[T3] '72 FI won't idle down

[Jim Adney]

On 13 Jun 2026 at 22:36, Keith Park wrote:

> Well it does run lean when warm with the AAR Open, used to see it when I
> had the AFR meter on it and it tends to miss then too, we'll see what
> happens when I close that off when the engine is hot.

Keith, you've added so many extra variables to an already crowded variable 
space, that it's impossible to guess what your problems might be. Have you 
considered trying to find a 914/4 2.0 L brain and pressure sensor to make a 
more self-compatible system? That's likely to be expensive, unless you get 
lucky, but it would probably get you to a better starting spot.

In the past, my own back-of-envelop thinking has suggested increasing the 
fuel pressure in proportion to the increase in displacement. That suggests 
that running lean would be a natural consequence of trying to simply 
increase the displacement.  

Then you'd be left with the question of what difference does it make when 
you substitute a different size throttle throat than the 914/4 one. I assume it's 
different. Off hand, I don't think it should make a difference, since, once 
again, the pressure sensor only responds to the pressure in the IAD, which 
is what is feeding the cylnder. OTOH, there's probably a difference in the 
pressure drops across Type 3 and Type 4 air runners, intake passages, and 
valves. There's a LOT to worry about.

-- 
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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