New crate motor dieseling

[need-for-speed]

"A lot" if the time, what you're experiencing is due to the timing being too low...which will make the idle very low. Instead of correcting the lpw idle by adjusting the timing, some folks will simply turn the idle screw up on the carb....which basically dumps more gas. So when the engine gets cut off, the carburetor is still sending gas down the hole and as it enters the combustion chambers, the heat keeps igniting it causing run-on or "dieseling". Start with your timing 1st and foremost.

^^ THIS.

Back in the 70s and 80s when I was working on my 350 Chevy’s, (without Internet help) I always noticed how much better the engine idled when I advanced the timing beyond the 8° recommended by the factory. I remember thinking that I was doing something wrong. Apparently I was not lol.

Who would’ve thought 40 years later it’s common practice to run a higher i.e. 12 to 14° base timing? When setting the timing at the crappy recommended level of 8° the only way to compensate is with increasing the idle screw so far that the engine can still suck fuel past the butterflies without you depressing the throttle, and hence dieseling.

OP, what’s your initial timing set at, and is your vac advance connected to manifold or ported vacuum?

 

[kdsracing]

I went through this last year after I put my AFR heads, cam and an Edelbrock 650 carb on my car. It would diesel almost every time that I shut it off. I always ran ported vacuum to the distributor. I now run manifold vacuum to the distributor. it never diesels and the idle is much stronger and smoother.
Timing the engine:
Warm the engine
Disconnect vacuum line and plug.
Set timing to desired setting (I am running about 15 degrees initial)
Plug the vacuum line into the distributor to manifold vacuum. (RPM will increase)
Turn your idle down to where you want it.
This worked for me.

Think about what you are doing when you use manifold vacuum to the distributor. When you open the throttle, manifold vacuum drops thereby taking timing away from the engine when it needs it to make power and accelerate. When you close the throttle, you advance the timing when the engine doesn’t need it. With ported vacuum, timing increases as Venturi velocity increases, thus allowing the engine to produce power and torque. When you close the throttle, ported vacuum drops. The engine doesn’t need advanced timing at this time because it is decelerating. Does this not make sense?

 

[kdsracing]

Think about what you are doing when you use manifold vacuum to the distributor. When you open the throttle, manifold vacuum drops thereby taking timing away from the engine when it needs it to make power and accelerate. When you close the throttle, you advance the timing when the engine doesn’t need it. With ported vacuum, timing increases as Venturi velocity increases, thus allowing the engine to produce power and torque. When you close the throttle, ported vacuum drops. The engine doesn’t need advanced timing at this time because it is decelerating. Does this not make sense?

Engine run-on is caused by idle speed being too high. The cure is in correcting the idle speed, not timing, not plug temperature provided you are running the correct plug and not waaay too hot of a plug. If the throttle is closed sufficiently when the engine is turned off, it will not have run-on.

 

[mallard]

Engine run-on is caused by idle speed being too high. The cure is in correcting the idle speed, not timing, not plug temperature provided you are running the correct plug and not waaay too hot of a plug. If the throttle is closed sufficiently when the engine is turned off, it will not have run-on.

Solid post. I was a mechanic back in the day and another issue was cars with carbon buildup in the combustion chamber causing “hot spots” leading to the dieseling from the increased chamber temps. It also was synonymous with pinging issues on acceleration too.

 

[Sbcking]

Long ago I was told that 90% of running issues are caused by the ignition system. But that was before computers and fuel injection systems. So it probably was something to do with residual energy in the system itself even though the key is off. However it is true that an engine might run on if it is hot enough to fire up any fuel that is still being sucked in by the natural aspiration of a gas engine. So if the lower idle doesn’t resolve the problem, then just try unplugging the alternator plug and see if the engine will still run on when you turn the key off

 

[kdsracing]

So I was installing a remanufactured 350 VC111P crate motor on my 1979 Camaro and I use to have a 305 with a edelbrock intake 2701 and 1405 carburetor and I reused alot of the stuff because it was all a little over a year old parts well come to my suprise the engine is dieseling I'm about 160 miles on the engine and I've been trying to figure this out the car drives really nicely so I feel like the timing is good but it still has this issue every so often it doesn't happen all the time after I changed out the ignition coil old coil it happened all the time on shut off now it only happens maybe 1 in every 4 shut offs of course input would be nice these forms never let me down on good info.

Long ago I was told that 90% of running issues are caused by the ignition system. But that was before computers and fuel injection systems. So it probably was something to do with residual energy in the system itself even though the key is off. However it is true that an engine might run on if it is hot enough to fire up any fuel that is still being sucked in by the natural aspiration of a gas engine. So if the lower idle doesn’t resolve the problem, then just try unplugging the alternator plug and see if the engine will still run on when you turn the key off

There is no “residual” energy after the key is turned off unless the charging system is malfunctioning. Once the key is off there is no power going to the charging system, no energy can be produced to feed the ignition.

 

[ZZ430]

Engine run-on is caused by idle speed being too high. The cure is in correcting the idle speed, not timing, not plug temperature provided you are running the correct plug and not waaay too hot of a plug. If the throttle is closed sufficiently when the engine is turned off, it will not have run-on.

^^ This is true, great post.

Everything else is fun to talk about, but the throttle blades being open too far at shutdown is what's happening.

 

[GoldenOne7710]

you don’t set idle speed with timing. You set the timing based on what the engine wants to run well, then set idle speed at the carburetor. Run-on is corrected with idle speed, not timing, not spark plugs.

You know that and I know that...but not everybody knows that. You sound like you're trying to "correct" me....and if so, you misunderstood what I'm saying.

Never assume the timing is correct without checking it with a light. As I and a couple other pointed out, some people try to correct a low idle situation that is often caused by low initial timing. Some folks who don't know to check the ignition timing will attempt to correct the low idle by turning the idle screw on the carb. What does turning the idle screw do? It situates the throttle plates from being fully closed to not full closed. So turning the idle speed TOO MUCH up at the carb, it will allow enough air and fuel down into the cylinders and the heat in the combustion chamber will continue to ignite the fuel...even with the ignition off...causing dieseling.

I can't tell you how many cars we've "fixed" that would run-on (or diesel) when the key is turned off by simply raising the timing to the correct setting (with a light)...which WILL raise the idle speed...then turn the idle screw on the carb to close the throttle plates and it idle it back down. Diesel/run-on was immediately corrected with zero other changes and the vehicle always ran stronger afterwards.

 

[need-for-speed]

You know that and I know that...but not everybody knows that. You sound like you're trying to "correct" me....and if so, you misunderstood what I'm saying.

Never assume the timing is correct without checking it with a light. As I and a couple other pointed out, some people try to correct a low idle situation that is often caused by low initial timing. Some folks who don't know to check the ignition timing will attempt to correct the low idle by turning the idle screw on the carb. What does turning the idle screw do? It situates the throttle plates from being fully closed to not full closed. So turning the idle speed TOO MUCH up at the carb, it will allow enough air and fuel down into the cylinders and the heat in the combustion chamber will continue to ignite the fuel...even with the ignition off...causing dieseling.

I can't tell you how many cars we've "fixed" that would run-on (or diesel) when the key is turned off by simply raising the timing to the correct setting (with a light)...which WILL raise the idle speed...then turn the idle screw on the carb to close the throttle plates and it idle it back down. Diesel/run-on was immediately corrected with zero other changes and the vehicle always ran stronger afterwards.

That’s how I read your previous post.

 

[2ndGenCrazy]

Ignition Timing is the most misunderstood thing across the board for hot rodders.
This has been discussed many times on this board.

Minimum spark advance to achieve maximum manifold vacuum at desired idle setpoint.

There is a reason that advancing the idle spark setpoint has eliminated the dieseling on the cars mentioned in this post.