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Dale Stockstill
Dale Stockstill, Automotive Diagnostic Technician
Category: Classic Car
Satisfied Customers: 1238
Experience:  40+ years as a technical expert, especially Corvettes.
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Ford F100 Pickup: 1970 ford 390 firing system, when at idle

Resolved Question:

1970 ford 390 firing system, when at idle it gets weak fire to plugs. I stick a test light to the negative side of the coil and the engine dies. What am I doing wrong
Submitted: 2 years ago.
Category: Classic Car
Expert:  Dale Stockstill replied 2 years ago.

Hello,

 

My name is Dale.

 

First of all, the negative side of the ignition coil should be going to the distributor. If you put a test light on that wire ( which goes directly to the points) you are grounding out the points. The voltage travels through the bulb in your tester to ground.

 

I would appreciate it if you would give me the drive ability condition for the engine. I understand that you say it has weak secondary (spark plug ignition) at idle but what leads you to believe that the spark is weak at idle? The secondary spark to the spark plugs should always be a BRIGHT BLUE color under any condition, If it is Orange or Red then there is either a low voltage input to the positive side of the ignition coil or a different problem causing this fluctuation in voltage or amperage.

 

Please let me know what the engine complaint is so I can help you diagnose the problem.

Sincerely,

Dale

PS- I do not know that you are going wrong anywhere. I just need the story starting from the basics.

Customer: replied 2 years ago.
Dale the motor has just been rebuilt, it run for about 500 miles, it had a miss so we change the plugs,plug wires, still had a miss, we changed distribitor , cap and rotor, next the coil, still have miss. At idle we pull plug wire on plugs one at a time on some cyclinders
no change We pull plug wire on cap and it seems we have a little spark. Spent so much now I need some help and do not mind paying
Expert:  Dale Stockstill replied 2 years ago.

Thank You for your response and good story. That helps me know where to start. You need to isolate the problem down to one of 3 possibilities. Being as it is a new engine, it could relate to any of the 3 so lets start with the easiest test and go to the hardest one. Take some carburetor in a can ( I use Chemtool B-12 Spray) graphic and give the engine a shot down the throat of the carburetor. Use the straw that comes with the product so you can control where the fuel goes. If she smooths out you know it is a fuel related problem. This includes vacuum leaks as well as carburetor adjustments, Vacuum leaks can include external hoses misplaced or not hooked up properly to a intake gasket that slipped and is leaking from the bottom internally. Spray along the edge of the intake, carburetor base plate gasket, etc. Use the spray like a portable fuel, BUT BE CAREFUL WITH THIS PRODUCT-IT IS VERY FLAMABLE AND CAN CAUSE A FIRE IF NOT USED PROPERLY.

 

Now if the spray makes no difference, the next step is to remove all spark plugs and run a compression test on all cylinders. I hope you do not have to go this far, but on a new engine many things come into play.

 

So, if I were there this is exactly what I would do. You may end up with a spark or ignition problem but I doubt it. I have to run some errands ( about an hour or two) and then I will check back to help you along. Never fear, WE WILL FIGURE OUT WHAT IS GOING ON. Do not be concerned about paying anything right now. I do not take advantage of folks that are in a bind. Once we get you straightened out then you can decide about that issue.

 

Have a Blessed & Productive Day !

Sincerely,

Dale

Expert:  Dale Stockstill replied 2 years ago.

Well, I am back my friend,

 

While I was gone I decided to send you a couple of schematic diagram that are specific to your 390 engine.

 

graphic

graphic

 

I just wanted to make sure we were on the same page with respect to the ignition system.

 

Please let me know ( at you convenience) where you are at with the testing and what I can do to help.

Sincerely,

Dale

Customer: replied 2 years ago.
We have 150 to 152 lbs in all cyclinders except #2. it has 0 lbs. I talked to the rebuilder, this is our farm truck, he said bring it to him. Who would have thunk it! Thanks for info and tell me how I pay you.
Expert:  Dale Stockstill replied 2 years ago.

I was afraid of that. You just click on the accept button and then if you wish you can add any bonus that you feel is appropriate. I leave that up to you. Sounds to me like a valve problem but no one will know for sure until he checks it out on #2. At least he is willing to stand behind his work.

Sincerely,

Dale

Dale Stockstill, Automotive Diagnostic Technician
Category: Classic Car
Satisfied Customers: 1238
Experience: 40+ years as a technical expert, especially Corvettes.
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Dale Stockstill
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