Chevrolet Forum - Chevy Enthusiasts Forums - Engine is not running smoothly

Hello,
I have a problem with my 1981 Chevrolet Caprice V6. While driving a constant speed, for example 35 mph. the engine is not running smoothly. It is like it is searching. I know it’s not an fuel/carburetor issue because my car is running on gas and I have a LPG (like propane) installation. While driving on LPG the carburetor does not do anything, just passes the LPG an regulates the amount of LPG by changing the throttle valve position. Both on gas and on LPG the problem remains. I replaced the sparkplugs, cables, rotor, distributor and air filter already. Also checked the timing, everything seems to be OK. I hooked the car up to a troubleshooting device. There are no trouble codes. I’m thinking about changing the ECM but since I am from the Netherlands it is not so easy to get the parts here. And because I am not sure I hope the is somebody who knows what causes this problem. Can anybody help me with a solution?<O:p></O:p>
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Regards, Erik Ganzevoort<O:p></O:p>