Disclaimer- I’m new at this

I’ve been tuning my 360 on my ‘72 f250 after it sat for 24 years. I got it running great while idling, 18 inches of mercury on the vacuum gauge with a rock steady needle, 40 lbs of oil pressure at idle, all new distributor cap, plugs, and wires as of October, new Holley 500 cfm 2 barrel carb. It was running great but the past couple times I actually ran it down the road the engine starts running rough. I revved it up and it seemed to run better but ran rough again when it got back down to idle.

I pulled the plug wires at the distributor. All of them had spark, but 1,4,6, and 7 made no difference in the idle. I’ve done some research and others have dropped 2,3,5, and 8, the exact opposite of mine. Someone suggested that maybe the carb jet that feeds that side of the dual plane intake is blocked? I have been running it without an air cleaner for testing purposes only and I’ve never driven the truck for more than 2 minutes at a time.

I think my next steps are, check plugs to make sure they’re firing and not fouled, do compression test, remove valve covers and check to make sure all valves are working, and if everything is good there, clean the carb? I’m thinking it’s a fuel issue because 4 of them just dropped all of a sudden, I’m fairly certain there’s no vacuum leak because I’ve checked with propane and the needle on the vacuum gauge is steady, and I don’t believe there is a spark issue because there is spark at every wire on the dizzy. If anyone has any other ideas or anything else to check I’d love to hear it. Thanks!

 

If that engine sat for 24 years and you didnt take the valve covers off and turn it by hand to see if everything was moving correctly, I would say there is a high chance there was stuck valves and now bent push rods. I learned that one when my engine sat for about 22 years and my dad wanted to crank it over and get it running. I ended up with stuck intake valves and spaghetti for push rods. I would pull the covers off and check everything there before moving on.

 

Thanks for the reply 1browski, I did have the valve covers off and cranked it by hand. That was about 6 months ago. I soaked the cylinders and made sure there were no stuck valves before cranking it up. Since then I’ve changed all the fluids and put a new motorcraft oil filter and diesel oil with all the zinc and additives along with some seafoam to clean up some crusty valve springs and rockers. Since then it’s been running great while stationary. No valve train noise at all.

 

Well thats good. A week ago, my dad wanted to put the push rods and rockers on his 428 in his 66 f250 and crank it over. I asked if he had some memory loss since my 390 and told him to just get that out of his mind. Then I laid down the law on what is going to happen if he want to get that truck up and going again. It was pretty hilarious. Im at a loss on your truck symptoms, I not good trouble shooter when the weird stuff starts happening. My 390 heads are standing up on my work bench right now actually. Ive had a low vacuum issue for years on the motor. Yesterday I just seen water leaking from my #4 intake valve. Fun fun. Im sure tomorrow some other will get on your problem. Good luck.

 

Could it have sucked some trash from tank into inlet of carb? My carb started dropping 4 cylinders on a dual plane last year and I had a fuel line coming apart that had affected fuel flow into the carb.

 

I suppose it could be, it’s a new tank but the fuel line has not been replaced and is pretty rusty. I’ll take a look

 

Here are some pictures of what I found when I took my carb apart. If I am not mistaken on mine it was dropping 2 cylinders per side because of how a dual plane intake carries air and fuel.

 

My hardline from my saddle tank to the fuel pump has an issue as well. My tank still looks brand new inside. Since I didnt want to make a new line right away, I ran a rubber fuel line in its place to see if it made a difference and it did. I think there must be chunks of rust in mine that moves and cuts off the fuel possibly. Havent had an issue since bypassing it. I will probably replace it when I make a new one for the 72 f250.

 

Took the carb off today and jets were almost completely plugged. Cleaned it up and put it back together. Gas tank is new so it has to be the line. I’m going to put it back on and see if it works tomorrow. Thanks everyone for the suggestions

 

Good luck, seems like you may be on the right path.

 

Problem solved. Jets were plugged. New hard line on the way

 

Good deal.

 

I just tore down a motorcraft 2100 2-bbl carb on an FE engine. One of the accel pump squirters was plugged tight.
Both rubber diaphragms on accel pump and power valve were petrified, and float level was too low.
Sure ran good today once putting the carb back on.

Good to hear your carb was plugged with debris, being a simple fix.