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Forum: Hot Heads Tech Talk

Forums->Hot Heads Tech Talk->Re: Spark advance

Re: Spark advance

Timed vs. ported vacuum advance is a deep subject.

There are well-formed schools of thought and hard-heads on both sides. The paper in this link provides a wealth of food for thought on timing. About halfway through, the subject of vacuum advance is covered.

I used to be in the "timed/ported" camp because I had been TOLD by many who I considered more experienced than myself, that ported was the only way to go with a modern carburetor. After reading several articles and doing my own open-minded experimentation I changed camps and I am now a staunch "full" vacuum supporter on vintage engines.

Pay close attention to the section that discusses WHY ported vacuum was adopted.

The two things that won me over in working with my 325 were better idle quality, and the significantly lower engine temperature at idle (it is a major factor if you're running a high performance engine with a stock radiator.) If you have a very high overlap cam, the idle quality issue may be the bigger advantage.

Just seeing for yourself what happens to your idle when you swap the hose from the ported to the full might give you an idea that full might be better. The idle speed will increase by 700-1000 rpm on the same carb idle setting. Rather than do what someone says (me or otherwise) try it both ways for yourself and see which you prefer for overall driving quality. Note: In changing from one way to the other, be sure to fine-tune the idle and air screws screws for best idle.

This all comes from my own personal experience. I'm not right or wrong. Do some research into the subject and reach your own informed conclusion.

  user offline
on: Tue 07 of Jan, 2020 [14:16 UTC] reads: 387

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