[roadmaster_56]

Thanks to all, for you feedback regarding the engine vibration on my 48 Chrysler flathead 8. I've been working on it and wanted to share my results. Better get a cup of coffee and settle in....this is a long one.

First, despite my efforts, I haven't been able to eliminate the vibration about 1200 rpm and to a lesser degree up through the rpm range as felt through the steering wheel.

I re-adjusted the valves (hot - engine running, that was fun). The intakes were ok at .008", but the exhaust valves were too tight. I decided to set them to the "high speed driving" setting at .012". The engine seemed more willing to rev and more relaxed during upshifts with the Fluid Drive.

I did a second compression test and noticed that the compression came up on several cylinders by about 5 lbs. ranging between 100 to 118. The #8 cylinder remained the lowest at 90 lbs, unchanged from my first compression test. I squirted some oil into #8 (toward the cylinder, not the valves) and it came back to 105 and gradually bled down to 95, then eventually back to 90.

I also noticed that my highest reading cylinder was #4 at 118 lbs. This cylinder is pretty close to the carb, has a short manifold run and possibly runs richer than the others. I thought it may have some carbon deposits that are upping the compression and may be causing a secondary firing/missing of that cylinder.

I'm now assuming that all my valves are OK, so I must have a worn or stuck ring at #8 and carbon deposits at #4....what to do? Get the Seafom!!!!!.............I ran some through the engine via the vacuum plug under the rear of the carb and put some in the gas, (fogging the neighborhood) then took a 30 minute drive at 55 to 60. Got any ideas on how I could further distinguish between and fix a worn cylinder and a stuck ring? I really don't want to remove the head and piston if I can get away with it.

I still had a very slight, intermittent engine misfire along with the engine vibration. It is noticeable by a slight engine quiver at idle and audible at the exhaust pipe as an occasional "putt". I never feel it when driving as a power loss, its' not as noticeable as say, when you loose a cylinder - but I do feel a vibration at a specific rpm.

I decided to check out the distributor. My points were new about 1000 -1500 miles ago, but since then, they had developed a slight pit / cone, (cone on the breaker arm), so in went a new set of points and a new condenser.

While I had the dizzy out, I replaced an original grounding wire to the breaker plate that had become frayed. Of course during reassembly, I dropped a tiny clip pin into the guts of the distributor, so off came the breaker plate. I took this opportunity to check out the distributor weights and springs. All looked fine - but dry, so I added a drop of engine oil to the pivot points and cleaned/lubed the breaker plate bearings. I never did find the damn clip.

With the new points gapped at .018" as per the book, I attached my digital dwell meter. The readout was very unsteady, ranging between 22 and 45 degrees. I did some research and learned that some digital meters take "snapshots" of the pulses unlike older analog meters (the kind with a pointer and scale) which "average" each pulse.

The digital meter was pretty useless in this case because of the wide variation, so out came the old "Sears Best" meter from 1975. That showed a dwell of about 45 degrees at idle with some slight variation. As I raised the idle up to about 1000 rpm the dwell dropped to about 37 to 40 degrees.

So what does this all mean? It means that my distributor cam is very worn, which I could see when I put on my special MAGNIFYING GLASSES. I'm guessing that this could account for some of the missing and vibration, but I'm not sure. I have a spare distributor from my 56 Desoto, but they're not the same...... so, I'm pondering my next move.

I put everything back together and took a test drive. The engine seems more relaxed and free revving, but I still have that same damn vibration at idle and up through the rpm range. Kinda frustrating but...... well, it was time well spent and I enjoyed every minute of it....just wish I had a better outcome.

As always, your comments are most welcome.

David
Santa Cruz, CA

[DavidAU]

If it was a Flathead Ford I would put a couple of pints of MMO in the gas tank and drive the hell out of it for a while. The side valve Fords can have a sticky valve problem if they are not used regularly and the MMO frees them right up. A lot of guys run some MMO in the gas all the time and have no problems.
If the MMO does not help ( give it time) it may be worthwhile putting a vacuum gauge on it when it is idling and see what readings you are getting. It will probably pinpoint where the miss is coming from.

David

[DeSoto Frank]

I second the suggestion about Marvel Mystery Oil and some running.

If you really want to determine the issue with the low compression on # 8, I would recommend a cylinder leak-down test (applying air-pressure through the spark=plug hole, with the piston at TDC , and listening for hissing at: the tail-pipe (leaking exhaust valve), carb throat (leaking intake valve), oil fill (leaking rings), and looking for bubbles in the coolant at the radiator filler (leaking head gasket or cracked head/ block).

This is probably the best diagnostic.

Is this vibration bad enough that you're worried about doing further damage to the engine, or is it "barely perceptible", but you've noticed it and now it bugs you ?

[roadmaster_56]

Hi Frank and David, I may go ahead and get a leak down test, but I'm pretty confident its not the valves (as you suggested ,I did the "oil squirt test" and I think its the rings on #8 (either worn or stuck).

I do occasionally add MMO to the gas, but I think i need something stronger. My vacuum reading is pretty good I think, its fairly steady at about 20 at idle.

I suspect the previous owner (a little old lady) did mostly low speed, back roads driving and only 26k at that! I say this because the bottom of the car was caked with about 60 years of mud and dust.

In addition, to the Seafoam, I've tried ATF through the intake manifold vac line, lots of smoke, but no change. I just got through changing the oil and now I'm going to add some Rislone to see if that will unstick the rings (if that, indeed is the issue).

I've had this problem since getting the car. The vibration is not normal from what others have said, but I don't think its going to kill the engine. Its mostly felt through the steering wheel.

I suppose I could live with it, but I prefer not to, as I've heard the Chrysler 8 is so smooth. I just replaced the exhaust manifold which was leaking and making some noise, so now its pretty quiet.... but that makes the vibration all the more noticeable.....ugh.

Thanks,
David
Santa Cruz CA

[hchris]

Here`s another thing to consider; what condition are the engine mounts in ? if they have gone soft / split or perished then any engine misfire will be exaggerated and show up as a vibration.
See if you can get a good grip or some decent leverage on the block and try to move it side to side ( engine not running of course ); dont forget to look at the rear trans mount as well.

[DeSoto Frank]

Good point about the motor mounts; most MoPar mounts are either fossilized or Jell-O gooey after 40+ years.

[roadmaster_56]

Thanks guys, I'm gonna check out the motor mounts next. Any suggestions as to the best way to check? Maybe get a strong buddy and try to rock the engine back and forth?
Had that problem in a 66 Riveria, but it was easy to check....just had to torque it up against the brake and the engine would twist/lift about 2" off the broken mount.
Thanks
David
Santa Cruz CA

[Rusty_OToole]

The dwell readings indicate a "wobbly" distributor due to wear, probably worn bearings although the cam could be worn and the advance mechanism too.

The solution would be a rebuilt distributor. In the old days every town had a shop with a Sun distributor machine, that would test and analyse your distributor in minutes. I don't know where you would find someone to do this today.

I don't know how bad your miss or vibration is. Usually those engines are very well mannered and stay cool, smooth and quiet even in an advanced state of wear.

The compression readings indicate that the engine is worn but not worn out. The problem could be tuning related and a good distributor and possibly a rebuilt carb will cure it.

[Rusty_OToole]

Another old time test is to slowly pull the wires out of the distributor one by one with the engine running. Each time you should hear a snap snap of the spark jumping the gap and when all the way out, the engine should slow down. If you find a cylinder that makes no difference, that is the weak one. From there you can check for the cause but at least you have narrowed down the search.

The reason to pull the wires out of the distributor not off the plugs, is you are less likely to get a lifter. Use insulated pliers or wear rubber gloves if you are nervous.

[Rusty_OToole]

This is almost too obvious but you might want to recheck your spark plug wires. It is very easy to mix them up, especially #4 and #5. I have caught myself this way more than once, on straight six and straight eight engines.