Best way to prevent vapor lock & air/fuel creep

stantaur · May 27, 2019

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.

SoCal805 · May 27, 2019

does your motor have the coolant crossover in intake manifold?

dhutton · May 27, 2019

Back in the day I had good success placing heat shields between the intake and carburetor. They were Holley carbs and GM even had one available. I’m sure they are available for other style carburetors.

https://www.summitracing.com/parts/hly-108-70/overview/

Don

stantaur · May 27, 2019

Thank you both. Site won't allow me to post the pics due to my other pics in threads.

It's a Vic Jr intake that has no EGR underneath. The coolant passage (attached to thermostat housing) is conventional, connecting both heads/passages.

The heat shield is just like that Holley one, except with GM part number & firing order stamped on it. I attached to that, a "wing" of folded 4" x 11" strip of DEI heat shield on passenger side and 4" by 5" on driver's side.

NashvilleCat · May 27, 2019

"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. "

Looks like you shouldn't be having any heating issues. I don't think you need a 1" thick spacer; a 1/2" delrin/phenolic spacer would work just as well and you could run a taller air filter than 3" - drop bases work better with a 3.5" to 4" tall filter.

I definitely don't think you need anything more than a stock high-performance mechanical pump. LS6 1970 big blocks used an acdelco pump. 40 gph is plenty.

SoCal805 · May 27, 2019

OP

I am "guessing" you have a sbc….and if it has the "exhaust" crossover that runs under the carb...if so many block off those ports or run the Edelbrock intake gaskets with the metal restrictor (small hole) plates

stantaur · May 27, 2019

Thank you. It's a built LT-1 w/ Edelbrock Vic Jr. intake (no exhaust crossover), AFR 210 heads, 253/259 @ .050" solid roller and 550 CHP @ 6500, with the Carter M4891 mech fuel pump. Looks reasonably stock or "Day 2" as some call it, others say "reversible restomod" that can be taken back to stock over a few weekends of bolting its original engine/trans/exhaust/suspension back in there.

The bowls have never drained far enough to starve the mains, but above 80F ambient they drain enough to drop fuel level low on the idle/cruise circuits & cause that air/fuel ratio to creep up from 14.7:1 to 17:1 if I'm sitting in traffic or doing stop-n-go crawl.

Thanks to years of wisdom from forums like this, the car's remarkably well-sorted and I may be splitting hairs. The carb delivers 12.5:1 WOT and 14.7:1 idle/cruise when it's 60F outside, so I'd hate to go EFI (and TBI at that) unless absolutely necessary. I can see myself going electric fuel pump & return line route IF we find proof that people with carbs have done this successfully (having no air/fuel ratio creep and no vapor appearing in test fuel lines in conditions from 40F through 110F ambient air temp).

My question is that once carb temp gets past 110F (easy to do given under-hood temps w/no hood scoop sitting in traffic), does it even matter whether we're delivering perfect 6 PSI between the regulator and the needle/seat, if all that's going to happen is fuel passing the needle/seat flows into a hot 110+F fuel bowl (at atmospheric pressure) and heats/vaporizes, resulting in inconsistent air/fuel metering of the idle/cruise circuits (and potentially starving the main circuits if the fuel level drops that low in the bowl / is uncovered during acceleration)?

Asked differently, is it futile to engineer a perfect in-tank pump w/return-style design with 6 PSI regulator mere inches away from the carb, only to be thwarted by the fuel boiling in the bowls anyway? If that's the case, then maybe that's the only remaining reason I'd consider migrating to EFI (if I were going to be driving the car consistently in stop-n-go traffic above 80F), which may be a pretty rare use-case for what's essentially an early-morning weekend toy that gets driven through the twistys of Oregon.

Thanks again - really appreciate your insights over the years. The car's been a ton of fun since its 2002-04 restoration, including 15 yrs of de-bug.

SoCal805 · May 27, 2019

Well carbureted cars will react to variable operating temp conditions, altitude. You will be chasing your tail to have a carb car stay dead nuts on AF ratio for example but if all your fuel delivery and carburetor is in proper working order, timing is good it will run within a variable "window" of operating conditions, including hotter engine temps

Yeah going to a EFI system eliminates most of the varying degrees of carbureted motor fluctuation has.

bucko · May 28, 2019

SoCal805 said:
Well carbureted cars will react to variable operating temp conditions, altitude. You will be chasing your tail to have a carb car stay dead nuts on AF ratio for example but if all your fuel delivery and carburetor is in proper working order, timing is good it will run within a variable "window" of operating conditions, including hotter engine temps

Yeah going to a EFI system eliminates most of the varying degrees of carbureted motor fluctuation has.

So true. Having a "carb computer" with a database that expands from idle to WOT mapping, plus sensors that read the amount of air coming in, plus temps, plus fuel pressure, make these database maps stored a perfect solution to any carb woes.

stantaur · May 28, 2019

I agree that EFI's combination of sensors, computation and dynamic metering will deliver more accurate & consistent air/fuel ratios across a wider range of elevations, temperatures and conditions. I'm a big fan of EFI, having done tons of tuning on AEM & Hondata systems on my own cars. For this car, I've considered Edelbrock's multi-point setup or Holley's new Terminator (were it not for their cost, challenges w/cams over 250 @ .050" and other complexities).

Until EFI's cost/benefit ratios improve though, I've been waiting for the tech & cost situation to improve... foregoing a fair chunk of EFI's versatility in exchange for the carb's simplicity and thus searching for proof that a return-line design with carb could reduce or eliminate fuel vaporization out of the carb's bowls. If more pressure up to the regulator would cure things, I'd be willing to do an electric return-style fuel routing, but so far it seems like the fuel's going to vaporize in a carb that exceeds 115F and is limited to 6-ish PSI being delivered to it. I even considered the composite intake AFR marketed a few years ago.

Absent proof in several forums & searches so far, I question whether a return-style design can eliminate enough vaporization to justify the effort of plumbing a return-style system that minimizes the distance between regulator and bowls and runs a line back to the tank (which needs to be dropped in order to install a return line into it, at which time one may as well add an in-tank electric fuel pump thereby eliminating the mech fuel pump's introduction of heat from the block).

Not having a ZL2 hood scoop (or willingness to cut the firewall to do a 1967 Z inspired cold-air setup that could encapsulate the entire carb & air cleaner, forming a cooler zone than the rest of the engine bay) makes it tougher for our Z to cool the carb and reduce vaporization there, as with our original flat hood & open element air cleaner there's no easy way to vent underhood heat raising carb temp above 115F in traffic (which seems to be most of my problem). With the car running fine over 40 MPH, there's enough airflow to get the carb cooled back down on mornings below 80F. So the current answer seems to be this is about as good as I can expect for now, unless I find a way to cool off the carb.

Someone will dig this thread up in 5-10 yrs and be surprised what we tolerated before companies made the cost/benefit ratio of EFI (TBI or SFI) superior to primitive-but-simple carbs.

Thanks again for your wisdom & ideas ...and especially for helping me avoid a ton of fruitless work (of re-plumbing a fuel system to fix something that may be most likely attributable to the carb).

stantaur · May 28, 2019

Also found this example showing an electric in-tank pump and return-style including flow-through using both sides of each carb bowl.

While not the perfect example with all factors documented (need to see the temps w/the car running w/hood closed through a variety of situations), if this mini-example is representative of cycling the fuel through the system, and it's not pulling heat out of the fuel in the carb bowls, then it reinforces my decision to "live with" my current setup until cost/benefit of going EFI intersects with the value I'd derive from being able to use the car in a wider range of use cases (incl stop-n-go traffic on hot days).

One more way of thinking about this is to imagine the polar opposite design... one that uses EFI that's fed by a mini-reservoir and electric pump module (Edelbrock 36032 here: https://www.summitracing.com/parts/edl-36032 ), which is fed fuel from mech fuel pump. If that approach has no vapor lock issues, yet a carb fed by electric fuel pump w/return line has vapor lock issues, then we can deduce that the carb (and today's E10 gas formulation) is likely to be the culprit and thus the most likely cure to solve vapor lock is to use EFI (which keeps fuel at a high pressure and high vaporization temp) all the way to the injector nozzle (whereas a carb houses fuel in bowls before it reaches the circuits & venturis, thus allowing it to heat soak and vaporize).

Thanks again for your insights & "pressure testing" (pun intended) these lines of reasoning.

Greg O · May 28, 2019

I had a similar issue when i tried to go boosted with a mechanical pump. It was a big honkin pump. During the summer in traffic the pressure would drop to 2 lbs. i went electric with return regulator and problem solved. However, i have a couple of theories about why it would vapor lock with the big mechanical pump.

1) the high volume pump is attempting to move a bunch of fuel per stroke. At idle with a dead head system the more volume it tries to pump, the more friction as there is nowhere for the fuel to go and therefore more heat. A lower volume pump may be less prone to this.

2) I also suspected the high volume pump may have damaged the fuel pump lobe on my cam. The pushrod would not come out as it was mushroomed. Flattened cam lobe also means less fuel pressure.

I never “proved” either theory as i really needed to go electric for the boost anyway but some things to think about.

Best way to prevent vapor lock & air/fuel creep

stantaur

SoCal805

dhutton

stantaur

NashvilleCat

SoCal805

stantaur

SoCal805

bucko

stantaur

stantaur

Greg O

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