Strange 66 289 misfire

Hi, I have a 66 Mustang with a 66 289 with a mild cam, 4 barrel edelbrock 600 tri Ys dual exhaust and a C-4 auto. I just fired her up a couple of weeks ago to get er ready for the season and the car was running fine. I took it out for a ride and it ran strong but when I got home it started missing at an idle. I just shrugged it off and bought a set of pertronix wires and autolite plugs. I started the car up today and it would barely stay running, so I decided to change the plugs and wires. well needless to say the car runs like poop now. Its missing horribly. I pulled each plug wire and came up with this, cylinders 2,3,5,8 make absolutly no difference(not firing), I could pull em off and it would run the same. I have spark coming outta the cap and I haved tried another set of wires and plugs but it runs the same, I have also checked my timing and rotor and distributor cap, all is new and in good shape. Now!! The strange thing is that if I revv the car up slowly it will be missing like crazy then all of a sudden whammo!! the idle jumps up and she roars like nothing is wrong, but if I hold the throttle there you can hear stumbling a little still then when I drop the idle down it goes back to idleing like poop again. I figured I would pick your brains before I pull the valve covers and start digging into the engine mechanical I.E. lifters/valves. Oh yea and I did a compression check and its about 150lbs per cylinder. What do you guys think???

Thanks, Jack

Have you tried looking for vacume leaks? just a thought.
[addsig]

Yea, I did, but then again I thought to myself how could that cause 2,3,5,8 cylinders not to combust?

Jack

Sounds like the idle passageway or idle airbleed on one side of the carb is plugged. On a dual plane manifold the two end cylinders on one bank and the two middle cylinders on the other bank are connected to the same side of the carb. So if one idle jet is plugged the cylinders you listed will be dead at idle. When you open the throttle plates enough to pull fuel from the main jets, those cylinders will kick in.

Hmmmmmmm,

Check for moisture under the cap?....

This is very thought provoking... Clears up if you slowly open her up? Then takes off all of the sudden like it's supposed to?....

Check your vacuum advance linkage for hangups inside the cap....
[addsig]

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Check your vacuum advance linkage for hangups inside the cap.... Then do what 67Coug said and clean out the carb...

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<font size=-1>[ This Message was edited by: FEandGoingBroke on 5/14/05 4:29am ]</font>

Ok, well I sort of narrowed it down, if I close the choke manually almost all the way, it starts running normal again, I have an Edelbrock 600 carb and a 289 performer intake on the car, and it was just sitting since about December, I think that thing that 67coug said makes a heck of a lot of sense, but does that mean a carb re build is in the near future? I was thinking about going back to a holley 650 because I have had nothing but problems since day one with the Edelbrock, and also I noticed a considerable power loss when I swapped over to the Edelbrock. Oh, how do I determine if I should be running a spacer plate under my carb???

Thanks a million for the ideas guys, Jack

You have a clogged idle passage in my opinion. Take out the idle needle for that side of the engine and shoot some carb cleaner in it and see if it helps.

Just get a bottle of cleaner like tha man said and clean out both passages and maybe buy an $18.00 paper rebuild kit and toss it in after giving it a good cleaning.


[addsig]

I'll third that. It's definitely a problem with one side of your dual plane intake. If there are no vacuum leaks you definitely have a carb issue on one side. A good cleaning of the carb will probably cure it, maybe throw a quickie kit at it.