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willysmjeeps.com :: View topic - Slow Speed Lugging - Missing
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Slow Speed Lugging - Missing

 
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jnissen
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Joined: May 12, 2018
Posts: 92
Location: Austin Texas

PostPosted: Sun Apr 19, 2020 6:42 pm    Post subject: Slow Speed Lugging - Missing Reply with quote

Running around this weekend noticed at low speed while lugging the motor it will miss badly. Clearly is ignition. Tried adjusting timing and it helps slightly but not much. The system is a 1955 M38A1 with a 12V conversion to an Autolite distributor.

Running a 3.5 ohm coil at 12V with points in the Autolite distributor. We are also running a ballast resistor in this setup. Seeing that the coil is roughly 3.5 ohms do we still need to run the ballast resistor? Thinking of removing it.

We had a Pertronix type ignition in there earlier and did not run the ballast resistor. It ran the same and had the same low speed high load miss. We wanted to try points and installed the ballast resistor back in. Any guide on coil resistance and points and when you would normally require the use of a ballast resistor (Ballast resistor is about 2 ohms).
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wesk
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Joined: Apr 04, 2005
Posts: 15109
Location: Wisconsin

PostPosted: Sun Apr 19, 2020 7:10 pm    Post subject: Reply with quote

The commonly used ballast resistors for jeeps were used to handle the original 6 volt coils when used with 12 volt jeeps. Eventually the industry switched from the ballast to internal resistors on the old 6 volt design coils. Those 12 volt Willys jeeps in the 50's and early 60's that came with a ballast resistor left Willys/Kaiser with the old 6 volt coil. The need for high energy ignition system didn't exist yet so the industry saw no need to change the original design of the 6 volt coil. The simple inconvenience of the external ballast type resistor led to the introduction of the 6 volt coil with the built in resistor.

Since your engine had this same issue with your Pertronix system that it now has with it's Autolite point system the odds favor a coil issue. Start with an ohm check both primary and secondary and both cold and hot. Then compare your ohm readings with the coil test specs sheet. If you are not already in possession of a test specs sheet for your coil please tell us exactly what brand/make and model/part number and we can probably find one for you.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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jnissen
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Joined: May 12, 2018
Posts: 92
Location: Austin Texas

PostPosted: Sun Apr 19, 2020 7:29 pm    Post subject: Reply with quote

Update: Removed the ballast resistor and when doing so cleaned up the coil. Printed on the side of the coil in faint ink is "No External Resistor Required"!

Removed ballast resistor and it's running much better but still a slight miss under lugging. Going to try a different coil next.
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wesk
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Location: Wisconsin

PostPosted: Sun Apr 19, 2020 11:47 pm    Post subject: Reply with quote

You would be doing yourself a favor to actually troubleshoot the coil with an ohm meter and determine it is bad before buying a replacement.

A simple ohm meter set to low ohms for the primary test where you want to see a 0.5 to 1.8 ohm resistance both with the coil cool and the coil warmed up. Then the set the ohm meter to cover the 10,000 to 20,000 ohm range and you should get a reading between 8,000 and 15,000 ohms both with a cool coil and a warm coil.

Coil testing link: https://itstillruns.com/test-12-volt-ignition-coils-7959727.html

What were your compressions reading using a leak down test set and did you hear any valve leakage during the test.

Leak down testing link: https://www.mobil.com/en/lubricants/for-personal-vehicles/auto-care/vehicle-maintenance/how-to-do-a-leakdown-test
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100

Mjeeps photo album: http://www.willysmjeeps.com/v2/modules.php?set_albumName=Wes-Knettle&op=modload&name=gallery&file=index&include=view_album.php
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jnissen
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Joined: May 12, 2018
Posts: 92
Location: Austin Texas

PostPosted: Mon Apr 20, 2020 12:15 pm    Post subject: Reply with quote

Itís a 3.5 ohm coil. Presently runs OK but will miss if under strain at low RPM. Tried insulating plug wires that ran near metal with no improvement. The standard points are being used to switch the coil. At any RPM lower than 1500 while under a load (steep hill) it will buck and complain as it drops out.

Going to look at ignition in a dark garage tonight to see if anything can be observed. Itís clearly finding a high voltage leakage path under load may need to chock wheels and test with a helper.

* Updated to show lower than 1500 RPM. Had incorrectly said higher than.
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jnissen
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Posts: 92
Location: Austin Texas

PostPosted: Wed Apr 22, 2020 9:01 am    Post subject: Reply with quote

Ignition wires are new but still appeared to be slightly leaky. The connections at the plugs were glowing in the dark. Tightened up and less but still same issues.

Adjusted timing and it affects where in the RPM run up the missing occurs.

The fact the engine starts and runs well in the 1500 to 4K RPM range clearly says the coil is working but may not be optimal. Tested at higher RPM while in second gear up a hill. I typically shift prior to high RPM but I let it rap out a bit. Itís clearly missing at higher RPM as well as you can hear the exhaust popping at the elevated RPM.

Ordered a different coil with primary resistance closer to 1.5 ohms. Will use an external resistor (0.7ohms, 1 ohm, 1.5 ohms, 2 ohms, and 2.3 ohms available) as opposed to the fixed resistor version in there now. The thought is we can tune the current in the coil. We know that the old coil with 3.5 ohms of DC resistance is giving us marginal results.

BTW I had the power resistors as Iíve done lots of tests for power supplies in the past.
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jnissen
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Joined: May 12, 2018
Posts: 92
Location: Austin Texas

PostPosted: Fri Apr 24, 2020 10:03 pm    Post subject: Reply with quote

Well after playing with resistors and the new coil I can safely say that we have an excellent ignition but still seeing the odd loading up and miss at low speed.

Decided to remove the air intake tube on the Carter YF carb we are running. The YS is the standard carb on the M38A1 but were using a YF for the manual accelerator pump that is built in. Once the air intake was removed the Jeep was able to climb our test hill with ease and no more missing!

So looks like it was our existing CJ5 style air hat and tube that leads over to the M38A1 oil bath air cleaner. We narrowed it down to just the air hat causing the issue. We have no tube and no oil bath air cleaner and can still experience the miss while lugging up a hill. Remove the air hat and that goes away! Yeah surprised us as well.

So now were looking at alternatives to a CJ5 style air hat and even considering building something with a bit of a plenum to overcome whatever restriction the original presents. May require us to test a few spacers and possibly a whole new hat design. Oh well glad to finally figure out the root cause.
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jnissen
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Joined: May 12, 2018
Posts: 92
Location: Austin Texas

PostPosted: Fri Oct 23, 2020 7:04 pm    Post subject: Reply with quote

Just a follow after several months of operation. The reason we suspected the air hat was when it was removed the Jeep ran terrific. Yes this is true but only half the story. My son managed to 3D print a different air hat that had significantly more volume. Things were really working well until we added the vent tube from the oil fill dip stick to the air hat. Once that was added back in then our woes started back to square one!

You can probably anticipate where this is leading. It seems the motor at low speed lugging speeds or at high RPM is producing enough oil blow by that when it becomes excessive the motor balks at re-burning that Smokey mess.

When we were going through everything we opted to not put new rings in it since it didnít blow smoke out the tail pipe. Well thatís still true but the ring to cylinder fit is worn enough at lugging or high rpm that we get more blow by gasses than we care for.

Perhaps in the future we will pull the motor and give it a quick dingle ball hone and new set of rings. The piston to cylinder wall as well as no ring ridge was present when we had the head off earlier. Hind sight is 20-20 but in this case a re-hone with rings would likely fix this.

Sometimes you can overthink a problem and in this case it was clearly overthought! BTW the Jeep runs fine in 90% of its operating rpm range so thatís a plus. Just enjoying it. Will attend a drive type rally tomorrow so itís going to stretch itís legs again.
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