Stop Go Throttle

Sounds like you had a shop pull the engine and replace the transaxle.  Assume dual carbs? If so suspect carbs (linkage and manifolds) were pulled to get the engine out of the bay (as it is generally too wide with them still on).  When it was all put back together, the carbs need to be synchronized so both are feeding the same fuel/air mixture at idle, low speeds and transition to high speed.  My guess is that one carb is "shut down" low at idle.  Also, what is engine idle RPMS (800-1100)?  If you have a carb "snail" or can borrow one, you can check and adjust.  (I have the old-school version called a Uni-Sync) from 1970).  On the snail, there is a readout and on Uni-Sync the level of the ball gets matched side to side. I've seen experienced old mechanics use a bit of hose to adjust by ear!  You want them to suck in unison (with same air/fuel mixture too).

Thank you very much for taking the time to write all of that.  You are spot on on all of your assumptions.  I’m guessing you are correct in that my suckage is out of alignment.  

@Cartod

Not sure if you said:

1.  Pull away from stop and engine seems to stall.

OR

2.  Pull away from stop and engine seems to 'self-accelerate'.

Just a thought:

If it is the 2nd choice, and since the trans was just replaced, perhaps the trans is loose in the mounts.  As the engine/drive-train torques up when pulling away from a stop; if the mounts are loose, the engine moves around and change the pressure on the throttle cable.  Jerkiness would be the engine jumping up and down as torque bounces up and down. (You pull away, the engine self accelerates, you pull your foot off the gas, the car decelerates, you put your foot on the gas, etc.) You may find it more prevalent in either fwd or reverse (dependent on which way torque is applied).  And, it would be more noticeable when, for instance, when pulling in or out of the garage than doing burn-outs.  Also, how the throttle cable is routed can exaggerate the effect.  There has to be a 'hinge-point' in the cable between the linkage and the fixed mounting point, because there will always be 'some' torque induced movement because the mounts are rubber.

OK, let's see if this makes sense:  When you first take off, it starts to accelerate, then you step a little harder on the gas pedal, the carb mixture goes super lean, the car bogs almost like you took your foot out of it, so you push harder on the gas and it recovers and takes off with a snap, right?  And then it repeats whenever you give it more gas?

What I think is happening, is that recovery snap is because pushing harder on the gas shoots a stream of gas from the accelerator pump into the carb throats to overcome the initial lean-ness and it takes off.  I bet that if you're cruising along at 2000 rpm and gently push the gas it will do the same thing.

I am assuming that your mechanic removed both carbs and their intake manifolds as assembled units to get the engine out - That's pretty common.   Hopefully, the carb was not removed from the intake manifold and the intake/carb assembly was unbolted right from the head.

This is important, so call them and tell them of your trouble and ask if/how they pulled the carbs and what they did for manifold gaskets when they replaced them.

If they pulled them as described and they re-used the manifold to head gaskets, or even if they used new gaskets, it sounds like you have an intake manifold leak.  Those are pretty common whenever the carbs/intakes are replaced.  

If you get a can of carburetor cleaner spray, like Gumout Carb Cleaner, you could get the engine idling and then using the little red wand on the carb cleaner, spray around the base of the intake manifold where it meets the head.  If the engine idle changes speed, up or down, while spraying and returns where it was when you stop spraying in any particular spot, you've found a vacuum leak.  Or, it may be leaking and you get no response to the spray - That happens, too, but it is an easy, cheap trick to try.

Or you could take it back to your guys, let them drive it and see if they can find a vacuum leak or whatever else is going on.  Usually, if it's leaking, you'll get strange noises like snaps and pops out of the carb throats (air cleaners off) if there is a leak somewhere, but not always.  If they're good, they'll find the problem quickly.

Just thought that, if you have a single carburetor in the middle and a vacuum-advance distributor, they may have left the hose from the carb base to the distributor off and that would give a similar symptom.

Good luck....

I’m not sure I fully grasp the issue, but here’s another one from the “90% of carb problems are ignition” files:

After I replaced my shroud, without touching the carbs (other than R&Ring them) my engine would either idle or go like stink at WOT. Nothing in between. After chasing carb problems for a month I re-checked my coil with my analog multimeter and the secondary circuit was out of spec. I was way late at night when I finished up and I mistakenly wired my coil backwards, which burnt my points and condenser up. The coil tested fine on my $10 HF digital meter, but when I finally checked it with my good one, it was bad.

Put a new coil on it and it worked fine.

I'm thinking y'all might be wrong here.

What was changed on the car? Carbs? Nope.

The trans was changed.

First thing, make sure new and non-Chinese mounts were installed. All three of them. If I got a new trans and the mounts weren't replaced I'd be mad.

Second, check the Bowden tube(on the clutch cable) for proper bow and clutch cable for freeplay adjustment.

Lastly, make sure the throttle cable has a little play in it. A too-tight cable can cause herky-jerky acceleration. While you're at it, have someone help you adjust the full throttle adjustment. If they're isn't one, put a cable stop on the pedal side.

@RS-60 mark posted:

@Cartod

Not sure if you said:

1.  Pull away from stop and engine seems to stall.

OR

2.  Pull away from stop and engine seems to 'self-accelerate'.

Just a thought:

If it is the 2nd choice, and since the trans was just replaced, perhaps the trans is loose in the mounts.  As the engine/drive-train torques up when pulling away from a stop; if the mounts are loose, the engine moves around and change the pressure on the throttle cable.  Jerkiness would be the engine jumping up and down as torque bounces up and down. (You pull away, the engine self accelerates, you pull your foot off the gas, the car decelerates, you put your foot on the gas, etc.) You may find it more prevalent in either fwd or reverse (dependent on which way torque is applied).  And, it would be more noticeable when, for instance, when pulling in or out of the garage than doing burn-outs.  Also, how the throttle cable is routed can exaggerate the effect.  There has to be a 'hinge-point' in the cable between the linkage and the fixed mounting point, because there will always be 'some' torque induced movement because the mounts are rubber.

Right on! The first thing I thought of when I read the original post.

Ok.  Checking on transmission mounts first, engine seems to self accelerate.  I bet this is it.