Tuning and de-bugging 7074

[Gumball]

I just passed 2100 miles on 7074 and, while I really like driving it, I've noticed a bit of a stumble and hesitation these past few months. It would start right up - even after quite a few hours, I could start it without touching the accelerator pedal - and had a nice lope at idle. It even drove really well just off idle and was returning 20 mpg on longer highway trips. But, when dipping harder into the throttle, the stumble was evident and was followed by a softness that I couldn't quite figure out.

I'm running an old OEM Ford dual point distributor that has a Pertronix module in it, as well as a Pertronix coil. The spark plugs were new-ish (about 800 miles) and the wires were Pertronix as well, but the cut-to-length version that I assembled myself.

Given the nature of the issues, I checked the cap, rotor, plugs, and plug wires. Here's what I found:

1) Distributor cap was suffering from what appeared to be a pattern of the rotor striking it right at the edge of the terminal, causing some arcing that was damaging the terminal and the surrounding cap material.

2) The rotor was actually hitting the cap terminals and had work an edge of the rotor tip flat and shiny, with an odd spark residue pattern that was only on one side of the rotor tip.

3) Six of the plug wires had good resistance numbers, but one of those had a loose end. However, the other two were showing infinite resistance; i.e., no reading on the meter at all.

4) The plugs looked horrible - crusty and dark... indicating very poor burn patterns (if any burn at all). Also, I had gapped the plugs at the spec for '94 (which is the year of the heads), which would have been a stronger, electronic ignition, instead of a more appropriate spec for the Pertronix-based old-school ignition.

I did some measuring and found that the cap / rotor issue was due to two things. First, the aftermarket cap had terminals that were too low. Compounding the issue, I didn't have the rotor all the way down on the shaft.

So, after a new cap, rotor, plugs, and plug wires (pre-made, this time), it was running even better, but still had the stumble and the miss or flat sound under load.

This time, I pulled the distributor out and dug into it a bit more. Since it's a mechanical advance unit, I found that the springs were too tight and weren't allowing it to reach full advance. Some lighter springs and a little testing on a friend's old Sun diagnostic machine and it was ready to go back into the car. It's now pulling 34* of advance by 2400 rpm and the static advance is at 10*

The final step was to pull the carb (Holley 650 dp) and check out what was going on inside it. The secondaries are 73s and look good, but the primaries were a bit small at 67. Most importantly, I found a small piece of something - appears to be tyvek or plastic packaging wrap - wedged in one of the jets!!!

Pic of the material that was clogging the jet:



So, after bumping the primary jets to 70 and getting rid of the blockage, the carb was back on and the idle mixture, curb idle, and float level were reset.

And now - the stumble is gone and acceleration under load is awesome. This car just keeps getting better and better as I fiddle with it and has truly been a joy this summer.

To all those in the process of building - especially those who may be a bit frustrated from time to time - remember that it's all worth it in the end!!!!

[rmiller64]

Hi Chris

I run a 1965 dual point in my Mustang. years ago I rebuilt it with new bushings which may help the rotor from contacting the dist cap electrodes on yours. I could never get mine to run with the electronic ignition module so I found I believe (Scott Drake) sells a complete dual breaker plate with new points and condenser all installed. Had it recurved and dropped that in and never have had another issue. setting total Dwell is a art.

Just drove #8442 for the first 20 miles yesterday, so far no issues for the new 331

[CraigS]

Looks good to me w/ one exception. I wonder if the original main jets would have been OK w/ the junk removed.

[Gumball]

Looks good to me w/ one exception. I wonder if the original main jets would have been OK w/ the junk removed.

Maybe, but once we pulled them out, he felt that a 67 was too small for a healthy 347... wanted to go with the 70s, instead. I left the turkey pan off for now so that I can continue to squeeze performance out of it, now that it's running pretty well. Next step is to take it to a dyno for some real tuning.