302 Timing and Misfire Advance

Bogar · Jun 23, 2018

I have a 74 AT 302 w points and it has a nasty misfire and backfire under acceleration. New wires, plugs, cap and rotor without improvement. Did a little more googling and the internet pointed me to timing. She seems to run well until I hook up the vacuum advance and then the nasty misfire cones back. Beyond frustrated and thinking about taking it to a shop (I know but it’s summer after all) and hoping the forum can help. What am I doing wrong or not doing? Thanks a bunch. Mark

bronconut73 · Jun 23, 2018

Set base timing to 10 degrees before TDC.

Make sure vacuum advance line is hooked to ported vacuum port on carb and not directly to intake or other carb vacuum source,

DirtDonk · Jun 23, 2018

Are you still running all the vacuum lines that came on the truck originally? Still running an EGR valve? Is it a dual-diaphragm vacuum advance can, or a single?

When you leave the vacuum line disconnected (and plugged of course) how does it run? Does the erratic running and funny noise go away completely?
Like said, if simply connecting the vacuum hose while the engine is idling makes for a funky running engine, it sounds like full vacuum advance is being introduced. So try connecting a single vacuum line to the timed-vacuum port on the carb.
This full vacuum is pointing to some of the vacuum modulators and timed valves are malfunctioning, if you still have all that stuff intact.
If you are just running one vacuum line from the carb to the advance can, you should definitely use timed, or ported vacuum.

When you do that there is no vacuum signal at idle and it won't act up when you connect the hose.

Did it just start doing this out of the blue?

Paul

Bogar · Jun 23, 2018

Thanks for the response Paul and I will do my best to answer your questions.

I believe running all stock vacuums lines with a single vacuum advance.

She runs great when the single line is plugged under idle and rev. When I connect the vacuum, she runs ok at idle and terrible under rev.

She was running great until I tried to replace the points with Ignitor II. I couldn’t get her to start with Petronius and ended up reverting to points and condenser and now have the misfire.

B RON CO · Jun 24, 2018

Hi, if it ran ok before you took the points and condenser in and out, I would check the point gap, or dwell. Did you set the gap with the points on the high spot on the cam?
Your trouble could also be the condenser. Is the wire screwed to the points, and out of the way? The rotor can snag the wire on some distributors.
I'm surprised the Pertronix gave you trouble. Did you check all the instructions, and set the gap?
Good luck

Bogar · Jun 24, 2018

Bronco - You made me think about the Petronix again so I went to Jegs this morning and picked up an Ignitor 3. Happy to report she is running great again! No more points to worry about. You guys are great and I still have time to enjoy the sunny Sunday!

DirtDonk · Jun 24, 2018

Were you able to return the original one, perhaps as defective?
When you installed the original Ignitor, did you make sure to run 12v to the Red wire? In other words, you can't use the original Red w/green Ford wire because it's a resistor wire, so is not 12v. Or did you connect the Red wire directly to the coil's positive?

Glad you got it running better! You sure won't miss (no pun intended%)) having to mess with points anymore.;D

Paul

bronconut73 · Jun 24, 2018

You have a Jegs near enough to just cruise by and grab a part?

Color me gd jealous, lol.

Bogar · Jun 25, 2018

Jegs was awesome about me exchanging the Petronix II and upgrading to Petronix III. I removed the resistor wire (pink) and spliced into the red/green at the ignition for both ignitors. For whatever reason, Petronix II wouldn't work for me. Possibly, the Petronix II was faulty or maybe the breaker plate was bad.

Jegs is only about 10 minutes away and they have a huge inventory so most parts can be picked up.

Timmy390 · Jun 25, 2018

Bogar said:
Jegs is only about 10 minutes away and they have a huge inventory so most parts can be picked up.

You lucky dog.......

Tim

Broncobowsher · Jun 25, 2018

That sounds very much like when I had two plug wires crossed. I fought it a month until I accidently found I goofed. The vacuum advance would advance just enough to start sneezing out the intake before the valve got closed. Most problems are generally very simple, once you find them.

302 Timing and Misfire Advance

Bogar

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bronconut73

Bronco Guru

DirtDonk

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Bogar

Full Member

B RON CO

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Bogar

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DirtDonk

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bronconut73

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Bogar

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Timmy390

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Broncobowsher

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