1966 289 Ford with C4 Edelbrock Performer intake Holley 650 vac secondary carb Pertronix Ignitor otherwise stock Yes this is the engine I just putting a timing set on a couple of weeks ago. It was running perfectly afterwards, then literally in just one day it went from being a nice runner to running like CRAP during part-throttle acceleration. It has a strong and crisp clean idle, great low throttle, great steady cruise, great full throttle. But at 5-3" of manifold vacuum (which is a normal intersection takeoff vacuum reading) the car is spitting and farting and burping to the point it will actually STALL if I don't let off the gas, or give it full throttle. It has 17" manifold vacuum at idle in Park, 19" at idle in gear. No apparent vacuum leaks, unless you consider a functional PCV a leak. I've always run only 93 octane. Plugs, wires, cap, and rotor all look to be in good shape and no older than a year. I jacked with the fuel system at first, assuming I have a bad bog due to fuel delivery. The line from the tank to the carb is clean and blows through easily. The fuel filter is clean but hasn't been replaced. I replaced the jets with a pair 3 sizes bigger, but I get exactly the same stumbling behavior - except it ran ridiculously rich. I replaced the power valve, no change. Locked the secondaries out, no change. Hell I replaced the whole carburetor and it still does exactly the same thing. I think I have ruled out the fuel system. So I turned to the ignition. Advancing timing makes it slightly worse, retarding makes it slightly better, but with 6* advance it has no power and it never eliminates the problem - it will stall with any base timing setting. I checked the vacuum advance diaphragm and it keeps a suction, and it moves the advance plate like it should. I even swapped with another known good vac advance unit and it still does the same thing. I have tried ported and full vacuum - it likes full vacuum slightly better but it still has the bad stumble on acceleration. I replaced the mechanical advance springs with new ones and checked the motion of the mechanical advance assembly, it appears to be good and smooth. The physical "stop" to the mechanical advance is in place. I have even replaced the coil because I had an old one laying around, with no change. The Pertronix module has the recommended .030" clearance from the ring thing. So far NOTHING I have addressed has made any change in the part-throttle problem I'm having. It seems to have too much advance in this range, but that is an assumption and I have no way to measure it while it's under load. What am I missing? Thanks for any help!
When I built the Volvrolet, I had a similar issue (don't remember if it ran well at full throttle though). After a couple of nights of troubleshooting everything from the intake manifold gaskets to the fuel pump (and even changed the carb in the process), I found that my coil was fried (I too was running a module in the distributor).
Don't play with Fords all that much, but if you have access to another coil, I'd swap it out in case it's breaking down under load.
Hmm, two votes for coil. I have tried a new one and a good used one already, but I will test them for resistance to verify that they are good. Is .7 ohms Primary and 10.8 kilo ohms Secondary a standard for most ignition coils? I read the response about blocking off vacuum advance. I meant to mention that I had tried that with no change in the part throttle acceleration.
If it was a grad. demise, from good to bad, I also would be looking for something electrical to be breaking down and approaching a complete failure. I dont know the history of your car but there has been quite a bit of disscussion on the HAMB about old gas, running fine for a short time and then causing problems that seem to drive owners to the edge of insanity
Your vacuum readings at idle while in park or gear sound a little weird, like they are backwards. Anyways if the coil is not breaking down then carb intermediate circuit might be gunked up. Not familiar with Pertronix stuff, so dunno if that could be bad, sounds to me like fuel issues.
It has fresh gas in it, no older than 1-2 weeks. I have 14.4v charging. One thing that was suggested was verifying voltage at the coil where the Pertronix hooks up, it should be full charging voltage. I will verify that. Plug gap is .035. I do have the vacuum readings backwards, sorry about that, 19" in Park and 17" in gear. I really appreciate all the ideas-
had somethin like that happen to my truck. turned out it was the fuel pump. might check that. mine was shy of 2yrs old.
Manual fuel pump. I haven't checked fuel pressure, but it will spray all over if you remove the fuel line right after shutting it off. I will measure it. Squirrel, I'll check compression. I do think it is strong, it hasn't lost any power on the top end.
Same thing happened to me and our 67 Buick GS 340, I ended up putting the points back in. Corrected the problem! Might be a bad Pertronix Ignitor? I think that early on they had a run of some that broke down after they had been through some hot/cold cycles? I never did send mine back in because it was out of warranty.
To properly check voltage at the coil, you will need to ground coil -. Run a jumper from coil - to a good ground. Hook up your volt meter and turn the key on for a few seconds and read the voltage. DO NOT leave the key on for more than abut 15-20 seconds or you could fry the Ignitor. Then check it without the coil grounded. What this will tell you is if there is a resistor wire or a bad wire supplying voltage to the coil. You can also run a jumper from the coil+ directly to the battery and see if that cures the stumble. Again don't leave it hooked up without the car running or you could fry the Ignitor. What i do is use a jumper with alligator clips, start the car and then hook the jumper up to the battery. Unhook it before you shut off the ignition.
I'd vote to try a different Pertronix, or put the points back in for a test. Sometimes if the Pertronix isn't indexed correctly (sometimes from the factory, sometimes not installed correctly), you get a spark generated when the rotor contact is right in between two of the terminals in the distributor cap. When that happens, you may get the wrong plug firing, and it will act a lot like they do when you have some moisture under the cap and get cross-firing. Depending on where the vacuum and mechanical advance come in, this can cause odd problems like this. Aside from another Pertronix or points, if you have another distributor cap you can sacrifice, file off the locating tab, and install it so that it is rotated on the distributor just a bit before or after where the now-removed tab would locate the cap. You might have to strap the cap down with a bungee cord if the clips/screws can't be used to hold it down. Temporary, but you can do some troubleshooting this way.
Sounds like you have the same pertronix system I have. Primary on that model I believe is 1.5 ohms.not .7. I had a similar problem with mine running like crap til it died. Replaced coil with an Accel super stock that runs a 1.2 primary and that took care of it. 1.2 ohm is on the bottom of the acceptance scale for pertronix but it worked and didn't change anything performance wise
Early on? so like 1975? (They started selling them in the early 70's) You'd also be surprised as to how good they are at warrantying the product, at least from my experience.
Been using them for 25+ years with no problems. Friends with the tech guys there and Over 3 million sold over the past 35 years and a less than 1% failure rate and most of those caused by voltage spikes, wrong coils, or ground issues. Think you are a bit off base there. JMO
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