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My car is converted to LPG but I still run it on gas. One of the reasons is to keep the carburator working, the other is the automatic choke, which only works when running gas.
When I switch over to LPG, while driving, the fuel suply is cut off by a valve and after a while the fuel bowl is empty and I switch over to LPG. When going back to gas, the LPG is cut off and fuel enters the carb. After a few seconds the car runs on gas again.
This works nice and the car is running very good on gas and LPG but.......after a long ride (< 60 miles) when the engine is hot the car runs very poor when I switch back to gas for a few minutes. It won't idle and it smokes like hell. I have to rev the engine to 2000 rpm to keep it running. After that, it's like a switch is turned and the engine runs like it should be.
What could this be ? I was thinking about a hot, empty carburator recieving fuel which evaporates and causing all kind of problems. Any more ideas ?
 

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Erwin,
I've seen forklift carbs freeze on LP, even on a hot day....wonder if the LP is freezing the needle/seat and or float and sticking them open until they get thawed out? Just some wild speculation..*G*
 

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I think you have a classic vapor-lock problem from the gas sitting dead in the line and heating up to the boiling point. Then when the gas solenoid opens it sends the boiling gas into the carb.

One way to keep the gasoline cool would be to use a return line system like the FI cars do. If you install a bypass regulator with a return line just ahead of the gasoline solenoid it will circulate a small amount of gas back to the tank and keep the gas supply cool enough to eliminate the problem. The regulator would need to be adjusted to bypass just a little gas all the time.

You could also do this by installing another solenoid on the return line and have it open up when switching to LPG, and close when you switch back to gas.
 

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You could try a thin insulating carb spacer - it should keep the carb fuel bowls somewhat cooler and reduce any boiling due to the temp of the carb (it makes about 25 degrees c difference for me, possibly more, but its the difference between instant burn and reasonable temperature to touch). Also the return line is a good idea as whisperer suggested. One other possibility is it is running way rich (taking some time for the lpg to empty from the mixer lines into the carb at the same time as having extra petrol put in) for a short period in which case you could add a timed switch for the petrol solenoid.
 
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