ATC 90 Running Strong

Kevin Forth — Sat, 20 Feb 2021 21:08:59 +0000

After a few months of work, the ol’ ATC 90 is back in action. This was not a restoration, but rather a series of long-overdue repairs needed to get it operational again. Over the past few months, the following repairs were made:

Rebuilt engine top-end with new intake and exhaust valves
Honed cylinder (it was still within spec)
Replaced muffler with aftermarket unit
Replaced original ignition coil and condenser
Replaced original ignition points
Replaced starter recoil spring
Replaced gaskets and O-rings where necessary
Cleaned and lubricated the drive chain
Replaced Hand Grips
Replaced damaged headlight and chrome bezel with new/old stock parts
Replaced worn kill switch with new/old stock replacement
Replaced portion of wiring harness where rodent damage existed
Changed engine oil

Once again, it starts easily. It’s just as fussy on pavement as it ever was, since this machine prefers loose ground. It’s been in the family for four decades and was deserving of some attention.

Back on the trail
Still looks good
New recoil spring (left) and broken spring (right)
Recoil starter is back together
Oscilloscope confirms magneto primary voltage
New headlight and bezel (left) next to original at right
New hand grips
Worn intake valve at left next to new one at right
New muffler is test-fit before the cylinder and head are installed

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

atc – Forths.com

ATC 90 Running Strong

The ATC 90 Gets Some Attention