I'll try to keep this short. I've got a 64 544 with an j-type overdrive. When I initially had the tunnel modified for the OD unit, everything seemed fine except that the OD fouled the tunnel. A few years later I took it to another shop and they modified the tunnel such that the OD unit clears everything. At some point after getting it back from the second shop (it's been many years so I don't remember any more if the problem was immediate, but I think it was), the throttle got really sensitive at least in 1st and 2nd. As long as the road is smooth, no problem, but the slightest bump resulting in an unintended input to the throttle will start it bucking. It doesn't seem to be as pronounce in 3rd, and everything's fine whe cruising at 30+ in 3rd or 4th. It doesn't seem like it would be a problem with the clutch or transmission. It acts more like a throttle issue. The fuel lines are only a few years old, the carbs are newer (reconditioned by Z-therapy maybe 10 years ago, and cleaned today), new plug wires, Fireball electronic ignition. Any ideas?
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'64 PVMoneypit '95 850

If this problem did in fact begin with the transmission tunnel work, you may very well have ended up with an engine/transmission mount issue, as Matt suggested. This would explain the incrementally worsening effect as you go lower in the gears, because the torque load on the mounts is directly proportional to the gear ratio in use. This means over 3 times as much load on the mounts in 1st as in 4th, for instance (assuming you're not spinning the rear tires).

Open the hood, set the parking brake, start it up and watch the engine for excessive movement as you begin to engage the clutch in 1st gear. An inch or less of movement is normal, three or four inches ain't. :)
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Gary L - 142E ITB race car, 73 1800ES
YouTube Racing Videos

Thanks for the responses. I spent some time today looking a little more closely at this and making a few adjustments to the linkages (the butterflies were completely open before the pedal hit the floor, so I fixed that), but it didn't change the jumpiness. What seems to be happening is that the carbs are just too sensitive to inputs from the gas pedal. The slightest little movement of the gas pedal moves the butterflies too much and starts the car lurching forward.
I also adjusted the vertical linkage on the carbs per the manual to allow for sufficient free play. Wiggling the throttle shaft (at the carbs)with the rpms up does exactly what you'd think it would. The rpms go up and down.
I've got damping oil (well, ATF) in the dashpots. Maybe a thicker oil to slow down the reaction of the slides?
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'64 PVMoneypit '95 850

The oil dampers don't act to slow the throttle response (well, they do slightly, when going through a quick transition to WOT, but that's a very small effect in a very large change).

What they do is prevent the slides from going up too quickly. When the throttle is opened quickly, it holds the slides down. More air still goes into the engine, but the velocity over the jets increases. This creates more vacuum (venturi effect), which in turn sucks more gas out of the jet. This acts as an accelerator pump function.

After a second or so the slide/piston rises to equilibrium, and the air velocity goes back down to the normal constant speed, and the mixture goes back to what the needle specifies for steady state running.

I'd tend to suspect some issue with the engine/trans mounts as well - it may be a case where the drivetrain is moving around, and this is ending up translating into some throttle actuation. Volvo tried to do a pretty good job of locating the linkages in a way that makes most of the engine's movement ineffective at working the throttle, but if something is loose, it could be happening. You might be able to notice the gear lever moving around more than normal?

And last but not least, there's the whole seat/foot interaction? My PV has a set of DCOE's on it, which have a razor sharp throttle response. You have to be pretty gentle with it in 1st gear. If you tip your foot down, the car surges forward. This pushes you back into the seats (the really springy original 1963 seats). This tends to pull your foot off the pedal a little bit. This cuts the power. This lets the seat springs rebound and push you forward. This tends to push your foot forward. Pretty quickly you can get into a jerky lurching as you bounce back and forth in the seat. Heh. I tend to rest the right side of my foot against the tunnel, and just angle the left side of my foot down - this gives a better frame of reference for throttle actuation that's more immune to the rest of my body being moved around by the car.

And that's after spending time on the throttle linkage geometry to slow the initial tip-in action down as much as possible!
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)

I think it comes down to the whole seat/foot thing. That's exactly what mine has been doing. The engine and tranny mounts all have only a couple of thousand miles on them and are fine (I double-checked). It'd be nice to figure out a way to tighten up the accelerator pedal linkage so it isn't so touchy.
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'64 PVMoneypit, '95 850

Just brainstorming here, but would retarding the ignition timing a little bit smooth things out? Who knows, the ignition may be a little advanced anyway. I don't think I've checked it in years.
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'64 PVMoneypit '95 850

Checking the timing a good idea. Did consider mentioning it when I penned my previous reply. Matching the timing to modern fuel may mean a small difference to factory figures. Make sure your advance mechanism is working smoothly. Sticking in an advanced position that doesn't match the engine speed can make for poor running.
Return springs on the carbs might be incorrect or weak. Pedal feel is proportional to the strength of the springs. Sometimes the mounting tabs at the manifold end are incorrectly mounted which can give a weak spring effect.

OK, so I retarded the timing a few degrees tonight from the (painted) mark on the pulley and it's instantly a lot better. The car still has a slight tendency to start bouncing along if I'm not careful with the throttle at low speed, but it isn't nearly as sensitive. I can drive down a bumpy road at 10 MPH and not send it into fits now. I think I'll drive it like this for a while and keep an eye on the engine temperature, fuel consumption, etc.
Thanks for all the help, guys!

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'64 PVMoneypit, '95 850

read this thread with interest..low speed bucking that clears up at higher speeds is usually poor ignition to me....it is not just timing however....check dwell and condition of points as well an consider aftermarket electronic ignition. i suggest this not because original points condensor etc were of poor design, but because replacement parts are very shoddy (caps and rotors and condensors especially)....with electronic ignition the ills of the caps and rotors are less glaring.

I would think a loose spot in the linkage is likely. And of course, throttle return springs should be checked.

One other thing I'd check, after making sure all is tight, is motor and transmission mounts. Not something I've seen nor heard of, but, if the engine is moving a little bit, and the linkage is not, it seems that could possibly cause the butterflies to open. Again, thats a wild a** guess.

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-Matt I ♥ my ♂

This can sometimes be down to the throttle rods being wrongly adjusted, especially the vertical one. If your pedal gives you full throttle before it reaches the floor, adjust the vertical rod until full throttle is "pedal to the metal". When fully to the floor it shouldn't put any strain on the mechanism at the carbs but still give you fully open butterflies. You'll probably need a small adjustment to the central push rod to the carbs. There should be a fraction of slack there when idling. Lever should be on the stop, not floating.

The overdrive to body work issue is most likely a red herring.

Describe 'bucking'.

When the car in nuetral, if you operate the throttle by hand, does the engine rev up smoothly with no hesitations or hiccups ? If you hold the engine at 2500 rpm and wiggle the throttle shaft, what happens ?

Greg