The grounds both within and outside the distributor need to be rechecked.  Unhooking the tachometer temporarily is a good suggestion.  If that doesn’t make a difference, then this sounds like a points or condenser issue.  As mentioned, point bounce could be the problem but also insure that the point gap is set to spec.  The point gap being on the big side can create issues.  A condenser not being tightened properly can also do the symptoms you mention.

Lorena, Texas (South of Waco)

I should have looked up “dizzy” ignition before I posted as most of what I put there doesn’t apply to your situation.

1955 312 T-Bird Warwick, NY

If you are running with a stock single point distributor the problem could be point float at 3500 RPM. I run a 55 T-Bird w/312 and a new Mallory Dual point tech drive distributor. The dual points cut the individual point open/close rate in half. If you are 12 volts and running a point distributor without a ballast resistor the points will burn out. Modern resistors made in Asia are just plain junk and will fail at some point, this happened to me, symptoms include momentary hesitation at steady speed and eventually severe backfiring. If you want to sleep nights and remain with tech drive I highly, highly recommend the Mallory Dual point distributor with an MSD 6A or 6AL ignition box. This little beauty will put only 3 volts to the points which it uses solely for timing (points will last forever just set gap and forget dwell), and it eliminates the need for a resistor as well as the condenser which is also complete junk, and give you a boost in performance that will light your fire. I put mine through an 85,000 volt epoxy filled coil with amazing results. You’ll love it!

PS a failing condenser will give same symptoms as above for the resistor.

1955 312 T-Bird Warwick, NY

3 spd with OD, while driving in 2nd gear if I put it to floor, pulls steady til 3500 rpm then can feel surges and tach is kind of goes crazy. If you stay on it the surging continues.
I do notice that there is no ballast resistor, do knot know if it has coil with that built in or not. This is in a 1955 Ford fairlane so wiring is 1955 wired to a1957 set if that makes a difference. That I don’t know.
Going to check to see if ground wire in dizzy is there. ( it is a 1957 Ford dizzy )

What happens when you try to rev past 3500 rpm?  Does it smooth out and pull on up to ....what rpm?  Joe-JDC

JDC

In your initial post,   are you saying the engine was surging around 3500rpm,   or just the tach?    Stick shift or auto?    You'd definetly feel a difference with a stick shift.   

Disconnect the tach and see what happens.

"FREE SAMPLE"

Did you check to see if the ground wire is still in the distributor?  This is often left out of 57 distributors when they are reassembled. 

It is a fairlane, std. dizzy not tach. Drive.

Is this a sedan or a Tbird.  Is it a distributor driven tach if not a TBird?


54 Victoria 312;  48 Ford Conv 302, 56 Bird 312
Forever Ford
Midland Park, NJ