Carb Questions

Hi guys, I'm having trouble finding what I'm looking for with the search function. I need to replace the Carb on my Bronco.

We have a 302 V8. I've done a bit of reading but can't decide on the best route to go. Can you guys recommend your best choice?

I'm also looking for a good walk through or tutorial on how to remove and re-install/tune a new carb, I've never done it before but want to DIY.

Thanks!

What's wrong with the carb you have now? If it's the factory 2 bbl they are real easy to rebuild.

What are you gonna do with the bronco? Like stated above the stock 2v is a good all around carb. If your looking to go 4v the edelbrock performers work good on the road. If you're gonna take it off road look at a q-jet.

Just trying to improve fueling and road driving. I'm sure I could clean and rebuild the existing carb but a new carb isn't that much so I figured it was NBD to just install a new one and learn how to do it.

carb manifold ect will run you $500 or more if bought new. Dont get me wrong I like aftermarket stuff but for the most part there is nothing wrong with the stock 2v carb it really cant be beat. Easy to tune as well. Learn how to tune what you have which may not need any tuning but you can fiddle with it then what you learn there will pretty much apply to any carb. If your only looking to replace the 2v with another stock 2v then its simple theres nothing to it. New carbor carb kit will usually come with instruction on tuning

Hi, changing the carb will not give you a major improvement in fuel mileage, and if your engine runs fine I would leave it alone.
Ford put some of the best carbs on these engines.
If you do want to improve the performance make sure the tune up is up to date, and most of us advance the ignition timing a little bit..
You don't say what year the Bronco is, but the later smog motors were kind of lethargic, but that is because of emmision requirements, not the carb.
Good luck

Thanks guys. I really don't know much. It's a 1975 with 302 but I'm told it isn't the original motor. It's also an aftermarket Holley carb on there now.

The truck starts fine, but feels very rich. It bogs heavily under full throttle and passengers in the back complain of heavy fuel smell.

Should I just look at tuning then?

Yes tuning is in order. You can download tuning instructions from edelbrock you will probably need a tuning kit edelbrocks are pretty easy to tune. Bad part about the edelbrock is they dont do good offroad but they are goos on road. Along with carb tuning you may also need to adjust igintion timing to get it running its best as igintion timng also affects the overall carb tune.
If your bogging under full throttle it sounds like the secondaries are opening to soon there is a adjustment for that. Or it could be the step up springs need to be changed because they are not allowing enrichment.

Pull some spark plugs and post some pictures. Also the carb and the numbers on the air horn. Using this we can make some recommendations to get you a little closer.

For example if it is Holley 600 vacuum secondary it is just about perfect for your engine. With a few adjustments you can make it run great and get great mileage. As someone said you need a few basics set right or it will feel and act lethargic.

Ignition - 10-14 degrees initial, check the total advance, should be 32-36. Vacuum advance connected? Try ported and manifold vacuum and see what feels better. For stick shift cars I like manifold vacuum and ported for automatics. When you pull the plugs you can see the timing impact by looking at the ground strap.

Carb - make sure the carb is clean and not gummed up with crap. Holley's hate that. Is the choke working correctly / manual? Measure your vacuum going down the road at your cruise rpm using a vacuum gage. Let's say it is 14" (once you get your timing sorted out). Pop open the carb and if it is stock 600 4160 it may have 66 jets and 8.5 power valve. Both of these are not correct for a stock 302. When you pull the plugs they may be sooty or dark. I would start with pulling out 4 jets sizes and going to 62 in the front - leave the rear alone. Take it for a drive and see how it feels at steady throttle. If it feels like it is miss firing it is too lean if it is ok leave it alone. Change the power valve from 8.5 to 10.5 . The power valve opens when vacuum drops to 8.5 inches of vacuum. Your truck probably cruises at 14 maybe 16 inches of vacuum so the throttle has to open pretty far before it sees 8.5 so the engine waits a looooong time to get the extra fuel to get accelerate under power. Going to 10.5 will pick that up earlier but still be closed when you are cruising. Both of these modifications will make the accelerator pump tuning more sensitive. These are easy just takes practice.

All of this leaves the secondary side unchanged because that carb has a plate that is more difficult to modify but also gives you enough fuel to prevent going over lean on full power.

Carbs .. I hate them but I have had a few apart...

Hi, go to the Holley website and learn about your carb.
X2 on the 600 cfm vacuum secondary carbs.
Before taking it off check the fuel level per Holley instructions.
Make sure your ignition tune up and timing is good.
Adjust the idle mixture screws.
Holley recommends a power valve at 1/2 engine vacuum so you will need a vacuum gauge.
You may as well work on your carb first. All the primary bowl jet and power valve work can be done with the carb on the engine.
Good luck

pcf_mark said:

Pull some spark plugs and post some pictures. Also the carb and the numbers on the air horn. Using this we can make some recommendations to get you a little closer.

For example if it is Holley 600 vacuum secondary it is just about perfect for your engine. With a few adjustments you can make it run great and get great mileage. As someone said you need a few basics set right or it will feel and act lethargic.

Ignition - 10-14 degrees initial, check the total advance, should be 32-36. Vacuum advance connected? Try ported and manifold vacuum and see what feels better. For stick shift cars I like manifold vacuum and ported for automatics. When you pull the plugs you can see the timing impact by looking at the ground strap.

Carb - make sure the carb is clean and not gummed up with crap. Holley's hate that. Is the choke working correctly / manual? Measure your vacuum going down the road at your cruise rpm using a vacuum gage. Let's say it is 14" (once you get your timing sorted out). Pop open the carb and if it is stock 600 4160 it may have 66 jets and 8.5 power valve. Both of these are not correct for a stock 302. When you pull the plugs they may be sooty or dark. I would start with pulling out 4 jets sizes and going to 62 in the front - leave the rear alone. Take it for a drive and see how it feels at steady throttle. If it feels like it is miss firing it is too lean if it is ok leave it alone. Change the power valve from 8.5 to 10.5 . The power valve opens when vacuum drops to 8.5 inches of vacuum. Your truck probably cruises at 14 maybe 16 inches of vacuum so the throttle has to open pretty far before it sees 8.5 so the engine waits a looooong time to get the extra fuel to get accelerate under power. Going to 10.5 will pick that up earlier but still be closed when you are cruising. Both of these modifications will make the accelerator pump tuning more sensitive. These are easy just takes practice.

All of this leaves the secondary side unchanged because that carb has a plate that is more difficult to modify but also gives you enough fuel to prevent going over lean on full power.

Carbs .. I hate them but I have had a few apart...

Click to expand...

B RON CO said:

Hi, go to the Holley website and learn about your carb.
X2 on the 600 cfm vacuum secondary carbs.
Before taking it off check the fuel level per Holley instructions.
Make sure your ignition tune up and timing is good.
Adjust the idle mixture screws.
Holley recommends a power valve at 1/2 engine vacuum so you will need a vacuum gauge.
You may as well work on your carb first. All the primary bowl jet and power valve work can be done with the carb on the engine.
Good luck

Click to expand...

Thanks guys. I'll do some research and will benefit from this advice. Plenty to keep me busy over the winter.

I have one other symptom to try and diagnose/solve

Our EB will almost always stall when we come to a stop, especially before the engine is up to full temperature. But it's become common practice to go to neutral, left foot brake, and always give a bit of throttle to keep it from stalling at stop lights. At a complete stop when the engine is up to temp I can usually let off the gas and it will then idle fine.

I'm hoping going through the carb cleaning process advised above would fix this, as the idle level itself feels fine.

That sounds like a carb problem but could also be really weak spark or meager timing. Check your initial timing as a first step just to get that set to 10-14. My favorite quote from this old school tuner was "most carburetor problems are fixed with a timing light!".

Do you have an electric or manual choke? Manual is all on you to adjust as you drive. Electric you may need to adjust the pull off speed - there is a dial on the side. Mark the current position then loosen the 3 screws and turn it toward the rich side (it says right on the black plastic which way) one line. Test it out and see how it runs. Once warm be sure the choke is fully open. Not right? adjust one more line. Repeat.

Assuming you set your timing to 10-14 and your vacuum advance is hooked up time to adjust your carb. You may not need a rebuild but you need some adjusting. If you are not going to take it apart and adjust the power valve and jets as I noted above this will get you idling decent.

Check the float level. On the passenger side of the front float bowl is a site screw with a flat bladed screwdriver head. With the engine running take this screw out. No fuel should run out. You may get a little trickle which is perfect. If more than a trickle comes out your float level is too high and needs to be lowered. If you do not have fuel to bottom of the hole it needs to be raised. Just check it - it is rarely wrong unless someone did something silly. But too high can cause stalling you described. If it needs to be adjusted you loosen the screw and turn the nut on the top of the carb (not the other way around like you would expect). Google adjusting holley float level for a pic if it is not correct.

Put your vacuum gauge on a fitting on the carb under the throttle blades - usually use the one on the front right side at the very bottom almost at the manifold. If something is connected to it use a T. Read the idle vacuum with the engine running - maybe 17" on a stock truck. Set the idle in neutral to something you like - maybe 650 - 700 rpm. Remember the vacuum reading. On the left and right side of the carb toward the front are two small screws one on each side just behind the float bowl. These are idle mixture screws. Turn these in a little at a time until it starts to die then start turning them out very slowly a little at a time. Keep watching the vacuum gauge and when it stops improving vacuum stop turning it out. Repeat on the other side. You may be able to get your vacuum to 17-19" of vacuum which is very high and super stable. Now shut the truck off. Gently screw each idle mixture screw in until it gently stops counting the exact turns out. Unscrew them back out to where you started. Each side should be very close to the same - maybe 2 or 2 1/2 turns out. You do not want one side 1 turn and the other 2 turns you want them even.

You may need to lower you idle speed now because it may climb as the vacuum improves.

Summary:
Set timing 10-14 degrees
Check choke operation
Check float level
Adjust idle mix screws

Now you are a Holley expert!

If you want to cure your rich stinky carb you need to change the jets and power valve.

At idle the vacuum reading should have a very steady vacuum needle. If it is not steady then you have some tuning to do.

... SO I'M THE SPOILER.. I'm the old fart here. & I owned a parts house from 1974ish to 1995. & I started to work there in 1967. So that's my History.
Now I say .. What ever you decide on trying .. Just don't give up your old CARB.... Don't turn it in for a core. '' ''keep it'' '' ...
YOU Know how that old carb runs. You can keep it for years & THEN Rebuild it & your good to go again. .. Sounds to me like yours is still good. .. YOURS IS STILL GOOD. But you'll not understand until you've gone through about a half a dozen carbs.
... SORRY But Myself & My employees ask a lot of questions about what the old carb was doing . SO the customer would tell us what was happening to his old carb.. Then my employees would tell me & WE'D REBUILD IT. & SELL IT AGAIN.
...

OK I am getting into this project. Decided I should first rebuild/tune the carb myself, install new spark plugs and a new distributor and wiring.

I took some photos of my carb, it appears to be a Holley 4412-3: https://www.holley.com/products/fuel_systems/carburetors/performance_2_barrels/parts/0-4412S

I think I found the right rebuild kit and instructions here:

https://www.holley.com/products/fue...omponents/rebuild_kits/renew_kit/parts/37-474

https://documents.holley.com/199r8338rev1.pdf



Here's some pics.

If you're not a carb guru you might be better off dumping the Holley.

Hi, it looks like an open vacuum port on the tree under the egr valve. Plug that and go from there.
I'm guessing you have a stick shift because I don't see a kick down lever or vacuum modulater hose.
I would plug the open vacuum port and get it running as good as possible. I would want to check the plugs after a couple of tanks of gas. The Jets in the carb may be a little too big. You want to run the engine the way it is, or get an air fuel ratio reading before going down a couple of jet sizes. Your carb came with a 73 jet, which is kind of big for a 302, better for a 351. But you should not just get smaller jets. You should go slow when trying smaller jets, and only go like 2 sizes smaller, then drive it and read the plugs. If your plugs are sooty and black the engine is running rich, but with the open vacuum port it it leans it. You want nice tan clean looking plugs.
Good luck

SteveL said:

If you're not a carb guru you might be better off dumping the Holley.

Click to expand...

Not a carb guru.

What would be a better replacement?

I think I see clutch linkage buried in there, so a manual trans?
If that open port is on the bottom of the vacuum tree like B RON CO was pointing out, that's at least a part of your issue right there.

What gear ratio and tire size too?

Lots of things still left on the table, so before you try a new carb, definitely fiddle with what you have. I don't disagree that some other carbs are easier to mess about with, but the Holley's are very tunable. Just fiddly and patience-trying.

Paul