Q: I have a Ford truck with a 390 FE engine. I had the heads redone and installed a new cam and lifter set, true roller timing chain, valve springs, single plane Edelbrock intake, adjustable rocker arms, and headers. I use the truck to haul my 12 foot camper.
When I reach highway speeds and keep the rpm steady, I hear a sound like a chain or marbles inside a coffee can. When I let up on the gas, the sound goes away, and when I hammer the gas hard, it goes away. I drove 1,000 miles with this condition. Is it the cam walking and hitting the timing cover? Could it be the lifters or valve springs?
A: Your sound does not seem to be caused by a mechanical problem. If it was lifters or valve springs, the sound would be constant, not just when you’re cruising. The problem sounds more like pre-ignition. There are several ways to cure pre-ignition:
- Run higher octane fuel. Premium gas rated at 92 or 94 octane is best for an engine with a compression ratio between 9.25 and 10.25:1. Using premium can be expensive, especially in vehicles that use a lot of fuel.
- Run the engine on the rich side. You can do this by using larger jets in the carburetor, or by running a colder range spark plug than the one you are using now.
- Try playing with ignition timing. Most engines like to run more timing than stock specs call for. If you run too much initial timing, you will get pre-ignition; too little and you lose performance. We recommend starting initial timing at 10 degrees BTDC and adjust from there.
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