an issue with this engine dieseling on shutdown,

Grumpy

Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
With the manual transmission, the low vacuum of this engine, and the emphasis on street driving, would I be better off with a mechanical secondary carb? I also have an issue with this engine dieseling on shutdown, which I have researched as best that I can, to getting two much fuel past the carb on shutdown. I cannot increase the timing to reduce the idle setting without risking detonation. Would a smaller carb with mechanical secondaries be a better choice? I agree with your idea of getting a good Holley tuner, but we do not have one within 3 hours drive of this chunk of the middle of nowhere. Thank you for your expertise. I have spent a LOT of time reading on your site and it has helped solve many problems.
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in many or most cases engine dieseling after the ignition is turned off is caused by a combination of a bit too rich of a fuel/air mix being drawn in and a bit too much heat in the combustion chambers this is common when a holley carbs ESPECIALLY IF transfer slot is badly adjusted to expose too much slot at idle or carbs OR with the float level set too high, or a fuel pressure regulator pressure set too high or a blocked return line on the fuel pressure regulator, or at times indicates a vacuum leak

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Tuning Tips for Holley Carburetors
Q: Holley 2300/4150 Tuning Tips
A: Float Level ? The fuel level in the bowls should be set at the bottom of the sight hole so that you have to jostle the car to get the fuel to come out the sight hole when running. We set the floats in the shop to a particular setting; but fuel pressure dictates fuel level. Therefore you must set this when you first install the carburetor. Finally, after setting the floats, always give the carburetor time to burn off enough fuel to reopen the needle/seat so that your new setting is really what you are seeing. Many times people lower the floats too much because they haven?t waited a sufficient amount of time for the fuel to burn off and the new setting to take.
Throttle Blade Adjustment ? The throttle blades should be set at an rpm that is as low as possible to keep the transfer slots covered at closed throttle position. Keep the primary and secondary throttle blades adjusted equally. If you should lose your setting just back it completely off, put 1 round in the primary and 1 round in the secondary and start there.
both front and rear float levels on holley carbs with site plugs should be adjusted to the level that just allows the lower edge of the site plug threads to get damp with fuel at idle

Idle Mixture Screws ? Always adjust the 4 idle mixture screws evenly. The settings on all 4 should be the same. Our original setting is 1 1/2 turns out. If for some reason your engine absolutely needs a different setting on one side or corner you can just about bet something is wrong with the engine causing it to have a different signal/vacuum on that side/corner.

Fuel Pressure ? 6.0 to 6.5 pounds. You should absolutely know what your fuel pressure is! Many racers have no clue what their fuel pressure is, and they chase engine problems for weeks when a simple fuel pressure gauge would have indicated the problem straight away.

btw if your not already registered on my home web site it would be to your advantage to do so
 
I have experienced engine Run on after the ignition key has been turned off Grumpy.
Its also caused by hot spots in the Combustion chamber.
I have found if available and the Right Reach AC Delco Truck Spark plugs with the Super Wide and thick Ground electrodes help prevent or all together eliminate engine Run On issues.
Keeping Base ignition timing at 5-10 drgrees BTDC At 600-900 Rpm idle engine speeds helps.
The rate of mechanical advance is not critical for Run on issues.
Of course the distributor stop plates must be modified as needed for Full ignition advance dedired. 27-40 degrees BTDC Typically .
 
Racing grade cold spark plugs work also but Foul up easy slow speed driving with a Hot cam with 64 degrees Overlap or more.
 
Also found true racing gas eliminates all Run On issues.
But not pertinent here because right now the Group is using available pump gasoline 87-93 RM/2 .
 
The AC Delco Truck Spark Plugs were used in all 366 ci & 427 Tall Deck Big Block Chevy Engines Stock.
 
Grumpy said:
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If you find that your throttle plates are open too far causing too much of the transfer slot to be exposed,
like Grumpy said, then the usual solution is to let bypass air into the engine, usually by drilling holes in the
throttle plates. I can tell you of a way to do this without doing any drilling to the carb. If this is your actual
problem, then you should also have a dead spot off idle, because you have already gone past the transition
circuits and actually are idling on the main jets.
It is possible to have a big cam and idle below 1000 rpm.
But all of your other settings must be correct and verified first, otherwise you will just add another variable
into the equation, further complicating the issue.
 
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Most common thing I've had to fix to stop run on in customers cars is improperly wired electric cooling fans for the radiator where the fan becomes an alternator as its freewheeling after the ignition key is turned off.
 
Not an issue with a Vertex Magneto Birdman in my 1963 Grand Prix.
I flip the Switch to off and the Olds 425 shuts off right now.
 
NOT A TA said:
Most common thing I've had to fix to stop run on in customers cars is improperly wired electric cooling fans for the radiator where the fan becomes an alternator as its freewheeling after the ignition key is turned off.
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Let me guess - DIODE?
 
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