75 Cold Start Problem

[mmoroge]

My 75 seems to require excessive cranking to get started from a cold state. Once it starts it will restart with just a bump of the starter either warm or at full temperature. The ignition and wiring are mostly stock except for the addition of a Mallory Hyfire control box that was installed years ago. I had the carburetor professionally rebuilt and have set every parameter of the choke to specification. Once the engine finally fires from cold, the carburetor high idles and kicks down as it should. This leads me to believe I have an ignition issue.

I have taken some voltage readings and was hoping some members might be able to spot an errant value or point me in the direction of additional diagnostics. Here is what I have:

Battery:
Cold stored volts-12.4
Cranking volts-10.2
Running volts-13.8

Coil (positive side):
Key on/engine off-12.2v
Cranking volts-9.2v
Engine running volts-13.6v

Based on these numbers, it appears there is not a ballast resistor present which I believe is correct for a 75 Duraspark ignition. The only number that looks off to me is cranking volts. Shouldn't that be the same as battery cranking volts? If not, is 9.2v adequate for the coil?
Thanks for any help!

 

[B RON CO]

Hi, my Bronco will start easily, in 2 seconds, every morning, if I drive it regularly. If it sits for a while I will have to crank it until the carb fills with fuel. This can take a while. Good luck

 

[mmoroge]

The cold start issue will occur next day after driving so I do not believe it is carburetor related.

 

[phyler]

If it cranks till it starts, I doubt it's an electrical issue. If you need to put a jump pack on to get it to fire, then it's a voltage issue.

I would be thinking it's an issue with the needle and seat where it doesn't quite close and then let's the fuel siphon back or a leaking accelerator pump that does the same thing. Basically, drains the bowl and you have to crank till the pump refills it.

Easy to check by popping the top of the carb after a day and look to see if the bowl is low on fuel.

Good luck! I know how frustrating this can be to figure out these gremlins.

Sent from my Pixel 5 using Tapatalk

 

[DirtDonk]

Where are you located mmoroge? Could your area bet getting a new type of winter blend of gas?
Just wondering if your issue stems at least slightly from a quick-to-evaporate fuel mix.

Another thing is just because the choke high-idles then comes down, does not meant it's still in perfect adjustment. If you have not yet, I would pull the air cleaner off next time it's sat overnight and as you manipulate the throttle lever, see what the choke plate does. For example, does it close all the way?
Your '75 should have a vacuum operated choke pull-off diaphragm I think, so when you flip the throttle lever for the test the choke blade likely closes all the way down. Then once the engine is being cranked a little bit of vacuum signal causes the choke pull-off to activate and open the choke a set amount to make sure it's not closed too far to run smoothly.

Some carbs take this adjustment/function very seriously and have only a narrow state where it all works great. Once it's slightly out of that adjustment range, things start acting up.
Otherwise, when was the last time you tuned it up or replaced the plugs, and plug wires?

Just a couple of thoughts then. Hope it's a simple thing. Good luck.

Paul

 

[B RON CO]

Hi, X2 on checking that the choke plate is fully closed and held closed by spring pressure. MAYBE you can simply adjust the black choke spring cover a little richer. This was a common adjustment every spring and fall. If you suspect a spark issue (I don't) I suggest an inline spark tester on a plug wire while a helper cranks the engine. Good luck

 

[mmoroge]

Yes, choke plate is set to fully close when throttle is pressed. The choke pull off is set to spec and the high idle screw is also set correctly. I have pulled the air cleaner after an overnight and I am getting gas when the throttle is moved.

This is my 8th Bronco over a 25 year period of owning them and the only one that has struggled to start. That is why I am leaning towards a low voltage issue during cold cranking. Do we have any electrical experts out there that could weigh in?

 

[Slowleak]

You should have 12 volts at the coil only when the key is in the start position. At that point it gets full voltage thru the starter relay to help with starting. With the key in the run position, the coil gets power thru the resistor wire which drops the voltage.

 

[73azbronco]

Sounds more like fuel issue.

Are you sure accelerator pump can pump fuel and you hear/smell it before a cold start is attempted?

Ignition problem would stay with you even when started and warmed up and restarted. Or get worse, not better.

 

[Rustytruck]

Turn the key on push the gas peddle all the way to the floor twice then turn the key to start. the engine should start and be on high idle. with the throttle position solenoid on the carb and the high idle cam it should stay on high idle until you move the gas peddle. allowing the idle to come down as the engine warms up. the 12.4 volts at the battery when cold in the morning is telling you your battery is nearly aged out. 12.7 volts cold in the morning is normal new Battery voltage. next time after you run disconnect a battery cable and see if voltage is still 12.7 Do you have the steel heat tubes to the exhaust manifolds hooked up? do you have the heat riser foil hose over the exhaust going to the intake on the air cleaner? rectangle intake hose from the core support to the air cleaner snout?

 

[DirtDonk]

While I was leaning towards fuel as well, spark issues such as weak or inconsistent spark often cause starting issues. Hence the questions about plugs and wires.
The fact that gas is still in the carburetor is a good sign.

You can check a couple of things in this regard.
Just because it’s electronic doesn’t mean it can’t change slightly over time. And maybe your older motor would like a little more advance anyway. Can’t hurt to check or experiment.

Another is the stator output. You can check its condition with an ohmmeter between the purple and orange wires at the connector to the distributor. Should read between 400 and 700 ohms. Anything out of that and it’s time to replace it.
The test is easy, but of course separating that connector can be a real pain.

Another is the wire you guys were talking about. Verify that the little brown wire on the starter relay has a good clean and tight connection.
You might check for excess resistance as well. The fact that the voltage of the ignition coil is less than battery voltage could indicate some of the wires are getting old.
If the hard starting when cold is consistent, you could test by connecting a wire between the positive side of the coil and the battery positive directly. After that immediately go in and crank the starter to see if it starts up easier. If it does then low-voltage could definitely be an issue.

And one more voltage test that you can do anytime. After the engine has been running for a couple of minutes check voltage at the coil again. Should be less than system voltage. I forget what the ohm reading is for a 75, but it should still have a resistor wire to the coil.