M4ME 17085580 primary circuit

[theraymondguy]

Since we are talking about a wet-flow system a working crossover is going to be beneficial.  It will heat the intake and improve efficiency allowing us to run a leaner mixture and greatly improve engine performance thru warm-up. 

Not having one is NEVER a good idea if you plan on using the vehicle in cold weather, so keep them open on your plow truck engine or you will absolutely hate it in the Winter months.

For part time, warm weather and casual use you can get away with a blocked crossover but it does not improve power or performance since the intake eventually heat soaks anyhow. 

You also loose your factory choke if it is divorced or "hot air" so plan accordingly.  F

So basically I'm recommending to keep them open......Cliff

I'm glad I left it alone then. 

I'm not having any luck with that warm weather I'd requested.  The carb is mounted, plumbed and connected - no fuel in it. 

Something that's always bothered me about this carb (parts carb is the same) - the port for the vacuum advance has vacuum on it at all times.   According to the underhood decal the circuit is plumbed correctly.  It's not full manifold vacuum at the port on the carb, but it's 70% of it or so.  It goes from the lowest port on the main body (drivers side) to the throttle plate and is exposed to vacuum via a horizontal rectangle port located beneath the primary throttle blade.  The port is actually just below the transfer slot as well.

Clearly this will provide vacuum at idle.  The car is equipped with a coolant temperature activated switch - I'm fairly confident it is stuck open.  This is meant to be some sort of emissions device, I'm guessing its purpose be to would close off vacuum advance until the engine is up to temperature?

[77cruiser]

My 84 truck had something like that, would turn the vacuum on or off when it warmed up, I can't remember which way it was.

[Cliff Ruggles]

The factory tried many ways to regulate vacuum to the distributor vacuum advance over the years.  None of them were overly successful especially the TSC systems used starting in 1970.

One of the problems they experienced was having manifold vacuum advance active at idle speed.  This thru emissions off the scale and with some engines they lost control of the idle speed with too much timing.

They also tried timing retard set-ups at idle speed which were pretty hopeless.  In the 1980's they used valves to reduce vacuum/timing at idle put not kill it off completely then bring it on with engine speed increases. 

I've had the best success over the years just running vacuum directly to the distributor and bypassing all those devices/components.  Big advocates of manifold vacuum advance will often jump in and say that it works better than ported vacuum, but there are simply engine combinations that don't like, want, need or respond to a chit ton of timing at idle speed. 

I tell folks here to use what works best for what you are doing.  If your engine likes 20, 25 or 30 degrees timing at idle then add it in with the appropriate VA unit to your base timing via manifold vacuum.  If your engine combo doesn't make enough vacuum at idle for that deal you can buy a "special" vacuum advance to reduce the spring tension so it applies at really low vacuum.  I NEVER go that route here, but it is an option and often required for folks who like really tight LSA cams, low vacuum at idle, and menacing idle quality........Cliff