Running Rough and no Power

[Mike Thomas]

All,

I'm having a problem with my 1976 Universal Atomic 4 and I hope someone can help.

The engine is running very rough with a distinct rhythmic shutter, almost like a cylinder is misfiring. When I give it more throttle it sounds like it's starved for fuel and dies in shutter. It will restart again. Oil pressure is OK but fluctuates down in-time with the rattle. It will idle very rough and continue to idle rough when the transmission is engaged. There is also a constant "whoosh" sound coming from the exhaust (that's new) as if there was a blower or hair drier in the exhaust. There is water coming out of the exhaust at good flow. No water in the oil or oil in the exhaust.

The only thing that was replaced before the problem arose was the coil. that was two weeks ago and it was running fine.

I have replaced both fuel filters, all spark plugs, spark plug wires, dist cap (electronic Ign), rotor, coil and squirted marvel mystery oil in each cylinder.

Checked that I have a spark at each plug and checked for compression on each cylinder. I don't have a compression gauge but held my finger over each sparkplug hole and had compression on every cylinder. #1 may have been a little soft but it didn?t feel like a valve was stuck.



Anyone have any ideas where to look from here


Thanks for any help
Mike T
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MERCEDES-BENZ OM611 ENGINE SPECIFICATIONS

[Don Moyer]

Mike,

The first thing I would do is to confirm that the plug wires are installed in the proper sequence.

1) Set the engine so that the first cylinder is at the top of its compression stroke (TDC).

2) Make sure the plug wire from the post in the distributor cap directly above the tip of the rotor goes to the first plug (at the flywheel end).

3) Then in clockwise rotation around the top of the cap, the wire from the second post goes to the second plug, the next wire to number the 4 plug, and finally, the last wire goes to the number 3 plug.

Don

[Mike Thomas]

Hi Don,

Now the engine will not run at all. It tries but does not continue running.

Just to clarify.

I checked the plug wires and they should be in the correct position. What I did was remove the plug closest to the transom/transmission and turned the engine until that cylinder was at TDC.

At that point I removed the distributor cap and confirmed that the rotor tip was pointing between 1 an 2 o'clock, replaced the rotor cap and put the plug wire on the post the rotor pointed to and on the plug closest to transom/transmission. The next post clockwise 4-5 o'clock on the distributor went to the next plug moving forward toward the fly wheel, the next post 7-8 o?clock went to the plug closest to the flywheel and the last post 10-11 o'clock when to the plug 2nd from the front that was left un-wired.

This would make (I assume) a 1-2-4-3 firing order as printed on the fly wheel cover (working from transmission (1) to flywheel (3)).

Because the engine ran worse over time and was not triggered (as best as I can tell) by anything, I assumed it was a fuel problem once I confirmed that I had spark. I removed the carburetor, dresses the float needle and cleaned all of the jets. I confirmed that the engine is getting spark. Confirmed 100+ compression at each cylinder and I am still not having any luck.

I seem to be burning through ignition coil's. I had to run to the local auto parts store and get a new coil because the one that was in there had oil welled up in the wire socket. Some weeks ago I did have a problem with the purple ignition wire that runs to start button. Could that part of the problem? The new coil says "must have an external resistor" Is there an external resistor that needs to be in the system? FYI I have the electronic ignition upgrade installed.

Thanks
Mike...
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GLASS BUBBLERS

[Don Moyer]

Mike,

I'm at a loss on several issues in your last report:

1) I simply can't follow the methodology you used to insure that your plug wires are in the proper sequence, so the issue of whether or not the plug wires are installed in the proper firing order remains a red flag in my view. Specifically, I'm not sure you realize that the cylinders are numbered 1,2,3,and 4 from the flywheel end of the engine.

2) You seem to be using coils that require external resistance which would easily explain why you keep burning them out. Unless you can find a large ballast resistor in the lead from the ignition switch to the positive terminal of the coil, you should be using coils that are internally resisted, meaning that they need no external resistor.

NOTE: External ballast resistors are quite easy to find (if you have one). They are made of white porcelain, approximately 4" long by 3/4" square, and will be located near the engine in the circuit leading from the positive terminal of the coil to the IGN terminal of the ignition switch.

3) As a separate but related issue to (2) above, your Ignitor electronic ignition system will be somewhat problematic due to the low resistance in the primary circuit of your externally resisted coils (1.5 ohms as opposed to the required 3 plus ohms). You can use externally resisted coils with the Ignitor, but I'd rather not confuse the issue with that discussion at this time.

3) The purple wire should go from the IGN terminal of the ignition switch to the positive terminal of the coil, not from the "Start button" to the positive of the coil as you report. This lead is arguably the most important connection between the cockpit and the engine, so if you're having troubles with it as you reported in a previous message, it could definitely be at the root of your problems.

Don

[Mike Thomas]

Don,

I double-checked everything the way that you suggested and everything is in its right place. I also added a ballast resistor to the coil/electronic ignition line and checked to make sure I have a minimum of 3ohms resistance. The ignition (purple wire) seems to be run correctly.

It cranks, fires and tries to run but doesn?t

I did notice that after the engine stops cranking there is excess pressure that hisses out of the exhaust for up to 5 seconds after the engine stops turning. Could the cause of this problem be because there is a blockage in the exhaust system?



Thanks
Mike Thomas
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Gm Family Ii Engine History

[Don Moyer]

Mike,

You're making a good case for a clogged exhaust system, though I don't quite understand how it could have been OK one day, and clogged the next.

In any case, the best way to check out an exhaust system is to remove the two bolts securing the exhaust flange to the aft end of the manifold so that you can move the flange away from the manifold far enough to allow all the exhaust gas to escape. The engine will make lots of noise and fill the cabin with exhaust fumes, so have all your windows and hatches open. The good news is that you won't have to let it run for more than a few seconds to determine whether the engine is running OK or not.

I would also check to be sure you have one end of the ballast resistor connected to the positive terminal of the coil, and the red wire of the Ignitor and the purple wire from the IGN terminal of the ignition switch both connected to the other end of the ballast resistor. If you have the red wire connected to the positive terminal of the coil, then the resistor, and then the purple wire, the Ignitor, is not receiving enough current.

Don