The best candidates for fast burn conversion are spark ignited piston engines with adjustable spark timing. Those with carburetors as original equipment do not have computers that control additional automotive functions besides fuel metering and spark timing. The fast burn engine can’t use computer management as a low cost conversion option. If an idle circuit similar to that of a carburetor is added to a fuel injection throttle body as a spacer plate, a throttle body might be able to serve as a basic carburetor for cold vapor fuel. Many modern fuel injected engines can be owner retrofitted with carburetors and aftermarket or used factory intake manifolds for use with carburetors. The traditional multi circuit carburetor is not required for vapor fuel, but is less expensive than producing new twin circuit carburetors designed specifically for vapor fuel. All carburetor air vents should be plugged so engine vacuum signals draw only from
unvented carburetor fuel bowl, which is connected to fuel bubbler in tank or racing fuel cell. The fuel float and needle valve assembly is removed. Any accelerator pump is removed and enrichment hole is plugged. In carburetors that use vacuum power valves, such can be used to fine tune vapor flow according to vacuum signals. The idle circuit must retain adjustment from barely open to wide open.
Rod and jet enrichment systems such as used on Carter and Rochester carburetors can also be used to fine tune vapor mixtures. Go as lean as you can without engine stumbling under load on power circuit and richen slightly for best throttle response. If max power metering is sufficient with selected main jets, which flow much more vapor than liquid, we can regulate maximum fuel supply by means of an adjustable valve in supply line. Start fast burn engine with idle mixture screw open wide and quickly close it after engine starts. Back adjustment screw off one turn after you stall engine for lack of idle fuel. Too much idle fuel could melt engine parts as easily as too much fuel in power circuit. If we never meter more fuel than can burn in half a power stroke, we avoid meltdown while maximizing power and fuel economy, minimizing exhaust emissions. With fast burn, we want to retard initial spark timing to between sixty and eighty degrees after top dead center. On a V-8 we can move each wire in distributor cap forward one position on cap in direction of rotor rotation. This will retard timing 90 degrees from standard initial timing. A timing light on number 8 cylinder will allow you to set initial timing with stock timing marks. Setting number 8 cylinder for ten degrees advance would have number one cylinder firing at 80 degrees after top dead center. 20 degrees advance would have number one firing at 70 degrees ATDC, etc. An aftermarket timing tape, when available, is another way to accurately set spark timing. Otherwise, one can remove crankshaft dampener or pulley with timing marks, measure circumference, divide by 4 and mark at 90 degrees ATDC, 85, 80, 75, 70, 65, and 60 degrees. Marking all way back to 30 degrees ATDC is a good idea. This is most advance likely practical for max fast burn power and allows one to check for total mechanical or electronic spark advance. Optimum timing for best performance and economy, like fuel mixture, must be worked out by road testing converted vehicle.
Fully vaporized fuel is likely to be combustible over a wide range of air/fuel ratios. As a safety precaution it makes sense to install a PCV valve or something similar. Should a timing chain let go or an intake valve bend, we don’t want fire in intake manifold to find its way back to our bubbler and explode it. Better that a fuel supply line disconnect or rupture under hood from combustion back pressure.
The bubbler where our vapor is made by engine vacuum, is a modified fuel tank or racing fuel cell. Our tank/cell must be vented to atmosphere. We want our air pick up filtered with something like a lawnmower air filter and mounted higher in or outside vehicle, than outside of fuel filler tube, which is most convenient place to tap our tank/cell for a very rich vapor.
On typical fuel tank, our air vent can be easily installed where original fuel supply line attached. We want another PCV valve near our air supply filter to seal tank from atmosphere when vehicle is not running and tank/cell is pressurized by atmospheric heat. Gas expansion in our tank now moves fuel through carburetor into intake system, where it allows instant starts, as with fuel injection.
Bubbler design is guesswork. Whatever supplies most consistent air/vapor mixtures from a tank 3/4, 1/2, 1/4 and 1/8 full, is what we want. Variations in vapor from tank will make engine tuning difficult and variable. Consistent mixture from bubbler is important. If air supply is opposite vapor supply line, we would expect a richer vapor than if air supply line was near vapor supply line. We want air supply line to lie on bottom of tank, below whatever fuel we have. We never want fuel tank full, so a drain in air supply line will be handy for accurately checking fuel economy. You add ten gallons to an empty tank and drive until a working fuel gauge goes below 1/4 tank and you then drain remaining fuel from tank into gallon jugs. Subtract remaining fuel from ten gallons you started with. Divide gallons used, into miles driven and we will know our MPG.
