Intake manifold gasket change = significant mixture change?

[co12294]

2.2L type 4, dual HPMX 44s, EMPI intakes/linkage, Pertronix 009 clone.

The engine builder used thick phenolic gaskets (prob what the HPMX kit comes with) and within a few hundred miles I had a bad air leak causing the idle to creep to 2.5k RPMs. So I replaced them with the HP gaskets from aircooled.

I didn't make any other adjustments to the carbs yet, but was surprised to see the mixture significantly different than pre-leak with the phenolic gaskets. Idle AFR used to be ~13.5, now it's ~16. WOT was previously ~12, now it's ~14.5.

I'm going to adjust carbs tonight, but was just curious if anyone had an explanation for why simply changing the gaskets might result in an overall much-leaner mixture.

[SGKent]

if the A/F is much leaner now then you likely didn't get a good seal, and have an air leak. Use a smoke test to find it. The only other cause might be a change in float height caused by moving the carbs around, or a hose is disconnected/in the wrong spot. The phenolic gaskets were probably there to help keep the carbs cooler in summer to slow the fuel in the bowl(s) from boiling.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[co12294]

[co12294]

Thanks for the sanity-check, it turned out to be a sudden exhaust leak (header cracked on my repair weld) with extremely coincidental timing.

[SGKent]

[co12294]

[Clatter]

There are thick phenolic gaskets that come with the 2.0 bus gasket set.
These are to be used with the factory fuel injection manifolds.

They are NOT to be used under a set of carb manifolds.
They’re too soft/weak and will result in air leaks.

If you want some heat isolation to help with heat soak, use phenolic spacers between the carb and manifold.
Gary Berg sells them IIRC..
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Bus Motor Build

What’s That Noise?!?

[co12294]