TB42 gas systems.

[GQPorkroll]

About to change my gas system, presently running an A300 type negative converter Impco system. I wasn't fully aware of how how gas worked, but after this system being explained to me, I'm not a fan and it will explain some of the issues I'm having.

First off, I didn't have any idea of positive/negative systems, and I had no idea that negative systems used vacuum to draw the gas through. I don't have a primer so it explains why it's impossible to start on gas. And because of the air restriction, it can't be good for it running on fuel.

I have just ordered a Holley setup for gas with gas lines going directly into the Venturi and a positive converter. It came from a gas specialist so I'm confident this system will work a lot better.

Has anyone had experience with this system? The dearest part was obviously the carby but if I'm able to run gas with petrol as a backup as opposed to the opposite, which I'm doing now, I will see an improvement in running costs. Also as there is nothing over the top of the carb I can go back to a standard airbox and run a snorkel.

[threedogs]

Not sure if this will help but my last truck a 75s troopy with 350 Chev ran a square bore 650 Holley, Kept melting the Venturi off it, my fault as try to get it too lean. What I did find out though is that you can get a mixer that fits under the Holley. Gas only needs a hole and butterflies. It had the mixer on top with air cleaner the works, damn but the name escapes me, only problem was it had an open air filter. How does your gas get in????

Name of the mixer was New Generation is TB42 efi????

[GQPorkroll]

No it's not EFI. presently the mixer sits on top of the air filter and sprays gas into the carb, relying on vacuum to draw it from the converter.

[threedogs]

Don't know how but try to find a mixer thats bolts under the carby, has to be one. not the best setup as squenctual best, have you looked at Gas Research, no power loss

[GQPorkroll]

Yeah this setup is a lot different to ones I've seen, will post some pics when it arrives,

I spoke to a gas fitter and he said that positive converters are a lot better, and using the modified Holley is a much better set up as the standard carby isn't very good to begin with, but when adding gas, there's a lot of compromises. He said they'll use a lot more fuel and the gas runs inefficiently. I've noticed that when I run gas there's a very strong smell of gas at idle I expect because the vacuum isnt drawing all the gas through, or it's drawing too much and it's not being burned. The new system is a lot more efficient in that regard and I expect better economy on gas.

It also allows me to use a snorkel and revert to the original airbox which is much better for me.

[buzzbro]

Impco systems work well on TB42 patrols but make sure thar it is a genuine 300A 1&20 series mixerthere were a few knock offs that never worked properly. If you are smelling gas at idle the idle mixture is too rich and a lot of people richened them up to over come starting issues. Starting with your foot flat creates more ported vaccuum and can help starting, most people are told never to touch the throttle when starting and this is certainly the case for forced idle systems and EFI's. I used to tell my customers to turn the ignition on for a couple of seconds then off then crank for cold starts. As far as the mixer causing a restriction on gas that can be true if the diaphram on the mixer is not aided. There were two ways of doing this one was vaccuum which was not the best or the one that worked was a cable primitive but effective. As far as running your original inlet piping there was a fully enclosed shroud that went around the air filter that could be plumbed up to your original intake tube.
300A mixers had comon problems with the 3 screws that held it together comming loose, they come up from the bottom and have the 3 nuts on them on top of the air cleaner plate. there are also ways to put in a primer valve to aid starting

[buzzbro]

If you go the holley option make sure it does not have a brass float (there was 3 different floats available) once the cold fuel hits it in the nice warm float bowl it will implode