Solving heatsoak with a Holley 4-barrel carburetor on a stock L28

[Camaden]

Hi all, I've been dealing with some heatsoaking issues on my setup. I have a holley 390cfm 4160 carburetor on an Arizona Z car manifold and aftermarket headers (unknown brand) in a '77 280z with the stock L28. I'm not sure who else is running this setup, but I know it at least used to be somewhat popular, so I figure most of the issues have probably been figured out, but I haven't come across much information on solving heatsoaking issues with this setup. 

 

There is no room for a phenolic spacer, as the air cleaner already contacts the hood when closed. It's a 2in air cleaner, and it doesnt look like theres a way to reduce that height to squeeze a spacer in there. I have a heatshield between the headers and manifold, but that only helps for continuous driving. If I ever park for between a couple minutes and an hour or two, it still heatsoaks and I get a super lean AFR. 

 

To mitigate this, the best thing I've found is just running jets that are way too big, but then I'm obviously running too rich when I'm not heatsoaked. Size 62 jets seem to put me at a healthy AFR when I'm heatsoaked, but I really should be running 52 or 53 jets for optimal AFR.

 

I don't have a fuel recirculation setup, so it dead ends with the carburetor. I'm doubtfull that this would help a ton, since the carburetor itself has no fuel outlet, and by running a lighter 8.5 power valve in the holley, I can lower the AFR under WOT sometimes, leading me to believe most of the vapor lock is in the carburetor metering plate. I'm considering trying this anyway, since I don't think it'll hurt anything.

 

I'm also hesitant to try a heatshield like this one, as it adds height, even if I skip the smaller plates and only add the main heatshield, and as I mentioned, I'm already contacting the hood at the vent.

 

How much has this been an issue for other Z owners running a 4-barrel setup? How impactful have fuel recirculation or the Jegs style heatshield been for other setups? What solutions have I not found yet? Thanks

[NewZed]

Do you have a heat shield between the headers and the intake manifold?  The carb is pretty far from the head so the heat would either be conducted through the intake manifold from the head or radiating/convecting up from the headers.

 

If it's the intake manifold that's getting hot maybe you could add some insulating spacers between the intake and the head.  And add a heat shield for header heat.

 

You might also look at a cooling fan like Nissan added to the 280ZX engines.  Might need all three.

 

Borrowed this from the AZC site.

 

[Camaden]

I do have a heatshield between the headers and intake manifold, but it's probably not great. Had to cobble something to gether with wire and some pieces of aluminum sheet I had laying around since I haven't seen anyone making a heatshield that fits this setup here. It's probably better than nothing, but it doesn't seem to help significantly. I havent measured intake manifold temperatures to see if it changed them at all. I'm considering wrapping or ceramic coating the headers to see if that helps a bit more.

 

My radiator fan clutch is bypassed, so it's always on and providing a breeze through the engine bay. I could set try to set it or another fan up to run for a while when the car is off I guess. That might help

Edited August 24 by Camaden

[NewZed]

I would even consider shielding the float bowls.  Wrap the bowls with some heat tape maybe.  They're sticking out there in the convection stream, the hot air transfers its heat to them as it passes by.  Lots of area to absorb heat.

 

The way to approach is to ask how the heat gets there.  Is it coming through the intake manifold metal, then the carb body, then to the bowls?  Or is it coming through radiation or air flow (convection)?  Conduction (metal transfer), radiation (line of sight), or convection (hot air flow).  Each mode has a different solution.

[Camaden]

I'll start measuring temps. That'll give me a better idea of what the issue is. I'll try insulating the float bowls too, thanks for the tips.

[Brad-ManQ45]

There was a discussion about wrapping exhaust vs ceramic coating vs high temp paint somewhere here.  One person noted that changing from VHT Black to VHT White on a Corvair engines pushrod tubes lowered oil temps.

 

You might try painting the bowls underneath white.  You may also want to try fabricating aluminum heat shields that mount under the carb hold-down nuts instead of under the carb base.

 

Wrapping the header or ceramic would help immensely.