Engine Misfires And More Misfires. Carb Guys Get In Here!

89'notchback · May 11, 2015

This is my first post on this forum and I'm really hoping you guys can help me!
My car has had a misfire for well over a year at this point and I'm now officially stumped.

Here is what is does:

-It misses at idle- It sounds like: normal,normal,bump,normal,bump. The tach drops a little each time.
-Misses horribly under load. If I take off it starts badly missing around 2,000rpms then clears up around 2,300rpms
-When cruising you can hear the same noise above.
-Loss in power and rich Idle.

Here's what I've fixed/replaced after noticing the misfire:

-New carb (holley 650 ultra dp)
-New fuel pump (holley "red" pump) 6.5psi and fuel filters.
-New plugs, wires, distributor. (HEI and getting 12+ volts) Yes, I'm using the correct firing order.
-Rebuilt the entire top end.
-Set timing at 13 degrees initial. (messed with timing many times with same results)
-New alternator (130 amp)
-New exhaust
-Checked for vacuum leaks with carb cleaner and couldn't find any.

My cars specs:
1989 Mustang Notch
302 HO Carbed
T5
3.73 rear end.
All new suspension (SR performance springs, Koni shocks and struts, tubular uppers and lowers)

Someone please lead me in the right direction as I'm out of ideas. Thanks!

89'notchback · May 11, 2015

Also, I did a compression test and it was fine.

CarMichael Angelo · May 11, 2015

I can tell you that the issue you are having is not your carb. It sounds electrical to me.

Whether it's firing order, something wrong w/ your distributor, or coil, you need to look there as opposed to the carb.

There are really good guys that will come along here in a bit to help you diagnose the issue,so be patient.

rbohm · May 11, 2015

the first thing to ask is,

is the miss consistent? meaning, does it happen with regularity? and under load, does i constantly miss, or does it run hard one time, and the next time you load it up it runs like crap?

as for the carb, as an aside, go back to a vacuum secondary carb and lose the double pumper.

modulistic · May 11, 2015

try a new coil.

n8rfastback · May 11, 2015

I would check all of your ignition grounds and leads. The wite that supplies 12 volts to the dizzy, check it every where for loose connections. Check to make sure your block is grounded well to the frame. Any crappy looking connections should be broken free and reconnected. Be sure to strip paint and make sure all bolts for grounds have good contact.

89'notchback · May 11, 2015

rbohm said:
the first thing to ask is,

is the miss consistent? meaning, does it happen with regularity? and under load, does i constantly miss, or does it run hard one time, and the next time you load it up it runs like crap?

as for the carb, as an aside, go back to a vacuum secondary carb and lose the double pumper.

The miss is constant. Under load is when its at its worst. It'll clear up if I push through it but it still misses just not as bad. And I have a vac, secondary carb and it did the same thing.

89'notchback · May 11, 2015

n8rfastback said:
I would check all of your ignition grounds and leads. The wite that supplies 12 volts to the dizzy, check it every where for loose connections. Check to make sure your block is grounded well to the frame. Any crappy looking connections should be broken free and reconnected. Be sure to strip paint and make sure all bolts for grounds have good contact.

All connections are fine. It doesn't lose signal.

89'notchback · May 11, 2015

I'm going to redo the valve lash tomorrow. Maybe I got them too tight.

rbohm · May 12, 2015

89'notchback said:
The miss is constant. Under load is when its at its worst. It'll clear up if I push through it but it still misses just not as bad. And I have a vac, secondary carb and it did the same thing.

that tells me you do not have a carb issue. i would start with plug wires, makes sure the have the proper continuity. next check the condition of the distributor cap. make sure there are no carbon tracks that run between the plug towers. one more thing to do, replace the starer solenoid on the fender. the entire electrical system runs through it, and if the disc inside is loose and touches the ground, it will create a miss.

89'notchback · May 12, 2015

rbohm said:
that tells me you do not have a carb issue. i would start with plug wires, makes sure the have the proper continuity. next check the condition of the distributor cap. make sure there are no carbon tracks that run between the plug towers. one more thing to do, replace the starer solenoid on the fender. the entire electrical system runs through it, and if the disc inside is loose and touches the ground, it will create a miss.

I've checked the continuity of the wires and they're fine. Distributor is brand new (3rd one no change). And I've never thought about the solenoid. I'll definitely try that thank you very much!

rbohm · May 12, 2015

not the distributor, but rather the cap. also check the plugs, sometimes they have problems that will cause a miss, even when new.

next run a compression check and make sure the numbers are with in 20-25lbs of each other.

rbohm · May 12, 2015

another thing ot check is all your grounds, and your ignition switch.

89'notchback · May 12, 2015

rbohm said:
not the distributor, but rather the cap. also check the plugs, sometimes they have problems that will cause a miss, even when new.

next run a compression check and make sure the numbers are with in 20-25lbs of each other.

Cap looks brand new. I pulled the plugs two days ago and they looked good. They're fairly new too but I've put about 3 different sets on it and nothing helped. Compression test was within 8psi I believe.

89'notchback · May 12, 2015

rbohm said:
another thing ot check is all your grounds, and your ignition switch.

I just remembered. My started does hang for a second or two after the car fires up. Could that be a symptom of a solenoid going bad?

89'notchback · May 12, 2015

I found this bolt hole behind my pass. Side head. It's on the block next to the bell housing. What is this??

rbohm · May 12, 2015

89'notchback said:
I just remembered. My started does hang for a second or two after the car fires up. Could that be a symptom of a solenoid going bad?

yes it can be. it can also indicate a bad bendix drive.

89'notchback · May 12, 2015

Little update. I got my volt meter out and the battery, starter solenoid are at 14.3 volts and hei, and choke are getting 13+ volts at all times

7991LXnSHO · May 12, 2015

HEI and Holley! Old school I can help with.
It sure sounds like an electrical issue, and the HEI system is simple, compact and effective. I know you tested plug wire continuity, but did you wiggle them while holding the probes on the contacts? How close were the readings on each wire? I think the rule used to be 5k ohms per foot, but as long as they are close and do not show breaks in continuity when wiggled and no physical damage, I bet they are OK. USE dielectric grease on the boots and caps.
Did you run it in a dark area and mist the wires with water from a squirt bottle? That's to help make any sparking to ground easier to see.

Next, it is time to dig into the HEI unit and its wiring. It is very easy to pinch a wire with the metal leads from the coil and cap. (I may still have electrical tape or shrink tube on one of my lead wires.) Then inspect the wires that attach to both ends of the ignition module and wiggle them while using your continuity tester.

I do not know how to test the pick up coil beyond visual inspection.

If all this is good, take the module to a place that specializes in auto electrics or maybe any place with a mechanic as old as your car. There is a little box that runs the modules through its paces. If it passes 4 times, you missed a step. (Like the pick up coil or a power supply problem.)

The vac carb will not fix this problem, but if you are on the street, it will give better mileage and drive ability, easier tuning, and you will not feel any power difference.

Here is someone's take with exploded picture of an HEI system. I will not re type the coil testing on page two.

Ignition Systems- HEI operation

89'notchback · May 14, 2015

7991LXnSHO said:
HEI and Holley! Old school I can help with.
It sure sounds like an electrical issue, and the HEI system is simple, compact and effective. I know you tested plug wire continuity, but did you wiggle them while holding the probes on the contacts? How close were the readings on each wire? I think the rule used to be 5k ohms per foot, but as long as they are close and do not show breaks in continuity when wiggled and no physical damage, I bet they are OK. USE dielectric grease on the boots and caps.
Did you run it in a dark area and mist the wires with water from a squirt bottle? That's to help make any sparking to ground easier to see.

Next, it is time to dig into the HEI unit and its wiring. It is very easy to pinch a wire with the metal leads from the coil and cap. (I may still have electrical tape or shrink tube on one of my lead wires.) Then inspect the wires that attach to both ends of the ignition module and wiggle them while using your continuity tester.

I do not know how to test the pick up coil beyond visual inspection.

If all this is good, take the module to a place that specializes in auto electrics or maybe any place with a mechanic as old as your car. There is a little box that runs the modules through its paces. If it passes 4 times, you missed a step. (Like the pick up coil or a power supply problem.)

The vac carb will not fix this problem, but if you are on the street, it will give better mileage and drive ability, easier tuning, and you will not feel any power difference.

Here is someone's take with exploded picture of an HEI system. I will not re type the coil testing on page two.

Ignition Systems- HEI operation

I've tried 3 different HEI distributors. How could the same problem be present in all 3? That's what makes me think the distributor isn't the problem. And yes when checking wire continuity I wiggled them when tested and every wire passed the test. I did run in the dark and misted water and could not find any arcing.

Engine Misfires And More Misfires. Carb Guys Get In Here!

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