Hey all,

I have searched the forums, the internet, and various other places, and can not come up with an answer.

I have a 1986 F-250 California truck with a 351. I have always been from the train of thought that the vacuum advance is supposed to be hooked up to ported vacuum. I found that the factory is supposed to be hooked to manifold vacuum, but then goes to a thermal switch with three ports. Is that port supposed to switch between manifold and ported vacuum, or is it supposed to be manifold vacuum all the time?

Also, are these trucks (carbed) as dependent on temperature for the vacuum switches as, say a computer controlled engine? I ask this because my temp gauge reads barely into the normal area (very low). I know it is a factory gauge, and probably not too accurate, but do not know what the thermostat is in there now. I will be replacing it soon, but was wondering if the above switch is supposed to switch back and forth between manifold and ported vacuum after the engine warms up.

Sorry if I repeated myself..........

Thanks,

Nick

 

There's a whole section in the factory manual on this which explains in great detail exactly what goes on, when and why.

But briefly, the 'retard' side of the vacuum unit on the distributor is always directly piped to manifold vacuum. There are 2, 3 port temp controlled vacuum switches in the cooling sytem, although the temp that they 'switch' at is not the same. On both switches the middle port is the 'master port' which hooks up to the top port when 'cold' and the bottom port when 'hot'.

The advance side of the distributor dual vacuum unit is piped to the middle port in the 3 port temp driven 'switch' on the thermostat housing. When the engine is cold or at normal operating temperature the middle port is open to the top port (which goes to the next switch in the system). When the engine is hot to overheating the middle port is open to the bottom port. The bottom port is piped to manifold vacuum so essentially when you get to an overheating situation there is manifold vacuum on both sides of the dual vacuum unit and you will get no vacuum spark advance.

So when the engine is cold or at normal operating temperature the middle port of the thermostat mounted 'switch' is open to the top port. This top port is linked to the middle port in another 3 port temp switch mounted somewhere else in the cooling sytem (not sure exactly where). The top port of this switch linked to the 'spark port' vacuum outlet on the carby thru a special valve which delays the release of vacuum. The bottom port of this switch goes directly to the spark port.

So, when the engine is cold, vacuum from the spark port is routed through a 'sustain' valve which retains the advance vacuum longer than normal, giving more sustained advance while the engine is cool. When the engine gets to normal temp the sustain valve is bypassed and the distributor sees instantaneous vacuum from the spark port in the carby

This spark port on the carby provides no vacuum at closed throttle (idle), high vacuum at part throttle reducing to low vacuum at full throttle.

Honestly, that was the brief version!...

 

Thanks, all this vacuum stuff is killing me. It is all there and all hooked up, but I am not sure if it is working right. My distributor only has an advance, not a vacuum retard too. I would pull it off (all the extra vacuum lines), but the truck runs well as it is, and I have finally learned that if it ain't broke don't fix it..... now with that said, I am wondering two things - do I have a bad thermostatically controlled switch? Or is it not getting warm enough for the switch to work right? Is it actually working correctly?

I just measured the temp with a probe. Putting the probe in the water at the radiator showed 130 to 145 degrees. Putting the probe on the area at the base of the switch showed 150 to 155 deg. I am thinking the previous owner put in a 160 deg thermostat, so things might not be working as designed. When the truck has warmed up as warm as it will, I am still getting manifold vacuum to the vacuum advance. Is this right? I am going to replace it with a 190 or 195 deg one on my days off but was wondering if I am on the right track.

I am just hoping to get this advance working correctly simply because I want it all working right.

 

Okay, the truck sat for a little bit, and when I got in and started it, the coolant was a little warmer since it had been sitting - it was almost to the N in normal.. I immediately hooked the vacuum gauge to the ported line and was getting a vacuum reading at part throttle with little reading on the manifold vacuum line. I got in for a quick test drive and it felt a little quicker off the line and a little more responsive. As the engine cooled down (the coolant circulated more), it felt like it lost a little of the peppiness. I will replace the 160 deg thermostat with a 195 deg thermostat on Friday and hopefully that extra 35 deg will make the little differece I need. I think I am on the right track.

 

From what I understand there is essentially 2 vacuum paths from the carby to the advance unit. Straight thru which it uses when warm or via the 'sustain valve' which will hold the vacuum advance on longer. You could try going straight from the carby to the vacuum advance unit and see what that does. Make sure it's on the right port in the carby too, there's often more than one.

Having said that it does sound like it's running a bit cool.

 

I don't think you should ever be getting manifold vacuum to the advance. The 'spark port' vacuum from the carby is from the air cleaner side of the throttle plates.

No throttle = No vacuum to spark port = No advance
Part throttle = High vacuum to spark port = Advance
Full throttle= Low vacuum to spark port = low to no advance (depending on the power of the return spring in the advance unit)

 

Thanks, I am guessing that I am still getting a little vacuum because the switch has not opened all the way since the engine is running cold. Of course, that is only a guess.

 

Your engine will last longer also with a warmer thermostat in it. It heats up the oil and and helps get the nasty stuff out of it.

 

http://performanceunlimited.com/cobr...hermostat.html

This site claims over double the cylinder wear at 160 degrees, than at 185.

 

Okay,

I put in a new 195 deg thermostat. It runs like before, but now that it warms up, I have less vacuum at idle going to the dist, but there is still some. My next gusee is that there is a problem with the ported switch. I looked on line and found that they come in different colors (I am sure it is not for looks). Since they appear to be color coded, is there a chance that MAYBE there should be a little vacuum at idle?

 

They are color coded for different temps. Each color switches at a different temp, they need to be replaced with the same color.

These ported vacuum switches PVS, should be changed and/or tested at regular maitenance intervals. Usually around the same intervals as you would change the thermostat, or every other time.

If these PVS's are Original, it wouldn't hurt to change them.

 

I think you'll find that the colour relates to the temperature which they switch over from the top to bottom port, 70°C is the rating for the one in my manual.

I suppose you could still see a little bit of vacuum at idle. The vacuum advance port on the carby is upstream of the throttle plates so it shouldn't see manifold vacuum, but due to the air filter there will be a lower pressure in the carby than the ambient air pressure. You could try running it without the air filter and see if you are still getting any vacuum then.

Also, if your throttle plate is open a bit much at idle then you'll see a bit of vacuum but it shouldn't be enough to activate the advance.