See Figure 1
This is a more effective form of heat riser which is vacuum actuated. It is used on all carbureted engines built in 1975 and later. It heats the incoming air/fuel mixture during the engine warm-up process, utilizing a ribbed heat exchanger of thin metal that is located in the intake manifold. This pre-heating allows the choke to open more rapidly, thus reducing emissions and improving cold driveability. Problems with this system might be indicated by poor engine operation during warm-up.
This valve should be checked initially at 6 months/7,500 miles, and thereafter, at 18 month/22,500 mile intervals.
To check, move the valve through its full stroke by hand, making sure that the linkage does not bind and is properly connected. If the valve sticks, free it with a solvent. Also check that all vacuum hoses are properly connected and free of cracks or breaks. Replace damaged hoses or broken/bent linkage parts as necessary.