Vacuum advance Question- 85 GMC
 

Got a question that I should know the answer to, but I cant seem to find it in the sea of vacuum lines.
The truck is an 85 GMC 1/2 ton SWB with the factory 305 and quadajet and all of the smog equipment in place. There is no vacuum hose connection to the distributor vacuum advance. All of the external ports on the carburetor are already taken.Where is the missing hose supposed to connect to on the carburetor??? Any ideas here? Thanks. Lane

Re: Vacuum advance Question- 85 GMC
 

Lane: Sounds like someone has eliminated the vac advance hose. Here is how you fix it. With the engine idleing find a vac port that is active, put a "tee" in the line and attach the vac advance cannister to it. Be careful here. There are 2 kinds of vac sources. 1 is manifold vac (vacuum present at any throttle position) and "ported" vacuum (vacuum present at one throttle position but not another) You want a source that is constant IE, manifold vac. To check for this before you "tee" in, put a vacuum guage in the line you choose and run the engine. Watch the guage as you slowly open the throttle to about 2000 RPM. If vac drops to near 0 you got the wrong line. If it stays above 10 to 15" thats the line you want to cut into. Set base timing then hook up the advance unit. How it works: Distributor runs at full advance at idle. (this is for power and acceleration). As transmission is upshifted load will increase and vacuum will drop retarding timing a bit to prevent detonation. As RPM increases and your up to highway speeds vacuum will have dropped to a point where the vacuum advance unit cannot tailor spark advance properly and the weights take over. This prevents detonation (spark knock) at higher speeds. Hope this helps, jim

Re: Vacuum advance Question- 85 GMC
 

Jim, you're response provided me with the information I'm looking for. I'll give it a try this weekend. Appreciate the help!