ATC 90 Engine Work

Kevin Forth — Thu, 12 Nov 2020 16:59:15 +0000

The ATC 90 engine work continued today, focusing on the head. I took the carbon-caked valves to the wire wheel, which made easy work of removing the carbon. While the exhaust valve looked good, the intake valve face was too worn to be used again. A new pair of valves was available online for under $20, so I opted to order them instead of reusing the original pair.

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.

Intake valve face is too worn to use
Rotary tool cleans carbon
The holey muffler will be discarded
Combustion chamber is cleaned
New exhaust is test-fitted

The ATC 90 Gets Some Attention

Kevin Forth — Thu, 29 Oct 2020 01:40:42 +0000

For 4 decades, our Honda ATC 90 has loyally served the family, providing 4 season fun and utility for all ages. For the past few years, however, it’s grown harder to start. This past Sunday, I towed the machine down-state for some long overdue repairs.

Grandpa and Ryan on the ATC 90 in 1988

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.

80 psi is not good
Camshaft looks ok
head is full of carbon
will take cylinder measurements soon
Piston will be examined more closely
Poor little beast

Heavy carbon layer is likely the cause of the low compression
Aside from carbon deposits, head is in good condition

honda atc 90 – Forths.com

ATC 90 Engine Work

The ATC 90 Gets Some Attention