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Manifold vacuum

Thatbroncokid

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Mar 5, 2015
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119
Trying to tune my carb properly, gonna hook up the ol vacuum gauge to get the job done right, just curious where i should hook the gauge up to? Can't find a definitive answer as to where to find full manifold vacuum. Stock 302 with autolite 2 bbl carb. thanks!
 

B RON CO

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Hi, you want a source of direct manifold vacuum. If you have an automatic transmission the modulater is a good source. Some engines have a vacuum tree behind the carb. Good luck
 

DirtDonk

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There is a vacuum fitting/port/nipple/thingy under the front of the carb body, near the base and adjacent to the idle-air mixture screws. I believe that is the full vacuum port on one of these carbs. The other one, sticking out of the passenger side, is (I believe) the ported vacuum often used for the distributor.

Was going to say the vacuum tree as well, as that will always have full vacuum. Not sure if the readings vary slightly between the tree (plumbed to a single intake runner) or the carburetor (plumbed to a common plenum area) but they should both be close and give at least semi-consistent up and down readings on the gauge.

Your '70 won't have a true "tree" (at least not from the factory) but would very often have had a block fitting with multiple vacuum hose ports sticking out of it, screwed into the intake manifold in the same area behind the carb that a later model ('73 and later) would have had a fitting that looked more like a tree.

Remember too, that the most vacuum is not always the best. Yes, it's most likely the best, but when it comes down to every individual engine, you will want to do the tune-up "dance" where you alternate between adjusting the ignition timing, idle-air mixture and idle speed screws.
You're looking for high vacuum, but also the most steady vacuum and best idle quality.
Which means that the two idle-air screws do not necessarily need to be the exact same number of turns out. This will vary by carb and engine.

Sorry if you knew all that already. Just wanted to make sure in case this was your first time.

Paul
 
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Thatbroncokid

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Maybe I'm blind or dumb or a little bit of both, but I can't find the vacuum port on the front underneath the carb area, and my little vacuum block doesn't have any ports coming out of it. Any ideas?
 

B RON CO

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Hi, I can't see it to well but the square block behind the carb, on the manifold runner will tap into vacuum. Doesn't it have any plugged up ports? I hate to be the guy to tell you to take it off for 2 reasons. First, it might break, and second, you will still have to find a barbed fitting to attach the vacuum hose into. Good luck
 

DirtDonk

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Wow, someone went to a lot of trouble to make any vacuum sources invisible!
Someone with more intimate knowledge of the carb you have will have to see if they can point one out hopefully. But I'm with you (great pic by the way!) and would have to agree that the fitting I was expecting is nowhere to be seen.

And the little cube in the back. Never seen one like that as a "plug" so I'm guessing someone didn't want to just go buy an actual plug/cap for the hole, but instead cut off and soldered all the vacuum ports in it.
I don't even see a vacuum advance hose on the carburetor. Is there a single tube/hose fitting sticking out of the side of the carburetor over on the passenger's side, just near the front?
What about the distributor itself? Is there a "can" on the front (vacuum advance pot) with a hose fitting that doesn't have a hose on it?
I don't believe that any Early Broncos were ever delivered without one form of vacuum advance or another, but I wouldn't be surprised if one existed and you found it!

I wonder if perhaps that particular model carb was intended to be used with a plate underneath with all the vacuum ports?
Notice the large 3/8" hose running from the passenger valve cover to the back of the carb plate? That's your PCV valve in the cover, and the vacuum for it from the plate.
These plates are used for various things over the years and are of various designs. Yours is probably just for PCV vacuum and as an intermediate port for the heat riser that shunts some hot exhaust temps to under the carb to help it warm up quicker to open the choke sooner, and to keep it from icing up in colder weather.

For now you may just have to tune by ear, and look for what settings work best.
But for the next step, you can drill and tap a pipe-thread hole in the intermediate plate for a vacuum fitting. This would be your vacuum test port for future reference.
But for "ported" vacuum to be used on a single-hose vacuum advance distributor, you'll need to find that specific port on the carburetor.
Hopefully this carb has one. If not, I can only guess that it's been replaced at some point with a version devoid of all the normal stuff we're used to.

Good luck. Interested to hear what the Ford carb experts say.

Paul
 

DirtDonk

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What Ron said. Probably easier to pull that thing and install a fitting than to do that in the carb plate. But either one has it's potential issues.

Oh, and thanks for the pics! Really helps out with diagnosing stuff.

Paul
 
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Thatbroncokid

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there is one bolt plugging the vacuum block on the side facing the firewall. Any tips on how to attatch a vacuum fitting? or what i even need to do that?
 

DirtDonk

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Pull the bolt and see what kind of thread it is. Chances are pretty good (but no guarantees with old Broncos!) that it's a standard 1/8" NPT (or something normal like that) and you can usually buy a hose-barb fitting with the proper thread at most auto parts stores even today.
They used to be pretty common, but you won't find that kind of fitting on most modern cars after the '90's I would not think. But I've seen them on the HELP! spinner racks, and some of the Dorman fitting racks and shelves at the local Autozones, O'Reilley's, and mom-n-pop shops.

Should be something you can unscrew the bolt, clean out the hole, screw in the fitting (maybe with a little careful application of teflon tape or other sealer) and attach your hose.

Sounds simple, right?%);D

Paul
 
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Thatbroncokid

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Nothing surprises me with this truck anymore I swear. I always try and think of the strangest, most unlikely thing that will happen, and then get proven wrong, its quite fun really. Ill let you know how it goes!
 
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Thatbroncokid

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Mar 5, 2015
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Can't for the life of me get that plug out of the intake, nor can i get the bolt out that is plugging it on the side closest to the firewall. Tried tuning it by ear, and it idles nice, but sputters and bogs in second gear, especially going up hills. Dont have a timing light, so maybe my timing is off? Im of the belief i should be able to at least get it to run pretty good without using a vacuum gauge. Any thoughts? Ive searched extensively for vacuum leaks and havent found a thing.
 

B RON CO

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Hi, I would like to see a picture of the distributor and see if it has a vacuum line going to the vacuum cannister.
You can set the timing by ear, which means gently advance the distributor until you get pinging when you drive, then retard the timing until the pinging stops.
You can only adjust the idle mixture screws and the idle speed on your carb. After the engine is warmed up, slowly turn in one mixture screw until the engine starts to run rough, then back it out 1/2 turn. Then do the other screw. You can repeat the adjustment if you want.
Next set the idle to about 650 RPM.
Since you mentioned a bog in 2nd gear, I would look down the carb when the choke is open, engine off, and open the throttle. You should see two nice accelerator pump squirts.
I would guess the bog is ignition related but you may as well check the accelerator pump.
If you haven't done a tune up I would check the plugs, points and condenser, and look at the cap and rotor. You can check the ignition wires by running the engine at night and looking at the plug wires for arcing between the wires.
Post what you see.
Good luck
 
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Thatbroncokid

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I'll get you a picture of the distributor asap. Definitely have 2 good jets in the carb. Just replaced plugs and wires. Cap and rotor look okay, but I'll replace them anyway. They haven't been changed for the years the truck was sitting.
 

B RON CO

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Hi, you can use the vacuum advance hose to attach a vacuum gauge, but it should read zero unless you raise the idle. Also you have to read vacuum at a steady RPM, so I would raise it with the idle adjustment screw on the side of the carb.
I would also check the vacuum canister for leaks by sucking on the hose unless you have a vacuum pump. Good luck
 
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