feels strong but when I shut it down it runs on and pukes out the carb. What causes this and how do I correct it. Its running around 195 to 205 in this heat up in jersey. I run 12 to 14 initial timing and 38 total. Is the run on or post ignition bad for the engine. help me understand this, also its a 9.8 to 1 running on what I believe is 92/93 octane.

Thanks Rich

Sounds like you are talking about "dieseling" after the motor is shut off? If that is the case, your throttle plates on you carb are probably open too far at idle. I had the same problem with my 489. Just advanced the timing a little more and was able to turn down the throttle adjustment on the carb and that took care of it. 12-14 intial timing is a little low I think. Mine runs best with 20 intial, some are even over that. Try advancing you timing to maybe 16-18 initial. Also running vac advance to PORTED MANIFOLD vacuum will allow you to cut down on your carb idle speed as well.

I would just try 36* total timing and see how that works. Your initial will change as you set the total. Probably around 12*.

His total is already 38. If he retards to 36, his initial will be decreased to 10-12, which will cause him to have to increase his idle screw to keep his idle speed up, opening the throttle plates farther, making his dieseling worse. I think he needs to increase his inital as stated earlier and adjust his mechanical advance to stay a total of 40 or less. I know the BBCs like alot of advance. Mine seems to run even better at 22-24 initial with total of 42-44 without detonation, but I'm to chicken to keep it there! I'd bet his performance would pick up a little too and his engine may run a bit cooler as well:thumbsup:

BB489-I think you may right about the initial timing. My problem is that if I try to advance the initial timing my total timing goes into the 40's and I dont want go there. Im using a pro billit msd distributor and havent figured out which bushing and springs will give me some more initial timing. My idle is at 1100 rpm if I can get it down to 900 Maybe that will stop the dieseling. What are the disadvantages of running 40+ degrees of timing?

Pinging,detonation, lack of performance and and broken pistons.

Lower your idle.

BB489-Im using a pro billit msd distributor and havent figured out which bushing and springs will give me some more initial timing. My idle is at 1100 rpm if I can get it down to 900 Maybe that will stop the dieseling. What are the disadvantages of running 40+ degrees of timing?

Use the black bushing supplied with your MSD distibutor and the lightest silver springs. you should be able to have 25+ initial degrees and total around 38-40. Do you have a domed piston in that motor?? If you do you will need to run 38-40 total. if you have flattops the motor will most likely like no more than 36-38. Every motor is different but big blocks tend to like a lot of timing.the only way to tell is keep turning that distributor !!!! when you go to far........IT WONT CRANK !!!!!!!!

if it is real bad, double check for correct plug wires to cylinder!

also, if you have MSD unit, trickle voltage going back to unit can cause partial engine firing after shut down from voltage reg charging lamp...
this solved mine awhile back.....
From MSD install manual...
"To solve the Run-On problem, a Diode is supplied with the MSD in the
parts bag. By installing this Diode in-line of the wire that goes to the Charging indicator, the voltage is kept from entering the MSD. Figure 5 shows the proper installation for early Ford and GM vehicles.
Note: Diodes are used to allow voltage to flow only one way. Make sure the Diode is installed facing the proper direction (as shown in Figure 5).
GM: Install the Diode in-line to the wire going to terminal "4".
Figure 5 Installing the Diode to a GM or Ford Vehicle.
GM 1973 - 1983 with Delcotron Alternators
GM Delcotron Alternators use an internal voltage regulator. Install the Diode in-line on the smallest wire exiting the alternator (Figure 7). It is usually a Brown wire. Most other applications: On other applications where engine Run-On is experienced, a Resistor can be put in-line to the MSD's small Red wire. This resistor will keep voltage from leaking through to the MSD unit."

Have you totally uncovered the transfer slot on the Holley with your idle adjustment set high if so it is time to bore some small holes in the throttle plates.

If your idle circuit and off idle transition are ok then the next set is a A/C throttle solenoid that is activated by the ignition. When you turn the ignition off the throttle bores close completely and avoid the dieseling problem by cutting off the air supply.


Larger Dave

Thanks for all the replies, I really appreciate it. The problem is not that bad, the post ignition lasts like 2-3 sec. I think I need to start with just lowering the idle from the linkage. It would be nice to get a couple more degrees of initial timing and then back the curb idle down so the transition slots are visible then adjust with the 4 corners. perhaps I will try playing with the bushings in the distributor.

Thanks again for all the ideas- Rich