A miss when idling and with throttle...

[77 Bronco]

I hope this is in the right section. I have a 1968 Ford F-100, Automatic, manual brakes, 360FE 2 barrel completely stock w/ 70,000 miles. The truck started missing today. If you let the truck idle and stand at the back of the truck it will miss now and then. If I take off the vaccum line going to the advance module on the distributer and plug it then it will run smooth. The module does hold vaccume so I know the diaphram is good. The linkage also moves when I put a vaccum pump to it. I thought it was the points and condenser...changed them...re-timed...reset fuel rich/lean...ran like the day she came off the dealer lot. Went right back to having a miss. I checked all the distributer/points connections. Isn't there supposed to be a resistor for the coil? I threw another coil I had off something else to make sure the one that was on it wasn't bad. No change so I changed it back. My specs that I got were:

Plug gap: .035
Point Gap: .017 (good stating point really)
Dwell: 26-31 sitting at 29 but point gap isn't .017 anymore to acheive that
Point Resistance is a little high - not sure exactly how to remedy this
Idle RPM: 550-600 ....is just under 600 when in drive..800 when in park
Timing: 6 degrees BTDC

I have a set of autolite plugs in there now....only because these parts stores around here don't have the plugs for these old trucks laying on the shelves. Got a set of mortorcraft on order...should be here soon. Plugs look kinda light colored...maybe white...which means lean...I am 3-3 1/2 turns out on my mixture screws for the 2 barrel. It runs off choke..no issues there. My vaccum gauge is broke so I can't determine what vaccum I have. I've ran 3-4 tanks of good fuel throught the tank by now....no I don't beleive it is bad fuel. I add a lead additive so as not to hurt the valve seats. I am a young guy in my mid 20's and tried to pickup as much as I could from my dad before he passed. I know better than to throw parts at it...most of the time it is something that can be tested or thought out...or even overlooked. Still learning. Any help is much appreciated. Sorry if I am all over the place with the info. Let me know what kind of information you need. Thanks everyone.

[Redmanbob]

Considering that if the vac line going to the carb is connected to ported it shouldn't have any vacuum at idle. Stumps me as to what effect the line being removed and capped could possibly have on the idle. If it's connected to ported vacuum and you have a vacuum at idle, something needs adjustment to correct this. As to it being part of the miss issue ?? Not sure but don't think it would be. Checked the plugs for cracks ? dizzy cap ? /carbon tracking.. Sorry not much help, "electrical is the devil" for this one... Born oblivious and seems it's gonna stay that way.

[77 Bronco]

My dizzy is a dual vaccum. The port towards the front is for manifold vaccume....off of the vaccum tee at the rear passenger of the manifold and the port closest to the body of the dizzy is vaccum from the carb...front passenger side port. Dizzy cap and rotor are worn but look good. But I was thinking of replacing anyways....but I don't see it being a problem since my issue is intermitent in a way. Thanks.

[77 Bronco]

Oh, just so everyone knows....it misses on both sides of the engine. It has true dual exaust so u can hear it miss on both sides...has to be ignition. Oh, and the autolites are brand new...but I guess I could have a bad one or 2....hmmm...changing to motorcraft anyways

[Bear 45/70]

Plug wires. I also think you have you dual vacuum plumbed backwards.

[77 Bronco]

Plug wires are new....but I'm going to check the firing order tomorrow just to play it safe....still doesn't explain the miss here and there. Seems right to me...hoses are formed and almost fit right into place. Any info on how a dual vaccum is plumbed?

[HOTWRENCH]

Manifold vacuum to the distributor will cause too much advance at idle and cause the symtoms you describe. Your vacuum advance is giving you too much total advance when idleing. Change to a single port vacuum advance and run it with ported vacuum and your problem will go away.

Have a good day-----Hotwrench

[77 Bronco]

Hotwrench....the dual vacuum distributer thas installed now is the original. I'd like to stay with that. Are these know for having issues? Newbie question...what is the difference between Manifold Vacuum and Ported Vacuum? I know Manifold is constant and vacuum from the carb is only there off of idle. I guess I am not familar with the term "Ported Vacuum". Thanks.

[HOTWRENCH]

77 as far as the dual port vacuum advance goes , it was used for emission control. Just use the port on the front of the canister, not the one closest to the distributer cap. Ported vacuum is a part time source , the only time vacuum is present is when the throttle is opened. This port is on the carb body on Motorcraft carbs. Passenger side near the choke assembly. Some engines will tolerate full time vacuum advance , butt!!! many won't. That port on the back side of the dual advance canister is to retard timing , as stated it was used in emission control setups.

Have A Good Day-----Hotwrench

[montana_highboy]

Have you checked for vacuum leaks? if the miss is only during idle and part throttle i'd suspect a vacuum leak.

[77 Bronco]

montana_highboy....I don't think it is a vacuum leak since when I pull the manifold vacuum hose off of the dizzy and plug it the engine smooths right out.

[Bear 45/70]

Originally Posted by 77 Bronco

montana_highboy....I don't think it is a vacuum leak since when I pull the manifold vacuum hose off of the dizzy and plug it the engine smooths right out.

Yep, you have the manifold vacuum to the wrong port on the vacuum advance unit.

[77 Bronco]

Bear 45/70...so I am assuming that the "Manifold" vacuum line goes to the closest port to the distributer cap and the "ported" or vacuum line coming from the carb goes to the port fathest away from the distributer cap....closest to the radiator? If I remember correctly i thought they may have been backwards...so I connected the "maifold" vacuum to the closest port to the cap and the truck died but I didn't have the "ported" or carb vacuum line on the other fitting closest to the radiator. I will give it a try again. Maybe I should post a picture in my gallery of how I have it set up. I'll try and do that tonight if I can. Thanks.

[Bear 45/70]

Like was said before, the engine will run better if you just drop the manifold vacuum line altogether. It's there to please the government emissions BS and is not preformance enhancement. The reason the motoried is that the manifold vacuum retarded the timing causing the engine to die (it won't run that slow) You need to recheck you initial timing again too.

[77 Bronco]

Bear 45/70 I beleive the vacuum is there for the advance to work. I don't want to go away from original. So something is either broken or not properly tuned or whatnot. Just trying to narrow it down. From whats been said here if I just do away with it I should be in the clear and I understand that. As I have said before. I am looking to stay original with everything working within reason. It's not my daily driver but I do try and take it out as much as I can. Real nice truck. I appreciate everyones input...just keep straying away from the issue of what is bad. In my mind....if it's original and worked foer some time and has gone bad or out of spec or whatnot...unplugging or disconnecting doesn't solve the issue in my mind. Could you imagine a doctor saying....sir..ur heart isn't functioning properly...we're going to disconnect it....lol Hope I don't come off the wrong way....I do appreciate everyones input. Just looking for a reason as to why I all of a sudden would have to disconnect it when it's been there it's whole life. I'd either like to replace/readjust/rebuild whatever is failing. But I am not looking to just throw parts at it. Thanks.