Running lean

[Good old number 29 158]

It’s getting to be time to rebuild or replace the tired, old, original 503 in my Kitfox 1. I freshened it up with new crank end bearings, when I bought it, at 196 hours. As of today, it’s at 480 hours, so it’s really close to the recommended 300 hours for a complete crankshaft overhaul. We didn’t separate the crank and replace the center bearings or big end rod bearings, when we did the outers, so it definitely needs to have the whole thing done. 
Aside from that, it has lost compression, due to a lean running event, this fall. I took a long cross-country trip to interior Alaska. I had the prop pitched for cruise, and jetted accordingly, but after an overnight in Talkeetna, I took off at -10°, saw over 1,200 on the EGT gauges, easing off the throttle to get cooler temps, but the damage was done. Since the EGT probes are calibrated at 70°F (a fact I had forgotten), that adds another 80° (the difference between 70° and -10°), meaning the temps were actually in excess of 1,300. Yikes! 
Anyway, it was a great trip and I made it home safely, but the loss of compression is fairly obvious. The prop keeps turning the engine over for a couple revolutions when I shut it off. I had to pitch the prop down to get enough RPMs for takeoff, but the takeoff run increased from 250 feet to 500, due to the lack of power. A compression gauge has both cylinders around 91 psi, which I’d consider to be low for this engine, but I don’t know what it was, before. 
After that incident, I pulled apart the top end and de-carboned the cylinders. No cylinder scuffing or melted pistons, so I put it back together. I might have honed the cylinders and replaced the rings, and possibly the pistons, but I know it’s time to do the crank, so I didn’t want to do the top end without taking care of the rest. 
While I had the engine off the plane, I also checked the points and changed the gear oil in the B box. Timing was in the middle of spec, but .005” different between cylinders, so I opened the points back up to max advance, the same on both cylinders. The oil in the gear box did not have any metal in it, but was dark, which it had not been at any other time. Having considered this awhile, I concluded that it was most likely caused by blow-by past the pistons, pressurizing the crankcase and pushing carbon past the crank seal, into the gearbox. 
So, like it or not, it looks like time to take care of the old girl. It is the original engine, ordered with the plane in late 1984. I do want to take care of it and keep it all together, but it is underpowered and a single ignition, so I’m considering an upgrade. Unfortunately, my pocketbook is also running lean, so options are very limited.
My first choice might be an MZ202, with less weight and more power, but a new one, at ten grand, is out of the question. New 582 engines are apparently still available, but similarly expensive. I like the Yamaha Phazer conversion, which would require more work on my part, but could be done for less than a new 582, if I was good about it. I’m wary about purchasing a used engine from someone I don’t know, but a provision 8, dual ignition 503 or 582 would be an improvement, with the bigger crank bearing and dual ignition. Or a 670, for that matter. 
For the cost, I might just rebuild the original motor and  run it, but it seems like a good time to upgrade. If anyone here has something available, that would be suitable, let me know. 
And, as always, be careful about running too lean.

[1avidflyer 1983]

What does your plane weigh empty?  An HKS would put out at least as much power as the 503.  Jabiru would do a lot more than the HKS.  Both would need a different motor mount though.  But a 582 might as well.  JImChuk

[Good old number 29 158]

It’s about 450 lb. Those would be great options. I kind of like the idea of going to a four stroke. 

[akflyer 1427]

Just seeing this.  I know Bob has a 503 or two sitting in the garage as well as a few 582s and 670s.  His son will be wanting to sell this stuff off in the near future.  LOTS and LOTS of KF parts, frames, wings etc from mod 1 -4.

[Good old number 29 158]

After pulling apart the top end and taking some careful measurements, it appears that the bore is well within spec, but the pistons are worn past maximum spec for piston to bore clearance. 
The original pistons are stamped 71.92 mm (not the 7.93/7.94 red dot/green dot spec), but measured at 71.755mm. The bore measured at 71.99 mm, still slightly under the original 72 mm spec. That put the piston to bore clearance at .235 mm, which is more than the maximum allowable clearance of .2 mm, and well beyond the .07-.09 mm build spec.
Ring end gap was at .65 mm, still less than the 1 mm max, which seems huge. The rings were a bit eaten up, with surface pitting particularly noticeable on the top rings of each piston. The exhaust ball joint had also broken apart, ending up in the muffler. Remember where it says to lube that with high-temp antiseize? Another good thing to remember. That allowed a minor exhaust leak at the y-pipe, which is likely to have contributed to burning out the rings.
There is minor scoring in the cylinders, only below the exhaust ports, just enough to catch a fingernail. 
I believe that the loosened tolerances of the well-worn engine helped to prevent a seizure during the high temp takeoff. Prior to my ownership, it had once been hot-seized on takeoff, caused by an improperly installed fan. The owner made the impossible turn back to landing. 
All things considered, not bad for an engine that’s going on 39 years since manufacture, and 500 hours. 
I’m tempted to hone the cylinders and just replace the pistons and rings, but that leaves me questioning the center crank bearings and big end rod bearings, which are still original. Even more to think about, since it is an early Provision 4 motor, with single ignition. 
It sure has been a good engine. I have to say that the simple, lightweight engines, like this 503, have really been underrated. People laugh when they hear it has 50 horsepower, but lots of planes were built to fly with that much power, and lots of them are still around. There might be something to it. 

[Dusty 126]

The 503 is an awesome motor. My old 503 was stripped and inspected at 600 hrs and released for service. At 1200 hours it is on condition and still going strong for the new owner of my trusty drifter