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Allis Chalmers Discussion Forum
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Allis B mag

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jeffalex2

08-05-2006 04:02:35




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What would make a mag consistantly miss on #3?...the last cylinder that fires. Plugs and wires check out. I have a spare cap that I'm going to swap out. Is there anything internal that needs checking? It does it at all speeds. Thanks.




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Ken L

08-06-2006 04:23:40




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 Re: Allis B mag in reply to jeffalex2, 08-05-2006 04:02:35  
Your saying that you get a good strong spark up to 1/8 inch on the other wires but you don't say if you are getting a spark on 3. If you can only get a 1/8 inch spark on all cyl., that's not good. You should get a good 1/2 inch spark or better. A good mag should knock you flat on your butt. A 1/2" or better blue spark should be should be no problem for a good mag. If you can't get that, get the mag checked out. If you are getting a good spark to 3, it's possible that a valve may be hanging up. I had an M Farmall years ago that did the same thing right after a valve job. Had a little miss when first started and got worse as it warmed up. The valve stem was warped a little (the guy doing the valve job should have caught that when grinding the valves)and as it warmed up and expanded, the valve would keep hanging up more. Screwed around with it for about four hours before I took the valve cover off and found an exhaust valve hanging up. Hope this helps

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jeffalex2

08-06-2006 05:55:06




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 Re: Allis B mag in reply to Ken L, 08-06-2006 04:23:40  
I think the fire's OK...it did about knock me on my butt...even through insulated pliers and a glove, when I was stupid enough to rest my hand on the radiator while holding #1. :) When you pull the #3 wire, and hold about 1/4" from the plug tip, it'll draw arc once every 20 seconds or so. The others hit every time. I had the valve cover off while hot and everything looked OK. I think the compression check would have told me if the valves were the problem. The compression's not ideal, but, she should run another year or so as is. I plan to do a complete rebuild in a couple of years.

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Ken L

08-06-2006 16:20:12




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 Re: Allis B mag in reply to jeffalex2, 08-06-2006 05:55:06  
If it's only firing once every twenty seconds, it's definatley electrical. Check the cap, pionts, condenser, rotor and, side play in the rotor shaft. If the bushing is worn in the shaft, it can do weird things.



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jeffalex2

08-05-2006 14:11:31




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 Re: Allis B mag in reply to jeffalex2, 08-05-2006 04:02:35  
I've got 80-85lbs compression on 2,3,4...and 78lbs on 1. I had the head reworked last month, thinking that was the problem. Valves, seats and rockers look good. It's definately a fire problem. I did swap plugs and wires and the miss stayed on 3. That makes me think it's the cap. It'll fire on 3 a couple of times and then miss for 10 seconds or so. All the others draw a good strong spark up to 1/8" or so off the wire and there's a noticable change when you pull the other wires while running...I pulled the cap and can't see anything obvious. The miss gets worse as it warms up and since 3 is the last cylinder to fire, I thought maybe there was something about mags I didn't know that would cause this. I don't think it's a coil issue since 1,2 and 4 still fire strong. I'll swap the cap out and let ya know what happens. I'm still open to other ideas though...Thanks

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old

08-05-2006 15:44:29




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 Re: Allis B mag in reply to jeffalex2, 08-05-2006 14:11:31  
One way to check a cap to see if its cracked or has carbon tracking is to spray it with starting fluid and then let it dry. If theres a crack or carbon tracking you will then see lines on the inside of the cap where they are. Learned that one from a guy years ago but hes long since gone



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Dick L

08-05-2006 15:03:32




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 Re: Allis B mag in reply to jeffalex2, 08-05-2006 14:11:31  
I have two caps that act the same way out on my work bench. I was going to pitch them and just had not taken the time. How about I sent them to you. You could then tripple your problem. {:^D Although I would have to find something else to fill the empty place on the bench. I dont think they missed on #3 though.
Sure sounds like you have pin pointed your problem.



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old

08-05-2006 08:30:18




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 Re: Allis B mag in reply to jeffalex2, 08-05-2006 04:02:35  
As Dick say it could be a bad plug or a valve problem. I would take to plug from number 3 and put it and the plug wire in another cylinder and see if the miss goes with the plug or stays on #3 if it moves you know its either the plug wire or plug. If it doesn't move you know its either the cyclinder its self or the cap.

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Dick L

08-05-2006 06:26:11




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 Re: Allis B mag in reply to jeffalex2, 08-05-2006 04:02:35  
If missing on just the same cylinder it could be a bad cap. I have used some solid wire ratty wires that didnt miss so if you have the solid wire plug wire that is probly not your problem. New plugs could still not fire so swap plugs and see if it is still #3. After that you need to do a compression check. A valve that does not seal tight will cause a miss. Check the rocker arm/ valve clearance first but still do a compression check.

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