390 won’t start after new carb

[MEDEVAC247]

Hey gang, Marcos here from the San Fernando Valley looking for some guidance. I have a 1972 F250 with a stock 390 in it. I decided to change the carb out to a Holley 500 CFM PERFORMANCE 2BBL CARBURETOR. Before the carb swap the truck ran fine but I wanted it to run great! After installing the Holley the damn thing won’t stay on. All hoses (booster, PCV, etc) were connected to the manifold and were left that way. The only two hoses that needed connecting (I think) to the carb were the fuel hose and the distributor vacuum advance. Everything looks buttoned up as it should. I’ve gotten it to start and I’ll hold the throttle open so it doesn’t die but as soon as I release it hoping for an idle it immediately dies on me. Did I do something wrong or is this carb junk? Im contemplating putting the old one back on Any help is greatly appreciated.

Also, before the carb swap I had changed out the fuel pump, fuel lines, filter, threw on MSD spark plugs with MSD wires, Blaster 2 and distributor cap and rotor. Truck ran fine with all that up until I switched the carb.

[DuckRyder]

At risk or being insulting, i assume you have set the choke (if electric) and base and fast idle speeds? What list number is the Holley (on the air horn)

[1972hiboy]

At risk or being insulting, i assume you have set the choke (if electric) and base and fast idle speeds? What list number is the Holley (on the air horn)

Sounds like the low idle speed screw needs adjustment. or at least start there. My uncle had put a 500cfm Holley 2bbl on his 390. I found that carb to be setup so far on the rich side that it would start cold with no choke and idle just fine.

[MEDEVAC247]

At risk or being insulting, i assume you have set the choke (if electric) and base and fast idle speeds? What list number is the Holley (on the air horn)

It has a manual choke and I didn’t mess with any base and idle speeds. I thought they came set from Holley so you wouldn’t have to mess with anything

[cep62]

Could also be a vacuum leak do you have the spacer under the carb with the P V C port ?
if you disturbed it or have the wrong gasket it'll suck air.

[cep62]

I didn’t mess with any base and idle speeds.

turn them all the way in , then out about a turn and a half .
that should let it run , then adjust it from there one side at a time.
It's not blowing black smoke is it , if so you need to set the float.

[DuckRyder]

This might help:

https://documents.holley.com/199r11515.pdf

[MEDEVAC247]

Thanks for the help guys. I was in Az but now I’m back home so I’m going to try all things mentioned and see how it goes. I wish I was more mechanically inclined but I’m working on it.

[zgerbic]

MEDEVAC247,

At this point I would not assume the carb is junk but rather you have a vacuum leak or the card will need to go through an adjustment process.

Be aware carbs are not really adjusted from the factory to run if just bolted in. Since there is no way for them to guess what the setting would be they just rough in the float level, set the default choke level, set the idle mixtures to a default level and assure the throttle is a little open. They will install a typical set of jets that should work in most cases, as well as a default power valve. Even a carb that was just running on one engine might not run right on an "identical" engine if it is just swapped.

There is no way for a vendor to preset a carb so it is left to the installer to figure it out. There is an assumed basic level of knowledge, and many books/postings to assist.

There are a general set of steps that most of us with experience go through when installing a new carb, and you will see some troubleshooting steps from the responses you get to this topic. DuckRyder's advice is a good starting point to read through the carb manual.

There are a few things to check when you first install a carb. Most carbs follow a similar process as shown below. Non-Holley carbs (like the Ford one I assume you took off) will usually have a significantly different way to check and adjust the float setting but should otherwise be similar. Below is an abbreviated set of things to check/do for your Holley carb:

- Before putting the carb on, make sure the mating surfaces are clean and flat, and install manifold gaskets. Look for any places that could provide a vacuum leak. You should not need any sealer if the gaskets are ok.
- Bolt down the carburetor but don't over-tighten it as this may distort something causing a vacuum leak.
- Attach the fuel line and make sure there are no leaks.
- Reattach any vacuum lines or plug those that are not used. Make sure you get the vacuum line(s) to the distributor attached correctly.
- Turn in the idle mixture screws lightly and back out 1 and 1/2 turns. This is probably where the factory set them to.
- Turn in the throttle adjust screw until the throttle arm stops moving. Turn the screw in until the arm starts to move then one to two turns more.
- Open the choke all the way. Make sure it can just barely close and will open completely. Leave it open at this point.
- Since this is a Holley carb, I would remove the float bowl sight plug (screw) which allows you to tell if the float is set right.
- Remove the power line from the coil so the engine cannot start, or sparks can occur around any leaking gas.
- Put a rag under the float adjust hole and spin the engine a few turns to see if the float level is correct once the float bowl has got gas in it.
- if gas pours out of the hole the float is too high and the engine will flood, or run overly rich.
- if gas cannot be seen by the edge of the hole, the float is too low and the engine will run lean, could pop back and overheat, or not run without the choke on.
- adjust the float level till when you crank the engine to get the fuel pressure up, the level is just at the bottom edge of the float bowl screw. Cranking the engine a little more should not pump any more fuel out of the hole. This adjustment process is much safer to do with the engine not running, especially if the float is too high and gas starts running/spraying out the hole. Also if the engine shakes (depending on cam or other engine issues) you cannot get the level right while the engine is shaking. Put the float bowl sight plug back in.
- Next, with the choke completely open, look down into the throttle opening and open the throttle to see that the accelerator pump is pumping gas down the throttle openings. Once or twice is fine, you don't want to flood out the engine before you try to start it.
- This is a manual choke carb so just set the choke to have the choke plate open about 1/16" to 1/8" max as a starting point.
- At this point all the preliminaries are done. It is not necessary to have the throttle or other linkages attached as you can do this later after the carb is adjusted properly. You can hook up the power to the coil and try starting the engine.
- It is assumed you did not move the distributor when the carb was replaces so the timing should be ok. If you moved the distributor or changed the vacuum advance hoses, you will need to recheck the timing after the engine is running.
- With the settings above you should be able to get the engine started and it should idle, though maybe a bit fast. Let it warm up and open the choke a bit as it warms up till the choke can be opened fully with the engine still running.
- At this point you can make some of the final adjustments. I would attached a tach and manifold vacuum gauge under the hood to make the final adjustments to the idle mixture screws and throttle adjustments. If you need more help as far as how to make the fine adjustments, post a request and some of us can give advice.

Assuming you have tried to make the fine adjustments to the carb and the engine will not idle correctly or stay running below about 1000 RPMs, you probably have a vacuum leak somewhere. It could be around the carb mounting gaskets or in a rubber line that you moved during carb installation. There are many ways to check this. Most people get the engine idling at a stable level and then go around the base of the carb and carb spacer spraying a little carb cleaner. Can also check around where the vacuum lines attach to the carb base or manifold vacuum fittings. If there is a leak, the engine will change speed as the cleaner is sucked into the engine. Others can provide other methods and advice.

While you can skip some of these steps, you can never be sure all is adjusted right with the carb and may have other problems down the road that can be a mystery to fix. If just removing and replacing a carb on an engine that was running ok, most major problems will be vacuum leaks and float levels. Smooth idle problems on an engine that is running ok with a new carb will generally be idle mixture screws or small vacuum leaks.
Keep in mind that as a rule of thumb, under acceleration you are using the main jets and power valve for fuel. But when cruising down the road or at low speeds on the highway you are using the idle jets for fuel. This is another reason to get the idle mixture set correctly.

Let us know how this goes.

[MEDEVAC247]

Hello everyone, I’ve been waiting on a 1” spacer for the carb and once I installed it the truck started and ran fine. I still have to fine tune the carb but it’s back running. I’m assuming that the spacer it had on there with the front and rear ports wasn’t really mating well with the Holley. I didn’t do anything different other than change the spacer to an aftermarket without ports and it eliminated the vacuum leak that it surely must have had. I appreciate you all. Now I have an issue with my 74 Ranger but I’ll look up threads to see if there’s one already for my stalling issue after the engine is warm

[DisneysPatB]

Hey gang, Marcos here from the San Fernando Valley looking for some guidance. I have a 1972 F250 with a stock 390 in it. I decided to change the carb out to a Holley 500 CFM PERFORMANCE 2BBL CARBURETOR. Before the carb swap the truck ran fine but I wanted it to run great! After installing the Holley the damn thing won’t stay on. All hoses (booster, PCV, etc) were connected to the manifold and were left that way. The only two hoses that needed connecting (I think) to the carb were the fuel hose and the distributor vacuum advance. Everything looks buttoned up as it should. I’ve gotten it to start and I’ll hold the throttle open so it doesn’t die but as soon as I release it hoping for an idle it immediately dies on me. Did I do something wrong or is this carb junk? Im contemplating putting the old one back on Any help is greatly appreciated.

Also, before the carb swap I had changed out the fuel pump, fuel lines, filter, threw on MSD spark plugs with MSD wires, Blaster 2 and distributor cap and rotor. Truck ran fine with all that up until I switched the carb.

Hey!! I am up in Sunland.