I have seen many Intake/Exhaust Manifold hardware loosen off over time causing vac. leaks. In turn this will cause some nasty back fires,

The mixer hardware tends to come loose on occasion also. It will cause the same problem.

During Pm's I now tend to quickly check the tightness of these things.

4y engines tend to gum up valves run a can of seafoam or intake cleaner through intake hope this helps

I was just talking to some Toyota guys and they suggested that you may have weak intake valve springs. They Installed stronger springs and it seemed to cure the problem. Check with you Toyota dealer for some tech reports on this issue. Hope this helps

Prentice...
I have a consignment an I use what they give me on the spark plugs. I do make sure they are gapped to toyota spec..028-.030.

I have a fleet of 7 and I agree that the regulators get tar build up and need to be cleaned. The other thing is the intake manifold gets alot of carbon build up. I end up removing and cleaning as best as I can. The other main trouble I have with them is the wires tend to break from vibration at distributor connection to harness.

Actually this problem started while trucks were still under warranty, Had local dealer send their tech out and that is how I found out what caused our problem. Had same problem on two trucks after warranty expired.
We had at the time nine 7fg's and at least five of them had this problem. Cooling system checked out ok and after we switched fuel all is fine. Have not had to repair any in over a year now since fuel change.
I was told directly from their tech that low grade fuel caused our problem.

Nash,
some one ought to tell you about how and why that goop gets there... and you should check the cooling system of the truck, and make sure you have a working thermostat, plenty of coolant, and maybe even an LPG coolant line thermostat, that stops flow above the regulator (return to engine) when that flow is above a certain temp. Good fuel helps too..

Had same problem and turned out to be the fuel regulators were full of oil. We were using a low grade LPG then went to better grade and haven't had that problem lately. Still had to rebuild the regulators. Cost was a little over $100.00 dollars but they ran great after that. Not saying that is your problem but just something to check.
There should be a plug/screw at bottom of regulator to drain out oil if any is in it.

Just a quick question. Do you have the right spark plugs in it, with the proper heat rang for a Propane Engine???????. I did a tune up on a TCM and it ran like crap. Changed the spark plugs the second time and it worked great. Sometimes you get faulty NEW parts. It's a quick, CHEAP Check. Hope this helps

That valve operates the fast idle. If you pull the vacuum line from the air cleaner side on a warm engine the idle speed will increase. It should be set at 1200-1250 rpm. When the tube is replaced, engine speed should drop to around 800 rpm, or normal idle speed. Transmission should be in neutral, of course.

It was "popping back through the carb/intake". I mentioned that I tuned up unit last month. I decided to replace wires/cap/rotor to see if it makes any difference. Surprisingly the problem went away... It has to been a bad wire or something.

If you don't mind... What is the purpose of the "valve, vacuum transmitting"? It looks like a filter that goes from carb to governor and has a red end and a black end. Why the color difference? Both directions appear to flow air the same way. How can you tell if its no good?
Thanks again.

when you say "backfire" are you talking about an explosion out the exhaust (maybe you have an exhaust leak?) or popping back through the carb/intake

I removed and inspected all vacuum lines by sight and hand. I checked timing with timing light and results were to book. The unit has 6000 hrs on it. The wires/cap/rotor/spark plugs were changed last month. Also the unit has "automotive backfire deflector" that runs to vapor box. If this helps trouble shooting.
Thanks for your time.

moisture in the distributor cap? valves out of adjustment? needs a complete tune up?
how did you "check the vac. lines"?