Posted: Mon Jan 24, 2022 12:09 am Post subject: Member's question on Ignition Noise Filters & engine rou
Got this in my email from one of our members and I wish to repost his question and my reply here so that all of you can benefit from the discussion.
Subject: M38 distributor filter
I see on the Jeep forum you offered to send Floys a wiring plan for the firewall distributor filter for early m38s. Could you please send me the information too?
My jeep SN is 19631; I donít know if this jeep was supposed to have a filter.
My M38 started to run rough intermittently, backfiring, and running slow. Now it runs like this all the time. I have been driving it daily for three years. I put the rebuilt distributor in exactly like the old one. I havenít removed the oil pump, so splines are exactly like it was when it was running like a top.
I had an electric shop try to fix the ignition, but after two months they gave up. I spent weeks on this; replaced coil, condenser, cap, rotor, wires, plugs and distributor. The timing at 5btd seems to break appropriately. I see a spark at the points. I even put a new carb on it too.
So I am totally stumped.
So I thought maybe it needs a filter?
Any other things I should check; assuming I have the timing correct and it still runs like timing off?
First let me say that asking these questions on the technical board on our mjeeps web site is always the best way to get info and have that info shared with all members. That is the purpose of the web site.
The radio noise filter used externally on the firewall was applicable to MC10001 thru MC 20599 (some sources use MC20601 as ending serial) and also required the use of the 1st (early) distributor IAU-4004-UT. If you have an IAU-4006-UT or -AUT series distributor it has the radio noise filter built in and is not to be used with the firewall mounted noise filter.
As for a wiring plan for the external firewall mounted noise filter I have not heard of one. There are plenty of wiring diagrams in the two applicable manuals (TM 9-804 pg 170 & 172) (TM 9-8012 pg 175, 177, 183 Par e) that address how it is integrated with the M38's firewall harness.
As for the engine running rough the info you have provided is not enough for me to offer any further troubleshooting choices. You would need to provide a lot more details of step by step procedures you have thusfar employed. My advice here for folks that do not have the experience to carefully troubleshoot these type problems is that you find a local person hopefully familiar with the military 24 V system and waterproof venting and carburetion systems to assist you in proper troubleshooting protocols.
It is imperative that you very carefully determine timing marker accuracy especially on early M38ís that use the flywheel timing marks which can be 180 degrees off very easily. It is imperative that you accurately determine #1 piston TDC withing Ĺ degree and #1 piston 5 degrees BTDC position with a Ĺ of a degree. With the L134 this requires head removal and the use of a dial indicator. My preference on the L134ís is to add the F134 type crank pully timing mark system and tab. Much more reliable and much more accurate.
Distributors and coils need very careful inspection and testing before employing them. IE coils must have their primary and secondary resistance ohm readings recorded accurately both with the coil cold and hot. Capacitor/condensers must be tested with a proper capacitor/condenser test set. If you are not mounting the ancient 1950ís military radios to your jeep then I always suggest eliminating the radio noise filter on the firewall or in the distributor. They are useless with modern radios. To eliminate the firewall mounted radio noise filter you simply remove it from the harness and splice the harness. To eliminate the internal distributor noise filter requires replacing the capacitor with a solid wire and is detailed in my ignition system photo album on mjeeps web site. Internal wires inside the distributors are very prone to aging and shorting out and should be replaced frequently. Carbon tracking and cracks are common problems with the distributor blocks and you should inspect and clean them carefully. Indexing of plug wires in caps are often in error causing an inability to swing the distributor housing enough for correct timing setting.
Troubleshooting is an art that requires careful planning, execution and recording of results item by item. Your TM 9-804 (Early M38 Operatorís Handbook) and TM 9-8012 (Late M38 Operatorís Handbook) contain outstanding quality troubleshooting guides that we all find extremely useful.
The list can go on quite a bit. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
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