Vacuum Advance

Lifted2x · Jan 26, 2009

I have an 83 with the duraspark conversion but I dont have a ported vacuum for the distributor. I have the truck running on propane and I just haven't figured out what to do about the vacuum advance. I heard the holly carbs don't have a ported vaccum and just wondered what others where doing to make it work.

Wicked_Sludge · Jan 26, 2009

there has to be at least 1 vacuum port somewhere. is your brake booster hooked up? you could "T" off of that. otherwise, there should be fittings in the intake manifold. if there arent you could drill and tap your own.

Lifted2x · Jan 26, 2009

I wasnt sure if it could have straight manifold vacuum I was told it had a "timed" vacuum port from the carb. So its fine to just give it manifold vacuum?

Wicked_Sludge · Jan 26, 2009

i cant think of any way the vacuum could be timed. im pretty sure vacuum advance dizzy's just use regular manifold vacuum.

Lifted2x · Jan 26, 2009

Alright that's kinda what I figured, Thanks

shadetree · Jan 28, 2009

It is not "timed" vacuum, but "ported" vacuum. It started being used in the 60's when emissions became a factor. Using manifold vacuum causes the distributor to be fully advanced at idle. This causes an increase in emissions.

By "porting" the vacuum, timing is retarded to presets during idle reducing emissions. This is done by having a vacuum port above the throttle plates connected to a dedicated port. During idle, the port is closed to manifold vacuum, and when the throttle plates are open, the port is exposed to manifold vacuum advancing the timing.

If you have used the Edelbrock carbs, there will be two vacuum ports in the base. One is ported, and the other is not. Holleys usually have the port up on the side of the metering block on 4bbls. Also, carburetors used on emission controlled vehicles are different than the earlier carbs as they are designed to operate with the huge vacuum leak associated with the PCV valve. This connection is usually one of the larger ports in the base of the carb, the other being for the brake booster.

shady

Lifted2x · Jan 28, 2009

shadetree said:
It is not "timed" vacuum, but "ported" vacuum. It started being used in the 60's when emissions became a factor. Using manifold vacuum causes the distributor to be fully advanced at idle. This causes an increase in emissions.

By "porting" the vacuum, timing is retarded to presets during idle reducing emissions. This is done by having a vacuum port above the throttle plates connected to a dedicated port. During idle, the port is closed to manifold vacuum, and when the throttle plates are open, the port is exposed to manifold vacuum advancing the timing.

If you have used the Edelbrock carbs, there will be two vacuum ports in the base. One is ported, and the other is not. Holleys usually have the port up on the side of the metering block. Also, carburetors used on emission controlled vehicles are different than the earlier carbs as they are designed to operate with the huge vacuum leak associated with the PCV valve. This connection is usually the large port in the base of the carb.shady

Ok thats what I have been told. My ranger has a propane carburator with no provision for ported vacuum. It does have the duraspark ignition so is there any way to make it work correctly?

Wicked_Sludge · Jan 28, 2009

thanks for clearing that up shady. i guess i havnt been under the hood of a carbed vehicle in longer than i thought :icon_thumby:

shadetree · Jan 28, 2009

If you have a Duraspark conversion, you may not need a vacuum port, depending on which system you have, and which carb you have. Give this a read, and check it against your system and see if it helps.

http://www.therangerstation.com/tech_library/2_8Duraspark.html

shady

Lifted2x · Jan 28, 2009

shadetree said:
If you have a Duraspark conversion, you may not need a vacuum port, depending on which system you have, and which carb you have. Give this a read, and check it against your system and see if it helps.

http://www.therangerstation.com/tech_library/2_8Duraspark.html

shady

It says to use the ported vacuum. I am using a impco 100 propane carb without any vacuum ports for a dizzy

shadetree · Jan 28, 2009

Lifted2x said:
It says to use the ported vacuum. I am using a impco 100 propane carb without any vacuum ports for a dizzy

Yes, if the distributor has not been changed. It evidently hasn't since you have a vacuum advance pot on your distributor, correct? You can run engines without the advance being connected. Race cars do this all the time, but you have to advance the distributor accordingly.

You can also hook the distributor to the intake manifold vacuum, This will probably require a distributor timing change. Check your timing as it is now, and if it is in specs, leave it alone, then connect the vaccum line to it. Basic timing is always set with with no distributor advance, either vacuum or mechanical unless it is stated otherwise in the manual.

Fully advanced timing at idle will sometimes screw up the idle. You can try it and see. If the idle is ok, I would just leave it alone.

The only other option is to get a carb with ported vacuum. I suppose you could get an adjustable valve that opens with a pre-set pressure if you want to go to the trouble.

What counts is total timing advance. You want total advance to be somewhere over 30*. Check the timing with the vacuum line disconnected and plugged at idle. Note this figure.

Rev the engine up and see if the timing advances. If it does, then you also have a mechanical advance in the distributor, which most do. Note this figure.

Connect the vacuum hose to the dist. Note how much the timing is advanced. Add the three figures together, basic, mechanical, and vacuum and see what you have. Let us know what you find.

Being that you are using propane, you can run more timing advance due to the octane of the propane.

shady

Wicked_Sludge · Jan 28, 2009

this may be dumb, but would it be possible for him to just drill and tap the side of the carb above the throttle plates, then screw in a hose barb? this would give him ported vacuum, right?

shadetree · Jan 28, 2009

Wicked_Sludge said:
this may be dumb, but would it be possible for him to just drill and tap the side of the carb above the throttle plates, then screw in a hose barb? this would give him ported vacuum, right?

Sure, if he wants to go that far. Keep it small, and stay away from the venturi. He really needs to find out what kind of Duraspark setup he has. But, I think it will run fine with manifold vacuum if it needs vacuum at all. At least give it a try.

shady

Lifted2x · Feb 4, 2009

Sorry it took so long to reply but I did pull the vacuum line of and it is doing way better. I think it has about 15 or 20 degrees of mechanical advance. Do you know why there is two grooves on the crank pulley? I havent had a chance to change the timing since I pulled the line off but it is way better.

martin · Feb 10, 2009

The reason for the vacuum advance is to retard the timing when the motor is under heavy load, therefore preventing detonation. If you do not have a ported vacuum on your carb you can use a vacuum soleniod on the vacuum line and a switch on the throttle arm that kills the vacuum at idle. I just wired mine to a switch on the consul that gives me two different idle speeds. Works great when I need to compensate for running the on board air

Vacuum Advance

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