[Tomcat]

I have a 1970 351C 4V, dual diaphram distributor. Very few miles on a rebuilt motor. All original equipment except for a new coil, points, distributor cap, rotor, spark plugs and spark plug wires. I also ran a new wire to the + side of the coil. Although the engine idles a little "loppy" it still runs fine at low speed. When I get between 60 and 70 mph is when the engine starts "missing". I've also noticed the tach starts acting strange. I'll run the car up to 70 mph and the tach will be at 3000 rpm then with the engine "missing" the tach will "jump around" from 3000 rpm to 2000 rpm to 2500 rpm all while I'm trying to hold the speed of 70 mph. Can anyone help me correct this problem?

 

[ClawIt]

If the tach is still in the coil circuit then jumper it out and try without. I've had two tachs go crazy over the years and the engines would run lousy.

 

[doctordesoto]

if its not the tach , i'd change the condensor
doctordesoto

 

[cougar1]

Your timing is not too far advanced?..or bringing in too much advance as the rpm's get past 2000 rpm.
Also, you said you ran a 'new' wire to the +side of the coil, a resistance wire?

 

[Tomcat]

Update. I've got the dash pad out, the tach is out of the circuit with a jumper at the connector so I'll be testing today. Cougar1, the new wire was run from the connector on the engine side of the firewall to the coil. The wiring diagram shows this wire to be an 18 gauge red wire with a green stripe. The wire from the same connector to the connector at the tach is a pink resistance wire. What I did find strange was the wire to the temperature sender in this three wire assembly is a pink resistance wire and not the red with white stripe wire as shown in the wiring diagram. According the wiring diagram the resistance wire only runs from the connector at the tach to the connector on the engine side of the firewall. Have I interpreted this correctly? Otherwise I finally got my dash lights working and will be testing to see why my fuel gauge isn't working. Any tips to test the fuel gauge sender and gauge would be appreciated.

 

[Tomcat]

Update. Jumpered around the tach, operated the throttle and the ignition still was "breaking up" between 2000 and 3000 rpm. Disconnected vacuum hose at the distributor and plugged the hose. Operated the throttle up to 4000 rpm and the engine didn't miss at all. Reconnected the tach and with the vacuum hoses to the distributor disconnected and plugged I put on 144 miles of highway and in town driving this weekend. The more I drove it, the better it ran. (This motor has less than 300 miles on total rebuild.) I now intend to check the total advance (mechanical + vacuum) and set the intial timing based on what I find. Traced my fuel gauge problem to the sending unit. I believe it is a grounding problem. Before putting the sending unit back in the tank I had cleaned and painted it and the retainning ring. Looks like I will be removing some paint to ensure I have a good ground. Then I'll test it again. Cougar1, looks like you were 'spot' on with the "too far advanced" suggestion. Thanks.