Best way to prevent vapor lock & air/fuel creep
Is it possible to prevent fuel from boiling in the bowls of a carburetor while engine is running at low speeds (stop & go traffic) when outside air exceeds 80F?
Even if you employ an ideal return-style fuel system with electric fuel pump and regulator within 20-30" of carb, it seems like the fuel boiling in the bowls would still be the problem despite being able to keep the fuel in the feed/return lines at near ambient temp. Return-style system could keep the fuel under high pressure in the line and reasonably cool, but once the fuel's between regulator and needle/seat it would be 6 PSI (thus lower boiling point than the 40+ PSI line behind the regulator) and once it passes by the needle/seat in the bowl, it's at atmospheric pressure & regular E10 pump gas is going to vaporize around 120F +/- a few degrees maybe depending on additives / formulations blended by refineries.
Reason I ask is the measures I've implemented (listed below) failed to stabilize "air/fuel creep" of the car delivering 14.5:1 in cruise, idle & stop-n-go driving when temps are under 70F, thus the air/fuel ratio creeps into the 16-17 range when temps are above 80F.
I have a clean tank & fuel sock, clean stock routed & sized fuel lines, a Carter high-flow mechanical fuel pump, Mr G 40 micron filter feeding a 750 Mighty Demon atop a 1" phenolic spacer and GM aluminum heat shield (with thermal side blanketing added) and this is all topped off by a stock Chevy drop-down 14" open element filter.
Hood is the original flat hood (wish I could do the ZL2 hood to vent hot air, but we don't have room to store an extra hood), 160F thermostat & 4-core cooling system w/puke tank keeps temps around 170F even on the hottest days in stop-n-go (and cruises at 160F over 40 MPH).
Handheld pyrometer shows 115F at fuel bowls (which show fuel in them via sight glass & vidcam I mounted) with fuel being a little lower on the mark when warm vs. when cold or in over 40 MPH cruise speed. 135F on header-side of mech fuel pump, 115F on forward non-header side. Vic Jr manifold registers 185F on its runners & base.
Over 75F, the car stays responsive to throttle during stop-n-go traffic and will return to normal when cruising above 40 MPH, but I'm concerned that as this summer's temps rise to 90-100F outside, the vapor lock symptoms will get worse in traffic.
I'm considering going to a return-style fuel system with electric fuel pump to bring 40+ PSI fuel from tank to regulator (20-30 inches from carb) and returning to fuel tank, however, I wonder whether that would cure the air/fuel creep or if the fuel would still be boiling in the bowls no matter what I did. Should just live with the air/fuel creep in warm conditions? The car's run like this for the past 15 yrs on this build by the way - so maybe I'm just obsessing about air/fuel fine-tuning in warm weather and should just live with it.
If I go electric/return, do people have recommendations on the quietest pump (Walbro, Aeromotive, Holley) and external or in-tank layout. I figure since I'd have to plumb a return-line into the factory tank, the in-tank pump would be the quietest so I'd want to get the most reliable pump as well since I'd hope this lasts many years & miles before needing replacement.
Thanks in advance for your ideas. I'll try attaching pics to show the current layout, but if this site limits filesize my apologies.
'69 Z/28 - Dad's orig. owner, 44K mi., match #s DZ302 & M20 preserved in glass case. LT-1 w/ 210 AFRs, 0.640" lift 253/259 @ .050" Erson roller and Tremec TKO600 in car for more fun & 11.7s @ 121 MPH on drag radials & pump gas.