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So my 1968 Olds 98 with the 455 4 barrel carb has recently started the dieseling issue when I turn off the ignition. I have to turn the ign key back-on, rev up the engine a bit and then turn off the key to kill it. A couple of months back the shop did a complete tune-up replacing the spark plugs/wires, etc as well as a carb tune-up. Do I need to add any kind of additive to the fuel. I use mid-grade fuel from either Shell/Chevron/Exxon.
If it ran fine before the tune up, I’m guessing something isn’t set correctly.
Usually dieseling is caused by the throttle blades being opened too far. Is the ignition timing set correctly? If the timing is retarded from where it should be, the throttle would need to be open farther than normal.
Are the plugs the correct heat range?
If the engine is carboned up, hot spots in the combustion chamber can cause dieseling. Take the car for a long highway drive. Be sure to give it a couple hard acceleration to blow the crap out of the engine.
A trickle of cold water down the carb while it’s running will also clean the crud out of the engine. This method works well, but you need to pay attention to what’s going on! With the engine fully warmed up, rev the engine while SLOWLY pouring water in the carb. The idea isn’t to drown the engine! If the engine is carboned up, you will know it from the black rings on the ground under the tailpipe.
Im guessing something was messed up during the tuneup, I’d explore what was replaced/adjusted first.
Leaving aside what may have changed in your engine, I find that using manifold vacuum for vac advance helps. The higher advance at idle requires a lower throttle opening, which reduces the tendency to diesel.
My guess is your previous tune had the timing pulled back, and your current tune is by the book. Meaning factory settings.
As has been mentioned, high test, premium gas, is a must along with an octane booster with MMT (lead substitute).
After years of trying this and that here is the magic combo for my high compression factory set 350.
One bottle of this (5.25 FL OZ) with each fill up of 93 octane minimum.
A cheaper alternative is to buy a gallon of Xylene at your local hardware store.
That will boost the octane rating considerably higher than 5.25 ounces will do, for a much lower price per ounce.
A cheaper alternative is to buy a gallon of Xylene at your local hardware store.
That will boost the octane rating considerably higher than 5.25 ounces will do, for a much lower price per ounce.
Same Ratio ? 5.25 oz to a tankfull ?
Last edited by Cosmic Charlie; November 24th, 2020 at 08:34 PM.
Turn off the engine with the transmission still in Drive (or Reverse). I've found that this usually prevents the engine run-on. It doesn't eliminate the cause, of course, but at least the engine stops.
I recall this same issue on my '69 Toronado. Back in the '80s I'd use a few gallons of 100+ octane racing fuel w/ the 93 pump gas. When that was not available, leaving it in Drive usually did it for me, it did not have a throttle-stop solenoid like my Cutlass that would close the throttle when the ignition turned off (wish it did). I do recall a time after the Toro ended up in my friend's hands when a half-shaft CV joint broke and putting it in Drive didn't help. We had pushed it home from where it broke down on the street, but still had to run the engine to have power steering. We ended up stuffing rags in the air cleaner snout and literally choked it to death.
November 23rd, 2020, 08:11 AM
