idle issues

[Big GMC truck]

Should my truck's, 1984 GMC K1500 305, idle be the same in park and in gear? Currently, the idle is 600 rpm in gear and increases to 1000 rpm when placed in park . My mechanic says the accelerator cable is the cause of this? Is this true or is there something else causing the problem? Thanks

 

[beater_k20]

sorry, but your "mechanic" doesnt know his ass from a hole in the ground. it is normal for an RPM drop when the trans is placed in gear, and it fluctuates with timing adjustment. check your ignition timing, and adjust your idle screw on the side of the carb. should be like 850 at idle, 750 in gear.