All the info Ive read, which admittedly did not some from a service manual, but from the net, suggests that manifold vac was used predominantly before the smog are.

I wasnt there, I couldnt say.

But. It only makes sense to me to use the full vac on vac advance. retarding timing means more gas is needed. Advancing allows a leaner mixture. At idle there is a leaner mixture. So advancing would keep it from having too much gas. And you can tell the difference when you switch. Maybe Im wrong. I dont know. But I will say with some degree of certainty, that you will burn less gas by connecting advance at idle and then toning the rpms and idle mix down a bit. Leaner mix with a little more advance seems wiser than retarded timing and more gas. At least efficiency wise. And performance wont be affected either way.

 

Yeah I'll have to dig into this some more, it has service info for a bunch of carbs and all kinds of vehicles, going back to at least the 1930s. There was some other service info in there too that I recall being surprised at because "everybody knows" ported vacuum is a smog era atrocity. The sentence quoted though, by itself indicates that is not the case in any event.

I think we done hi-jacked this man's thread pretty good huh? LOL (sorry)

 

I guess we did.

I would be interested to see. because it makes no sense in my sleep addled brain for the engineers back then to port the vac, when vac at idle seems to be better. Would love to know for sure either way.

 

Well, the fact is, ported vacuum is only an issue at idle, (so they say?) the vacuum signal is identical when cruising around town, highway runs, etc. Others are pretty forceful in saying that one should always always use manifold vacuum. Once I try this, maybe I'll be a True Believer, lol.

I've been meaning to try it, (manifold vac) because it's fun to get these old engines running well. I spent quite a while setting up the distributor mechanical advance curve, and read everything I could find about vacuum advance as well. Ford had a LoadoMatic distributor, that ran exclusively on vacuum advance, ported or not I dunno, but it wasn't received very well.

 

I imagine it wouldn't be very good to have ONLY vac advance. What happens when you reach WOT? You'd have almost no advance...

 

Good question. Not that familiar with it, it was used pre-1957 Y blocks is about the extent, it means a particular carb has to be used. Most owners have opted to install a different distributor probably.

 

So I think I have that LoadOMatic figured out.

It uses standard vac advance from full manifold vacuum to sense engine load. But instead of centrifugally operated weighs and springs, it has a orifice in the smallest part of the venturi that allows it to detect engine speed. The amount of air passing through the venturi is pretty much proportional to the engine speed (more air=higher engine speed), so the vac can advances the timing using the venturi "vacuum" and the Mani vac.

I think. 😁

 

This is part of the issue, it would seem there's a major yet intermittent vacuum leak or wacky solenoid or somesuch?

Bet you're not the first to have this exact same bogging issue, has an internet search came up with anything?

 

I haven't had time to read the explosion of replies on here just yet, I'm on my phone waiting for a meeting. However, I replaced a lot of vacuum lines today, set timing at 10 dead on, and adjusted the mix screw for max vacuum which is at 19 now. I pulled it out for a test drive and it was in a bog right away. It hasn't come out of that for an hour of varied driving now.

However, it's acting a little different in the bog now. First of all the bog is only from 1/2 throttle through wot now. Also it seems to be better when I turn the mix screw counter-clockwise. There's at least a full turn of the screw where vacuum isn't affected. That's a lot right?

When I turn the screw in further it gets worse.

Do I have the wrong jet sizes in this carb?

 

"Explosion of replies."

I should apologize, and take responsibility for the topic getting off track. But I won't - it's clearly Cstephen's fault.
Heh...

 

Guilty as charged.

Its the ADHD. Sorry.

 

Well it's staying bogged today. Hasn't let up for hours of errands now. It had a fit for about ten minutes where I only made it about a hundred yards.

I limped it to a mechanic friend at his shop. He had his old school carb guy test drive it with me. It acted bad but not like it can when it gets angry. That mechanic and I agreed that it seemed like it was running rich. However, after we explained it to my friend the shop owner, he made a better argument for lean. And I have to agree, based on the new symptom of falling on its face above 1/4 to 1/2 throttle on up it seems like it's not getting fuel.

Unless one of you has a suggestion in going to check the fuel lines all the way back for a collapsed line. (I live in south Dakota and while we have no emissions checks we have a lot of ethanol).
And I'm going to drop the tank and check the filter in there. I'm hopeful that I'll get this fixed!

 

If you think its running lean, can I suggest, and Im just thinking out loud here, cuz honestly Idk if its viable or no, but when it is bogging, if you think it is too lean a mix, close the choke plate about halfway. This will draw less air and more fuel into the engine. Lean issue could be tested that way Id think.....or just some carb cleaner into the top see if it comes out of the bog briefly.....


If you try that and it bogs more, it may well be an excess rich mix.

 

Will a lean condition or fuel issue, cause an instant 4 Hg. drop in manifold vacuum? Maybe, I dunno.

 

Both good suggestions and i've already tried them.

Increasing choke makes it worse, full or near full choke kills it immediately.

Carb cleaner also makes it worse.


So does this sound like a stuck float to anyone? I'm just confused because it didn't improve with a new carb.

EDIT:

Tonight it's cold again and the truck isn't requiring any choke to start. One more point for rich mixture!