• Welcome to ClassicBroncos! - You are currently viewing the forums as a GUEST. To take advantage of all the site features, please take a moment to register. It's fast, simple and absolutely free. So please join our community today!
    If you have problems registering or can't log into your account, please contact Admin.

Carb Issues... Rebuild time?

OP
OP
NashBronco

NashBronco

Sr. Member
Joined
May 9, 2013
Messages
396
Loc.
Flowood, MS
what should the vacuum gauge read? Mine is reading 15 off the manifold.... should I be checking it somewhere else.
 

thegreatjustino

Contributor
Red Head Grease Monkey
Joined
Jan 23, 2002
Messages
15,624
Loc.
Stockton, CA
First photo say 13. If your needle is steady while the engine is running, there isn't a vacuum leak. If you had a leak, the needle would be sweeping back and forth through several pounds of vacuum. The steady needle is a good sign. Although 13 pounds is low.
 

Rustytruck

Bronco Guru
Joined
Feb 24, 2002
Messages
10,875
First you want to make sure your points are properly set. then set your Idle speed. then check and set your timing. then reset your idle speed. now that that's set you can adjust the carb to the highest steady vacuum reading. Make sure both idle mixture screws are the same number of turns out. then open both screws a 1/4 turn out and see of the vacuum reading goes higher or lower and remains steady. if the vacuum drops then they are too far out. If the adjustment raise the vacuum then turn another 1/4 turn out on both screws. Adjusting until you get to the highest steady vacuum reading. Once you get close adjust by making smaller adjustment turns always adjusting both screws the same amount and letting the engine work through the adjustment. you want a few minutes between adjustments. You may have to adjust the idle speed screw to keep the engine at the proper engine rpm while making these adjustments. Once you get to the highest steady vacuum reading at the proper idle speed and timing then turn the 2 mixture screws in 1/8 of a turn and reset the idle speed. Your carb should be running a higher vacuum reading and the vacuum needle should be rock steady. If the vacuum cannot be steady then you have an engine miss in the ignition, a vacuum leak or if the needle is really wild swings then you have a cylinder issue. There is allot of online documentation on using a vacuum gauge.
 

WheelHorse

Bronco Guru
Joined
Nov 22, 2004
Messages
2,491
Post a picture of your spark plug---you pick one. Let's see if she looks rich or lean.

You could have a bad power valve.
 
OP
OP
NashBronco

NashBronco

Sr. Member
Joined
May 9, 2013
Messages
396
Loc.
Flowood, MS
So here is where I have things set. Warmed up for 15 minutes. No choke. Idle screw adjusted ALL the way in to keep it running. Fuel moisture screws 3 turn out. Vacuum gauge reads between 12-15 off the back of the manifold.

Sorry for all the post on this but I am still trying to convince myself I can do this myself. I should have the dwell tach this weekend.
 

Slowleak

Bronco Guru
Joined
Sep 12, 2013
Messages
3,732
Loc.
Georgia
When you had the vacuum gauge connected to the hose to the distributor and saw 8 pounds of vacuum, was the engine idling? You should have no vacuum on that hose at idle if hooked to ported vacuum on the carb.

If the idle speed screw is cranked all the way in, the throttle plate will be open enough to create a vacuum to the vacuum advance which is the 8 pounds you are seeing. With that much throttle you will be pulling in more air than fuel and running lean. That explains why the idle mixture screws are out so far. It needs extra fuel to balance the mix and run.

If that 8 pound reading was at idle, I would unplug the hose at the distributor and cap it off until you get things sorted out. The throttle should not be open enough at idle to create a vacuum to the distributor.

With it running, try slowly turning the idle mixture screws in. If the engine speeds up then turn the idle speed screw out to slow it down. If that does not work, then I'm guessing that your timing is way off.
 
Last edited:

WheelHorse

Bronco Guru
Joined
Nov 22, 2004
Messages
2,491
Plug is indicating a rich mixture.

Do the idle mixture screw make much of a difference? If you're positive the choke plate is opening all the way, I would check the power valve.

I believe the idle screw having to be cranked so far is to compensate for a leaky power valve as it is dumping a ton of additional fuel requiring additional air to compensate.

If the plug looked clean then I would go after timing, but you have also mentioned other signs of a rich mixture...starts easy when cold, harder starts when hot where the choke made it worse.
 
OP
OP
NashBronco

NashBronco

Sr. Member
Joined
May 9, 2013
Messages
396
Loc.
Flowood, MS
Thanks Wheelhorse. The idle mixture screws make no real difference when I move them in and out. I will check the power valve.

I finally got it to idle low (normal) last night. but choke had to be pulled half way out. Also, when I step on the gas, it stutters a little and then finally picks up.
 
OP
OP
NashBronco

NashBronco

Sr. Member
Joined
May 9, 2013
Messages
396
Loc.
Flowood, MS
thanks WheelHorse.

Maybe I am making some progress. Here is where I am at as of today. I made these changes over the weekend after I rebuilt my carb.

Vac. gauge reads 11-14 off the back of the manifold. holds pretty strong at 14 but occasionally dips to 11 and then back up to 14.
Once its warmed up I am able to fully open the choke!! Finally!
F/M screws are 2 turns out.

However, I give it some gas and when I let up on the pedal, it starts popping pretty bad. Is this a lean or a rich symptom? Also, doesn't seem to have a lot of power when I test drive it down the road. that also leads me to a rich mixture. Should i go "IN" on the F/M screws? I have played with it and it doesn't really impact the idle or vac reading.

One guy told me it may be a compression issue with one of the cylinders? Or too much pressure from the fuel pump.

EFI is moving up on my list..... but still pretty far down.
 

Rustytruck

Bronco Guru
Joined
Feb 24, 2002
Messages
10,875
That dip to 11 is not good. Usually a miss fire. First thing I would do is check each plug and wire and your coil connection. If you have an infrared temperature gun aim it at each exhaust port and look for one hole colder than the other. Ohm check the wires and swap plugs and see if the problem travels with the changes.
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,355
...Vac. gauge reads 11-14 off the back of the manifold. holds pretty strong at 14 but occasionally dips to 11 and then back up to 14.

That's too low. I'm going to go out on a limb here and say your ignition timing is too retarded. Among perhaps other things, but this is a critical point to check before chasing other tails.
The steady is a good thing, as mentioned, but a stock cammed 289/301 should pull a minimum of 18, and more like 20 at idle. Mine runs 21 when I have my timing set to my preferred 12° BTDC. Don't remember what it was when timing was at the stock settings.

Before you do anything else, find the #1 spark plug wire at the distributor, and then checking each one counter-clockwise from there, find out what firing order you're sporting. Looks like the more modern 13726548 from here. Yours "should" have the older 15426378 of a 289 unless the cam was changed out. Sorry if you already mentioned that, but I didn't go back over every post.
If it's off, it "should" also be running rough as well as weak and with a weak vacuum. But it might be so well balanced that it's still smooth and just weak.
I'm thinking it would be odd to have the wrong firing order and a steady vacuum signal, but I've seen stranger things.

Once its warmed up I am able to fully open the choke!! Finally!
F/M screws are 2 turns out.

Hey, a result!
But more to do still...

However, I give it some gas and when I let up on the pedal, it starts popping pretty bad. Is this a lean or a rich symptom?

Popping out of the exhaust, usually too lean. Popping out the carb, incorrect timing.
Again, "usually" but as you'll find out there are few things etched in stone.

Also, doesn't seem to have a lot of power when I test drive it down the road.

It wouldn't at this point. Too low of a vacuum signal, and too lean (theoretically) and unknown timing I think?

that also leads me to a rich mixture.

Possibly.

Should i go "IN" on the F/M screws? I have played with it and it doesn't really impact the idle or vac reading.

Neither. Not only do they have ZERO effect on how your engine runs under load, but if they're not making any difference at idle, then they're not making any difference.
This is likely because your carburetor is still running with the idle speed screw turned up to keep it running. Doing so also eliminates the idle/air mixture screws from the equation.

One guy told me it may be a compression issue with one of the cylinders?

Certainly could be. But too many other things as well, so still need to check. You can do a load test in each cylinder when you get it running normally, or you can rent a compression tester and know for sure. But I wouldn't at this point only because there are too many other questions still in need of an answer.

Or too much pressure from the fuel pump.

Too much pressure could make it run rich. But we don't know it's rich yet. Yes, that plug made it look that way, but if you have a super lean mixture causing what's known as "lean-misfire" then you are still going to have unburned fuel in the cylinders making soot.
Again, hard to say at this point...


The idle mixture screws make no real difference when I move them in and out. I will check the power valve.

They can't. If you still have the speed screw turned in so far, they're inefective at best.

I finally got it to idle low (normal) last night. but choke had to be pulled half way out.

Too lean. Too retarded.

Also, when I step on the gas, it stutters a little and then finally picks up.

Also too lean.
 
Top