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co12294 Samba Member
Joined: May 15, 2022 Posts: 19 Location: SoCal
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Posted: Tue Sep 27, 2022 10:52 am Post subject: Intake manifold gasket change = significant mixture change? |
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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. |
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SGKent Samba Member
Joined: October 30, 2007 Posts: 41031 Location: Citrus Heights CA (Near Sacramento)
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Posted: Tue Sep 27, 2022 11:13 am Post subject: Re: Intake manifold gasket change = significant mixture change? |
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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. _________________ “Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin |
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co12294 Samba Member
Joined: May 15, 2022 Posts: 19 Location: SoCal
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Posted: Tue Sep 27, 2022 11:21 am Post subject: Re: Intake manifold gasket change = significant mixture change? |
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SGKent wrote: |
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. |
I did test by spraying starting fluid at the base of the intake and no uptick in RPMS (significant idle jump before I replaced the gaskets).
Good point about jostling the carbs around, I'll let things settle down a little and see what it looks like in a bit. |
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co12294 Samba Member
Joined: May 15, 2022 Posts: 19 Location: SoCal
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Posted: Tue Sep 27, 2022 3:31 pm Post subject: Re: Intake manifold gasket change = significant mixture change? |
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Thanks for the sanity-check, it turned out to be a sudden exhaust leak (header cracked on my repair weld) with extremely coincidental timing. |
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SGKent Samba Member
Joined: October 30, 2007 Posts: 41031 Location: Citrus Heights CA (Near Sacramento)
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Posted: Tue Sep 27, 2022 5:20 pm Post subject: Re: Intake manifold gasket change = significant mixture change? |
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co12294 wrote: |
Thanks for the sanity-check, it turned out to be a sudden exhaust leak (header cracked on my repair weld) with extremely coincidental timing. |
so the high A/F ratio was dilution. _________________ “Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin |
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co12294 Samba Member
Joined: May 15, 2022 Posts: 19 Location: SoCal
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Posted: Tue Sep 27, 2022 5:53 pm Post subject: Re: Intake manifold gasket change = significant mixture change? |
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SGKent wrote: |
co12294 wrote: |
Thanks for the sanity-check, it turned out to be a sudden exhaust leak (header cracked on my repair weld) with extremely coincidental timing. |
so the high A/F ratio was dilution. |
Yep. I have a set of VS headers waiting to go on, the ones on here are basically rusted through...there was enough material at the crack to weld, but they cracked again right next to it. |
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Clatter Samba Member
Joined: September 24, 2003 Posts: 7549 Location: Santa Cruz
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Posted: Wed Sep 28, 2022 8:56 am Post subject: Re: Intake manifold gasket change = significant mixture change? |
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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.. _________________ Bus Motor Build
What’s That Noise?!? |
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co12294 Samba Member
Joined: May 15, 2022 Posts: 19 Location: SoCal
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Posted: Wed Sep 28, 2022 2:30 pm Post subject: Re: Intake manifold gasket change = significant mixture change? |
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Clatter wrote: |
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.. |
Great info, that makes a ton of sense, thanks. The phenolic gaskets JCS installed between the manifolds and the heads lasted all of ~500 miles. |
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