Hi, I recently put an edelbrock carb intake and MSD cap, rotor, wires, and new spark plugs on my 68 CDV. I adjusted the air/fuel mixture screws and now my engine has a knock when idling. It goes away when you step on the gas. The car starts,drives, and runs awesome other wise. Any ideas what this could be????

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I’ll take a quick guess. If you have the stock engine, it has a 10.5:1 compression ratio. Your Carb’s idle adjustment valve may be adjusted for an engine with an 8.5:1 compression. You also may have put in spark plugs with the hotter heat range made for low compression engines. And you might have regular vs. premium gas in the tank. You may also have too much spark advance (high compression engines require retarded timing). Combine all of that with a hotter spark from a powerful ignition system and the cylinders could be burning all of the gas in a lean mix way too soon- and the knock could be detonation. At a higher RPM – if you are detonating- you may not hear it, but the detonation would still be there in a lean burn situation.

A part throttle knock with no knock when I gave it gas was the symptoms my 70 Eldorado had the last week I drove it. This is because a mechanic changed the float in my carb and accidentally adjusted it to have a lean burn. I was side-by-side with an 80s Porsche 911 that I was racing at 85 MPH and we were accelerating rapidly when one of my pistons blew in half because of that lean burn detonation that I could not hear when I gave it gas. ~ Funny thing was; the engine did not die and I continued to drive the car home trailing white/blue smoke like an old steam train. (max speed with blown engine was 50 MPH… That was back when I didn’t know any better. The whole engine was filled with metal “sand” from the piston. Check for lean burn- even if that is not your knock- lean burn will kill the motor.

You need the coldest heat range plug you can get if you want that thing to live on today’s gas and you may need to retard the timing a couple of degrees more than what the manual calls for. Run a colder thermostat in the 180 degree range--- make sure it has the special plunger-like piece on the bottom (a Chevy thermostat will not block water flow to the bypass when needed and you will over heat).

I've never heard of an engine having detonation at idle. It should really rattle at part throttle. Maybe it's a lifter problem.

Well its weird...i could hear it at the front of the car when i was under it. Then I changed oil and now it is gone.

I'm baffled????

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Glad the knock is gone. Maybe you were just low on oil.



I've never heard of an engine having detonation at idle. It should really rattle at part throttle. Maybe it's a lifter problem.

Oops: :cookoo: I was thinking “idle speed” – not “idling” when I gave my advise in the former post. Yeah, I’ve never heard of detonation when the engine is not under any load either.