Propane system documentation?

[Reddeye]

My ‘72 C20 big-block was converted to a dual-fuel propane and gasoline setup in the 1980s. It has an adapter between the air cleaner and the carb made by Independence Carburetor Systems which is no longer in business. My question is does anyone have any documentation on adjusting this system? I’m having a hell of a time trying to get my truck to run properly across the rev range.

[Ol Blue K20]

I wish you luck, I'm no help. I don't have know anything about them. Have you talked to a fork lift mechanic? I would think the basics would be similar.

[Reddeye]

The only documentation I have is this crude system diagram, but I have nothing on adjustments.

[gearheadperkins]

Maybe check on some off road forums, as alot of offroaders use this setup for the extreme angles they run on trails and rocks.. Best of luck.

[truckster]

It's been years since I worked on propane systems, but the most common problem I found was that the diaphragm in the vaporizer would tear or get so stretched that it wouldn't meter correctly. Is there a brand name on the vaporizer? Most of them were made by Impco and you should be able to find parts for that brand.

[Reddeye]

Yeah, it's an Impco, but the problem started when I removed the air cleaner to fix the wire that goes from the cab to the diaphragm mechanism in the adapter, something with the adjustment bolt in the top of the air cleaner.

[Caddylackn]

What size Impco? If its a 425, that is only rated up to about 300 hp. You say this is a big block? It probably is not big enough to support your horsepower so I would imagine its running out of fuel at higher rpms. The Impco 425 is equivalent to a 2-barrel carb for a stock V8.

https://propanedepot.ca/wp-content/u...Carburetor.pdf

Watch your temperature gauge, as the motor will get hot and burn up valves if you are running it lean for long periods under load.

[Reddeye]

Correction: It's not an Impco, but a Borg-Warner Marvel-Schebler/Tillotson (seriously, five names guys?).

[chevmn]

Looks like same system that was on my big block truck when I bought it, and it had sat for well over 30 yrs. I never even attempted to resurrect it, and had no intent on running the truck on propane, so I removed it all, and sold the parts to someone who does propane. One thing about it, the truck only had about 62,000 miles when parked, and had been ran mostly on propane. Engine is extremely clean, and the only issue I ran into was timing gear was brittle, so I replaced (glad I checked).

[Reddeye]

Poking around and did find this guy who has a similar system on a 1974 Chevy: http://forum.73-87chevytrucks.com/sm...?topic=23550.0

[Caddylackn]

Propane is a great fuel to use, its cleaner burning and cheaper if bought in bulk. A propane only fuel system is good, but the gas/propane set ups not so much. To take advantage of propane you need to bump up the compression ratio to 11 to 12+ to one to take advantage of the higher octane (slower burning) of propane and also run more timing and a different timing curve on the distributor. There is 30% less amount of energy in propane than gas, so you need to take advantage of the higher compression ratio to make a propane system close to equivalent to gasoline in power and mpg. If you are at a stock 8.5 to 1 CR, the propane will make about 20 - 30% less power than gasoline. So there is no real reason to run propane on a stock V-8 unless its a fleet vehicle with really cheap propane available.

A truck setup that runs well on one fuel will run poor on the other fuel, so there are a lot of compromises made in compression ratio and timing. A dual fuel system makes sense if you are running higher CR on iron heads and don't want detonation. The propane can be run on hills and during towing and the gasoline can be used for starting and cruising. A propane fueled motor runs much cooler since it soaks up all the heat in the intake to vaporize the fuel. I have heard that it is very difficult to start vehicles in the winter on propane, so they start the truck on gas, then switch over when its running.