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Won't run below 20deg BTDC


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I have a bizarre problem where I hope someone can provide some enlightenment. I have a 283ci Small Block Chevy with a Crane Fireball II 302H camshaft. It's a hydraulic cam with a rough idle and good amount of overlap. The problem is it won't idle well if initial advance is under 20deg BTDC. I have verified that mechanical advance is not active at idle and that the timing chain is installed correctly. I have also confirmed TDC using a piston TDC indicator so I know my damper and timing chain cover mark is correct. I have not degreed the cam but I have never had a SBC require so much initial advance. Total advance is 42deg which is a bit high.

 

Could a big overlap cam require 20deg BTDC initial advance? Is there something else I should check?

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With that big a cam in a 283, 20 deg BTDC is quite reasonable. A little more, 22-24 deg won't hurt if it cranks OK and you can keep the total advance where you want it.

 

jt

 

Thanks for the response. The car seems to run and crank well at 20deg BTDC. I've never, in my experience, had a SBC require so much intial advance so I was skeptical.

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I would try to run even more timing at idle. As long as the engine cranks ok when warm, then turn it up.

 

however, 42 is too high. You need to reduce the amount of mechanical advance in the distributor.

 

25 to 30 initial and 38 total (if you are using older style heads), or 36 total if you are using modern heads (fast burn vortec style).

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I would try to run even more timing at idle. As long as the engine cranks ok when warm, then turn it up.

 

however, 42 is too high. You need to reduce the amount of mechanical advance in the distributor.

 

25 to 30 initial and 38 total (if you are using older style heads), or 36 total if you are using modern heads (fast burn vortec style).

 

We are using cast "Fuelie" heads so I'll try to set it up for 38 total. Thanks for your input.

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