High Idle Issue

[Georgeb]

Thanks for the lesson Rich! So to make a dizzy set up for ported work like the earlier ones would it help to reduce the mechanical advance to the 20 deg mentioned in the engineers write up or just offset for the additional advance. Like set it around 4 deg and let the vacume pull it up to say 19 at idle? With a total at the top being around 27 or so?

 

[rich weyand]

The stock distributor setup (20 degrees mechanical and 15 vacuum advance (the can will say 7.5 -- that's camshaft degrees, it's 15 crankshaft degrees)) is fine. That would give you 16 degrees BTDC as base timing. I just set mine at 16 degrees and went over to the nearby steep hill, stopped on the hill and punched it, No knocking, so that's where it sits.

The other alternative is to subtract whatever you have for mechanical advance from 36 -- in your example of 23 degrees of mechanical advance, that would give a base timing of 13 degrees. You want base + mechanical to be 34-36. I aimed at 36, which, if it doesn't knock, is better. Running 23 mechnical gives you a bit less advance at low rpm, which is a protection against knocking, but with these engine is seldom required. It's a "performance" setting, but that is for a performance engine, not a stock one. For a stock engine, you will get better performance with the stock mechanical advance value.

 

[Georgeb]

In your first example is the 16 degrees base timing with or without the vacuum advance connected?

 

[rich weyand]

In your first example is the 16 degrees base timing with or without the vacuum advance connected?

Base timing is always measured/set with the vacuum advance disconnected. People say to plug it off but you don't need to just to set where the sparky goes.

So you have base timing 16* BTDC, mechanical advance 20*, vacuum advance 15*. This will give 31* BTDC at idle, 16* BTDC at low rpm with your foot in it, 36* BTDC at high rpm with your foot in it, and 51* BTDC on decel from high rpms.