Engine mid/high rpm stumble, idles good.

Rynb15

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Nov 21, 2005
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Hey guys, got an 85 gt, carbed 302. I took over someone elses project. Stock 95 f150 bottom end, with the 85 gt top end. Edelbrock 600cfm carb. bbk shorty headers. Last owner did install new distributor, durapsark and accel coil. Then it sat several years after this. I have the right firing order, verified by watching the intake valves move while hand turning the motor. Timing is set about 10-12 degrees. Also has a holly red fuel pump (7psi, no regulator but i have one getting ready to go on). I have done new plugs, wires and rebuilt the carb. Car starts up and will idle all day. A quick stab at the throttle and it will rev right up to about 2500-3k. If i try to go higher, it stumbles and pops out the carb. By doing some research on this, symptoms of a bad coil will do all of this. Just wanted to get some more input before i start throwing parts at it. Appreciate any input. Thanks
 

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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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I agree with replacing the coil.
take a timing light and check for advance in timing when revving it up, that will also tell you if the ignition is breaking up.
 

Rynb15

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Nov 21, 2005
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Its cheap enough I'll just replace it to see what happens. I'll try to get a timing light on it tonight. What should it be, around 30-35? 3000rpms? Thanks
 

Rynb15

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Nov 21, 2005
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well i went and replaced the coil. Then it would not even try to start. Just crank. So I went to put the old coil in just to see if it would start and see if the new coil was bad. It would not start. And then starter got stuck cranking even with the keys out. Luckily I just installed a battery disconnect . Could this be related to not starting at all now? Starting to regret buying this car lol.
 

jrichker

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I can answer some of your questions since the stating circuity is almost identical to the 86 and later model cars.

No Crank checklist for 5.0 Mustangs

Revised 05-Oct-2010 to update Fluke references.

No crank. slow crank and stuck starter solenoid problems have the same root causes – low battery voltage and poor connections. For that reason, they are grouped together.
Use the same initial group of tests to find the root cause of both no crank and stuck solenoid problems.

Since some of the tests will bypass the safety interlocks, make sure that the car is in neutral and the parking brake is set. Becoming a pancake isn’t part of the repair process…


1.) Will the car start if it is jumped? Then clean battery terminals and check battery for low charge and dead cells. A good battery will measure 12-13 volts at full charge with the ignition switch in the Run position but without the engine running.
A voltmeter placed across the battery terminals should show a minimum of 9.5-10 volts when the ignition switch is turned to the Start position and the starter engages or tries to engage. Less than this will result in a clicking solenoid, or slow cranking (if it cranks at all) or a starter solenoid that sticks and welds the contacts together.

Most auto parts stores will check your battery for free. It does not have to be installed in the car to have it checked; you can carry it with you to the auto parts store.

The battery posts and inside of the battery post terminals should be scraped clean with a knife or battery post cleaner tool. This little trick will fix a surprising number of no start problems.

The clamp on with 2 bolts battery terminal ends are a known problem causer. Any place you see green on a copper wire is corrosion. Corrosion gets in the clamped joint and works its way up the wire under the insulation. Corroded connections do not conduct electricity well. Avoid them like the plague...

If the starter solenoid welds the contacts, then the starter will attempt to run anytime there is power in the battery. The cables and solenoid will get very hot, and may even start smoking. The temporary fix for a welded starter solenoid is to disconnect the battery and smack the back of the solenoid housing a sharp blow with a hammer. This may cause the contacts to unstick and work normally for a while.

A voltmeter is handy if you are familiar with how to use it to find bad connections. Measure the voltage drop across a connection while trying to start the car: more than .5 volts across a connection indicates a problem.

See http://assets.fluke.com/appnotes/automotive/beatbook.pdf for help for help troubleshooting voltage drops across connections and components. .



2.) Check the battery to engine block ground down near the oil filter, and the ground behind the engine to the firewall. All grounds should be clean and shiny. Use some sandpaper to clean them up.

3.) Jump the big terminals on the starter solenoid next to the battery with a screwdriver - watch out for the sparks! If the engine cranks, the starter and power wiring is good. The starter relay is also known as a starter solenoid.

The rest of the tech note only concerns no crank problems. If your problem was a stuck solenoid, go back to step 1.

4.) Then pull the small push on connector (small red/blue wire) off the starter solenoid (Looks like it is stuck on a screw). Then jump between the screw and the terminal that is connected to the battery. If it cranks, the relay is good and your problem is in the rest of the circuit.

5.) Remember to check the ignition switch, neutral safety switch on auto trans and the clutch safety switch on manual trans cars. If they are good, then you have wiring problems.

Typical start circuit...
Diagram courtesy of Tmoss & Stang&2birds



6.) Pull the starter and take it to AutoZone or Pep Boys and have them test it. Starter fails test, then replace it. If you got this far, the starter is probably bad.


Starter solenoid wiring for 86-91 Mustang

Starter solenoid wiring 92-93 Mustang or earlier Mustang with upgraded high torque mini starter.


Electrical checks for the switches and starter solenoid

Remove the small red/blue wire from the starter solenoid. Use a screwdriver to bridge the connection from the battery positive connection on the starter solenoid to the small screw where the red/blue wire was connected. The starter should crank the engine. If it does not, the starter solenoid is defective.

If the starter does crank the engine, the problem is in the clutch safety circuit (5 speed) or Neutral Sense Switch (auto trans) or ignition switch.


Typical start circuit...
Diagram courtesy of Tmoss & Stang&2birds



You will need a voltmeter or test lamp for the rest of the checks. Connect one lead of the voltmeter or test lamp to ground. The other lead will connect to the item under test.
Look for 12 volts on the white/pink wire when the ignition switch is turned to the Start position. Check the ignition switch first.
No 12 volts, replace the ignition switch.

The next step will require you to push the clutch pedal to the floor (5 speed) or put the transmission in neutral (auto trans) while the ignition switch is turned to the Start position.
Good 12 volts, check the clutch safety switch (5 speed) or Neutral Sense Switch (auto trans) for good 12 volts on both sides of the switches. No 12 volts on both sides of the switch and the switches are defective or out of adjustment. Check the wiring for bad connections while you are at it.
 

@408Stroker

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Oct 12, 2019
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well i went and replaced the coil. Then it would not even try to start. Just crank. So I went to put the old coil in just to see if it would start and see if the new coil was bad. It would not start. And then starter got stuck cranking even with the keys out. Luckily I just installed a battery disconnect . Could this be related to not starting at all now? Starting to regret buying this car lol.
Total timing should be 31* at 3000 rpm. Check plug gap should be .035. Check to see if plugs look lean. Is the miss under wot at 3000 or tip in.
 

Rynb15

Member
Nov 21, 2005
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miss starts over 3k. i gapped the plugs to .045, thats what i read for a 85. Im in the process of getting this car running. plugs are black but not wet. still adjusting the carb. but it would always start right up and idle no problem until i swapped the coil.
 

@408Stroker

New Member
Oct 12, 2019
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It sounds like a spark problem to me. I used on of those HEI slyle china made distributors one time and had nothing but problems. Went to an MSD ready to run and Blaster coil problem solved. Good luck
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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High rpm miss fire is going to be ignition problem. Make sure your cap and rotor are good. Use a timing light to see if you are losing spark.
Does it have a lot of miles on the engine? Have you had this car long? Any modifications?
 

Rynb15

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Nov 21, 2005
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Its currently has a cheap generic distributor on it now. About 5 years old but no miles. Im sure the motor has lots of miles, not original to the car. Motor is stock, i just got the car. Trying to put it back together. I might look into a ready to run distributor. I plan on getting a new motor, so i dont mind buying good parts. I just wanted to get everything back together first before i build a new motor.
 

Rynb15

Member
Nov 21, 2005
132
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It sounds like a spark problem to me. I used on of those HEI slyle china made distributors one time and had nothing but problems. Went to an MSD ready to run and Blaster coil problem solved. Good luck
So does the ready to run distributor get rid of the ignition control module? So just a distributor and coil?
 

Rynb15

Member
Nov 21, 2005
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so today after work i went and got a new ignition switch. I took out the old one and it fell apart in my hand. So put the new one in and it didnt start. Then i put a new ignition module and nothing. Checked for power with a test light, flashes when cranking, light dims when on run.(Should have done this before throwing parts at it) So now im confused. Put a spark plug in the new coil lead wire i installed and nothing. Switched back to the original coil lead wire i was using when this whole thing started and i got great spark to a sparkplug in the coil wire. Didnt try to start the car because its getting late and lots of stuff needs to get put back. Im guessing the main problem was the ignition switch, and then the bad coil wire i replaced the original with. So now hopefully it starts and i can get back to the original problem i posted about. thanks for all the help so far.
 
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Rynb15

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Nov 21, 2005
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Update on this for anyone following, still was not getting spark at the plugs even though I had spark at the coil wire.Tthe cap on the distributor wasent fitting right, seemed a little loose. It had an adapter to make it HEI style. I found the other cap in a box of parts with the female socket top and swapped it over. Fit real snug. Fired right up. I can rev it up without hesitation. Now just a million other little things to fix and ill be on the road. Thanks everyone for the help.
 
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