OK I have a 73 F-250 with a 390 4bbl Edelbrock carb and intake. It started missing at Idle speeds but is worse when Cold.Let it warm up and its better but still not wright. What I have done so far is change plug wires with no change. I have Done a Compression Check and They range from 130 to 120. Installed new Plugs Points Condenser Cap Rotor Checked timing and it is 14 Degrees. Still no change. Misses at idle even en warm but when speed is increased missing mostly goes away but not completely. Pulled valve cover and inspected valves and springs for any abnormalities and all look good. Still misses.I don't understand I am at My end of know-in what to do? I always thought If you have fuel spark and Compression It should fire! Nope #6 cylinder is mostly dead.Has Dual glasspacks and makes a whoof whoof sound outa the drivers side tailpipe when it does not fire on that #6 Cylinder. Oh checked with a vac Gauge and its a steady 15 inches vac at idle? Anything else to check? Engine only has about 6000 miles since rebuild.The Cylinder heads were rebuilt when engine was redone with new valves and Hardened valve seats,Carb Is new and intake. Any Ideas I would sure like to know what to look for?

 

Pull a few pugs, in particular, #6 and compare them. What do they tell you? All the plug wires fitting tight to the plugs.....especially #6? What was the compression on #6 when you did the test?

Vacuum is 15" Hg at idle? Too low unless you have a "lopey" aftermarket cam.....you're looking for 18 - 21" Hg at idle. Could be your carb's mixture needs adjusting.

What is the plug gap set at? And the points - set correctly? And the lobes on the distributor that the points set on....are they the same..one or two is not rounded over worse than the others?

Then there's the lifter pre-load, especially on your "dead" cylinder. How is it compared to the others?

Hate to suggest a leak down test when it's probably something simple...........

 

Compression on #6 was 120, Plug Gap is .035 Point gap is .017 Distributor is a Reman with prolly less than 2000 miles on it and yes #6 is sparking You can hold the wire away from the block and jump sparks. I tried adjusting mixture screws No change.Only thing I can think of if the pushrods are to long if the HEADS were milled during the rebuild.Lifter Preload? how do I check that? Not familiar with what to do?

 

Couple things - not trying to beat up on you but you should clear some things up. How do you know #6 cylinder is dead? A bum cylinder would result in a very unsteady vacuum reading, I'd think. Pull a plug wire at idle to observe this.

15" vacuum is a little low. I see you're in Dakota though. If above 1000' in elevation the corrected figure might be subtract 1" per 2000' of elevation, something like that. So not great at sea level - but right in the ballpark depending on your elevation. STEADY needle is important, and at a reasonable lope. Maybe 600 RPM, no more. Sometimes people like to goose the throttle to 3000 and measure that way.

It sounds like ignition is good - BUT, you are running points & condenser - and about once a week or so, and we're due for another, somebody asks about a rough running engine. And they are always using ****ty imported condensers. They are NO GOOD. Grab an old Autolite or Standard out of the junkbox and try that. And even junkyard used points are better than the recycled beer cans they make them out of today. Swap out the condenser though and see if that helps.

Also as a diagnostic disconnect and plug the vacuum advance canister. With a timing light connected run the engine up and down the RPM range, 3-4k and see that it is advancing and retarding smoothly and cleanly, and without "scatter" on the damper marks. A defective distributor can also cause a miss that is tough to track down.

 

I have discovered the #6 Cylinder does not fire at Idle but when give-in a little bit RPM starts to fire and miss-fire almost goes away. The altitude where I live in South Dakota is about 3500 feet. I run my idle about 650 RPM. As for as the ****ty Condensers go I have run into a Kinda similar problem before. I replaced the distributor with a Re-man one and It would work fine until it got warmed up for a while the start to run like crap. Well finally went to NAPA and Bought a quality Echilin one and problem went away. Looked at the points and condenser that came with the Re-man and they were cheap Chinese made stuff. Yup as far as the timing advance check I did that and it advanced smoothly. All cylinders are between 120-130 compression. I talked to one fella who was a ford mechanic and said it sounds like Valves But how can that be with that kinda compression? It almost seems like when running at low RPM the Hydraulic lifters or Lifter on cylinder#6 is holding ex valve open slightly to create a low compression issue. only at low Idle. But when sped up it goes away!??? I have been rack-in my mind all day on this issue with no answers. I took it to the Re-builder that Rebuilt the engine and he was Baffled also. He said he will figure it out. So now waiting game to see what he finds.

 

Sounds like you need a new HT lead for cylinder number 6.

You can check the resistance on it with a multi meter.
It should be <500ohms per foot.

A steady 15'' at your altitude isn't too bad. (I assume the vac advance is ported though)

Compressions of 120-130 reflect a decent, but low compression, engine.

 

Those spark plug wires I have are barely a year old and have prolly only 2000 miles on them. And when I held the wire away from the engine with it running would jump a spark a inch long. Got a feeling its something with lifters or valves.

 

That sounds like classic symptoms of electric choke malfunction. Been there too many times

 

Choke works perfectly fine. Starts up in high Idle and after a few min kicks down to slow Idle. Does not load up or flood when choke is partial closed.

 

Well, I think it;s time to pull the valve cover on the bank that doesn't fire. Start her up and watch the valves on #6 cylinder...well, all of 'em really but #6 is still misfiring, that's the one to watch.

 

Lalberts, the other guys are probably right. However, just because the choke is kicking down once it's warmed up or when you blip the throttle, that doesn't mean it's working fine. It could be staying partially closed. Point is, don't discount it. It's easy to swap an Edelbrock carburetor for test purpose.

 

At any rate its outa my hands as of Monday because i Took it down to the guy that rebuilt the engine to let him figure out whats wrong. Cuz I feel I tried everything

 

OK Here it is, The guy i took the truck too called and said you'd never guess what the problem was! Ok and he said the carburetor! He said He put a different carb on that he uses to test engines and it ran perfectly. He says that means that there is something plugging up the carb I had on it. Its a new Edelbrock #1406 600CFM 4 bbl was put on less than a year ago, still under warranty so he says he may just send it back and get a new one. will see?? But strange a carb to make just 1 cylinder miss?? The other cylinders seemed weak also so maybe there is something to it. I also have a Edelbrock dual plane intake on my 390.

 

It's kind of a PIA to check lifter preload on a 390, but it should have been done when they rebuilt it.

 

Still waiting for him to finish it!