C3 Detonation or Pre-Ignition???

[Tommy Boy McCabe]

Howdy,

I have a 69 Roadster 350/350 4spd and I just recently started having an annoying problem. After the car gets warmed up, when I turn off the ignition, the car keeps sputtering & chattering before spews it's fianl gasp and shuts off. What is this? What damage can it cause?? What do I need to do to make it stop??? Thanx for the help once again you Corvette gurus!

Regards,

Tommy Boy

[Joe L.]

Re: C3 Detonation or Pre-Ignition???

Tommy Boy------

This is a fairly common problem. There are several causes for it. First, you may have the idle speed set too high; the correct idle speed for your application is 750 RPM. Second, you may have the timing set improperly. The correct timing setting for your application is 8 degrees BTDC.

Another common problem causing this condition is carburetor primary throttle valves ("Butterfly" valves) which are not closing completely. This may be caused my misadjusted valves.

[Jim T.]

Re: C3 Detonation or Pre-Ignition???

Before shutting of the ignition let out the clutch enough to pull down the rpms some and turn the key to off. This will stop the sputtering and chattering.

[Donald T.]

Re: C3 Detonation or Pre-Ignition???

Tommy,

Detonation can cause engine damage. Assuming you have the car properly tuned up, and your cooling system is working properly, as Joe suggested, then you need to increase your octane. A product like lead supreme 130 will be very effective. Don't waste your money on the octane boosters at the local parts stores. Raise the octane just enough to stop the detonation.

Good luck!




Lead Supreme 130

[Michael W.]

Re: C3 Detonation or Pre-Ignition???

Is your car supposed to have an anti-dieseling solenoid?

[Duke W.]

Re: C3 Detonation or Pre-Ignition???

You're experiencing "engine runon", which was common on early emission controlled engines. GM "solved" the problem with a solenoid on some models that reduced throttle opening when the ignition key was turned off, which starved the engine of air. Check to see if your engine should have one.

As long as you are not hearing detonation when driving, you don't need more octane.

As stated you can get in the habit of letting out the clutch in gear as you shut off the engine AFTER applying the parking brake.

You can also solve the "problem" by converting to full time vacuum advance and bypassing TCS if installed, and this MUST be accompanied with a new vacuum can - NAPA/Echlin VC 1810 or equivalent - which cost about ten bucks.

Duke

[Marc R.]

Re: C3 Detonation or Pre-Ignition???

TB,
As Joe Lucia said, it's a common problem. I have an L-46 and have experienced such. It was 'cured' with a timing and idle adjustment. My car has the smog pump, etc.--original set-up. I wouldn't panic about the problem, but I'd certainly fix it. It results in problems such as trashing your air filter. Joe Lucia always knows. Marc