Setting the Timing

[racertb]

Do any of you use a piston travel gauge, or anything else, to get the correct TDC of the #4 cylinder when adjusting the timing as indicated in the Plymouth Instruction Book?

I recently put on new wires, changed the spark plugs (back to Champion D16), installed new points, cap, rotor and condenser on the car.  After gapping the plugs correctly (.27) and setting the points (.20), the timing seemed way off, as the motor was idling slow and barely running. It had been running rougher lately anyway.

As it was running, I loosened the distributor and turned it counterclockwise a little bit to advance the timing.  The motor slowly smoothed out and sounded much better.  I took the car for a ride and it drove well.

I will probably do a little more "fine tuning", but do any of you time/tune the car by "ear" as I did or what is your method?

Thanks...

[chetbrz]

I guess I am guilty of tuning by ear.  Basically turn it left until the motor slows then right until it slows, then set it in-between the two with just a hair advanced.   If it pings retard it a hair.  Then if your timing light is working, give it a check.

With these old cars the spark advance cable is key.  Pulled out the cable the car should run as you first described 'slow' with the plugs firing after TDC.   This makes it easier and safer to start when using the crank. Little chance of kick back.   Pushing all the way in is your hot run position.

My 2 cents.  PS... Matchbook cover for the point gap.  ~.018

Chet...

[Old Man]

I think basically we forget that the engine wears in all it's parts and the setting in the book is for a brand new tight engine. Many years ago I came across a Mechanics Illustrated  where Tom McCahill said he used a vacuum gauge to set his engines because of the wear factor. Rather than turning the carb air bleed screw to the book setting he hooked up a vacuum gauge to the wiper hose port and, with the idle set slightly above idle, he turned the screw for the highest vacuum reading. Usually on a "good" engine you will get close to 15 psi. (If you have a firing problem or a valve problem,it will show up as a 'jumping' of the needle.) He also went on to say that he sets his timing that way as well. Again because of the wear in the distributor,drive, etc. He turns the distributor while again watching for the highest vacuum reading. (On later engines you have to disconnect the vacuum advance.) He then goes back to the carb and resets the air bleed screw again etc. This he said was the only way to actually "tune up" an engine to it's present condition. Otherwise you are just setting it to the factory settings,an engine you no longer have. Works for me.   

[racertb]

Thanks for the replies...

Although the engine is not new, it probably has no more than 1000 miles on it since it was rebuilt in the'80's, but what you describe Old Man makes sense and is worth a try.  I was tuning by ear, like Chetbrz, and got the car running pretty well by that method.

I might try the point adjustment to .18 to see how that goes as well.  Also, are there timing marks on these motors for a timing light?  I have no idea...since it wasn't mentioned in the instruction book, I assumed there wasn't.

Old Man - should the vacuum technique start with the carb, then distributor, then carb again?  Or, distributor then carb?

I think with the tuning by ear and feel, I can get it pretty good, but I'd like to experiment.

[Old Man]

I start with carb but it's really a matter of tweaking. Once you set the distributor, I assume the burning of the mixture is better than it was before. Then going back to the carb you can tweak it for even better burning again. I have also done this on old motorcycles and made an amazing difference in performance. With an engine so young as yours I suppose this method won't give much in return. I don't know. It was designed more for older engines that had strayed from their original condition. Sort of making the most out of what you have left. The lash and backlash in the distributor system and the abrasive wear in the carb internal ports would make the original book values not match your engine. By 'tuning' this way you match the timing and mixture to your engine's present condition. Just turning the carb air bleed screw will make an immediate difference in vacuum and speed up the engine requiring you to turn down the idle screw. Happens every time. I don't know how to put it any other way. The original settings are only good for a few thousand miles and then you will find a difference in the way the engine starts,runs and gas mileage. I believe,but I'm not sure, it makes up for slight vacuum leaks in the system. In other words if there are minute leaks in the intake manifold,carb mounting etc., tuning the carb and distributor this way would set the air bleed and timing to 'that' air/fuel mixture and that's why I believe there is an increase in rpm. You can't be sure that the old gaskets on every surface are air tight. Makes sense? There are no timing marks on a '28,'29 or '30 Plymouth that I know of. In fact I don't think timing marks showed up until 1933. (They are hidden behind a cover on the bell housing and are painted in red on the edge of the flywheel.) Most flat head CPDD engines however had a removable machine threaded screw,slot headed drive, above the #4 or #6 position on the cylinder head and you could drop a long piece of coat hanger wire down the hole and watch the wire move up and down as the engine was turned over by hand sans spark plugs. Generally the point break was set at some point before the wire reached TDC. It varied with engines. However this again is setting your engine to the book settings and your engine may be miles off those. (pun intended.)                       

[racertb]

Thanks Old Man for the information...I will keep fine tuning and may experiment with a couple of things to see what's best for my motor.  At this time, she runs and drives fine (to my ears) and I will use the current set up as a benchmark that I can always go back to. 

Ted

[chetbrz]

I picked up this "Mile-O-Meter" a while back.  Based on this discussion I will have to put it to the test. 

Now I need to figure out what I did with it    Using it to tune the carb mixture sounds good to me.



Every day is a learning experience.   Tks Chet...

[Old Man]

Chet, Mine has only the one scale on it for vacuum in psi. But that guage will do fine for setting the carb and dist. It has the psi  scale on it with 15 psi being about center. (It has something to do with atmospheric pressure being a nominal 14.7 psi. Any higher than 15 psi and you would be working on a boosted or supercharged engine with the supercharger engaged. The stuff I worked on in the RCAF had a single stage blower on it and would put this reading of 15 psi up to 29 psi at full throttle or as we used to say "29 inches".) You just need a 1/4" (internal hole size) rubber hose,the same size used on wiper motor hook up, and put it on the wiper port at the carb/manifold and hang it from the rad stay facing out. That's for the carb setting and then you have to face it to the left for the dist. setting. Mine came with the hook built in but a piece of coat hanger wire taped to the guage would do.     

[chetbrz]

I think the Mile-O-Meter was designed to help the driver improve his driving skills to increase the drivers miles per/gal.  I don't know when this was out as a product but I would guess late 50's early 60's.  Of course it is a very good vacuum meter and has a large scale to detect leaky valves and other vacuum related issues.   I will have to pay more attention to vacuum readings next time I tune my old engines.  All that has been discussed makes good sense to me.

Thanks,  Chet...

[racertb]

All:

Is there a way to check the point dwell on our 4 cylinders?  I'm sure I can hook up my old dwell meter that has a choice for 4 cyl, but what would a "good" range be for our motors?  I know there's no spec in the book, but I'm just curious.

I also have a vacuum gauge that I'll be using to try the previously discussed method mentioned by Old Man.

Thanks,

Ted

[chetbrz]

I found this explanation of point dwell.  I thought you would find it interesting:

At ordinary engine operating speeds, the points open and close a couple of hundred times per second, the exact number depending on the number of cylinders and the engine RPM. The points need to be closed for an appreciable time in order to build up the maximum magnetic flux in the ignition coil core.

The period of points closure is specified by the ignition system designer and is typically expressed as degrees of distributor rotation. In a four cylinder engine, the angle between each ignition cam lobe is 90° and the period of points closure or "DWELL" is usually a bit over 45° of distributor rotation. In a six cylinder engine, the lobes are 60° apart and the dwell time is 30° to 35°.

The dwell is adjusted by setting the points gap to a specified distance at maximum opening. A narrower gap gives more dwell and a wider gap gives less. Taking it to extremes, excessive dwell means that the points close too soon after opening, cutting off the magnetic field collapse before it delivers all its energy. Too little dwell gives the magnetic flux insufficient time to build up to the maximum.

Both conditions give a weak spark which gets even weaker as the engine RPM rises and produces misfiring at normal operating speeds. The dwell, as well as spark plug gap, do have an effect on ignition timing. The later the points open, the later the spark comes and retards the timing. The earlier the points open the sooner the spark comes and advances the timing. That is why timing is the last thing to be set in a tune-up.