Sounds a little like something else is going on, the vacuum gauge should read rock steady at 19" to 21" vacuum on a stock engine at Sea level. At factory idle and ignition timing.

Here's a chart on reading a vacuum gauge, there are others you can find elsewhere online, and a couple webpages, they are all slightly different in the diagnostics, you can learn a lot about internal engine health using one. Although readings are generally supposed to be taken at factory RPM, even at a high clip the vacuum should be steady.

 

Tedster9--Thank you for the picture of the vacuum gauges. That helps a lot. I got on youtube and watched a few videos on vacuum gauges and what each needle movement means. The needle movement that most resembles what I'm experiencing is carb adjustment. But the confusing part is I've adjusted everything just like all the videos and advice I've received has instructed me to. However, none of these vacuum gauge symptoms mention anything about the motor not wanting to idle and stalling. So I'm still partly guessing as to the problem.

So I soaked the carb for another two days AGAIN and got it all cleaned up tonight. After rinsing all the Chem-Dip off the carb, I took some carb cleaner spray and sprayed out all the orifices then took my air compressor and blew out all the orifices. Just got it put back together, but that's as far as I got since it's only 25* outside. I'll put it on the Bronco tomorrow and fire it up and see what it does.

On a side note.....before I screwed the base plate back on the bottom of the carb I put a straight edge on it and found it was warped in the center. I thought maybe, just maybe this is why it's idling like it is and eventually stalling out (small vacuum leak). In a previous post I mentioned that when I took the front metering block off and changed jets, I also sprayed some carb cleaner in the orifices and then blew the orifices out with my air compressor. When I did that, I noticed some liquid seeping out between the base plate and gasket. Well now that I found that the base plate is warped a little, this makes sense.

So before I put the base plate back on tonight, I put a little bit of silicone on both sides of the gasket, hoping to fill the void where the base plate is warped. I made sure to keep the silicone far away from any holes that fuel or air flows through. I then screwed the base plate back on and cleaned up the little bit of silicone that seeped out on the edges. So I'm hoping that I have a small vacuum leak from the base plate and the silicone seals it up and cures my problem. I'll find out tomorrow night what happens.

My plan is if I still have the same problem, then I'm going to take the carb and install it on my red 79 Bronco and see if it idles the same. If it does, then I'm sure it's the carb that's the problem. If it idles fine, then that tells me there's something wrong with the engine in my green Bronco or a vacuum leak or something else. Does this sound logical to you guys?

I know you're probably freaking out about putting silicone on the base plate, but I figured I have to try something since it's not working correctly anyway. It couldn't hurt to try. I'll let you know what happens tomorrow when I get the carb installed; barring the weather cooperates.

 

Warped carb baseplate is a common problem, probably caused from overtightening. By a previous owner of course. There isn't much torque on the nuts, usually just snug. Maybe 50 or 60 inch-pounds. You can check with a straight edge. A large piece of glass is flat - flat enough anyway, lay a piece of wet-dry emery paper on it, then run the base of the carb in a nice lazy figure 8 pattern. If it's not real bad can clean slight warpage up and make a flat surface. Use new gaskets, and make sure all the old stuff is off the intake. Vacuum leaks will screw up a tune.

Definitely, slap a different carb on there and see what happens.

 

Well putting silicone on the base gasket didn't work out so well. When I put the base plate back on and tightened it down, the silicone spread and covered some fuel passage holes because when I got it back on the motor and fired up, I didn't get any fuel squirting when I touched the throttle.

So I got my 670 street avenger carb off my other 79 Bronco (red), that's also been sitting for a very long time, and installed in on my green Bronco and I assume the gas has varnished up some of the fuel passages because I'm not getting any fuel squirting when I push the throttle on this carb either. So I currently have it tore apart and soaking in Chem-Dip carb cleaner. I ordered a rebuild kit from Summit along with another base gasket for my 570 carb today. When it arrives (probably Wednesday), I'll clean up the 670 and spray out the orifices with carb cleaner and blow it out with my air compressor then put it together with all new gaskets. Then I'll throw it on and see what happens. I also ordered more gaskets that go between the carb and the intake manifold. But the carb is sitting on a 2" spacer, so maybe the gasket on the bottom side of the spacer has a small vacuum leak. Or maybe the slightly warped base plate is the problem, or maybe a combination of both. Who knows, but I'll put a new gasket under the spacer when I install the carb.

Based off the activity of the vacuum gauge, it seems like there's a small vacuum leak somewhere and I can't find it yet. But that's my focus because both of these carbs are completely rebuilt. We'll see what happens later this week once I get the 670 put back together. I'll keep you updated. Thanks for the help...

 

No sealant is used on carb base and spacer gaskets afaik. You'll need two if using a spacer, but you knew that already.

Make sure everything is clean, and flat, check for warpage. It is very important that there are no vacuum leaks. You can coat the gaskets with chapstick so they don't stick or tear, and can reuse them a couple times. One technique for locating vacuum leaks is to use an (unlit) propane torch.

With the engine idling, temporarily lean out the idle mixture screws so the engine struggles a bit and is idling extra lean, this will help with the test. Direct a stream of (unlit) propane around the carb base and intake manifold gasket areas. If the idle RPM picks up, you've found the leak.

 

I have an update on my carb issue.....So I got the 670 Street Avenger rebuilt and put it on the Bronco today. Fired it up and it run perfect!!!! So then I put the 570 back on (after replacing the base gasket; no silicone this time) and it still stalls while idling. So I've determined there's got to be something wrong with the carb. I've soaked it in Chem-Dip carb cleaner twice for days at a time and also sprayed out all the orifices with carb cleaner and blew the orifices out with my air compressor. Every single orifice in this carb is spotless clean and clear of any debris. It has been put back together with a brand new gasket kit along with all the little parts that come in the kit. So I've give up on it. I will probably get rid of it because I don't know what's wrong with it. I was considering contacting Holley and see how much it would cost to have them troubleshoot it if I sent it to them, but I have a feeling by the time I was done I could probably by a new carb. Who knows.

So although the 670 is probably a little too big for the stock 351m, it's all I have so I have to use it. But I'm happy I finally got it running because I need to drive the Bronco and park this death trap of a 71 Jeep my wife bought until I can replace the tie rods and wheel bearings.