A tiny wire wheel mounted to a rotary tool took care of the carbon removal from the combustion chamber. Gasket mating surfaces were cleaned too. The original exhaust pipe and muffler were long overdue for replacement. A hole in the bottom of the muffler was visible next to an old “temporary” patch. The original exhaust parts will be discarded since the aftermarket exhaust arrived last week.

Once the new valves arrive I will continue to reassemble the engine. Until then, I’ll clean and re-lubricate the chain and throttle cable. I’d like to see the ATC 90 back in good running condition before the snow flies.

When diagnosing engine issues, the best place to start is with a compression test. I suspected that cylinder compression was not up to spec, so a compression test was the first step. The specification is 142 – 170 psi. I connected the compression tester, opened the throttle completely, and pulled the starter rope several times. The gauge topped out at 80 psi … not great, as expected. The engine would need some top-end attention.

I pulled the engine head and cylinder for closer inspection. A late evening meeting with work put an end to the analysis, but I was able to get the top end disassembled. As expected, the cylinder head combustion chamber had a heavy carbon build-up. The cylinder didn’t look too bad, and the piston rings were not frozen into the grooves. I’ll look at the parts more closely over the next few days and determine what I need to order. I have already ordered a new exhaust pipe and muffler, mounting studs for the exhaust, a gasket set, and a few miscellaneous parts. Not too bad. I have no plans on tearing apart the bottom end of the engine at this time.

I removed the piston and took measurements. Both the piston and the cylinder are within spec, and visually look good. Therefore, I currently plan on reusing the piston. I’ll lightly hone the cylinder prior to assembly. The valves were heavily caked with carbon deposits, but can be cleaned, lapped and used again.

This is a good start toward getting the ATC 90 back in service.

Typical for Honda engines of the period, there is no external oil filter, but instead a centrifugal oil “slinger” which removes particles from the oil by spinning a finned chamber at high speeds, causing the particles to be thrown to the outside of the chamber instead of circulating through the engine.  There was a heavy accumulation of crud in the chamber when I opened it, which required heavy scraping to remove.  I suspect that this chamber wasn’t cleaned with any regularity, if ever.

I have yet to clean all the head parts, but so far I’m pleased with their condition.  The valves look great, as do the valve seats.  At this point I plan to clean and lightly lap the valves into their seats during reassembly.  The valve guides are also in excellent condition.  After taking measurements with a micrometer, the camshafts are all within spec, and at the high end of the range, too.  I already have a new camshaft chain, so I didn’t bother measuring the original.

I’ve removed the iron cylinder sleeves, and will simply retire them.  I’ve ordered new forged pistons and iron cylinder sleeves, which should arrive by the end of the year.  The engines cases, cylinder block and head still need to be cleaned and either bead blasted or vapor blasted to return them to original appearance, after which I’ll have the cylinders machined and honed to fit the new pistons.

These engine rebuilds are lot of fun.  I’ll post updates as progress is made.