CBX Stripdown - page 3
(Vol. 51 - January 1992)
Preparation and top end strip
Editor Bob Berry's Honda-6 engine was delivered
to Tadcaster-based expert John Wyatt for its rebuild.
What did John find upon stripping down the 33,000
mile motor? Brian Crichton's words and pictures
JOHN Wyatt has been working on CBX models for ten years.
He bought a new CBX in September 1980 and has it now - in
such pristine condition it has to be seen to be believed. Since
he started showing bikes four years ago John has won at least
one trophy for each bike he has restored.
Bob's engine is being stripped by John simply because he rang
us offering to do it after Bob mentioned in the magazine that he
had bought the bike. It wasn't the intention to have the engine
rebuilt at all - but such was the interest in the big Honda that we
decided to go ahead.
USUAL problems with the Honda six cylinder engine are:
- Burnt exhaust valves. If you use a bad six-into-one exhaust
system the exhaust valves will burn out. They can also bum out
with a standard system if the header pipes are not securely fixed
into the head and properly sealed. The motor then draws in air
and runs weak. This can happen on any cylinder.
- Oil consumption. All the original unfaired CBX models suffer
from high oil consumption at sustained high speed, especially at
over 100 mph. The cure is included in the stripdown story.
- Cylinder head gasket leak. Easily cured by the new, type
Honda gasket (Part No 12251422010). The new one has
adhesive around the oil gallery holes and around the cam chain
tunnel. A blown gasket can be a reason for high consumption
because the crankcase can be pressurised and blow oil out
through the breather.
- Head studs. Early engines could snap the tops off the
cylinder head studs when the engine was switched off after a
long hard run. This was caused by the extra heat from the
engine, deprived of cooling air, expanding the stud and
snapping it at the top.
Honda introduced stronger types of studs for the three
different lengths of studs. 220mm - part no 90031422003.
217mm - part no 9003442203. 222mm - part no 90039422003.
- Chain wear. The camchains wear, also the primary,
chain. This is normal wear, but Honda introduced an improved
chain, ie the top chain that joins the two camshafts for longer
service life. Average life for these chains (all three) is about
- Alternator brushes. They wear out extremely rapidly. You
can tell when the brushes are going because the voltmeter
starts to behave in an erratic manner. But even this isn't a
guarantee that the brushes are worn out. The only way to
properly check is to remove the alternator which is an easy job
(done in the frame) and check them with the best method known
to man - eyesight!
So, if you have a flat battery all the time or erratic charging,
suspect the brushes.
- Poor starting. Especially common if the bike has been left
standing for a long time. It’s inherent with all the CBX models.
John has never found a satisfactory reason for it but puts it
down to the vacuum fuel tap. It seems to take a lot of cranking
to fill all the fuel bowls. To alleviate this slightly, apply full choke;
knock the kill switch to 'off’; spin the engine until the oil pressure
light goes out; pump the accelerator pump by twisting the throttle
fully open half a dozen times; then leave the throttle off, turn the
kill switch to 'on', keep choke fully on, and within 15 seconds it
should start to fire. This may. not be immediately on all cylinders
but they soon fire up once two or three are going.
- Clutch chatter. An inherent problem that can be cured
completely. but it's expensive. It can be done by using a
different clutch outer off the CBX-B or C models (which don't
But first balance the carbs. This can cure up to 75 per cent of
the rattle usually. If it doesn't cure it completely, then the B or C
clutch outer, which features cush drive springs rather than
rubber is the answer, plus new rubbers on the primary chain
hydraulic tensioner oil feed.
On the non-B and C engines the rubbers tend to harden
because of the heat of the oil, and play develops in the clutch
outer which then causes it to start chattering - a horrible noise.
Also, the tensioner can crack or become hard and as a result
oil pressure is lost to the tensioner. Thus the primary chain is
not properly tensioned and this aggravates clutch chatter. With
worn rubbers pressure can go down from the direct 64-74 psi at
4000 rpm to about 30 psi.
- Gearbox. Fourth gear is a problem, mostly fixed under
warranty by now. If your bike shifts well, leave it alone. If it shifts
badly, the box will have to be inspected. The problem is with
fourth gear which will need replacing. On early engines the track
on this gear was not machined correctly and wears rapidly.
Some bad shifting can be put down to the gear pedal pivot
which wears rapidly on all machines. This takes only an hour to
fix and is very cheap. Replace the change pivot bolt (Part No
90101422000) and this should improve the gear change.
- Oil change. John recommends changing the oil filter every
2000 miles and the oil every 1000 miles to keep these engines
in top condition.
Some engines he has stripped (even as high mileage as
90,000 miles) have been factory, fresh because the oil has been
changed regularly. Other 20,000 mile engine without the same
treatment have insides that are black.
The CBX can suffer problems, especially if not cared for, but
generally there are very few real problems with this magnificent
and smooth six cylinder engine.