Jump to content

Adjust timing / Missing Spark - 1940 L Head 6


Ian.Harrold
 Share

Recommended Posts

General question, but how do you adjust timing and dwell on these motors?

According to the service manual "Breaker point dwell (cam angle)" is 38 degrees.

Is adjusting the timing as simple as loosening the "Advance Control Arm" and twisting the distributor slightly till timing is set?

The issue I am seeing is that I am seeing a miss every so often in a cylinder. I have new plugs, wires, and rotor. So I purchased a new Distributor cap and the problem still exists. I tried adjusting the points to 0.020 at the top of distributor cam lobe. Hasn't solved it.

I am using the Lisle 26900 Spark tester and can see every so often a missed spark.

Any Ideas? Thanks in advance.

Link to comment
Share on other sites

Dwell and point gap are two different ways to measure basically the same thing. If the lobes on the distributor cam are not worn then setting the factory specified gap should result in getting the factory specified dwell. I typically set my gap with feeler gauges on the bench (easier to work on than with the distributor on the engine) then check it with a dwell meter.

The dwell/gap sets how long current flows through the low voltage windings on the coil. If the dwell is too short a time (gap too big) then you won't get full saturation of the coil and the spark will be weak. If the gap is too small (dwell too long) then there will not be a clean opening of the point contacts and the field in the coil will not collapse quickly again resulting in a poor spark and erratic operation. But in either case the action should be consistent and not result in an occasional "missed spark".

Regarding setting the timing: I generally set the plate at the zero point on the block, loosen the distributor clamp and then statically time to TDC per my year's "operators manual". If I want to advance or retard slightly from there then I can move the plate based on its degree indications. I've heard that with the higher octane gas we have today you can go with significant static advance (determined by "road timing", basically advancing it so that there is just a little incipient ping when accelerating from low speed at full throttle in high gear) but I just leave mine on the factory recommended setting.

You might want to check that the plate the points are mounted on is electrically bonded to a good engine ground. There is a small flexible wire that should perform that purpose. If it is broken or missing then the points will be grounded through the mechanism that runs the vacuum advance and that might not be a good and stable connection.

Another item that might cause an occasional missed spark could be a condenser that is going bad.

Edited by ply33 (see edit history)
Link to comment
Share on other sites

I find that these old engines work better if you do what the old mechanics used to do. I use a vacuum gauge and put it on the vacuum nipple that the wiper motor goes on. Run the engine to warm it then adjust the idle bleed screw for the highest vacuum usually around 15 psi. Then leaving it connected ,but disconnect the vacuum advance vacuum line if you have one and plug that line,turn the distributor around back against the rotation until you get the best vacuum that way. It will be very little. Make sure of course you've already set the points gap. Go back to the carb and adjust the air bleed screw one more time. You will end up with the highest vacuum the engine can produce and i.e. the highest horsepower the engine can produce and the best gas mileage versus horsepower. Doing it this way 'matches' the settings to the condition of 'your' engine at this point in it's life. The factory settings are nominal to get the engine off the line and out the door. But as the engine wears they are no longer valid. Using a vacuum gauge is the best way to get the most out of your engine at the mileage it has now.

Link to comment
Share on other sites

Dwell is the points adjustment. You need a dwell meter to most accurately adjust points. If you dont have dwell meter then simply adjust the points to the recomended gap on a high lobe. It is important that you find the highest point of a cam lobe.

I usually take the distributor out of the engine to change points as it is much easier to work on it on a bench than in the car. And you will be setting the timing anyway. If you do have a dwell meter you will have to do the final adjustment to the points by hooking it up and running the engine. It might take a few trys to get the dwell where you want it.

Edited by JACK M (see edit history)
Link to comment
Share on other sites

Something missed here in the general sense. Always clean your new points contacts with carbon tetrachloride and if your using the antiquated setting points by feeler gauge method clean the feeler gauge with carbon tetrachloride too. Points come with a little film of oil so they wont rust on the shelf and if left on will burn prematurely. Always check the ground for the breaker plate. Clean the contacts in the new distributor cap and rotor with carbon tetrachloride too. Use only about a match head or less of distributor grease on the cam and whatever is left over from that put behind the point rubbing block. A intermittent engine miss is best to be seen on a oscilloscope to determine if the miss is coming from just one cylinder or random misfiring.

D.

Link to comment
Share on other sites

Thanks again everyone. As an update, I tried starting the truck without the vacuum advance (with the manifold nipple capped) and it would not start. I reattached the advance and it started. So I removed the advance, capped the manifold nipple and checked timing.

It appears my timing is about 3 degrees advanced according to the balancer measurements, not that bad. Idle was very rough without the advance. I am going to find a dwell meter to ensure the points are set correctly, then I'll recheck timing. Thanks again.

Link to comment
Share on other sites

  • 3 months later...

Update. So the truck is running rough again. Dwell is set to 39, pretty close to the 38 recommended.

At idle it feels like it is missing, though I am having difficulty tracking it down to a particular cylinder. I am also hearing a knocking sound at idle. It sounds like the driver side of the motor. I'll try and post a video or audio of it.

Out of curiosity if it were a rod or crank bearing, can I replace them with the engine still the truck? Can I just drop the pan and replace the Rod bearings? Or do I need to remove the head aswell? The shop manual was unclear. One final question, are the head bolts torque to yeild or can they be reused?

Link to comment
Share on other sites

Update. So the truck is running rough again. Dwell is set to 39, pretty close to the 38 recommended.

At idle it feels like it is missing, though I am having difficulty tracking it down to a particular cylinder. I am also hearing a knocking sound at idle. It sounds like the driver side of the motor. I'll try and post a video or audio of it.

Out of curiosity if it were a rod or crank bearing, can I replace them with the engine still the truck? Can I just drop the pan and replace the Rod bearings? Or do I need to remove the head aswell? The shop manual was unclear. One final question, are the head bolts torque to yeild or can they be reused?

It is possible to replace the rod bearings with the engine in the car and the head installed. Also possible, if I recall correctly, to replace all but the front main bearing as well. As to whether or not it is a good idea to replace bearings without pulling the crank to inspect and/or grind it is another question. At the least you should visually examine for damage and measure for roundness.

Torque to yield for head bolts on a Plymouth L6 engine? I think they stopped making them a decade or two before that concept came in. Examine the bolts for damage, especially the ones that go through the water passages, replace if stretched or corroded. I've heard that there is a pretty good match to the original head bolt with one of the Chevy 350 head bolt lengths. See Engine head bolt. however if you need to replace one that has been drilled and tapped on the top for your throttle linkage you may be in for a search.

Link to comment
Share on other sites

Thanks Ply33. I have never done much engine work before. And when I have it was always on new fuel injected motors. The head bolts on these were torque to yield, thus use and toss. I had to investigate how set points. :o I was used to rewriting the computer air/fuel maps and delay of timing.

Right now I just want to investigate why my engine is sounding rough and by my best guess missing. Your advise on measuring the roundness is worthy. Any other ideas as to what it could be? On the Passenger side all I can hear is the oil pump, it sounds like it is coming from the driver side of the motor. But inside the cab, it sounds like the Passenger side. Valve train?

EDIT: I have been listening to rod knock video's, and it does not sound like an audible metallic "knock"

Edited by Ian.Harrold (see edit history)
Link to comment
Share on other sites

Might be a valve tappet adjustment issue...

I'm guessing that if this is the first engine you've worked on with point type ignition then it might also be the first with adjustable mechanical lifters on the valves. :)

If loose they will make a ticking noise, if correct or too tight then they'll be quiet. Supposed to be adjusted hot and running which means burnt knuckles especially when working on the back cylinder. One bit of old time advise on valves was "if you can hear them then they aren't burning", so people often left them with more lash than the manual called for.

Link to comment
Share on other sites

  • 2 weeks later...

I'm going through similar pain right now on my flathead. Take time to adjust your valves to perfection. There are many things it can be: loose valves, bad wrist pin, broken valve spring, rod knock, etc... Did you do a compression test? My motor has 3 cylinders at only 80 lbs so I'm stopping and doing a rebuild. Stay with it and keep us posted.

Link to comment
Share on other sites

Now that I listen more, it sounds less and less like something in the bottom end more like something in the valve train.

Below is a video:

Thoughts?

What I hear there is typical Tappet noise and perhaps some piston slap common to these motors with high mileage, let me also add that from here none of it sounds threatening.

As to the miss, having done the basic ignition items perhaps remove a plug wire one at a time whilst idling, what should happen is an even bigger miss occurs as each plug is disconnected, if you find that there is no change on a cylinder as that wire is removed, you have at least pinpointed the area; NOTE have yourself a thick pair of rubber gloves whilst doing this to avoid a boot!!.

At a guess you may be looking at valve clearances that need redoing, possibly even a poorly seated valve, have you tried a compression test ?

Your earlier statement regarding the spark tester might also indicate a dodgy plug, the wire removal excercise might show up this. As well have a look at all the plugs after the engine has been running at normal temps, specifically you are looking for a noticeable difference in the color of the of the firing end of a plug that is misfiring. A bit of internet surfing will show some pics of good / bad plug colorations

Link to comment
Share on other sites

So I checked the compression after letting it idle and warm up. 100, 90, 83, 90, 90, 90. I put a bit of oil in #3 and it popped up to 90.

So the compression is right in the ball park of minimum acceptable. Though #3 does indicate perhaps the rings aren't the best after it had sat for 12 years.

Vacuum was around 18 psi at idle and wobbled by about 1/2 to 3/4 of psi. Plugs looked normal, and I regapped them prior to checking vacuum.

I still need to adjust the tappets.

EDIT: I was thinking about my Dwell Meter, Equus 3400. It has two leads, + and -. I am wondering if my measurements are 90 degree out of phase... i.e. 90 - "MY READING" = real. I noticed that when I was checking the points and filed them I could fit a pretty large file through the gap when it was at the top of a lobe. So if I am reading 39, then 90 - 39 = 51 actual which may explain such a rough idle and the large gap. I'll try readjusting with feelers back to 0.020 and measure with Dwell Meter?

Edited by Ian.Harrold (see edit history)
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...