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Farmall M Popping

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Farmall65

Member
I noticed my M likes to make a popping sound, when it has had time to warm up and then it is brought to an idle. I checked the valves and they needed some adjustment cause they had way more than .020 clearence. After I adjusted them, i took it for a drive and didnt seem to change much. Is this an issue with the rings or possibly in need of a carb rebuild? Tanner
 
Sounds like you are describing a backfire when the throttle is returned to the idle position. A pop or sometimes a BANG is completely normal. In fact I start looking for something wrong if they don't do it. If this is the case don't worry about it
 
If you mean the rain cap bounces and pops, ignore it. I have never seen one on an M or H or other in that era that didn't. It is normal. If it stumbles and misses, and does not get to speed, then fix it. Jim
 
The reason the rain cap dances is small misfires and Hydrocarbons igniting in the exhaust. The tractor is OK. A tractor or vehicle made before emissions control of any type, with a carb. will absolutely make noises caused by those Hydrocarbons burning in the short direct path to air. Unless there is a performance issue, ignore it. Leaning out the carb at idle will minimize the tendency to do it, but that is about it. A steady misfire, or fowled plug is an issue. Jim
 
when ever you want to know how an engine is for shape, check the compression and read the spark plugs. that tells the story with no guessing. nice clean burning spark plugs lightly tanned and you have an engine in good shape. but... none of these old tractors get worked in operating temperature to find this. most will be black and sooty looking. but if you have plugs all fouled in oil and carboned up you have an oil burner, or poor engine.you can have 3 nice clean plugs and one sooty one and this can very much mean a burnt valve, if the ignition system is all good. biggest thing is put the tractor to medium or even heavy work first for a few hrs. then do the checking and you will see exactly how the engine is.and if its your tractor monitor the oil useage between oil changes, that tells a lot too.if you got something burning a gallon or two between oil changes its o/h time.
 
I hate to disagree with you,Jim, but the leaner the mixture the hotter gas burns, just the opposite of a diesel. So the leaner you have it set the hotter that carbon will be, that's sitting there on the valves and domes.
 
Well the answer lies some where in between. my comments were to lean the mix to near ideal. not to take it into ragged edge lean. Just to limit the un-burned HC as much as possible at idle without harming drivability or response. Jim
 
I will throw another theory in here, an engine with siamese manifold will never have ideal mixture on all cylinders due to overlap on the intake valves on the shared runners. The leaner cylinders will occasionally misfire causing unburned hydrocarbons to enter the exhaust and potentially be ignited by the hot gas from the adjacent cylinders. This will happen occasionally when he mixture is leaned out, but is of little concern. Backfires through the exhaust are almost always due to unburned hydrocarbons.
 
I missed something on possibilities that I saw a long time ago. An engine had an issue with the ignition advance and would muck up the timing when the throttle was pulled back quickly. I missed this and had another mechanic catch it, so this sticks in my mind. Put a timing light on the engine and watch how the advance works when you change RPM's on the engine. It should move between at or above TDC when at idle to about 26-30 degrees when at full RPM. The change should be smooth when changing RPM slowly and should change quickly when abrupt changes are made, but should not go outside these boundaries.

Others can (will) post if my numbers are not accurate.
 
Had a M that started popping like that two years after complete overhaul. I went back through and checked settings on everything and could not find anything out of adjustment or plugged so I found by upping the idle a little and it would come out of it.
 
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