Hello everyone! I've had a handful of Slant-6 cars over the years, a Diplomat and a Cordoba. For the last 4 years, I've faithfully searched the internet looking for a "really nice" Cordoba LS. And by really nice, a car that needed very little. (I've purchased a fair number of basket cases, a story for a different thread.) Anyway, a couple months ago I found a real cream puff: A 1981 Cordoba LS with only 9,000 miles on the clock. (Really!) Here's some pictures: http://www.functionbox.org/Darlene/ Anyway -- I was considering an upgrade to a small block, but the car is so wonderfully original and runs just fine... Easy starts, good idle, etc. I was having memories of my last 6 that would run out of breath at 75 MPH, but this one pulls fine to 85. Here's my question: With the cruise set at 75, no one would describe the engine as smooth. Is there anyway to smooth out high-rpm operation? I presume the long stroke of the 225 precludes this, but wanted to check in with the experts. Thanks!

Hello! Congratulations on the purchase of a beautiful and rare car.

My first thought for high RPM rough running would be ignition related, specifically if the car has some variant of the lean-burn carburetor or ignition control. Does it?

Slants are generally very smooth at all RPM ranges in stock form and properly tuned up.

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Casually looking for a Clifford hyperpak intake for cheap.

Lean-burn free. Hmm.... I don't believe there is anything wrong, per se -- just doesn't go down the road as smoothly as a V8. As a test to make sure the vibration wasn't wheel or tire related, I shifted in to neutral at 80 MPH and let the engine idle... Perfectly smooth.

Family friends had an Aspen Wagon back in the 70's, that had the 4 speed. It was never smooth, from day one. The engine was just not balanced - or something like the pressure plate or something was just slightly off.

In any case, make sure the engine mounts are good - as in not collapsed or stressed to the side and the trans mount is not stressed or otherwise not right.

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'81 W150 on Propane... Oversize valves, Oregon Camshaft cam, 10:5 static CR, Distributorless ignition, megajolt timing controller, PowerTrax lockers.

We need more or better description of not smooth.

Are you experiencing surging where engine feels like it slightly looses power or backs off for a moment than catches back up and not as pronounced when not using cruise then suspect cruse control problem.

Cruise control on this vintage car are crude mechanical vacuum controlled devices compared to todays black box fly by wire devices. Problems with cruse can be electrical, vacuum, and mechanical. In the past I have had cruise electrical problems where connections in stalk and at device under hood had gotten cruddy and intermittent. Working switch back and forth will help improve connections in stalk, cleaning connections under hood, and increased unit use will help to restore smoother engagement and function.

If engine feels like it has dropped a cylinder under light load or steady cruse than suspect ignition electrical problems, and or incorrect ignition advance as in too much. You car is geared quite high, and at 80 mph is not turning over all the fast, and well within factory designed operation parameters.

Have the plug wires been replaced, or are factory fresh wires still in use? Cap & rotor in good shape?

One needs to look at lean burn system as major contributor to drivability problems. Lean burn monitors several engine conditions via sensors than adjusts timing and A/F mixture. If any of these sensors are malfunctioning the old adage: "garbage in = garbage out" applies. Also lean burn brain may not be fully competent after all these years, its components degrade over time regardless of millage driven, but don't be quick to blame the brain. When these devices were the norm, sensor malfunction usually was the cause of drivability problems.

You need to check your factory service manual for diagnostic methods of lean burn problems. Folks here will recommend eliminating lean burn system converting back to basic electronic ignition, but before you think about pulling the plug on LB, make sure all vacuum lines, electrical connections, and sensors are functional and connected correctly.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Thanks for the replies thus far! The issue I'm describing may not be an operational problem at all: The carb is in good tune, no surging, no performance issues to speak of. The car drives out, accelerates, and cruses just fine. The vibration I'm describing feels like a first-order rotational vibration. It is a high-frequency vibration that is synchronized with engine RPM. It's not rocking the car, or anything. It feels like a "course" engine. I suspect I don't really have an issue.... I'm probably not used to the feel of this six. My other cars are modern units with the characteristics of the engine engineered out of the cabin.

Do you have a tach? What RPM is the engine turning at 75 MPH? Does the engine run rough at a lower speed but at the same RPM?

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Casually looking for a Clifford hyperpak intake for cheap.

No tach. Super-smooth at around-town speeds. It idles smooth as can be. It's smooth until about 75 MPH. It becomes real noticeable at about 78+ where the engine is getting to the higher side of normal operating speeds. I don't often drive the car that fast, but I do like to take the car back and forth to my hometown.... It would be nice to keep up with traffic without worrying about shaking the drivetrain apart.

Very Cool looking Car! I really like those wheels! My only guess would be driveline (Driveshaft or old u-joints)?

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1964 Dart GT

I agree. If the engine does not vibrate at the same RPM but at lower speeds, I would look into rotating objects that are not tied directly to engine RPM, such as driveshaft or wheels. Could be a failing u-joint or a wheel out of balance. Or a bad trans mount. Or a bad tire.

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Casually looking for a Clifford hyperpak intake for cheap.

I'll check into those suggestions, but I don't think that's the case. I can replicate the vibration in neutral by simply revving the engine.

Ah. You said earlier that it was smooth driving around town, so I assumed that revving the engine to the same RPM without putting a load on it was smooth.

If you are positive that the ignition system is in good condition, my thoughts then turn to several of the more "exotic" and lesser-seen causes of high-RPM rough running.

(1) disassemble, clean, and lubricate the distributor. The weights or springs may be sticking causing the mechanical timing advance not to work or causing the the timing to fluctuate. If the advance spring with the long loop sticks against a hard blob of 30 year old grease then the weight may be able to bouce around in the long loop and cause the timing to fluctuate.

(2) your ctalytic converter(s) may be clogged if they are original. Catalytic converters adn carbs don't get along very well due to the carb not being able to control the fuel mixture precisely enough to ensure catalyst longevity. You may need to replace the converters with new three-way high flow versions. If the catalytic converters have failed, then the catalyst may have degraded internally and blocked the exhaust stream. This leads to a clogged exhaust, increased backpressure, and poor performance.

(3) failed muffler due to age and rust. Same symptoms as a failed catalytic converter.

(4) failing coil


If I think of more, I will post them.

I once had an 81 Cordoba powered by a slant. Beautiful car and a pleasure to drive, but the doors were too long for me and the lack of a drip rail drove me nuts. I live in rainy Washington State and I hated getting dripped on every time I rolled my window down.

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Casually looking for a Clifford hyperpak intake for cheap.

Thanks everyone for all of these suggestions. I'd like to reiterate that the car doesn't run rough at low or high RPM. I really believe this is first-order rotational vibration and not a fuel, exhaust, electrical, or timing issue. It's difficult to explain, and as I said earlier, I suspect there really isn't anything wrong. You know that coarseness you can feel when you're on a garden tractor at full rpm? It's kind of like that, obviously not as strong -- you're just aware that the engine is doing it's thing. I actually don't know that much about the balance of long-stroke Slant-6, but is it possible to balance them to make them smooth as say, an internally-balanced 318?

Someone hasn't done something silly and fitted a 360 balancer or similar external balanced unit? How about the engine mounts? Are they those poly units? I've never used them,but I've read that are pretty harsh. Apart from that I think others have covered most likely suspects. To answer your question,sure,you can balance a slant,it's actually a simple balance job compared to a V8. You can even do part of the job yourself ,buy some scales and check the weights of pistons,rods etc. the cranks are usually not too bad from what I've been told,so if you check the weights of the rods and pistons you are probably good enough....let's face it, how many engines get built that turn far more RPM than a slant and never get balanced and run smooth...I think your really looking for something fairly significant,not just a gram or two. A slant will never be as smooth as a well balanced V8, but you should be enjoying relatively smooth high rpm operation ...something's out of whack.

Car is 100% stock the way it came from the factory. I think what I need is a second opinion (other than my own) to take the car for a spin. I strongly suspect nothing is really wrong. My other cars with 225s felt a bit labored at modern interstate speeds, too.