Auto tranny experts...help a manual guy out

[73k5blazer]

So my only automatic transmission equipped vehicle, the 1970 Impala (400 small block TH350 trans), at cruise the trans, well I guess it could be the engine, but it seems more like transmission to me, surges slightly.
Now this car is bone stock original, 60,000mi on it, I'm the second owner. I've had it for 3 years or so now. I only drive it occasionally. It's well cared for, I flushed every fluid in it when I got it, including the trans fluid and filter.

It started doing this late last summer and it's doing it now. It accelerates great, very smooth, shifts perfect, firm, but not too firm, but if I get into cruising mode, I can feel a slight surge. And it's constant,like as long as I'm at cruise and not accelerating or decelerating, you can feel it. It's nothing horrible, like my wife doesn't really notice it, I notice it though, and experience tells me, it's not gonna go away all by itself.

Any suggestions here? Am I headed for a rebuild? Anything I can adjust? Is it going to grenade on me any day? Every auto I've ever owned has stranded me, so I've developed a complex about automatics now. Admittingly, I don't know much of anything about auto's.

 

[Thunder]

Sounds like a tune up problem to me. When was the last time you tuned it.
A bad condenser can cause a surge at cruise. Plug wires?

 

[MattK]

Almost sounds kinda like what my 4Runner has been doin lately. Like you say cruising, it kinda surges...almost jerks. Also got a bit of a stumble. Doin plugs and wires today and going from there.

 

[73k5blazer]

Well, it could be engine. I'm no stranger to a 400,but the points distributor is not in my list of know-how. He said he just had it tuned with some new distributor parts right before I got it. It had new plugs and wires as well right when I bought it. That was 3 or 4 years ago and 3,000mi or so.
I'll have to read up on how to adjust and test that distributor...
I've got the old school tach/dwell meter thing and a good timing light..

But it just doesn't seem like the motor is doing it...hard to tell I guess, it could be. Would be nice if it was a simple distributor part...I'll check that out....thanks!

 

[70jimmy]

Nice old Iron! I had a 70 Chevy Biscayne 4door for a while with same setup.

I would agree with the others on Tuneup. If you are going to keep the car and would like better performance you might consider an HEI swap.

Otherwise I would do plugs, plugwires, cap, condensor, and points. Check timing. Been a few since I had to play with points but IIRC Dwell and timing have a one way relationship. Only one affects the other. I think Dwell affects timing but timing has no effect on dwell. So you set the dwell first and then the timing.

If I am backwards I am sure somebody will set me straight.

Another possibility would be Fuel filter, not a bad idea to change anyway or screen on carb where fuel line enters.

With inflation my 2 cents are only worth half a penny.

 

[pauly383]

Pertronix makes a nice electronic conversion that fits under the points ditributor cap . No one would know at first glance .

http://www.pertronix.com/new_products/ignition_press/ig_pg21.htm

 

[Mudstud]

I suggest fule filter check/replace too. You guys think the modulater valve could be doing something wierd? Maybe a slight leak on the vacuum tube that goes to the modulator?

 

[pauly383]

I have blown up two rwd th350's of that vintage , and none of them did the surging thing ( I was a car guy before Pauls death , and ressurection as Pauly )

I had my vacuum line come off before . They don't like to upshift automatically when they lose vacuum .

 

[Mudstud]

I had pinched mine off once and after driving about 10 miles it started surging/lunging and actin all wierd. Turning out I had screwed the air cleaner down on top of the hardline and it pushed it to the side of the intake T- pinching it off. Scared me!

I also had surging experience after wheelin all day long with no probs so on the way home I floored it for a while- Kicked in the 4 barrels and let it rip. After stopping at a light and starting again the tranny was surging and shifting wierd/not shifting and I took the lil dealio off the passenger side and pulled the stuff out of it and put it back together and that solved that- Thank God. That lil rod in there was stuck.

 

[76zimmer]

The way the engine is still in stock configuration, I would go with a tune up also, a couple years on that set up is plenty.

 

[73k5blazer]

Yep...as soon as the Tornado warnings and watchs blow thru, I plan to break out the timing light and tune up meter with the dwell thing.
I was doing a little reading on points distributors, so do people just replace the points all the time, or can you adjust them. I found the reciept from the previous owners service records he left, brand new points and condesor, AC Delco brand, just a couple months before I bought the car, about 3.5years and 3100miles ago.
So can I just use my dwell meter and reset the points. They are a wear item, I get taht, but once they wear, can you adjust them a few times before they need replaced again. They are cheap enough, I'd buy new ones, but if they are just supposed to be adjusted a few times before replacing, I'd rather just adjust them.

 

[Chevy305]

Avery just had this exact same problem, but I don't recall the solution...

Drop him a PM

 

[Jimbo*]

I would bet you have a pinhole or a crack in a vacuum line somewhere. Can you rig up an inline tee, and run a line into the car so you can watch it as you drive?
Also, the early cruise control units used a "transducer" to set speed. It's that box looking thing on the driver side fenderwell that has speedo cables in and out of it. The gears inside are plastic or nylon, and as old as they are, might need some lubrication. Sticky gears might cause your fluctuation at speed.
Jimbo