Engine is dieseling badly on shutdown - Seeking advice

Since it's newly rebuilt, that would eliminate carbon buildup on the pistons, etc.  And changing carbs recreates the issue which kind of eliminates improperly adjusted mixture screws.

I assume you are saying the idle is too high which makes it diesel?  and if you lower it, that stops?

You should try cooler sparkplugs

Why do you say 9.5 or 10 ratio? You're not sure of the volume of the combustion chamber?

Look for a vacuum leak.

recheck timing.

It diesels with idle set low, even 800 RPM.

Compression ratio is estimated because of the dished pistons.  The heads are 1970 4V 351C D0AE closed chamber 62.8cc.  I might try Autolite 24 spark plugs.

A vacuum leak is a possible root cause.  The engine idles with 14 inches vacuum.  I worked on another similar engine and it had 17 inches.  I will start looking for a vacuum leak.

Thank you all.

And I will recheck timing.  Thank you.

Turn the idle speed down 800 rpm or less. Be sure the timing is not over 8 degrees advanced. If your cam is such your engine cannot idle under 1000rpm, then switch to fuel injection where you can shut the fuel off or you can install an idle stop solenoid that closes the butterflies upon ignition shutoff.

My hunch is that it's a vacuum leak at the carburetor.  It has the thick insulating gasket and I will put in a regular gasket.

Forest, is this problem typically associated with too much advanced timing or too retarded?

With the engine at idle spray starting fluid around the carb base. If there is a vacuum leak engine rpm will rise. Disconnect or vise grip all vacuum lines to be sure the brake booster line is not the issue.

Time to mention the old potential gremlin of a freewheeling radiator fan turning into a generator after engine shut off.

easiest way to test is next time you shut it off and it starts to diesel hit your brake pedal.

Larry

One alternate hypothesis is the radiator fans can act as generators and cause engine run on.

Updated: looks like Larry posted just before me, so reinforcing the theory...

Low octane fuel and hot engine?

If ignition is off there is no power to make a spark (excepting the rad fan possibility), timing adjustments would be moot, no?

I put 5 gallons 87 into the car once in a pinch, it dieseled so badly when I got home I had to put it in gear and release the clutch to stall it.

5 gallons of 110 race gas fixed it. Never ran 87 again.

This car is a new build and I am trying to get it up on its feet.  I have not yet produced enough heat to get the radiator temperature sensor to engage the fans.  Tomorrow I will focus on vacuum leaks and ignition timing.

For testing I even bypassed the entire wiring harness and hot wired the coil to run the car.  Here is a photo of the temporary ignition switch.  The black wires run from the battery to the coil through the temporary switch.  The factory ignition switch on steering column is still fully functional and usable.

The engine is dieseling because the combustion chamber temperature is too hot, possibles roots causes : bad spark plug timing, too low octane fuel, bad timing (generally too much advance), too hot sparkplug or vacuum leaks.

The another possibility is that the ignition coil is still powered when you turn off the ignition, possibles roots causes: radiator fans running or bad electrical connection (perhaps try to disconnect the orange wire from the starter relay).

Look to the vacuum setup to the power brake booster. The booster itself can have an internal leak or the plumbing  from the carb to it may have developed a leak.

The tube itself is steel and can develop a pinhole from rust where you can't see it or the booster diaphragm itself can be leaking.

As far as the timing causing this, it can be but if the timing light is showing correctly be aware that with the 4v balancers it is VERY COMMON for the balancing ring to have slipped. That can account for the actual timing being retarded while the balancer actually showing that it is ok.

It only needs to be off a few degrees to do this. A 351c set at 0 to 2° can cause this.

The only way you can find this without pulling off the balancer and having that ring position checked is to set the timing up, i.e., advance it and see if the dieseling stops. With an original untouched balancer you have a high likeliness that it has slipped.

You can have them rebuild so no need to panic but it will just be "down" while it is out for a rebuild.

For your information, the 4v balancer that was supplied new on the 351cj engines is not the best balancer to use. The D1ZZ Boss or the D2ZZ HO balancers would be.

The difference between the two would be the factory balance on them. The D1 is matched to the factory domed pistons while the D2 is for the dished pistons. You can run either together since the difference in balance is not something you normally will see until high rpm's.

The D1's are expensive and rare but the D2's occasionally come up used for sale. There are more 351 HO's out there then you would expect.

I assume you are not actually driving the car so the fans are not spinning so you can eliminate that.   The 14" of vacuum seems low.  Is the distributor advance plumbed to ported or full vacuum?  I believe it should be full vacuum, no?

The balancer is aftermarket and brand new.  We checked that the TDC marking on the balancer matched to cylinder #1 at the top of its stroke.

The car is being driven but only for a short distance while sorting things out.  The distance is not long enough to get the fans going.  When the car is stationary I can run it with my hot wire setup and the ignition switch off.  This helps to eliminate electrical issues as the source of the dieseling.  

Vacuum advance is plumbed to the full vacuum port on the Holley carb.

Within a few hours I should be able to try a new carb gasket and/or find a vacuum leak.  

I should also add that the brake booster is not fully plumbed yet and its intake manifold port is plugged off.

is the vacuum you are pulling a steady constant value or is the needle fluctuating?