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Hey all - wanted to get some takes on this. Had my 23rd series for its longest drive today since I've had it, about an hour and fifteen minutes. Mostly 50+ mph driving, about 86 outside. Not CONFIDENT on the engine temp, but stays about 5/8 of the way to H (I know, I need a proper gauge). A little background:

- 327 w/ Ultramatic
- Fuel: tank is brand new w/ vented cap; lines are relatively new, mechanical pump moving about 3.75 psi at idle, checked lines for leaks/kinks (possible air intrusion - nothing found). The carburetor (WDO 643-SA) has the float set per the Daytona rebuild kit instructions (3/16" w/ their float valve). Metering rods and and anti-percolator valves set per the Carter instructions (metering rods adjusted with the proper T109-113 tool).
- Spark: vacuum advance and distributor were rebuilt in the past 250 miles; plugs, wires, condenser, points, coil are all fairly new. Points gap set to .015". Car is 6v+. Initial timing verified at 7 degree.
- Intake: replaced oil bath filter with paper; no noticeable difference or performance; heat riser is free; no vacuum leaks found
- Exhaust: it's exhausting

The issue: after accelerating to cruising speed and I back off the pedal, 50 or so mph, the car has a tendency to buck somewhat. It's not aggressive, and it doesn't sound or feel exactly like the engine is actually cutting out, but feels like you're going over a wavy road. If I depress the accelerator, either a little or lot, the engine just goes and it all smooths out. It's not always there, but enough to be generally kind of annoying. It also only does this once it gets to operating temperature, but it's been an issue for a while. When the engine temp is below the halfway mark, operation is smooth at all speeds and conditions.

My question - is it something in the carburetor that can cause this issue (should I revisit the metering rods?), or is my transmission slipping? I don't know what the transmission (either in the high range or direct drive) would feel like if it was slipping. The transmission is in relatively rough shape compared to the other mechanical elements of the car - very leaky (I do verify it's full before every drive), and it has no low range (behaves like it's in neutral), so I would think it might be suspect, but I'm not sure.

Any thoughts on what I might be chasing here? Thanks in advance!

I have a 26th series, 327 & Ultramatic.

Transmission:
Mine is also very leaky and I top it up as necessary. The low band operates independent like the reverse band, so it not working is probably not affecting high. Have you tried adjusting your low band? It's is rather simple and that may be all it needs.

Are you running Type F transmission oil? The Ultramatics, unless they've been rebuilt do not like modern ATF.

I would think you would be able to tell if it was your transmission slipping as it should do it under load, such as pulling a hill or giving it more throttle.

You could try adding some Lucas Transmission fix to see if it made any difference, but what you're describing doesn't sound like a transmission issue to me, but maybe.

Bucking:
I would be more inclined to think carburetor issues would be more prominent when the engine was cold, rather than when warm and cruising down the highway.

I recently had a similar issue, which occurred when warm, and was erratic. It ended up being a bad coil, which wasn't even a year old. Engine was fine when it was cold and started acting strange when it warmed up. The coil had leaked it's oil, which I discovered because my plug wires were oily under my coil. Replaced the coil and it's been good ever since.

Those are my only thoughts at the moment....

Are you sure the bucking is from the engine? It could be the feeling of the DD clutch engagement, if your throttle pressure is set far too high. Sometime drive the car accelerating at low throttle and see when it engages. Pay attention to the engine tone and try to feel a bit of a "thump" as the engine and drivetrain rev-match. If this occurs much above 30 mph then you will want to adjust the throttle pressure. Simply move the respective linkage about an ⅛" at a time towards the carb until it feels right when driving.

Torque tighten the low range brake band to specifications (per the manual) then re-test. If that doesn't cure it, or you are slipping in Low, just forget about it unless you want to drop the trans. But if you're leaking fluid, this should be on your to do list.

Keep in mind that the transmissions are the #1 neglected item on these cars due to difficulty in finding parts and service pre-Internet. But once in good shape and kept in adjustment, they should last probably 2 decades or 100,000 miles between major service.

Humanpotatohybrid makes a good point. The service manual, along with this thread, talks about adjusting the throttle linkage and the transmission throttle valve.https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?topic_id=28363

My throttle linkage and throttle valve were all whackadoodle, but I adjusted everything back to spec and the direct drive engages just like it's supposed to. It was engaging too soon before.

Having said that, at the speeds your talking about, I would think the governor would be overriding the throttle valve and keeping the direct drive fully engaged, which it should be doing when you let off on the throttle so it's using the engine for braking.

The more I get things back to spec on my car the better EVERYTHING works.