Honda ATC90 - 1977

Quick info on rebuild (not restoration):

The piston was seized, valves rusted, starter cord was seized, carburetor full of sand/grit and corroded internally and externally, fuel tank full of rust, no spark (points welded together and broken wiring at a few spots), broken headlight, and main headlight switch and kill switch not working. It has a "megaphone" type of aftermarket exhaust commonly found for sale now, but is older and is a straight pipe with megaphone/baffle cone on the end. The air box and filter assembly is missing and replaced with a cone shaped air filter/screen tucked under the left side cowling.

I ended up having to to find an old used barrel and cylinder head in order to be able to have the cylinder honed and new valves installed in the head. The barrel that was on the ATC90 had been overbored/bored out to some new capacity and was well beyond service limit, so replacement was needed. I replaced the starter pull cord and recoil spring with NOS parts. I rebuilt the carb (see below), points, condenser, and repaired wiring as needed to get spark and lighting working. The headlight switch had a broken solder connection at the switch for power, so that was soldered. The sub-transmission lever was frozen, so I had to remove/replace the lever. It now usually starts one pull, hot or cold, and runs great.

For the carb (a Keihin 556A), a K&L Jet/Carb Kit (stock replacement) - kit #39546421449 and Honda OEM carb kit 16010-102-305 which did not have jets but had the fuel screen and o-ring, petcock gaskets, float needle and seat, slide needle, etc. A combination of both kits enabled rebuild of the carb. The used barrel and cylinder I found also came with a carb of slightly better condition of emulsion tube and float bowl, so I assembled "one good carb" that took a lot of cleaning and is still suspect on internal passageways with the amount of corrosion noted. Ultimately the slide needle was set to 4th groove down from the top (stock is 3rd groove down). Idle air screw is set to 1 1/2 turns out (owners manual range 1 1/8 to 1 3/8, service manual range 7/8 to 1 1/8, Clymer manual 1 turn out). Float height (20mm) is stock setting. Idle RPM set to 1400 rpm (owners manual 1200 rpm, service manual 1300 rpm, Clymer manual 1300 +/- 100 rpm). These settings work best so far for my carb and my air filter and exhaust pipe and motor, as well as the carb condition and K&L Jet Kit, which could have very small differences in dimensions compared to original items replaced.

I am at sea level, temps for setup and testing ranged from 50 to 75 degrees, with 50-90% humidity. I tested over a several week period after arriving at settings (and left them alone after making final settings) to see how it started/ran.

Intake/Exhaust Valves .05mm clearance.

Points gap .014" clearance. I used the 6V "light bulb" method to set points opening timing at the "F" mark. I used a regular 6V flashlight battery with wires/clips/bulb, as well as Gardner Bender GCT-304 Continuity Tester light tool. Spark Plug NGK D8HS .028" gap.

Lisle 33500 piston ring installation tool.

Earthtec 751-045 ring compressor kit.

Gas Tank Cleaning - Evaporust for several days then rinsed thoroughly - cleaned out rust perfectly.

Remote digital tach for setting pilot air jet and idle speed.

Sources for NOS parts:

NOSPartsNow.com (various parts)

Common Motor Collective (CT90 gaskets)

NipponMoto.com (gaskets)

Hondanuts.com - Northeast Vintage Cycle (various parts)

DML Moto in Coos Bay did the cylinder bore/hone and valve job - excellent work!

CMSL worked well to confirm part numbers, both NOS and superseded.