“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!

Published by

tracey pooh

i like digital video and road cycling

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

  1. hello,
    don’t know if the blog still running anyway. 🙂
    i’m about de buy a CB400A. and i wonder what do you think about this bike.
    is it reliable bike? any big problem (except the one you explain on this page. how many miles ? )

    thanks a lot and keep on riding 🙂

  2. hi there!
    personally, I loved it. If you get a decent candidate one (look for oil leaks or signs of it especially and check all electrical would be my advice) they are pretty rock-solid and _very_ simple — both great things.

    So when we blew one of the three main gaskets (due to oil too full, age, and running hard on highway on a blistering hot day — bad recipe!) and took the engine apart, we found that the insides literally looked close to new (jaws hit floor). We had some carbon buildup on the piston heads (expected) and cleaned them and replace the piston rings (pretty standard practice) but the transmission below and all the rest seemed super solid. Look for rust too — just in terms of indicators of how it was cared for and body/parts integrity.

    I _did_ find every once in awhile, esp. taking off from a stop sign on hard left/right, gunning reasonably hard, the footshift gear up could slip (which was jarring) but that might have just been my bike. I dont think “automatic transmissions” (this is basically a two gear version of it) are expected to last 40+ years so maybe that’s something to just be aware of.

    Bikes this old will have little problems here and there. In the case of old carbureted single or dual cylinder bikes, though, they are so simple that most issues can be traced back and sorted. They dont have high tech computers in them regulating, and electrical issues can be sorted out.

    Hope that’s helpful — sorry for the delay!

    Keep on riding and most importantly stay “up” and safe out there!

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