Please help! Honda problem is driving me nuts

 

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 more
subjective feeling 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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no guru here but have you replaced/removed the paper air cleaner element?
i' did have a problem with one that seemed to act 'funny" .
it may be worth trying removing for a test.
also, i have had a problem once or so with a cb750 single cam, that got carb
ice
in cold weather.
the idle circuits use air from a bit different source and they don't need
much air
to function.
good luck, sam

 

wrote:


I haven't tried no filter yet.  The air filter has been changed two or
three times since the problem first occurred - but that doesn't rule
out something hinky like a paper element that is hygroscopic and
swells when moisture is coalescing on the paper.

I'll have to try that too.  Thanks.

Carb ice is still possible.  I seem to remember that carb ice is more
likely at around 80 F - the air is able to retain enough moisture at
that temperature to cause icing and it is still cool enough to drop
below freezing under vacuum and fuel vaporization.  This problem
requires actual droplets of water and doesn't hurt idling, so while I
can't rule it out, I don't think it is likely - but the filter removal
might be a good test there too.

snipped

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Only suggestion(s), I'm not guru either.

It does sound like you've covered most everything, including
the air/box/cleaner subsequently mentioned.

But, no where in your water torture diagnosis(as I interpret
it) do you describe soaking anything above the tank. For
example, the electrics contained/possibly exposed in and
around the head, handlebars, switches, etc.

You're the EE, this should be a piece of cake.

I'm not real familiar with your bike, but am quite familiar
with Honda's. As such, they have a number of cutout
switches to control operation/safety.

. What about simply the right bar on/off switch triggering?
. Or, a similar condition on the starter button. It has a cut-
    out circuit to facilitate starting. Although not your problem
    almost all the old Valkyries have problems with this start
    button, as does mine.
. Where's the ignition key, have you drowned that?
. And, last but not least, if your model has one of those nifty
    kickstand down switches(sarcasm noted), perhaps the
    stand circuit is acting up.

Yeah, I know - "Well everything mentioned would simply
cause the entire ignition circuit to quit". And that's exactly
what I would think, but unless you've covered these items,
I wouldn't write them off.

The only other thing I can think of is to check the tank
venting to see if it's somehow sucking in the moisture.
I've never heard of something like this, but your comment
about a 5-15 second restart doesn't sound electrical.

You might also visit www.vjmc.org for consultation.

Good luck, and post your conclusion(s).

 



I didn't mention water above the tank but did douse it there.  The Low
Voltage circuits should be able to withstand being immersed in water
and still work until corrosion brought on by electrolysis ate the
wires - and the LEDs I added should wink out if that's the case.  It
did occur to me that perhaps something expanded when wet causing the
primary ignition circuit to open - hence the LEDs.  Two LEDs are "AC
coupled" they can only blink when a pulse on the primary of each coil
is present at ~100-400 volts.

That was my opinion three years ago - electrical and piece of cake.

Added leds monitor those circuits and applied water doesn't affect it.

I keep an open mind when troubleshooting and have revisited the
electrical from time to time as ideas occur to me.  I think those 5
leds should be pretty conclusive even if I hadn't already replaced
parts.  One monitors the kill switch circuit, two the positive voltage
to the trigger modules, and two the coils themselves for pulse
activity.  Only thing left there might be the ground connection at the
modules themselves.

Tank venting . . . yeah that was one of my brain storms. Figured the
tank might be filled with warm vapor and gas - hit rain and it cools
and contracts, pulling a vacuum and prevents gas from going to carbs.

I vented the tank with a toothpick to prevent the rubber from sealing
and added clear plastic overflow/drain tubes to the carb float bowls
to check for water by opening the drain valves - no water.  And I
suspect it wouldn't cause all four to drop out - running with the
petcock off causes cylinders to drop out one at a time over a mile or
so.

If and when . . .

Nothing is sacred - if it was easy or immediately obvious it would be
fixed by now.
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