run on

havoc_squad

Senior Chief Petty Officer
Joined
Mar 5, 2011
Messages
694
For the amount of carb diagnosing I did in the past on the 3.0 IL4, the three biggest contributors the engine run on was shutting off engine too quickly, timing too high, and idle circuit too lean.

Unless it's fuel injected or must use a sealed idle fuel mixure to pass emissions, a very slight rich idle mixture might not be a bad thing.

Gas vapor ignition without spark definitely hints of a lean fuel mixture at idle or engine temp too high for the fuel into the combustion chamber.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
There was never a true "Weber" carb..it was either a rebranded Q'jet or AFB/Edelbrock. Seems the later engines used a "swirl port" head with a ramp in the intake port. Has to be a mechanical/temperature issue to be auto-ignition.

The Weber 9600 series and the Edelbrock 1409 are copies of the original Carter AFB (which is just short for Aluminium Four Barrel)... Completely different to the Rochester Quadrajet...

Chris...
 

Joecef

Cadet
Joined
May 14, 2018
Messages
20
So basically if my timing is okay, my plugs are good and my air and fuel mixture is good, the only way to prevent/fix this is just to let it idle for a min or less after a run to burn of any excessive unburnt fuel?

Also in my Tahoe I believe its the gen+ bbl carb, if that makes any difference
 

havoc_squad

Senior Chief Petty Officer
Joined
Mar 5, 2011
Messages
694
So basically if my timing is okay, my plugs are good and my air and fuel mixture is good, the only way to prevent/fix this is just to let it idle for a min or less after a run to burn of any excessive unburnt fuel?

Also in my Tahoe I believe its the gen+ bbl carb, if that makes any difference

You let it run for a minute or so at idle to richen up the mixture at low idle, NOT to burn off excess fuel.

It's a LEAN (too much air, not enough fuel) condition that cause your engine to continue running after you turn it off, just as several people mentioned in the possible causes.

When fuel/air mixture is not rich enough, it can pre-ignite without the help of a spark plug due to the engine heat. That is one primary cause of engine run-on.

Fuel injection systems do not suffer from this because the injectors(assuming near the cylinders) instantly turn off when the motor is shut down.
 

bman440440

Petty Officer 1st Class
Joined
Apr 4, 2011
Messages
266
here is the TSB I found when I was having this problem

Printed in U.S.A. 97-17 JULY 1999



TO: SERVICE MANAGER TECHNICIANS No. 97-17
PARTS MANAGER
= Revised July 1999
Engine ‘Dieseling’ or ‘Running-On’
Models
MCM 4 Cylinder, V6 and V8 with Carburetors.
Situation
Under certain conditions, carbureted engines may ‘run-on’ when the ignition key is turned off. Normally, it
will take a combination of several of the following conditions to make the engine ‘run on’. These conditions
include but are not limited to:
1. Type and formulation of gasoline available in the area.
2. Type of boat and its engine compartment ventilation system.
3. Outside air, inside engine compartment air and engine coolant temperatures.
4. How quickly the engine is shut off after running at cruising or higher RPMs.
5. How the boat and engine are operated.
6. Engine idle rpm set too high.
7. Wrong heat range spark plugs or spark plugs with deposits on them.
8. Deposits in combustion chamber.
Carbureted engines, unlike EFI, do not shut off the fuel supply to the engine when the ignition key is turned
off. As the engine is coasting to a stop, air is still flowing through the carburetor into the manifold. This air
flow can still draw fuel from the carburetor, through the intake manifold and into the combustion chamber.
A hot spot in the combustion chamber will cause this fuel/air mixture to burn. The engine now ‘diesels’ or
‘runs on’ although the ignition system is not firing. EFI engines do not have this problem because the injectors
do not inject fuel once the ignition key is turned off.
Suggestion to Minimize ‘Run On’
Items number 1 ,2 and 3 can correct or reduce most of these ‘run on’ problems. Letting any engine idle a
minute or two before shutting it off is desirable. This helps the engine to normalize at idle rpm before being
shut down.
1. Be sure engine idle mixture, idle rpm and ignition timing are correct. On 4 cylinder and V6 or V8 engines
with Thunderbolt IV ignition, setting idle rpm as low as possible can help stop ‘run on’.
2. Adjust throttle cable barrel so that the carburetor’s throttle lever returns against the idle speed stop
screw every time the remote control throttle lever is returned to neutral, idle.
3. Idle engine 1 to 1-1/2 minutes before turning key off after running at cruise or higher engine rpm.
4. Try a different brand of regular fuel or try some premium fuel.
5. Increase idle mixture by 1/8 turn ‘richer’.
6. Go to next colder heat range spark plugs.
7. V6 and V8 only: Install a 140 °F(60 °C) thermostat.
 

aimlow

Petty Officer 2nd Class
Joined
Nov 7, 2018
Messages
180
So basically if my timing is okay, my plugs are good and my air and fuel mixture is good, the only way to prevent/fix this is just to let it idle for a min or less after a run to burn of any excessive unburnt fuel?

Also in my Tahoe I believe its the gen+ bbl carb, if that makes any difference

In addition to the above..idling allows the combustion chamber to cool enough to prevent auto-ignition. Advanced timing at idle allows for a leaner idle mixture, which is undesirable. Makes sure the idle speed is within specs. If you still have issues, put it in gear and shut the engine off.
 
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