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Discussion Starter · #1 · (Edited)
After the fuel system was sorted out I had poured some E0 (Ethanol free) gasoline into the tank and attempted to start the engine for the first time. Predictably it did not start, it didn't even sneeze once. I bought it in the no-run condition, so I didn't really expect it to start. The newer aftermarket CDI unit dangling on its harness and the old, weathered CDI unit in the dirty box with old parts were hinting of some kind of ignition problem. Pulled one of the high tension wires off its spark plug and connected it to my ignition tester. No spark, just as expected.
Motor vehicle Gas Auto part Automotive super charger part Automotive engine part


At that point it had become obvious that the CDI was the likely culprit. The newer aftermarket unit installed by the previous owner's repair shop had no p/n on it and it was both wider and longer than the original unit. I swapped the new aftermarket unit for the old original one just for the peace of mind, and predictably the tractor had failed to start either. I would hazard a guess that when the tractor originally died, the repair shop had correctly identified the CDI as the likely culprit but incorrectly ordered the replacement CDI with AC excitation instead of the DC one. The engine with the wrong type CDI in it once again had fail to start. At which point the owner decided to pull the plug. The tractor had sat like this for a few years before finally making it to FB Marketplace just 40 minutes away from me. I had been on the lookout for a H4518 for over a year both on FB and CL so I jumped into my minivan and snagged it for a song.

The rest was easy. I cut a groove on the perimeter of the plastic case a couple of mm deep and carefully pried the box open with a 1"wide hard putty knife that I keep around for such occasions. Wire wheeled the rubberised conformal coating compound off the PCB, and lo and behold ... the problem that had turned this tractor into a lawn ornament stared me in the eye. Here it is, in all its glory, circled in blue.
Rectangle Wood Gas Font Circle


Since I was not in the least impressed by the sloppy quality of the original solder job of this PCB, I had gone through each and every solder joint with my digital pencil, fresh AgSn solder with a dab of quality industrial water soluble flux that I use in the lab.
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Coated the PCB with hot glue, reattached the previously cut off lid and let the CDI sit in the vise overnight.
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The next morning I attached the newly repaired original CDI box to its harness and cranked the engine. This time around bright and solid sparks were cheerfully zapping inside the spark tester. Pulled the spark tester out, reattached the high tension wires to the plugs and reattempted to start the tractor. The engine took some cranking, but eventually started and rewarded me with kitten purr. Such a stark contrast with the loud air-cooled Kohlers and the Onan in my previous tractors.

... to be continued.
 

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'81 Gravely tractor, 50's 60's 70's 80's 90's Gravely tractors Various Honda Power equipment
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Good job, you even re-soldered all of the other pin terminals.
 

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Discussion Starter · #4 · (Edited)
Thanks, folks.
The reason I decided to retouch every solder joint was their poor initial quality. IMO this board should have not passed the QA control at the factory.

The vias on this board are of the same size. This is probably because the manufacturer had decided to save some money on tooling by not drilling different vias for different through-hole components. The unfortunate consequence of this is that all vias need to be as large as the largest pin found in all of the components in this circuit. This in turn leads to some pins sitting in their vias as a pencil in a cup. In order for this board to make it through the production floor w/o components falling out in the process (this looks to be a hand soldered board, not machine re-flown) the pins needed to be bent. That's OK if the pins are either cut short (~1mm above the foil) or they are bent alongside the tracks, not across them. From the looks of it, the person who was manually populating this board with components either did not understand this or did not care, so some of the pins were bent across the tracks, and in a few instances were almost bridging the gap between the adjacent tracks, potentially creating a short. Here's one example thereof: the original portion of the PCB and the same portion after I had trimmed the offending pin with an Exacto knife and re-flown the joint.
Human body Wood Tints and shades Font Glass


Font Automotive tire Pattern Metal Close-up


The second issue with this board is the scarcity of solder on the joints. Granted, I am used to adhering to stringent MIL spec requirements in my day job, but this tractor also sees some harsh vibrations and temperature swings, especially if it sits all year long outside in a 4 season climatic zone. Even here, in the Mid-Atlantic, we often see +100°F in the summer and a few years ago we had a fortnight of -14°C in February if I recall it correctly. Vibration and thermal expansion/contraction cycles inevitably lead to solder joint fatigue and subsequent cracking. IMO there is simply not enough solder. Also, the original coating was rubber like substance which is OK for moisture insulation, but adding some hard clear varnish conformal coating for harsh environments IMO would've dramatically increased reliability of this board.
 

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I think I need to send you the circuit board for the instrument panel in my truck...
 

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'81 Gravely tractor, 50's 60's 70's 80's 90's Gravely tractors Various Honda Power equipment
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I think I need to send you the circuit board for the instrument panel in my truck...
It sounds like you have a GM truck, which have a failure rate of almost 100% with the circuit boards in the dash. Join the club. They are a little bit expensive and have to be reprogrammed at a dealership before they can be installed depending on what year you have.
Good luck with it, I had mine replaced for the same problem.
 

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It sounds like you have a GM truck, which have a failure rate of almost 100% with the circuit boards in the dash. Join the club. They are a little bit expensive and have to be reprogrammed at a dealership before they can be installed depending on what year you have.
Good luck with it, I had mine replaced for the same problem.
No, mine doesn't have a bad circuit board, just needs the pins to the LED display resoldered, as one or more of them has an iffy connection (sometimes it works, sometimes it doesn't, when it doesn't, I can usually make it work by pressing on the IP). I just haven't made time to do it myself, but I'm not nearly as good at soldering as the O.P...
 

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'81 Gravely tractor, 50's 60's 70's 80's 90's Gravely tractors Various Honda Power equipment
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No, mine doesn't have a bad circuit board, just needs the pins to the LED display resoldered, as one or more of them has an iffy connection (sometimes it works, sometimes it doesn't, when it doesn't, I can usually make it work by pressing on the IP). I just haven't made time to do it myself, but I'm not nearly as good at soldering as the O.P...
That's what happened to mine, the LED went out for the gear indicator because of bad pin connectors plus a few other gauges started having problems.
We had it apart and found the bad pins and felt it was better to get a rebuilt dash assembly from GM.
They improved all of the pin connectors because of that problem.
 

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Discussion Starter · #9 · (Edited)
Next I decided to test the compression, so I let the engine warm up to the working temperature before shutting it off. Pulled the plugs out and discovered that they were only finger tight. This explained the long cranking time: the compression must have been low because of the loose plugs.

The other thing that surprised me was the plugs' brand and model. They were Champion RN9YC whereas the specs were calling for NGK BPR5ES-11. I don't care much for Champion plugs and feel that NGK are vastly superior. I like NGK so much that I install them in my Audis instead of the OE Bosch. The other thing that disturbed me was the one step colder heat range of those Champions. This explained lots of soot on their skirts. Why would anyone install colder plugs in a low revving, low compression, low power atmospheric tractor engine is beyond me. The only engine where I had ever installed colder plugs was in the car that I tracked at club events. That engine ran on a 3bar Motoren Technik Mayer brain and on a cold day was able to reach 25psi of boost and would ping on anything less than Sunoco 94 so colder plugs were justified. In this little Honda OTOH the colder plugs had only led to excessive Carbon deposits since they never reached the temperatures needed to burn them off.
Wood Audio equipment Glass bottle Insect Wine bottle


Screwed in my compression tester into the 1st cylinder, opened up the throttle wide and cranked the starter. The compression in that cylinder had climbed to 170psi which was the maximum for a new engine. Wow! Not bad for a 29 y/o tractor.

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However, the 2nd cylinder had only reached 90psi which was below the acceptable minimum.

Since there were no leaks of coolant or oil on the outside, both plugs had equal amount of Carbon deposits, there was no foam on the dipstick and the coolant had no traces of oil in it I had ruled out the blown gasket. This left only two plausible explanations: a hanging valve or a stuck ring. Since I knew that the tractor had been sitting motionless for a few years I decided to test the stuck ring hypothesis first. I pulled the plug out of the 2nd cylinder, squirted some Kroil into the combustion chamber and let it sit overnight. Came back the next day, reinstalled the plug, ran the engine up to the operating temperature and retested the compression. This time around the 2nd cylinder had reached 170psi as well.

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It was a lucky guess indeed. It appears that this tractor has had very few hours on the clock despite it being only #53 of the first year of production.

Installed a pair of new NGK BPR5ES-11 specified by the manual and torqued them to 25Nm. From that point forward, with the proper plugs and the compression at the maximum for a new engine, the tractor has been consistently starting from a quarter turn.
 

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That's the best kind of repair, the kind where you can go to bed and have a nice rest, wake up, and it's all better.
 

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Actually back when I had the 4514 it had the same NGK plugs but one had gone totally bad and it ran on only one cylinder. Did much the same for troubleshooting because the machine gave me no indication at all the previous run (purring like a kitten). So I too feared the worst. I am sure BTW the plugs I pulled were the ones it was born with. I tried to find the proper NGK ones everywhere. No dice. I looked up a Xreference chart and it Xed to the same Champions you saw. I too did a compoosh test (though I have an aversion to pulling plugs out of a warmed up aluminum head) so I did mine cold. 220 PSI in both jugs. Threw in the Champs and it was back to purring away
 
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