jackwabbit703

I've had dieseling issue with my vette for few months. I've checked everything from timing to mixture and still dieseled after shutoff. I've been using local Shell V-power 93 octane on my vette since I've moved in October since local Shell gas station was the closest one from my home. I've even used octane booster with the same gas and still had few episodes of dieseling with their gasoline.

Since nothing was curing my dieseling issue, I gave Exxon gas station a try which is 2 more miles from the Shell station. Filled it up last week and I have not had single dieseling since. Maybe it's different types detergent in their fuel caused dieseling...

Can gas station get shady with gasoline they are selling? I know about 4 years ago, local Sunoco gas station was in the news for tampering with their fuel got busted.

REELAV8R

The 93 was ethanol "enhanced" and the new fuel your using is not? Or the 93 was really 87? Hard to say. I wouldn't buy fuel there anymore.

jackwabbit703

Both were 93 octane with 10% ethanol. After this episode with Shell, I don't think I'll ever pump gas from ANY SHELL station.

cor66vette

I only use Shell for my Silverado (diesel)- and haven't used their gas in any of our other vehicles for years. I was told a long time ago that it's beneficial for the car to alternate, so I switch between Mobil and Sunoco.

69427

What's your idle timing and speed? Dieseling is almost always an easy issue to resolve or avoid.

jackwabbit703

Intial timing is set at 14 degrees at 750 rpm.

But I seriously think the issue was with the gasoline sold at the local Shell. I have not had any dieseling after switching to Exxon 93.

calwldlife

I never use Shell.
spark plugs get too much white soot.

69427

Ninety three octane to stop dieseling sounds rather extreme to me. I've got a decent CR in my engine, and mine won't diesel even with 91 octane.

jackwabbit703

Well I'm running 10:5 to 1 CR. But regardless, it had to be the gas. Don't think timing or mixture will fix itself and no dieseling is happening since switching to Exxon 93.

Maybe dishonest station owner is scamming people selling 87 for 93.. who knows..

69427

I've got somewhat similar compression (10.25). Runs fine on 91 (can't find 93 locally) and never diesels. Most every case of dieseling is caused by incorrect/retarded idle timing, that's why I was asking about your timing and vacuum advance setup.
You may have gotten some bad gas at the previous station, but you still shouldn't have to run 93 octane to prevent dieseling.

jackwabbit703

14 degrees is initial timing and vacuum advance.

Another thing to note, before I moved, I've always used the 93 octane from the same Exxon gas station since it was virtually across the street from my previous home. Now going back the previous years, I've never had dieseling problem using their gas on my vette. This all started since I moved to my new house and started using local closest Shell gas station 93 octane.

Also, in my area, we have no gas station that sells 91 octane gas. It's either 87, 89 or 93 in all stations. So I've always used 93 because that was what my engine builder told me to use.

69427

That's not much advance. There's a good chance that a bit more initial timing (or moving the vacuum advance source to manifold vacuum in case it was connected to ported vacuum) will improve the combustion efficiency enough to allow you to close the throttle blades further (while still retaining your 750 RPM idle target), and this reduced throttle opening is the key to eliminating dieseling.

jackwabbit703

Vacuum advance is definitely using full time manifold vacuum. I will try advancing the timing to 16 degrees if it ever diesel again and go from there.

Thanks for great replies and advises.

alswagg

dieseling is due to having a "hot spot" in the combustion chamber. When an engine is carbureted, and ignition is shut off, the motor is supposed to stop. But, if there is a hot spot in the combustion chambers, such as a glowing valve, glowing carbon deposit, or the such, the motor will run on or "diesel". Since you switched fuels and now no longer have a Dieseling situation without changing any differant items. I would suspect the Shell gas had deposits or differant detergents in the gas. Ethonal has a lower BTU than regular gasoline. Some scenerio's could be the ethonal in the fuel was not allowing enough of a clean burn to remove the carbon deposits in your combustion chamber. Or another, the shell gas just has more additives which did not completely burned. Either way, stick with the gas you are currently running and have a happy motor. Al

gcusmano74

'74s used an idle speed solonoid the set the normal idle speed. The small screw on the carb body is used to set a very slow idle. That is how the factory adressed engine run on.
On my smallblock/4 speed car, the regular warm idle is supposed to be 900 in neutral, with the soloniod energized. Idle speed is set by turning the stem of the solonoid with a 1/2" wrench. The slow idle is set with a screwdriver to something like 500 or 600 RPM. That's slow enough to eliminate the "dieseling". You have to unplug the solonoid to set the slow idle.

7t9l82

i think you are the victim of a shady station owner. i wouldn't be concerned until it happens again

jackwabbit703

I don't have a stock motor and solenoid is definitely not being used.

I ran her again today and no dieseling. I serious believe it was shady gas station.