consistent dieseling

[low ufo]

My 428 fe has started to diesel 90% of the time @ shut down.I thought it may be bad gas but i don"t think thats the problem maybe carb ?
any thoughts
mike

[wtm442]

Dieseling is sometimes (most times) caused by incorrect timing. Try different initial timing. Of course, many motors back in the 60's and early 70's had electric anti-dieseling solenoids on the carb, and they solve the problem, especially motor with radical cams.

[low ufo]

Hi warren
any recomendations on the timeing
mike

[wtm442]

Just back the timing off by 2 or 3 degrees and see if it helps.

Also, check the condition of the spark plugs.

Let me know what happens.

[David427]

Also try backing down your idle a bit. 1000 or below should eliminate runon if timing is close.

[Excaliber]

The typical cause of 'run on' is low octane fuel. Small red hot deposits accumulate in the combustion chamber of ALL engines. With time, the deposits increase. These 'glowing red hot' deposits ignite the fuel\air mixture STILL present in the combustion chamber when you turn the key off. Because the motor is still running it draws in more fuel\air through the carb, it's ignited and the cycle continues.

By it's very nature and design high octane fuel is more difficult to ignite, it is more resistant to 'detonation', thus 'run on' or deiseling. It takes a 'spark plug' to get it to burn, glowing red hot carbon deposits generally cannot set it off.

If the idle speed is low enough when it's shut down the engine cannot pull in enough fresh fuel\air through the carb to support 'run on'. High compression ratios also contribute to the problem, but generally it's a high idle speed that is the culprit.

My engine idle rpm is best around 1100, but this is 'just fast enough' to support a bit of 'run on', sometimes it does, sometimes it doesn't. I'm in the habit of leaving the engine in gear and letting out the clutch (while holding the brake) when I turn the key off to BE SURE 'run on' won't happen. It can be very damaging to your motor under the right conditions.

With an automatic trans another 'trick' is to turn off the motor with the car in DRIVE, when the idle rpm is at it's lowest point at shut down time.

[Michael C Henry]

What I've seen is not enough initial timing results in too much idle screw setting . More initial advance and the engine races at the same carb setting .Then lower the idle by backing out the idle screw. Start by putting a timing lite on the engine. Too much idle opening and you are suppling fuel every time it turns over, the hot engine just keeps light it off , may not be accurate timming, but it keeps running. In 73 ,when Ford realy started with the emmissions, they had an electric dash pot on the carbrueator that held the idle open while running and let the throttle plates close once the ignition was turned off. Once that dash pot came loose and the engine deiseled like crazy.

[SCOBRAC]

What's the compressio ratio? How many miles on the engine? All the suggestions listed are good enless you are running 10.5:1 or more compression. If it's always diesled on shutdown I suggest trying all the tips listed... Then go get some 96 octane fuel. Problem solved.

[low ufo]

Thanx every one for those helpful tips. i have switched to some chevron 91 oct. the best I can get around here with a 25% blend of 100 LL av gas,no more deiseling !!!any comments on the custom blend
mike

[Excaliber]

I also used a tank or two of mixed AV and pump gas, which immediatly cured any hint of deiseling. I couldn't feel any difference in performance. Due to the cost and hassle of mixed gas I set my idle rpm as low as I could and let the clutch out when I turn off the key. It's become such a habit to turn off the engine that way I don't even know if it DOES deisel anymore...

[SCOBRAC]

well 5 parts 91 octane and 5 parts 100 octane yield a fuel of 95.5 octane. Remeber your 427 needed 96-98 octane back in the day.

Octane doesn't improve performance. It doesn't make your car more powerful or rev higher, faster or longer. Those are functions of cam and ignition timing. It doesn't boost cylinder pressure or compression, but it allows you to run more compression and higher cylnder pressures. Octane will let your engine live longer and not rattle apart, detonate or preignite and may allow you to make changes which will allow for performance increases