# Coil wiring help '74 GTO



## Red74Goat (Jul 31, 2015)

I may need to replace my ignition coil. Still have original distributor and coil. Only option in '74 was the 350 cu. Looks like two different condensers attached to the coil, one against the coil inside the bracket and one attached to the firewall (I think). The one on the firewall doesn't have a wire attached any more and the coil has a black wire attached to a post that is broken off from somewhere. I can't find any details in the Pontiac service manual for the coil and all of the wiring attached to the terminals. Any help or direction would be appreciated.


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## Red74Goat (Jul 31, 2015)

*Pics of 74 GTO ignition coil condensers*

This pic shows the "condenser" that is attached to the firewall and seems to have a spot on the bottom with a wire broken off. This is where I assume the black wire attached to the primary terminal of the coil was originally attached.









This pic shows the condenser that is attached to the coil bracket and has a black wire connected to the primary terminal of the coil, with the end apparently broken off from something.









The coil has 3 thin black wires attached to one primary terminal and three wires attached to the other primary terminal. One is yellow and 2 are black. One of the black wires attaches to the condenser on the coil bracket and one attaches to the distributor, the other wires feed into the wiring harness. 

I was wondering about the purpose of the condenser on the firewall and if there are replacements available. I have suddenly had some problems with starting and it seems there may be an electrical problem that has developed and I need to start troubleshooting the cause.

Thanks


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## PontiacJim (Dec 29, 2012)

I am assuming that that is the condenser/capacitor used for radio suppression interference. It has nothing to do with the ignition operation, just eliminates/minimizes the ignition noise that can be transferred over your radio signal. Here is a little generalized info Identifying and suppressing radio interference | How a Car Works


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## Red74Goat (Jul 31, 2015)

PontiacJim said:


> I am assuming that that is the condenser/capacitor used for radio suppression interference. It has nothing to do with the ignition operation, just eliminates/minimizes the ignition noise that can be transferred over your radio signal. Here is a little generalized info Identifying and suppressing radio interference | How a Car Works


Thanks for the link Jim. I was assuming the same about the elimination of ignition noise interference, but confused that there are what appear to be two capacitors for the coil.


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## Red74Goat (Jul 31, 2015)

PontiacJim said:


> I am assuming that that is the condenser/capacitor used for radio suppression interference. It has nothing to do with the ignition operation, just eliminates/minimizes the ignition noise that can be transferred over your radio signal. Here is a little generalized info Identifying and suppressing radio interference | How a Car Works


I need to do a basic determination of the electrical ignition system to see if I have any spark coming from the coil. I just did a simple rebuild of replaceing the gaskets on the original Quadrajet and the intake manifold gaskets. It started and ran, a little rough at first, but did a little adjustment of the Qjet and it seemed to be okay idling, but bogged or stalled under load. Then after a few times, I wasn't able to restart it, like there wasn't any ignition. (I ran out of time to check the electrical system, as I was supposed to be packing for an international trip!) The coil came loose and was laying on the top of the backside of the manifold/head, with one broken black wire coming from the top of the coil. 

I'm not sure where to start next on the ignition. I was going to crank it and see if there was any spark at the cylinders. They move back from there to the coil and distributor. Any suggestions on the approach.

Thanks


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## PontiacJim (Dec 29, 2012)

Get the coil mounted and reconnect/secure the black wire coming off it or your coil won't work as it should. Make sure the wire is not broken internally even though the outside looks good. I don't know which wire it is, but if it is the wire to the points you can check it with an Ohm meter for continuity to make sure it is not broken. If it is the power wire side, just make sure you get voltage when you turn the key on. You should get 12 volts when you hit the "start" and then it will drop (I think its 9 volts or something - so the points won't burn) when the key is in the "run" position due to the resistor wire built into the system.

You just rebuilt the Q-jet and now have that stumble. First off, make sure you have no vacuum lines off or split, or a vacuum leak somewhere like the carb not seated correctly on the intake/wrong carb gasket for your intake. Look for the obvious and simple first. 

Next, this could be a bad accelerator pump, but I assume you installed a new one. Take a look down the carb and make sure you see the gas squirt into the carb when you open the throttle. If you get a weak/no squirt, the car will act like that because it is getting too much air without the additional gas squirt to correct the air/fuel ratio needed when you open up the throttle blades. You could also close the choke a little and see if that makes any difference. This would act the same as if you were richening the air/fuel mixture.

If everything looks good, it is also possible that the stumble/engine stalling and not starting is that your cab just flooded out - too much gas. Your float level could be off, or you have an internal leak somewhere. Q-jets are know to leak at the plug found at the bottom of the center section of the carb. The fix is to put some fuel resistant epoxy over the plug, but I have never experienced this personally, but many have. Not saying this is it, but something to look at.

Also, timing can play havoc with the engines performance. If it is not set correct, it could create a bog situation, but I am leaning toward the Q-jet since you say you rebuilt it. 

Just thought of this one. I don't know what year GM started using the EGR valve. If this goes bad/sticks, it can create problems with engine operation. See if you have one, if not, forget I mentioned it.


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## Red74Goat (Jul 31, 2015)

Thanks for the suggestions on next steps, Jim. They are much appreciated

I too was thinking about the Qjet rebuild and it certainly is out there as a variable, but I'm hoping to find something besides my carb handwork as the main culprit. ;-)

I plan on using your suggestions as my list of potential problem areas to work through. I can't get to it for the next week, but plan to be back working on it the following week. 

Thanks!


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## Red74Goat (Jul 31, 2015)

Red74Goat said:


> Thanks for the suggestions on next steps, Jim. They are much appreciated
> 
> I too was thinking about the Qjet rebuild and it certainly is out there as a variable, but I'm hoping to find something besides my carb handwork as the main culprit. ;-)
> 
> ...


I got back from my trip and got to work on checking out the ignition coil before reinstalling it. The primary winding was out of spec (it is the original 41 year old Delco Remy coil), so I replaced the coil and the condenser on the coil mounting bracket. Then took a bit of time to get the coil mounted onto the backside of the driver's side head. Started right up. Jim, thanks for suggesting that my first efforts be to properly mount the coil back on the engine. 

Then I adjusted the fast idle and both primary idle mixture screws. I adjusted the primary idle mixture screws by looking for the highest idle speed from the adjustment of each screw. Was idling fine and was warmed up. When put in gear it had trouble staying running, but I was able to drive it once underway. So it performed the quick test that Lars recommended in his Q-jet tuning bulletin to verify a lean condition by holding my cupped hand lightly over the choke air horn area while the engine is idling to restrict the air flow. The idle speed and quality both increased, so I assume I have a lean air-fuel condition. Getting that resolved is the next order of business.

Thanks.


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## BearGFR (Aug 25, 2008)

Sounds like you're on the right track. Another aid in setting idle mixture is to get yourself a good vacuum gauge and connect it to unported manifold (not carb ported) vacuum. Lightly seat both idle screws, then open them up a couple turns. Adjust in equal increments to find the point of highest vacuum.

Bear


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## Red74Goat (Jul 31, 2015)

BearGFR said:


> Sounds like you're on the right track. Another aid in setting idle mixture is to get yourself a good vacuum gauge and connect it to unported manifold (not carb ported) vacuum. Lightly seat both idle screws, then open them up a couple turns. Adjust in equal increments to find the point of highest vacuum.
> 
> Bear


Thanks Bear. I have a vacuum gauge and will give that a shot to fine tune the adjustment from my original settings.


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## PontiacJim (Dec 29, 2012)

Keep in mind that if you are using ethnol blended gas, the ethanol can cause the engine to run leaner. The higher the octane rating, from what I have read, the more the ethanol - so it may run even leaner. I know when I run mid-grade or premium, it trips my O2 sensor into thinking there is an emissions problem and my check engine light comes on -ie it is running lean. Put regular back in it, a 100 miles or so and the check engine light goes out. I've got the OBD I computer system on my car and this seems to be one of the flaws in the early system.

Just for fun, try some Stabil 360 gas additive. Might help run better - I use it in my '97 Toyota which was not designed for ethanol and it makes difference. 

Next step may be to go with a couple sizes up on your jetting, but there could be other fixes as well. I might adjust my idle screws out a bit more to richen it up as the factory settings were for the "good 'ole" gas and see if that improves anything.

If you REALLY want to know your carb better and how to adjust it up, purchase the book "How to Rebuild and Modify Rochester Quadrajet Carburators" by Cliff Ruggles. Great photos and text, step by step disassembly, assembly, modifications, and adjustments. Worth the money and may be helpful to you.


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## Red74Goat (Jul 31, 2015)

PontiacJim said:


> Keep in mind that if you are using ethnol blended gas, the ethanol can cause the engine to run leaner. The higher the octane rating, from what I have read, the more the ethanol - so it may run even leaner. I know when I run mid-grade or premium, it trips my O2 sensor into thinking there is an emissions problem and my check engine light comes on -ie it is running lean. Put regular back in it, a 100 miles or so and the check engine light goes out. I've got the OBD I computer system on my car and this seems to be one of the flaws in the early system.
> 
> Just for fun, try some Stabil 360 gas additive. Might help run better - I use it in my '97 Toyota which was not designed for ethanol and it makes difference.
> 
> ...


Jim thanks for the advice about mid-grade or premium gas causing older engine to run leaner, I hadn't heard that before. I will plan on filling with regular next fill up and then try backing out the idle screws a bit.


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