I have a 79 Bronco with a 351m. The carb is a Holley Street Avenger 570 cfm that I bought from a guy a few years ago. The Bronco has been sitting for a few years because it needed some TLC and I hadn't got around to fixing it until now.

So upon firing it up, the darn thing won't idle nor will it stay running when trying to feather or pump the throttle. So I remove the carb, tore it apart and soaked it for a couple days in some carb cleaner I bought from Autozone (chem-dip carb & parts cleaner). I bought a master rebuild kit, put it back together and installed it on the Bronco. I got it fired up and had to adjust the floats and got it pretty close to where it should be, but might still need a little adjusting (it's sitting on an incline).I can keep it running if I work the throttle and it seems like it tries to idle on it's own, but still won't. It's running much better but still having issues. It's sputtering &/or missing (sounds like it's not getting enough fuel, but not sure) and it doesn't want to idle it's own. I have increased the idle via the throttle set screw, but still doesn't want to stay idling. I'm sure this is just a fine tuning issue since it's completely rebuilt with all new parts, but not sure what to adjust.

One thing to note is the fact that when I had it tore apart I noticed that the primary jets were 50's and according to the Holley website, the factory size primary jets are supposed to be 54's. So someone has put smaller jets in this carb. I'm not sure if this is why it's sputtering and not wanting to idle or if it's some other issue. Can someone help me out and give me some idea of why it's sputtering and not wanting to idle? What do I need to adjust? Thanks for the help.

 

The deal with carburetors that is almost always overlooked, it's all about the preparation. They are (mostly) simple, but if the prep and setup isn't correct you'll end up pulling your hair out. One old saying is "90% of carb problems are ignition related." And it's true.

If the timing is off, or the ignition is weak, numerous vacuum leaks etc, it will never run right. Ever. Worse, compensating a carb adjustment downstream because of a major fault upstream just makes things worse. That's why the prep is so important. The fuel pump/tank is where you start to make yer money.

The fuel pressure delivered at the carb, and volume, has to be within spec or the carb cannot be adjusted. The float height is important, it affects everything downstream.

The jets actually have nothing to do with idle or even transition. You could remove the jets completely and drive around the block, if you kept the speed down. Jets are only in play for steady cruise above around 40mph. There's no telling what a previous owner did to an old carb. The best adjustment you can make is to be sure the carb is the right one to begin with.

Anything that has sat for a long time probably would need a new fuel pump, mechanical or otherwise, just on general principles. Once everything is known to be good - clean tank and clear hard lines, fresh fuel, good filter, float levels set, fuel level etc, ignition timing correct, no vacuum leaks, then you're ready for carb setup and adjust. That part only takes a few minutes. Again the preparation part is what takes the longest but you'll be rewarded with easy quick starts, rock steady idle, no bog or hesitation, good performance and near EFI reliability. Carbs were replaced in part because people can't be bothered to maintain or adjust anything. Hell, they'll probably stop letting people drive before too long.

Here's a link to a really good overview on the most common carb setup mistakes. Let us know what you find.

http://www.junkyardgenius.com/holley/tune01.html

 

That is great info and I really appreciate it. You're absolutely right about the prep work. And thank you for that tuning guide link. I read through it and will be using this guide tomorrow during my troubleshooting. With that said, let me touch on some of the advice you mentioned....

The fuel pump (manual) is new as of July, 2016 as well as the clear fuel filter. There's no sediment or dirt from the tank, as there's nothing in the fuel filter. So that part is good. The fuel is also from July, 2016, but it might also have some old fuel mixed in. There's only about 1/8 of a tank of gas, but it still may have some old gas. But even with old gas, I would think it should still idle. It may not run the greatest, but it should still idle. So I'm assuming there's another issue.

The carb gasket is brand new (came in the rebuild kit). All the vacuum ports on the carb that aren't being used are plugged off. With that said, I will verify other vacuum ports on the intake manifold to make sure they're not leaking.

I didn't know the info you mentioned about the jets. I didn't realize you could drive without them at low speeds. With that said, I'm pretty sure that's not my problem, so I won't focus on that.

The air cleaner is 4" x 14", so that's not an issue.

I will double-check the air mixture settings. I'm not sure what they are set at. I also have a vacuum gauge, so I will use that to do some fine tuning, if I can get it to idle.

I think I will continue to lower the float level a little bit and see if that helps. When I first installed the carb after rebuilding it, the float level was so high there was fuel flowing out of the air tubes on both the front and rear tubes. But as I lowered the float level, the overflow stopped. I need to get the vehicle perfectly level first before I make this adjustment. Question....if the float level is too high, will that cause the sputtering and idle issues I'm having????

I will also verify the accelerator pump arm clearance.

I'll report back sometime tomorrow after I have a chance to play with the carb. Thanks again....

 

Definitely, fuel height in the bowl is important, if too low it can lean out on the highway at cruise or under heavy acceleration, if too high it can flood and start a fire too. It should be pointed out always have a full size fire extinguisher handy in the truck even when not working on carb tuning. Old hoses rot from the inside, they look OK on the outside. Even 3 or 4 psi can send fuel spraying far and wide in the engine bay! Not Fun.

The float height or dry setting is really just a ballpark to get it running, the actual fuel height ought to be verified or checked because it will vary a bit depending on the specific fuel pump output, and that varies a bit (within a range). This is a bit tedious but a little attention to detail here will pay off.

The fuel level in the bowl in turn even affects the cruise air/fuel ratio to some degree. Maybe that's why the previous owner dropped jet size by 4 points? Just sayin'. This is why it's adviseable like you did, to start all the way back at the tank itself and make sure everything works as advertised. The fuel pump has to be able to supply at least a pint of fuel every thirty seconds at all times, under all conditions.

That's 15 gallons per hour! Of course (we hope) the engine won't consume this much, but the pump must be able to feed that much volume. Then, the carburetor needle and seat + float has to throttle all that fuel back to a very specific range +/- of fuel height in the bowl. This gets it into spec for the fuel well, annular boosters or PVCR/metering block and all those orifice/gee-gaws that carbs have.

If you had a bucket of water for example, and punched two holes in the side, the lower hole would shoot water out farther because of greater pressure. This is sort of why the fuel height in the carb bowl is important to get in the ballpark, and also be consistent, it sets the baseline for the rest of the carb tuning. Let us know what you find.

 

Here's an update. I spent all day today working on this carb and am still having problems. Here's what I've done.

I installed a fuel pressure gauge and when I got it started (after making some other adjustments first) it was reading 5 1/4 psi. At one point it was reading 5 1/2 psi. So the fuel pump is working, but is this enough psi? My other 79 Bronco, which is red, (I have two 79 Broncos, one is green and one is red) has a fuel pump that needs replaced, but it still works. It's weak and won't even keep the fuel filter full, but it runs and doesn't miss or sputter like this green Bronco is doing.

I changed the spark plugs just to eliminate that problem. The old plugs were black.

I adjusted the air mixture screws on each side of the metering block. The one on the drivers side was screwed out 2 1/2 turns and the passenger side was screwed out 3 turns. I adjusted them so they were screwed out 1 1/2 turns.

So after making these adjustments, I fired it up and moved it to level ground. It was a son of a gun to keep it running when it was cold, but it seemed easier to keep it running this time (after making these adjustments) then it was before the adjustments. So I think I did something right with these adjustments, but it still wouldn't idle nor would it stay running if I held the throttle steady. When I tried to hold the throttle steady without pumping it, it would sputter and slowly die. It acts like it's starving for fuel. I could keep it running if I constantly pumped the throttle a little bit (just enough to squirt fuel into it), but once that shot of fuel burned off it would sputter and die.

I also made some float adjustments until I could see the fuel in the sight window. I would make an adjustment and then go start it and keep it running for about 30 seconds to 1 minute. Then I shut it off (or just let it die) and then go check the fuel level, make an adjustment then fire it back up and repeat the process. At one point it was way too high and now it's way too low. I spent probably an hour trying to get the float adjustment right and am still working on it. In the mean time, now I can't get it started for some reason. I squirted a shot of starting fluid in it and it would run for a second or so and then die, no matter if i pumped the throttle or not. I didn't have this problem all day until this evening. Now I can't get it started and I have no idea why. I had my son watch the fuel pressure gauge as I tried to start it and it would pulsate anywhere from 1-3 as I was cranking the motor, so the fuel pump is still working (I thought the fuel pump went out). When it was running it was reading 5 1/4 psi. But now that I can't get it started, the fuel filter is only half full. When I had it running, it was totally full. Not sure if this means anything or not.

Since I can't figure out why it won't start now when it's been running all day, I'm not sure what to do now. Is this possibly a float adjustment issue? At this point, can't seen any fuel in the sight window even when I rock the vehicle. Maybe the fuel level is too low and that's why it won't start now? I thought once I made a float adjustment, I would be able to see the change in fuel level right away, but it seems it's not responding very quickly, even after running the vehicle for a minute or so. Is there a way to set the level (or get it close) when I take the fuel bowl off? Is there a dry adjustment or setting I can set the float at to get me in the ball park so I can get it started? Now that I can't get it started, I can't see the results of a float adjustment.

One thing I'm going to do is order some factory size jets (.054) and get rid of the .050 jets that someone put in it. I'm hoping this solves my problem because I can't figure out what else is causing the sputtering and refusal to idle. When I have the fuel bowl and the metering block off, I'll take my air compressor and blow out the holes in the metering block. I'm hoping the reason it won't start now is because maybe there's something clogged, even though I've tore this carb apart and soaked it in carb cleaner for 2 days. One thing I didn't do was to remove the air bleed screws when i soaked it and I didn't remove them when I used my air compressor to blow out the holes in the carb and metering blocks. So maybe this is where the problem is and why I can't get it started again????????

Sorry for the rambling on and on, but can anyone give me any ideas on what the heck is going on with this thing? I'm really frustrated right now. If it still won't run after I change the main jets, I'm going to install the 670 street avenger off my red bronco and see if it has the same problem. Other than that, I'm at a total loss on what to do. Thanks for the help....

 

Ok guys, I have an update....After posting my previous message, I've been sitting here trying to figure out why all of a sudden the Bronco wouldn't start because nothing was making sense. Knowing me, I would not have been able to sleep tonight because of this confusion. So I grabbed the flashlight and headed back outside. I figured it had to be a float issue because it was running and that's the only thing I've been adjusting. So I crawled up into the engine compartment and looked down into the carb and pumped throttle and there was no fuel being squirted into the carb. So I pulled the fuel bowl off to inspect the float needle. When I went to take the fuel bowl off, I just happened to notice the manifold vacuum port underneath the fuel bowl was not plugged off. I'm speculating that this is why it wouldn't idle (vacuum leak) after I rebuilt it. So after getting the fuel bowl off, I tried to remove the float needle and it wouldn't budge. I eventually got the float needle out of the fuel bowl, but while doing so I discovered why I suddenly wasn't getting any fuel. The float needle was stuck all the way down and wasn't allowing fuel to flow into the bowl.

So between the float needle and the unplugged vacuum port, I'm hoping I found the problems and it will run good once I get it back together. But before I put it back together, I ordered the correct size jets (.054) and a tranny kickdown lever from Summit Racing. Once they arrive I'll install them then put the carb back on and fire it up . I'm guessing it should idle and then I can hook up the vacuum gauge and do some fine tuning. But we'll see what happens once I get it back together. Probably won't be until Monday or Tuesday when I get my parts.

For the record, this is not the reason it wouldn't run before I rebuilt the carb. It was getting fuel to the fuel bowl and all the vacuum ports were either being used or plugged off. I believe it was just gunked up from sitting for a long time. After rebuilding it, even though it wouldn't idle (probably because of the vacuum port not being plugged off), it still run better than before I rebuilt it.

I feel a lot less stressed because I'm thinking I might have found the problem. I pray I'm right. I'll update you after I get the jets installed and back on the motor. At least I'll be able to sleep tonight....LOL

 

The jets aren't the problem, they have nothing to do with the idle circuit. The transition circuit provides fuel above idle speed and speeds and part throttle around 35 miles per hour. The accelerator pump provides a shot of fuel everytime the accelerator is pressed so the engine doesn't bog or hesitate. The power valve (economiser) is held closed by engine vacuum, and opens to provide a much richer fuel mixture for heavy acceleration and passing. The main jets just determine how much fuel is provided at steady highway cruise, careful selection here will determine fuel economy. The proper jet size is determined by the carb not the engine, per se. If using a different carb than was factory, some trial and error might be necessary but it should be close.

Don't bugger the jets up when removing or installing, a hollow ground blade screwdriver (like Gunsmiths use) works well here. You'll have to learn to read plugs.

A vacuum port not plugged off will cause all kinds of problems. This would have been a good time to use a vacuum gauge, as it would have indicated a low engine vacuum at factory idle speed right away. If it's the large vacuum port at the rear of the carb, it would be a good source of constant full time vacuum to hook up the gauge. A healthy stock engine in good tune at Sea level should draw 19" to 21" steady needle, at say 550 or 600 RPM. You can use the vacuum gauge to set idle mixture screws as well.

If you've been ragging on the battery and starter all day put the battery on a slow charger overnight. 6 amps would be perfect let it cook for a while. Cranking an engine that won't start is hard on the solenoid and starter itself too. Then if it does start, the alternator is tasked to charge up a dead battery. This normally takes about 18 hours. They don't like that.

A well tuned engine will start quickly without requiring excessive cranking. Not trying to beat up on you, trying to save you money. Say every 10 seconds of cranking, let the starter rest for 20 minutes. Keep the battery fully charged between your wrenching and start attempts. I bet you'll find a big ole vacuum leak fix will help a whole bunch. One of the common errors. Ask me how I know.

 

Thanks for your advice. Interesting info about the battery & solenoid. Yeah, I put the starter and solenoid through their paces yesterday. I'll put the battery charger on it today and let it charge....

Regarding the jets.....I know you're right about the idle circuit vs jet size. My train of thought here is the same as I had with replacing the spark plugs. I replaced the spark plugs whether they needed it or not just to eliminate them as a possible contributor to the problems I'm having. I'm using the same reasoning with the jets. This carb came factory with .054 primary jets so that's what I'm going to put in it....for now. I'm trying to develop a baseline set up and get it to run and then fine tune from there. I can't really do any fine tuning with the vacuum gauge until I can get it to idle on it's own without me having to operate the throttle to keep it running. First thing's first....

I don't remember what engine my friend had this carb on, but it was in a 60's Camaro and it had either a 305, 327, or 350 motor. My motor is a 351m. I have a 67 Cougar with a 351w and it has a 600 cfm Edelbrock and it runs perfect. The heads on the 351m have larger valves, intake & exhaust ports (flows more air) than the heads on my windsor and the Holley carb is a 570 (vs the 600 Edelbrock) so I'm assuming .054 primary jets will be better than the .050 jets, but I'm just guessing at this point until I can fine tune it with the vacuum gauge.

And I didn't take it as you beating up on me. I'm looking at this with appreciation for the info and advice you're providing. Thank you. Keep it coming. I'll update you when I get it put back together once my jets arrive.

 

You can use a vacuum gauge for jetting provided you run it into the cab to observe engine vacuum at steady cruise down the highway, I haven't tried this yet. Jet down/up for highest vacuum reading. I'm not sure that's what you had in mind. Just keep in mind that there are several circuits that have to be specifically tuned to your engine and altitude. Idle mixture, accelerator pump shot, cruise circuit (jetting) and power enrichment. Carbs are not "bolt on and go", Ford had a specific size carb in mind, with specific jets and booster cluster, power valve etc, for a specific truck, and depending on gearing, transmission, and accessories etc. Anything that will bolt on will probably work, but not necessarily very well. Ask me how I know! haha

Vacuum gauge can be used for setting the idle mixture as well.

 

I have an update and I have good news and bad news.

First the good news....I got the .054 jets yesterday and got them installed tonight. I also plugged the vacuum port under the fuel bowl that I forgot to plug and fired it up and it ran. So I let it warm up and then adjusted the float on both the front and rear and have them set at the perfect level. I even stabbed the throttle and no hesitation whatsoever. It's very responsive, even when I floor it it responds very well with no problems. Very happy with it so far.

Now for the bad news.....It won't idle. It sounds like it's either missing or not getting enough fuel because it won't run smooth when the rpms are low. When I let up off the throttle, the idle will come down and the motor will keep getting slower and slower until it stalls out. However, whenever i just touch the throttle it comes back to life and runs, then the idle settles down again and slowly stalls out. When the rpm's are low, the motor is not running smooth; the rpm's are fluctuating

I've turn up the idle set screw on the carb and no matter how much I increase the throttle on the idle circuit, it will slowly stall out. However, once I turn the idle set screw so that it will run at idle, it's actually on the cruise circuit; that's how high I have to set the idle before it will stay running. I don't have a tach, but I'd say the idle is somewhere around 1500 or more before it still stay running on it's own.

I figured the problem was the idle air mixture screws needed adjusting so I hooked up my vacuum gauge and tried to make some adjustments. FYI...the idle screws were set at 1 1/2 turns out to start. So when I hooked up the vacuum gauge, I noticed the vacuum gauge kept fluctuating, bouncing between 17 and 19. When it does this, the rpm's are also fluctuating up and down and won't run at a steady rpm. So I couldn't make any adjustments because I couldn't tell if the adjustments were changing anything because the motor won't run steady.

So then I figured I'd try to check the timing. So I hooked up the timing light, disconnected the vacuum advance and plugged the port and tried to check the timing. Again, the rpm's kept fluctuating and as it did the timing kept reading a higher number when the rpm increased and a lower number when the rpm's decreased. So I wasn't able to verify the timing because of the fluctuation of the rpms. With that said, one thing I thought was odd is the fact that the timing would fluctuate as the rpms fluctuated even with the vacuum advance disconnected. Is this normal? I thought with it disconnected, it would stay at a set rpm no matter if the rpms increased or not. However, one thing to note...the timing chain is not wore out because this motor was rebuilt no more than 10,000 miles ago. Every time the timing light fired, it was not jumping around like a worn out timing chain would show. The fluctuation with the timing light was gradual and smooth as the rpms increased or decreased. You can tell the difference between what my motor is doing and a motor with a worn out timing chain. The timing chain isn't the issue.

So as a last resort I figured maybe, just maybe I had a small vacuum leak somewhere, so I disconnected all the vacuum lines going to the intake manifold and plugged the ports, including the brake booster. So now I don't have anything drawing any vacuum other than my vacuum gauge. I fired it up and still the same problem; it won't idle and when it does run, the rpms fluctuate. To get it to run without operating the throttle to keep it running, I had to turn the idle up so high it was on the cruise circuit instead of the idle circuit. If I tuned it down, it would slowly stall out.

So I gave up for the evening and thought I'd come on here and ask for some help. My only guess is that possibly there are some holes in the metering block that are still partially dirty or clogged. Does this sound logical?? Am I on the right track? When I had the carb apart and soaked it in carb cleaner for two days, one thing I forgot to do was take out the air idle screws. Maybe this passage is partially clogged with old gas (varnish) or something???? This is my only guess. Can you guys shed some light on this idle problem and what to do? Thanks, I really appreciate the help...

 

I agree that is sounds like the idle circuit is not satisfying the needs of the engine. I would take the idle adjustment screws out and spray some carb cleaner through there a couple of times. Then put the screws back in and adjust them out 3-4 full turns. To finish cleaning them out with the engine running, use a heavy shop rag and cover the top of the carb's air intake while opening the throttle to raise RPM. This will cause a higher than normal low pressure surge to draw out anything in the passages. We called this a Poor Man's Carb Cleaning. Then go back to adjusting the idle mixture screws and see if anything has changed. On old non-cleaned carbs it might take a couple of times doing this to get them to idle and run. Since yours was just cleaned, it shouldn't be too hard. Be prepared for a bunch of black exhaust smoke as the engine burns off the extra fuel that is pulled into the intake.
I know that it sounds 'redneck' but I've used this method many times to get an old dirty carb working again.

 

hmm... ok.. just shooting in the dark here.... bad carb gasket? warped carb insulator base plate? Intake manifold not flat on the carb mounting area? Intake manifold gasket bad or warped manifold at the heads?

I would for sure check these looking for a slight leak. If your fire extinguisher is close by.. use an unlit propane torch or you can spray carb cleaner.. or.. akk quick start around the base of the carb to see if it is sucking air in there. Maybe spray around the intake manifold to heads gasket and bolts to see if somehow it is sucking air in there.

 

Interesting technique. Never heard of or done this before, but I'm definitely willing to give it a try. There's more than one way to skin a cat, right?

So trying to understand this technique, I have a couple questions. You mentioned to be prepared for a bunch of black smoke from extra fuel being pulled into the intake. Why is there a lot of extra fuel? Is this from having the screws screwed out 3-4 turns?

Won't covering the top of the carb with a rag cause it to die from lack of O2? Do I allow just enough air in to keep it running a little bit, or cover it entirely allowing no air in until it dies?

I'll give this a try tomorrow after work. Thanks for the advice. Appreciate it.

 

It takes a couple of trys before you get it to not kill the engine as it will be flooded with fuel and hard to restart. Blocking off the air intake at the top will draw fuel out of every opening that is in the low pressure area under the rag. It is a two handed operation as you rev the engine with the throttle just as you cover the air intake. If you prefer to not use a heavy rag, you can also manually close the choke plate. It kinda does the same but doesn't cause as much fuel to be drawn out as the rag does. I have had times doing this that the bottom of the rag will be wet with fuel.
Think of this as operating the choke in a much larger way. It is hard to explain here without seeing the operation. On motorcycle carbs I just use my hand to over the air intake on the carb. It will pull fuel out of the air jet also as my hand completely seals off all air intake. And yes it does leave a mark.

 

Worked on the carb again today. So freaking frustrated and confused and I'm about ready to throw this freaking carb in the garbage!!!! Here's what I did. I ran out of daylight Friday night so instead of doing the rag over the carb technique I pulled the metering block off and soaked it in the Chem-Dip carb cleaner again and let it sit until today (Sunday). I took the air bleeder screws out this time before I soaked it. I took some some carb cleaner spray and sprayed out the metering block and the body of the carb, then I took my air compressor and blew out all the holes in both the metering block and body of the carb. One thing of concern I have is when I blew air in one of the holes in the body of the carb, I could see some sort of cloudy liquid (assume it's gas, but wasn't as clear as gas), being blown out between the body of the carb and the base plate between the gasket.

So then I put it all back together and fired it up and I still have the same problem, although not as sever and seemed to idle better just a tad bit. But it's still not idling without eventually dying. And the motor is missing or hesitating and the vacuum is still jumping around 1-2 hg's.

So then I opened the air bleed screws 4 turns, fired it up and took the rag and covered the carb and tried to keep it running as long as I could until it tried to die, then just before it died I took the rag off the carb to keep it running. I kept doing this over and over for probably 15 minutes or so and it didn't seem to make any difference. I also didn't see any black smoke coming out of the tailpipes either (one of you mentioned I would have a lot of black smoke when I did this).

So after getting frustrated and out of ideas, I pulled the carb back off and tore it apart again and it's now soaking in carb cleaner (Chem-Dip). Probably won't get to work on it for a couple days and I'll let it soak that entire time. Then I'll take the carb cleaner spray and spray out all the holes again and then take the air compressor to it and blow out all the holes. Then put it back together and see what happens.

I've never had any of these issues with an Edelbrock carb, which I currently have on my 67 Cougar, but every Holley I've ever owned I've had nothing but problems with. They seem to make more power than an Edelbrock, but they also seem to be a lot more problematic and finicky to get to run properly. I really want to get this Holley running good. I feel I'm almost there, but just can't get this figured out.....

Any of you guys have any ideas?