Running on

[bigmal 225]

Hi guys, twice in the last week I have turned the engine off and it ran on for a few seconds like an old clapper with coked up cylinders. It's never done this before.

Engine is recently (6 months ago - 500 miles) rebuilt 351C with 10:1 and 600 Holley. No carbon build up.

The only thing that is different is I recently changed from 98 to 95 unleaded at a suggestion on another forum.

Engine temp was sitting just a little higher than previously (middle of the gauge) as I just fitted thermos fans and this is where they cycle.

It has me puzzled. Any suggestions?

[Brian Conway 264]

May want to retard the timing a degree ?  Brian

[bigmal 225]

Just now, Brian Conway said:

May want to retard the timing a degree ?  Brian

I have it set at 7 deg. I recently reduced it from 9 due pinging when hot. I'm not sure how this would be a factor when the ignition is turned off?

[RPM 1,190]

Ya gotta make sure the butterflies are adjusted so the slotted port forms a square. Carb needs to be off to check under there. Then adjust your idle with the idle jets. If you use the butterfly screw to set idle, it allows too much fuel in after the engine is shut off, causing run on, or dieseling. 

If you're running an auto trans, you can turn of the ignition while in gear to keep it from running on.

[LoneWolf2U 136]

Do you have the right thermostat and bypass restrictor in place?

A improper stat or missing plate will allow head temps to be much higher and cause fuel to flash when turning engine off.

[bigmal 225]

22 minutes ago, RPM said:

Ya gotta make sure the butterflies are adjusted so the slotted port forms a square. Carb needs to be off to check under there. Then adjust your idle with the idle jets. If you use the butterfly screw to set idle, it allows too much fuel in after the engine is shut off, causing run on, or dieseling. 

If you're running an auto trans, you can turn of the ignition while in gear to keep it from running on.

Thanks Bob, this has just started since I adjusted the idle so this could be it.

[bigmal 225]

2 minutes ago, Mach1Rider said:

Do you have the right thermostat and bypass restrictor in place?

A improper stat or missing plate will allow head temps to be much higher and cause fuel to flash when turning engine off.

When I rebuilt the engine I replaced both to make sure they are correct so they should be ok. Thanks

[LoneWolf2U 136]

Just asking to insure you used the C stat they are made for the Cleveland only with lower hat to close off the bypass hole.

[bigmal 225]

5 minutes ago, Mach1Rider said:

Just asking to insure you used the C stat they are made for the Cleveland only with lower hat to close off the bypass hole.

I was made aware of this before the rebuild so made sure I got the correct Cleveland stat and plate. Thanks.

[LoneWolf2U 136]

Then recheck your float levels and blade positions for fuel leaking into intake.

 

[1969_Mach1 333]

I'm a little confused as to why it was pinging in the first place with either 98 or 95 octane fuel and only 10:1 static compression ratio.  Is the motor running too lean with that 600 cfm holley carb? Or running too hot?

[jmlay 80]

Do you have a wiring diagram depicting how your fans are wired? Is a diode in place to prevent reverse current when the fans are costing?

[bigmal 225]

1 hour ago, 1969_Mach1 said:

I'm a little confused as to why it was pinging in the first place with either 98 or 95 octane fuel and only 10:1 static compression ratio.  Is the motor running too lean with that 600 cfm holley carb? Or running too hot?

It's quite possible it is jetted too lean. I need to check that as it was pinging before the rebuild and change from 4V to 2V heads. Thanks

[bigmal 225]

2 minutes ago, jmlay said:

Do you have a wiring diagram depicting how your fans are wired? Is a diode in place to prevent reverse current when the fans are costing?

It is currently wired through a conventional relay and temp switch. Interesting you bring this up as I have noticed that at 60 to 70 mph on the freeway the temp starts to rise now I have the thermos fans. I was thinking that they could be restricting airflow at these speeds. I was going to put a manual off switch in temporarily to see if turning the fans off on the highway reduces the temp. If it does I was going to fit a airflow switch in the circuit.

[barnett468 418]

7 hours ago, bigmal said:

I have it set at 7 deg. I recently reduced it from 9 due pinging when hot. I'm not sure how this would be a factor when the ignition is turned off?

it actually is a factor, but it's complicated to explain.

 

also, you may benefit from more idle timing but less total timing. its called "setting the advance curve".

 

 

[barnett468 418]

8 hours ago, bigmal said:

The only thing that is different is I recently changed from 98 to 95 unleaded at a suggestion on another forum.

Engine temp was sitting just a little higher than previously (middle of the gauge) as I just fitted thermos fans and this is where they cycle.

It has me puzzled. Any suggestions?

yeah, put the same brand of 98 in it and see if it stops.

[barnett468 418]

8 hours ago, bigmal said:

Engine temp was sitting just a little higher than previously (middle of the gauge) as I just fitted thermos fans and this is where they cycle.

 

Never, ever, rely on the gauge unless you have compared it with an infra red gun.

what is the temp in farenheit?

 

[bigmal 225]

Thanks guys. Some great ideas where to look. I will hopefully try them in t he next week or two and give an update.

[jmlay 80]

I do not know if the lack of diode is your issue but it certainly can cause fun on. Turn the fans off and stop them from rotating the shut the engine off. If that helps take a look at adding a diode to the current wording, assuming one does not exist today. In the end it may not be related but it is certainly something easily eliminated.:

 

https://electronics.stackexchange.com/questions/284090/the-concept-of-protection-diodes-across-a-fan-motor

 

https://www.jalopyjournal.com/forum/threads/electrical-problem-electric-fan-is-causing-a-slow-engine-shut-off.1073169/

 

[barnett468 418]

42 minutes ago, jmlay said:

I do not know if the lack of diode is your issue but it certainly can cause fun on. Turn the fans off and stop them from rotating the shut the engine off. If that helps take a look at adding a diode to the current wording, assuming one does not exist today. In the end it may not be related but it is certainly something easily eliminated.:

 

https://electronics.stackexchange.com/questions/284090/the-concept-of-protection-diodes-across-a-fan-motor

 

https://www.jalopyjournal.com/forum/threads/electrical-problem-electric-fan-is-causing-a-slow-engine-shut-off.1073169/

 

That's interesting and good info but it won't cause "dieseling" which is also sometimes referred to as "run on".

 

 

[Vicfreg 771]

RPM is right. The carb idle adjustment using the "curb idle" screw can expose  the "transfer slots" on the primary butterflies.  The only proper way to adjust the idle is with a vacuum gauge.

Hook a vacuum gauge to the manifold (full vacuum) port on the holley.   Get the car up to operating temperature.  If you have a traditional 600 CFM Holley 4150, there is an idle mixture screw on each side of the front bowl.  If you have a later model performance Holley, like an Ultra Street Avenger , there are 2 additional screws on the sides of the rear bowls.

Once the car is heated up, start with the drivers side (just preference) idle mixture screw, and turn it clockwise to see which way the vacuum needle moves.  The idea is to SLOWLY turn the idle mixture screw to obtain the maximum vacuum possible.  It is an iterative process, and once you get to the vacuum "peak", back off slightly.   

Then go to idle mixture screw on the opposite side and repeat the process.   

When you have adjusted both sides to obtain the maximum vacuum, then go back over to the curb idle screw and you will likely find that you can adjust it down to the point where the primary throttle plates are almost "flat".  Adjust to the curb idle specs for your car, and you are done.  (for automatics, get an assistant, and set the curb idle with the brake on and the car in drive.)

When RPM referred to "square", what he is referring to is the transfer slot will appear square at the bottom of the primary butterflies.  You can only see this if you flip the carb over.  

Not having it square can lead to an off idle stumble, or running on after shut off.

Hope this helps.....

First pic is the idle mixture screw

Second is the transfer slot

 

[Vicfreg 771]

Oh to be clear, on some  carbs the clockwise adjustment will decrease vacuum, on others it will increase.  So, the initial adjustment is just to determine the convention for your carb.

[bigmal 225]

20 hours ago, Vicfreg said:

Oh to be clear, on some  carbs the clockwise adjustment will decrease vacuum, on others it will increase.  So, the initial adjustment is just to determine the convention for your carb.

Thanks for this. Very helpful stuff and I will save this for later. As I am starting to suspect the correct main jets were not fitted when I got the car. I have booked it into a local shop on Monday to get the carby set up properly and also put on the dyno.

I'm interested to see what the output is since the rebuild.

[barnett468 418]

2 hours ago, bigmal said:

Thanks for this. Very helpful stuff and I will save this for later. As I am starting to suspect the correct main jets were not fitted when I got the car. I have booked it into a local shop on Monday to get the carby set up properly and also put on the dyno.

I'm interested to see what the output is since the rebuild.

The main jets have zero to do with it dieseling.

[bigmal 225]

18 minutes ago, barnett468 said:

The main jets have zero to do with it dieseling.

Yep, understand that. More about setting the carby up correctly to start with. Thanks