View single post by Esprit2
 Posted: 11-14-2016 07:19 pm
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 500

Lean mixture may cause the engine to run a little hot, but it shouldn't cause full blown over-heating. I suspect there's a cooling system issue at work as well. What thermostat is installed?

Balance the carbs first. That's critical, and any tuning on unbalanced carbs is a waste of effort.

Ignoring balance for the sake of conversation...
It sounds like the idle circuit is too lean. When the jetting is correct, tuning the Idle mixture screws for PEAK MANIFOLD VACUUM will result in them being 3.0-3.75 turns out from seated. If they end up out more than that, then the idle circuit is too lean, and you're attempting to (incorrectly) compensate by turning the mixture screws out further. Similarly, if the screws end up further in than that, like 2.0-2.75 turns, then the idle circuit is too rich and you're (incorrectly) compensating by turning the screws in too far.

The Idle Circuit feeds the engine from idle up to 3200 rpm, at which point the Main Circuit takes over. The jets and air correctors contrrol the mixture, and the IDLE Mixture Screws are only for making a small final tweak "AT IDLE". They are not for setting the overall Idle Circuit mixture. Get it right with the jetting first, then see where the IDLE mixture needles end up.

At 4.25 turns out, the tip of the needle is just about fully withdrawn from the orifice. Anything more than 4 turns has gone beyond the effective taper on the needle (ie, you've pulled the needle out of the hole), and will make no additional change. Anything beyond 3 3/4 turns means richer idle jetting is required (Idle Jet and/or Idle Air Corrector). Anything less than 2 - 2 1/2 turns means leaner idle jetting is required.

What Idle Jets and Idle Air Correctors are in ypur carbs now? There's really nothing more to talk about if we don't know where the jetting is now. The jets are all stamped with ID numbers... pull them out and look. A good starting point would be 7850-7 Idle Air Correctors with 52-55 Idle Jets.

But all that is pre-mature. Balance the carbs FIRST, then re-adjust the idle mixture screws for strongest/ peak manifold vacuum. If the mixture screws are still out 4.25 turns, then the idle circuit is too lean, and needs to be re-jetted. I recommend starting with 7850-7 Idle Air Correctors, then seeing where the Idle Jets take you... probably 52-55.

Tim Engel

Last edited on 11-14-2016 07:27 pm by Esprit2