dead cylinders

[mustang_gt88]

cylinder 4,6, and 7 are dead when the car is at idle, but when it to 2 about 3-3500 the car sounds normal and picks back up. I put new plugs in and check the spark, also pulled valve cover and removed the push rods from the dead cylinder and they were ok, also the compresstion on the dead cylinders is 130.
any ideals?

[MFIELD]

When you say dead, no fire? no fuel? How do you know they are dead?

[mustang_gt88]

I'm getting spark, i plug those plug wires off and no change in how it idled. I'm carrying about 3-5 psi fuel pressure.

[MFIELD]

I'm getting spark, i plug those plug wires off and no change in how it idled. I'm carrying about 3-5 psi fuel pressure.

I assume with that amount of fuel pressue its carb'd?

[mustang_gt88]

yes its carbed

[MFIELD]

What intake? What carb? I would up your base pressure to 6-6.5 psi. What does the plugs in those cylinders look like? Are they wet with fuel or oil? What is done to the motor? Just trying to get an idea what you have. Have you checked for vacumn leaks?

[mustang_gt88]

the intake is edelbrock performer, the carb is 650 double line holley, has the street cam, ford motor sport shorty header msd box and coil. I drove the car about 100 miles before this started happening, the plugs are not wet and do not have oil on them, they look fine

[MFIELD]

So this happened all of a sudden with nothing being changed? I would start with the basics. Check cap and rotor make sure all is good. Check plugs and gap. Check for vacumn leaks.

[mustang_gt88]

i did notice in the cap the points where the rotor sparks is corroded.

[MFIELD]

Try a little sand paper and see if that helps clear it up. If so replace it. O would be supprised if that would be bad enough to completely kill a cylinder especially if you had been driving it with no problems.

[mustang_gt88]

new cap and rotor button, nothing changed, also checked all the spark plugs. today it is 1,7, 4, 6 that are not firing, checked and the cylinders for compresstion and they are about 120 psi at 170 degree f.

[mustang8998]

What distributor are you running?

[mustang_gt88]

a ford dura spark

[redfirepearlgt]

Some one here was saying you can start the engine and spray some throttle body or carb cleaner around the intake base for detecting a vacuum leak. They say if there is a leak, the engine will surge up from idle when the cleaning agent gets sucked into the intake where the vacuum leak is present. Pretty clever I thought. Guess you could try that if you don't have a vacuum gauge to test it. Guess you would have to be careful it didn't pull down the throat of the carb though. Good luck. I've been down on my luck with mine this year as well.

[mustang_gt88]

i tried the vacuum test with spraying starting fluid around the intake and no change.

[redfirepearlgt]

Can you bypass the MSD box (I assume its a 6AL-2)? IT could be the MSD box. Can't believe a carbed setup would starve 4 of 8 cyclinders being a common delivery setup vice multiport F/I. I would be inclined to next swap wires such that 1 goes to 2, 4 to 3, 5 to 7, and 6 to 8 both at the plug and the distributor and see if the thing now misses on 2358 or still misses 1467. If 2358 then its likely wires and if 2358 its the plugs OR possibly pointing back to your MSD box causing the problem.

[mustang_gt88]

i tried switching plug wires and it got worse, could it be the idle circuit on the holley?

[redfirepearlgt]

Would have not expected it to get worse only change which cylinders were missing or stay the same. You did swap the wires two at a time and switch them both at the distributor and at the plug? Just checking. If that be the case when you switched back and it stayed worse I would suspect wires and/or how they are loomed or firing order has gotten crossed up. Verify firing order of wires. Read the MSD document on the 6AL-2 (still assuming its a 6AL-2 box). It is very specific about how they want you to loom the wires. Additonally it is also very specific how they want you to gap the plugs. As for the Holley idle circuit I don't know, I have no experience setting up a Holley, but would submit you rule out the electrical side completely before going to the carb. Just my .02 when it comes to basic theory of troubleshooting on anything you are working on. Rule out a suspect system in sequence before jumping to another related area of the problem or you will find yourself chasing your tail.

[mustang_gt88]

i did not swap them at the dzzy and the plugs, will try tonight.

[mustang_gt88]

the msd box 6al, not the 6al-2, I pulled the plug on the dead cylinder and grounded them to the body and the spark like crazy.

[1987coupe]

i had the same prob with my ford truck it would not idle and had no power (missing like crazy) untill 2500 rps then i changed the carb and it fixed my prob so if you have one laying around try that.

[2Fast4You]

You could have a blown power valve in the carb, It controls the idle circuit.

[NXcoupe]

You said 'street' cam. what cam is it exactly, as in part number and manufacturer. What year is the engine? Pistons stock? You said the affected cylinders had 130, what do the others have?

[mustang_gt88]

I believe the cam is the x303, stock pistons, all cylinders have the same compression

[NXcoupe]

X cam with stock heads? Stock springs? What year is the engine?

[mustang_gt88]

Stock heads, stock spring, the engine came out of a 91 gt

[mustang_gt88]

i had it wrong the cam is the e303

[mustang_gt88]

i put a different carb on and she runs great, thanks for the help

[redfirepearlgt]

i put a different carb on and she runs great, thanks for the help

Outstanding. Glad you got it isolated and figured out. Hope to see it out and about sometime. !!!!!!:bigthumb

[2Fast4You]

Good to know you have it back on the road.