Should there be a vacuum at the port in the carb for the vacuum advance at idle and part throttle?
thanks

 

yes.. you want to use manifold vacuum, not the ported vacuum.. if you use ported vacuum, it'll idle crummy, and will backfire when you hit the 4bbl.

 

No, there should not be vacuum at the ported vacuum port on the carb at idle. This port only sees vacuum after you crack the throttle(it's above the throttle plate). You do not want to ever use manifold vacuum to feed the dist. advance, you get full vacuum advance at idle. A big no-no. Can cause overheating and very slow back to idle from open throttle. It should never backfire or idle crummy if you have everything else set up properly(Intitial advance set properly). BTW, when you get the secondaries open your manifold vacuum will drop to close to zero, meaning you won't have any vacuum for the dist. advance from either manifold or ported vacuum ports.

 

ok, I'm having a prob. with my rebuilt 2bbl - with the vacuum advance hooked up or the hose plugged, the truck idles normally but sort of chokes out and dies if I give it some gas. It runs fine with that hose disconnected. Any ideas?

Thanks, Dave

 

Check your the vacuum dyaphragm in the dist. If it is blown, you have a vacuum leak when you crack the throttle. I usually pull the cap and push the vacuum lever arm towards the vacuum unit, put you finger over the port on the vacuum unit and the lever should not move back much. Pull your finger off the port and it should move back to the start point. If it moves back when you finger is over the port the vacuum unit is bad.

 

Thanks Bear,

the advance is good - I've already torn the dist apart and freed everything up - timing is ok, float is ok - I'm stumped.

 

it's weird - only time the engine even runs is with a ported vacuum leak (flowing free)

 

Try running the idle mixture screws all the way in. If it stays running you have a blown power valve. This happens when you backfire the engine. When you replace the valve get a power valve protector kit to prevent it from happening again.

 

You do not get full advance at idle. You get about 4 degrees + the baseline at idle.

Just because you have full vacuum at idle doesn't mean you have full advance. vacuum only modifies the timing by a few degrees, the vacuum is mainly for fuel economy. The rest is done by the mechanical advance in the distributor (racers use for mechanical advance, b/c they dont care about mileage)..

If you use ported vacuum at idle, you will be set at the baseline timing which, on most trucks, is 10 degrees. (ie no vacuum). However, when you look on any ford tag on the radiator, it says "PULL VACUUM LINE OFF OF DISTRIBUTOR AND PLUG". Why would you have to do that if there is no vacuum? That is b/c ford has them into the manifold vacuum from the factory.

Like I said, on my truck, with the distributor on the ported vacuum, my truck will idle rough, and it wanted to backfire thru the carb if you dumped the 4bbl from idle.

With the vacuum advance tied into the manifold vacuum, it sets about 13-14 degrees BTDC, and doesn't backfire.

BTW, if you don't believe me, throw a timing light on a EEC-IV truck with full electronic advance. at idle, it definately won't be on 10 degrees BTDC. That is why you unplug the spout connector.

My advance to the guy putting in the distributor is to try both. Manifold vacuum worked fine on my 351W and 300-6.

for the record, in case any one doesn't know, ported vacuum means that you only get vacuum when you start openning the throttle. at idle, you have no vacuum at all. Manifold vacuum is on all the time..

 

Bremen242; I'm sorry but you are wrong. I've seen as much as 10 degrees of advance from vacuum depending on the application and you do not ever want manifold vacuum at idle to you dist.. The ported vacuum port on the carb will have no, nada, none at idle if the throttle plates are closed. It is physically impossible due to design of carb. I've been racing cars, motorcycles and boats for better than 40 years and have forgotten way to much about carbs, dist.'s and engines in general for you to train me. I have even been a certified mechanic in my past. I have even raced 2 strokes successfully and know about non-accelerator pump carbs too. You never want any vacuum advance at idle because when you open the throttle the timing goes from an advanced state back to intitial and then advances again. This can cause stumble, hestitation and misfires, a no-no. To properly get your initial advance up high you need to shorten your advance curve in the mechanical advance in the dist. Otherwise you get strange stuff happening. You never got the info to use manifold vacuum form any certified source, cause it isn't the proper way to do it. If your truck is doing what you say it is you have several problems that can be easily addressed and fixed. You should never have a backfire when you get into the throttle, if you do you need to fix it and don't tell others how to do the wrong thing.

 

perhaps this is the case, however, my ford repair manual show this:


Original pic

perhaps you know better than ford, but my dizzy is saying on the manifold vacuum, not the ported vacuum.

I'm not going to saying you are 'wrong', but I think a picture says a 1000 words. Especially since that diagram is off of a emissions stick from an F-250.

Here is another site I found that explain vacuum advances. It also says that GM distributors are backwards from fords. perhaps you 'raced' gms?

http://www.nationaltbucketalliance.c...uum/vacuum.asp

 

The diagram offered in evidence does not show hose from vacuum canister going directly to manifold vacuum but it has a vacuum control valve (VCV) in the hose controlled by a series of other VCVs which hookup up to the Air bypass and and the ported vacuum port (S) on the carb. Complicated system which I have no further desire to figure out.

 

The only time the manifold vacuum is used is when the truck engine gets to hot in traffic and to spin the water pump and fan faster the system switches to manifold vacuum to advance the timing which makes the engine idle faster. Under normal operating conditions the ported vaccum runs the advance unit. That unit called a VCV(vacuum control valve, on my engine it's the PVS or ported vacuum switch) is a temp switch and only gives you manifold vacuum when the engine reaches high temps. Trace it out on the other leg of the switch, you will see you only did half you homework, also find out how the VCV works before you tell me I'm wrong. Being able to prove your point with out all the facts don't get it. "A man's got to know his limitation" as Dirty Harry says.