In my opinion dual fuel is a poor compromise. You won't see optimum performance or efficiency on either fuel unless you have a fully electronic controlled set up.
With my dedicated propane conversion I get the same 11 mpg I got on gas but it starts and runs great no matter how cold it gets or how long since it last ran. I've never used the priming feature even at 5 degrees last winter. There's a reason they call it "Poor man's EFI"
Get a copy of this...
Another one of the very best sources of information out there is...
Franz charges for his trouble shooting guide but it's worth it. I'd got mine years ago when they were free but I'd recommend it even if it was $40!
I got my major propane parts from..
Prices at helmar were the best I could find when I did my conversion. Go with silicone diaphragm in the mixer for durability.
Here's a link to the CARB approved propane kits and accepted modifications. This should help if you have to get smog tested.
The Very Best info anywhere is here...but you have to register to use the site and you're not allowed to copy the info unless you pay. If you want to be street legal you'll have to follow these instructions. If you want to be safe you'll want follow them even if it'll be a trail only rig.
Search for "nfpa 58"
Click on the first link..."NFPA 58: Liquefied Petroleum Gas Code"
Down about 3/4 you'll see..."Additional Information about this document:"
Click on..."View the 2008 edition of this document."
Like I said, you'll have to register and you can't download the file but just about everything you'll need to know is here.
Propane Conversion of Cars, Trucks & RV'S. If you can find one buy it. They come up on eBay once and a while. It has just about everything you need to know in one place and will save you tons of research. It was published in the early eighties when converting to propane was popular. Some of the market data listed is obviously out dated but the actual conversion information is still good. Not much changes with traditional propane systems. The newer 425 mixers are factory set and have tamper resistant adjusting screws to meet federal regulations but everything else is the same as the book describes.
I went to Amerigas for most of the small parts I needed. They even got me a fuel level sending unit that works with the original Jeep fuel gauge on the dash. they had all the hose, fitting and threaded adapters I needed in stock. They even had a silicone diaphragm repair kit on the shelf for the 450 mixer.
Here's basically what I did.
Impco 450 mixer with silicone diaphragm,
Model E converter,
Edelbrock Performer intake manifold,
Holley throttle plate,
Mallory HEI dist. with one standard spring and one of the lighter springs to advance the timing a little sooner,
Initial timing at 12 degrees btdc,
Disconnected the vacuum advance to prevent over advancing.
I didn't block the exhaust crossover in the intake manifold as recommended but I eliminated the valve in the exhaust to prevent manifold heating.
My tanks are under frame mounted and hold 23 gallons. (250 mile range) I put my fill connection where the gas filler was originally.
Closed loop computer controlled feedback system.
I mounted the mixer, converter and control solinoid on a custom plate on the firewall just behind the engine where the charcoal canister used to be. You'll see converters mounted in all sort of positions but you'll find that black gunk separates from the propane and will cause problems eventually if not mounted correctly. The discharge port is supposed to be pointed down to allow drainage of the "Gunk". The outlet of the converter should be higher than the inlet of the mixer but I couldn't make that work so I disconnect the hose at the converter outlet every couple of months to let it drain out.
Here's what my set up looks like from the top.
The small black box that is nearly cropped out of the picture just to the right of the converter is the Autotronics Dual Curve controller. It uses a standard O2 sensor to adjust the mixture by putting a vacuum to the atmosphere port on the converter. I had the bung welded in just behind the y.
We mounted my tanks on a hinged skid plate so I can pull a couple of pins and drop the tanks down to service the valves.
Mark at Amerigas found the sending unit calibrated for the Jeep's original factory fuel gauge so my old gas gauge is now my propane gauge.
72 Wagoneer on Propane w/Family cage and 3" spring drops.