“Save the Decaf!” — rebuilding and renewing the engine of 1978 CB400A Honda motorcycle (named Decaf)

Saving my 1978 Honda CB400A “Decaf” bike from a near fatal blow — lower gasket blowout and huge oil leak! (like, dumped a quart+ of oil in minutes while riding fast on highway at high temps). It was inadvertently overfilled with oil (my bad) and driven too hard (twin brother’s not so good) combination that killed the Beast!

Work with my twin brother (a very gifted self-and-internets-taught car and motorcycle moonlighting mechanic!), I as assistant especially for the entire reassembly and much of the cleaning work.

We had to take the top 3 layers off the engine stack (AKA “complete top-end rebuild”),
remove the pistons, and strip all the way down to the gearbox.
Layers: [valve cover] gasket [valves] gasket [cylinders and pistons] gasket [gearbox and crankcase]
Very neatly Honda makes them all stacked and connected via 8 super long screws that compress, when properly torqued, the entire set under the valve cover.

We:

  • replaced the piston rings (very thin split rings circling the pistons that make them “float” up/down the cylinders),
  • re-honed the cylinders (you use a “flexible hone tool” to clean, slightly resurface, polish and add a light “cross-hatching pattern” to them),
  • cleaned the pistons completely (lots of carbon buildup after decades!) and installed 3 new gaskets — especially paying attention to the lowest gasket that tore and blew oil using some “gasgacinch” sticky product around the oil jets).

Then we did a full valve job (gets the valves ideally aligned to the 4-stroke engine stages),
timing belt adjustment, oil change, and more.

In addition, the air filter was trashed, so replaced that and fashioned an air filter custom holder to keep it from slipping back into the air box.

Bonus points: exhaust and carbeurator gasket replacements.

About $150 in parts (piston rings alone were OEM Honda and $75+), but probably the equivalent of $800-1000+ in labor.

A challenging and very educational labor of love!

Fixing 1978 Honda CB400A always on neutral light — “decaf” project bike

I’m about to sell, with some sadness, my 1978 Honda CB400A “Hondamatic” motorcycle.
I have one last problem with it — that’s been baffling me for over a year.

When I consolidated the speedometer and “idiot lights” to a svelte new cafe racer style chrome LED-based single instrument, I found that its neutral light was always on, no matter what gear.

So I finally figured out a clever (if I may say so) way through research to use the “proverbial $7 part” from in-stock Radioshack — a RELAY switch!

Here, I describe and show the problem, step through a bit of the bike’s electrical system, and show you how I used a relay to fix the problem.

I’ll be sad to see you go, little Decaf!

The *awesome* video learning tutorials I used on Youtube to determine relays were for me!:
RELAYS – How to wire and how they work TUTORIAL !!
How An Automotive Relay Works and How to Wire ‘Em up
(thanks, guys!)

decaf front brake fix — fluid flush and bleed — 1978 Honda CB 400A

The front brake of my 1978 Honda CB400A (baby cafe racer — AKA “decaf”) went almost dead.

Here, we fix it in about 30 minutes — my first time doing more than just an oil change or electronic reworkings! ūüėé

We assume somehow my brake fluid has deteriorated/aged from viscous fluid to closer to water (this naturally happens over time) and/or somehow has taken in air (which will sack your brakes entirely).

We open my brake master cylinder, inspect, flush clean brake fluid through (making sure to never get air into the line — AKA “brake bleeding”), and see how the results are.

today I did my first oil and oil filter change, ever! (Honda CB400A “decaf”)

Russ and I changed our motorcycle’s oil and oil filters today. ¬†This was the first time I’ve ever changed oil in a vehicle — I guess it sort of was a life goal I figured I’d never get around to — part of understanding cars/bikes more! ¬† So it was a nice way to kick off the start of our 3-day holiday weekend.

The actual work on my bike was quite simple:

  • 1 ratchet-able bolt as the drain plug to release most of the used oil into oil drip pan
  • 1 ratchet-able bolt underneath to release the bike’s oil pan and oil filter section

 Russ helped figure out those bolts which I could confirm with a shop manual PDF.  Harder was sorting out what oil and filter to get/use and getting them.  I went with:

  • Valvoline SAE 10W-40 Motorcycle oil (for 4-strokes; non-synthetic; ideal for wet clutches) (AutoZone/Walmart)
  • K&N KN-111 oil filter (local motorcycle shop)

Putting everything back couldn’t be easier — slipped on new filter to bolt/cover, ratchet-ed the two bolts back in, and poured in ~3 quarts of oil. ¬†I was happy parts were all in good shape and aside from some relatively dirty oil, no badness or surprises for my 35 year old bike. ¬†Phew!

my first oil change
my first oil change
removing bottom bolt and filter
removing bottom bolt and filter
garage work overview
garage work overview