Advice on Honda MC problem?

 

I've tried everything I can think of with this problem and have a fair
degree of confidence that it isn't ignition related.

I have a 1981 vintage Honda dohc CB750C.  It won't go in the rain.
The problem occurred gradually over time
with first one cylinder cutting out when wet then two now all four -
and they pretty much die all at once.

The problem is worse when cold or occurs faster when the engine hasn't
reached operating temperature.  
Choking sometimes seems to help (but not always, and that may be my
subjective feeling rather than fact).

It runs for a bit after the rain starts, but once it starts cutting
out, it is likely to fail again with little or no delay.  
May spit and sputter for a second or two then acts like the ignition
was just switched off entirely.  Once it is having the problem, even
the mist from a wet road thrown up by trucks will cause a failure.

If the bike is stopped it can't be restarted immediately, but will
restart in 5-15+ seconds or so, and idle fine and rev, but
won't accelerate and will cut out again.

No amount of water poured over the engine with the tank removed will
cause the problem.  The bike will
continue to idle and will rev fine.  I've hit it with a spray bottle
in all the nooks and crannies, a garden hose and
even dumped buckets of water on the running engine.  It has to be
under load, and wet before it dies.

Hoses are all good and most replaced.  The airbox is good, the
connections between carbs and cylinders and box
are good.

All the usual ignition suspects have been replaced - coil wires, both
coils, spark plugs, boots.  I haven't changed
the spark modules but have added 5 LED's to tell me that the system is
getting voltage and the modules are firing
when they should (even if the problem is there).

The intake is stock, the exhaust is a 4 into one low restriction
header/muffler.  I added the after market exhaust
after the original exhaust got a few holes in the mufflers.

I recently rebuilt the carburetors & accelerator pump, with basically
no change in anything except it needs to be
synchronized now, and it vibrates a little more.  (ordered four vacuum
gauges and will build a carb synchronizer when
the parts get here).  BTW: no need to drop $100 on a vacuum
synchronizer - four new vacuum gauges only cost $21.

The bike seems to be running lean judging from the color of the plug
insulators - but doesn't overheat noticeably.  But I did experience
some vapor lock problems on 100 degree days when sitting at a traffic
light . . . that could be heat or gasoline related.

I plan to drill out the main jets a tad when I can locate some
suitable drill bits.  The local stores only have
fractional sizes and I want to start with something smaller than
1/16".  (still have the old jets from the rebuild so
I'm not burning any bridges - current jet size is stock: 1.02mm.)

My training is in electronics but I've been riding and tinkering with
motorcycles for 40+ years, and have rebuilt a
few engines in that time. - valve jobs, cylinder bore, rings,
transmission bearings, rewound alternators, built voltage regulators,
fork seals, etc..

I don't know how many total miles are on the bike.  It was bought by
someone who fancied themselves a MC
rider - crashed once then sat in a garage for 15 years.  I bought it
from the second owner, who rode it very little,
and it worked fine for about three years for me.  In spite of its age
the mileage is something on the order of 10K miles - odometer died in
original crash.

I'm pretty much convincing myself that this is related to lean running
as a result of the original muffler having holes and now the new low
restriction muffler.

Any guru out there with some other suggestions, things to try, and
would lean running cause a problem like this?
--


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Have you considered the possibility of carburetor icing? I've got
a tlr200, and it quit on me in the rain. Two or three times.

Some bikes, in particular sv650s in England, seem to come with
electrical carb heaters. Could your bike be equipped with heaters
that are now quitting?

bob prohaska

 

On Mon, 04 Aug 2008 05:15:51 GMT, bob prohaska's usenet account


Sure I considered it - I'll grasp at any straws on this one. There's
no perceptible signs of it (like butterfly valves hanging up or
getting hard to operate)

I have no good way to rule out carburetor ice - can't look into the
carburetor bores when the bike is all put together (or going down the
road).  From my flying lessons, they said carb ice is most likely to
occur at ~80 degrees with humid conditions.  This problem doesn't
occur with just humid air - it requires falling water or a very heavy
mist (fog isn't enough - BUT fog does require a longer warm up for
smooth running)

No provision for carob heat on the bike.

And it did work for years without a problem in rain . . . although gas
mixtures could have changed in that time, affecting the cooling of the
intakes.  (so that isn't positively ruled out)

All ideas appreciated . . .
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In another post, it was suggested that you have a low-voltage problem around
the "dash board".
I concur with that general idea.  You need to inspect ALL connectors and
connection points, looking for corrosion......often green.  That corrosion
attracts moisture like a magnet.
Give special attention to ground (frame) connections in your inspection.

 


Grasping at straws, I've hear of paper air cleaners getting wet and
over-richening the mix til the motor quit - mostly on cages - but never
actually seen it.

I more expect rain water is getting into somewhere you havent thot of yet.
Rain can get places you'd never expect or find whit a garden hose or spray
bottle.  I've had turn signals come on in a hard rain!  It's prolly
something simple yet stupid. After years of no problems, my H-D began
refusing to start if left out overnite in the rain. I finally found that a
wrench who'd done some warranty work had installed the ignition 'black box'
upside down with the wires coming out the top instead of the bottom. Hooda
thot? I flipped it back and never had the problem again.  Electric shit is
weird!

I can't imagine anything rain could do to an otherwise good running fuel
system. Supercharged cars (used to?) inject water into the intake to prevent
detonation.