OT motorcycle won't work in rain, any pilots out there?

[default]

I posted the problem with my 81 Honda back in April and it has only
just rained once since then, so I took the bike out to see if the new
wires and spark plugs and caps helped.

I did eliminate one possibility before the rain stopped - venting the
fuel tank didn't correct the problem so it wasn't a vacuum forming in
the fuel system.

This time I was starting out with a cold engine, full tank of gas, and
it was really raining hard. I didn't go more than two blocks before
the engine started running rough, applying choke caused it to smooth
out and run normally, turning off the choke caused it to run rough
again, that worked through several cycles. Eventually I let it get
too rough and it died. I coasted for about 30 seconds engaged the
engine and it fired right up. This seems to work about as well as
stopping to rest - engine will always start after a short rest.

I should have tried riding at very slow speed to see if it would keep
going - but didn't think of it when I had the chance.

No amount of spraying water over the engine, ignition etc.. will cause
it to fail in the driveway. I can dump buckets of water on the
ignition and engine without it dying. Even a mist of water from a wet
road will cause it to die and rain causes it to die faster.

At this point I'm almost ready to give up on an ignition problem since
all the ignition except the transistor switches and pick up coils have
been replaced. And the likely hood of two independent electrical
systems failing seems too extreme. All the electrical parts have been
drenched with water and it runs if the bike is stationary.

My latest and greatest theory is carburetor icing. From what I read,
the conditions are favorable - humid between 20-70F sometimes up to 80
F. The problem is more likely to occur when the engine is cooler. It
is more likely to occur when backing off on the throttle.

Where it doesn't fit the theory is that using the choke should
exacerbate the problem - it doesn't seem to, and while the engine is
relatively cool and the problem is occurring it actually helps. When
the engine is at full operating temperature the choke is less likely
to help it.

I can't get the problem to occur when the bike is stationary, it has
to be moving through airborne water in the form of rain or mist (we
don't get enough fog here but I suspect a heavy fog might kill it
too).

My engine is adjusted too lean according to the color of the spark
plugs. With "normal" carburetor icing this shouldn't exacerbate the
problem - according to what I read. I didn't change the jets but did
put in a less restrictive exhaust when the old one had holes in it -
the problem did occur with the old exhaust but with holes in it so I
can't say for certain mixture isn't involved somehow.

Any one with more ideas on troubleshooting this problem? Can it be
carb ice with richening the mixture making it better? All I read says
it should get worse as the mixture is rich, since they assume that the
airflow is too restricted. But as the mixture is made rich, I could
see where there may be less evaporative cooling from the gas since the
gas should evaporate slower with humidity and gas concentration
rising.

Have gas formulations changed in some way that might account for it?

I used to be able to keep going in torrential 30 foot visibility
without it missing a beat - now it won't go more than two blocks in
rain.

Once at highway speed, especially when going fast it seems less
susceptible but backing off the throttle then, in rain, will almost
always kill the engine.

 

[Jan Panteltje]

I used to be able to keep going in torrential 30 foot visibility
without it missing a beat - now it won't go more than two blocks in
rain.

Does it have an air filter? If so did you ever replace or clean it?

 

[default]

Does it have an air filter? If so did you ever replace or clean it?

Will check it tomorrow. I have a couple of new ones I'll change it
and wait for rain. It is a relatively large pleated paper filter -
but yeah, I haven't checked it in awhile.

Thanks

 

[Jim Thompson]

I posted the problem with my 81 Honda back in April and it has only
just rained once since then, so I took the bike out to see if the new
wires and spark plugs and caps helped.

I did eliminate one possibility before the rain stopped - venting the
fuel tank didn't correct the problem so it wasn't a vacuum forming in
the fuel system.

This time I was starting out with a cold engine, full tank of gas, and
it was really raining hard. I didn't go more than two blocks before
the engine started running rough, applying choke caused it to smooth
out and run normally, turning off the choke caused it to run rough
again, that worked through several cycles. Eventually I let it get
too rough and it died. I coasted for about 30 seconds engaged the
engine and it fired right up. This seems to work about as well as
stopping to rest - engine will always start after a short rest.

I should have tried riding at very slow speed to see if it would keep
going - but didn't think of it when I had the chance.

No amount of spraying water over the engine, ignition etc.. will cause
it to fail in the driveway. I can dump buckets of water on the
ignition and engine without it dying. Even a mist of water from a wet
road will cause it to die and rain causes it to die faster.

At this point I'm almost ready to give up on an ignition problem since
all the ignition except the transistor switches and pick up coils have
been replaced. And the likely hood of two independent electrical
systems failing seems too extreme. All the electrical parts have been
drenched with water and it runs if the bike is stationary.

My latest and greatest theory is carburetor icing. From what I read,
the conditions are favorable - humid between 20-70F sometimes up to 80
F. The problem is more likely to occur when the engine is cooler. It
is more likely to occur when backing off on the throttle.

Where it doesn't fit the theory is that using the choke should
exacerbate the problem - it doesn't seem to, and while the engine is
relatively cool and the problem is occurring it actually helps. When
the engine is at full operating temperature the choke is less likely
to help it.

I can't get the problem to occur when the bike is stationary, it has
to be moving through airborne water in the form of rain or mist (we
don't get enough fog here but I suspect a heavy fog might kill it
too).

My engine is adjusted too lean according to the color of the spark
plugs. With "normal" carburetor icing this shouldn't exacerbate the
problem - according to what I read. I didn't change the jets but did
put in a less restrictive exhaust when the old one had holes in it -
the problem did occur with the old exhaust but with holes in it so I
can't say for certain mixture isn't involved somehow.

Any one with more ideas on troubleshooting this problem? Can it be
carb ice with richening the mixture making it better? All I read says
it should get worse as the mixture is rich, since they assume that the
airflow is too restricted. But as the mixture is made rich, I could
see where there may be less evaporative cooling from the gas since the
gas should evaporate slower with humidity and gas concentration
rising.

Have gas formulations changed in some way that might account for it?

I used to be able to keep going in torrential 30 foot visibility
without it missing a beat - now it won't go more than two blocks in
rain.

Once at highway speed, especially when going fast it seems less
susceptible but backing off the throttle then, in rain, will almost
always kill the engine.

Vacuum leak screwing up distributor advance? Change out all your
hoses.

...Jim Thompson

 

[default]

Vacuum leak screwing up distributor advance? Change out all your
hoses.

No vacuum to distributor, no distributor. Two independent ignition
systems firing four cylinders - wasted spark on one cylinder when the
other is firing.

Only one external vacuum line and it is used to operate a fuel shut
off valve so that gas can't flow down (gravity) unless the engine is
turning over or running (vacuum turns on the valve).

 

[Jeff L]

I posted the problem with my 81 Honda back in April and it has only
just rained once since then, so I took the bike out to see if the new
wires and spark plugs and caps helped.

I did eliminate one possibility before the rain stopped - venting the
fuel tank didn't correct the problem so it wasn't a vacuum forming in
the fuel system.

This time I was starting out with a cold engine, full tank of gas, and
it was really raining hard. I didn't go more than two blocks before
the engine started running rough, applying choke caused it to smooth
out and run normally, turning off the choke caused it to run rough
again, that worked through several cycles. Eventually I let it get
too rough and it died. I coasted for about 30 seconds engaged the
engine and it fired right up. This seems to work about as well as
stopping to rest - engine will always start after a short rest.

I should have tried riding at very slow speed to see if it would keep
going - but didn't think of it when I had the chance.

No amount of spraying water over the engine, ignition etc.. will cause
it to fail in the driveway. I can dump buckets of water on the
ignition and engine without it dying. Even a mist of water from a wet
road will cause it to die and rain causes it to die faster.

At this point I'm almost ready to give up on an ignition problem since
all the ignition except the transistor switches and pick up coils have
been replaced. And the likely hood of two independent electrical
systems failing seems too extreme. All the electrical parts have been
drenched with water and it runs if the bike is stationary.

My latest and greatest theory is carburetor icing. From what I read,
the conditions are favorable - humid between 20-70F sometimes up to 80
F. The problem is more likely to occur when the engine is cooler. It
is more likely to occur when backing off on the throttle.

Where it doesn't fit the theory is that using the choke should
exacerbate the problem - it doesn't seem to, and while the engine is
relatively cool and the problem is occurring it actually helps. When
the engine is at full operating temperature the choke is less likely
to help it.

I can't get the problem to occur when the bike is stationary, it has
to be moving through airborne water in the form of rain or mist (we
don't get enough fog here but I suspect a heavy fog might kill it
too).

My engine is adjusted too lean according to the color of the spark
plugs. With "normal" carburetor icing this shouldn't exacerbate the
problem - according to what I read. I didn't change the jets but did
put in a less restrictive exhaust when the old one had holes in it -
the problem did occur with the old exhaust but with holes in it so I
can't say for certain mixture isn't involved somehow.

Any one with more ideas on troubleshooting this problem? Can it be
carb ice with richening the mixture making it better? All I read says
it should get worse as the mixture is rich, since they assume that the
airflow is too restricted. But as the mixture is made rich, I could
see where there may be less evaporative cooling from the gas since the
gas should evaporate slower with humidity and gas concentration
rising.

Have gas formulations changed in some way that might account for it?

I used to be able to keep going in torrential 30 foot visibility
without it missing a beat - now it won't go more than two blocks in
rain.

Once at highway speed, especially when going fast it seems less
susceptible but backing off the throttle then, in rain, will almost
always kill the engine.
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That sounds like icing in the carb is a possibility. It is possible that
frost buildup in the carb can plug off or cover up a jet, causing it to run
lean (or rich). It depends on the carb design and where the frost is
forming. Cool damp air is when carb icing normally occurs, especially since
the engine is also cooled by the cold air, so the carb does not heat up much
from the engine. Were any air ducts or fairing removed / modified / added?
Partial constant throttle just after idle is when most icing occurs. Proper
jetting is one fix, as is getting heat to the carb. Some carbs (mostly a few
snowmobiles and a few rare outboards) have electric heaters built in.

To throw another wrench into things, a rich mixture can be easier to ignite,
so the choke could also be compensating for a weak ignition system. The next
time it fails, pull a plug wire before it cools down and stick a spare park
plug on the wire and crank the engine over to check the spark intensity.

 

[mpm]

I posted the problem with my 81 Honda back in April and it has only
just rained once since then, so I took the bike out to see if the new
wires and spark plugs and caps helped.
Weird.
From what you say, it definitely seems to be caused by weather.??

I know you said you drenched the engine while running and it would not
stall.
I'm curious if you ever tried reving the engine and then aspirating it
with saturated (wet) air?
Then let off the throttle.

If that proves useless, then perhaps it is a combination of things.
Moist air, engine load, and physical weight on the bike.
Also, what altitude are you at? Could that be a variable here?

I'm not a bike mechanic, but had my share of Yahamas in college.
(Not enough parking on campus for cars, etc.. You know the drill.)

Good luck!

 

[Jeff L]

=----



That sounds like icing in the carb is a possibility. It is possible that
frost buildup in the carb can plug off or cover up a jet, causing it to run
lean (or rich). It depends on the carb design and where the frost is
forming. Cool damp air is when carb icing normally occurs, especially since
the engine is also cooled by the cold air, so the carb does not heat up much
from the engine. Were any air ducts or fairing removed / modified / added?
Partial constant throttle just after idle is when most icing occurs. Proper
jetting is one fix, as is getting heat to the carb. Some carbs (mostly a few
snowmobiles and a few rare outboards) have electric heaters built in.

To throw another wrench into things, a rich mixture can be easier to ignite,
so the choke could also be compensating for a weak ignition system. The next
time it fails, pull a plug wire before it cools down and stick a spare park
plug on the wire and crank the engine over to check the spark intensity.

You may be able to mimic the icing conditions in your driveway by applying
a constant light throttle (high idle - say 3000+ rpm) for a few minutes
while misting the air intake area with a garden hose.

Also note that when the carb warms up, the ice melts, and things go back to
normal. You may try some (eg a bottle or two in the tank) gas line
antifreeze in the tank for an added performance boost, and to help prevent
ice from forming in the carb. Note that if the bike is running lean, this
may lean it out more.

 

[GPG]

I know you said you drenched the engine while running and it would not
stall.
I'm curious if you ever tried reving the engine and then aspirating it
with saturated (wet) air?
Then let off the throttle.

If that proves useless, then perhaps it is a combination of things.
Moist air, engine load, and physical weight on the bike.
Also, what altitude are you at? Could that be a variable here?

I'm not a bike mechanic, but had my share of Yahamas in college.
(Not enough parking on campus for cars, etc.. You know the drill.)

Good luck!

Check/replace the CDI charging coil.

 

[Homer J Simpson]

This time I was starting out with a cold engine, full tank of gas, and
it was really raining hard. I didn't go more than two blocks before
the engine started running rough, applying choke caused it to smooth
out and run normally, turning off the choke caused it to run rough
again, that worked through several cycles. Eventually I let it get
too rough and it died. I coasted for about 30 seconds engaged the
engine and it fired right up. This seems to work about as well as
stopping to rest - engine will always start after a short rest.

Fuel filter?

--
..

--
..
..
..
..
..
..
..
..

--

 

[Roger Hamlett]

I posted the problem with my 81 Honda back in April and it has only
just rained once since then, so I took the bike out to see if the new
wires and spark plugs and caps helped.

I did eliminate one possibility before the rain stopped - venting the
fuel tank didn't correct the problem so it wasn't a vacuum forming in
the fuel system.

This time I was starting out with a cold engine, full tank of gas, and
it was really raining hard. I didn't go more than two blocks before
the engine started running rough, applying choke caused it to smooth
out and run normally, turning off the choke caused it to run rough
again, that worked through several cycles. Eventually I let it get
too rough and it died. I coasted for about 30 seconds engaged the
engine and it fired right up. This seems to work about as well as
stopping to rest - engine will always start after a short rest.

I should have tried riding at very slow speed to see if it would keep
going - but didn't think of it when I had the chance.

No amount of spraying water over the engine, ignition etc.. will cause
it to fail in the driveway. I can dump buckets of water on the
ignition and engine without it dying. Even a mist of water from a wet
road will cause it to die and rain causes it to die faster.

At this point I'm almost ready to give up on an ignition problem since
all the ignition except the transistor switches and pick up coils have
been replaced. And the likely hood of two independent electrical
systems failing seems too extreme. All the electrical parts have been
drenched with water and it runs if the bike is stationary.

My latest and greatest theory is carburetor icing. From what I read,
the conditions are favorable - humid between 20-70F sometimes up to 80
F. The problem is more likely to occur when the engine is cooler. It
is more likely to occur when backing off on the throttle.

Where it doesn't fit the theory is that using the choke should
exacerbate the problem - it doesn't seem to, and while the engine is
relatively cool and the problem is occurring it actually helps. When
the engine is at full operating temperature the choke is less likely
to help it.

I can't get the problem to occur when the bike is stationary, it has
to be moving through airborne water in the form of rain or mist (we
don't get enough fog here but I suspect a heavy fog might kill it
too).

My engine is adjusted too lean according to the color of the spark
plugs. With "normal" carburetor icing this shouldn't exacerbate the
problem - according to what I read. I didn't change the jets but did
put in a less restrictive exhaust when the old one had holes in it -
the problem did occur with the old exhaust but with holes in it so I
can't say for certain mixture isn't involved somehow.

Any one with more ideas on troubleshooting this problem? Can it be
carb ice with richening the mixture making it better? All I read says
it should get worse as the mixture is rich, since they assume that the
airflow is too restricted. But as the mixture is made rich, I could
see where there may be less evaporative cooling from the gas since the
gas should evaporate slower with humidity and gas concentration
rising.

Have gas formulations changed in some way that might account for it?

I used to be able to keep going in torrential 30 foot visibility
without it missing a beat - now it won't go more than two blocks in
rain.

Once at highway speed, especially when going fast it seems less
susceptible but backing off the throttle then, in rain, will almost
always kill the engine.

The reason that 'carb ice', is rare in cars and bikes, compared to in
planes, is twofold. The first is that it is normal in these vehicles to
heat the carburator to prevent it (this has the 'downside' of slightly
reducing engine efficiency), and the second is that it is rare to run for
prolonged periods at steady throttle settings (and particularly at low
throttle settings).
Fuels have changed, modern stuff, often contains ethanol, which can make
this much worse. However your problem sounds 'extreme'.
Has the fuel system been fully drained (if the fuel was left standing for
a while, it can pick up moisture).
Does the bike have any system to heat the carburettor, or the incoming
air?. Is this working?. If the bike is water cooled, then it may be like
the common system in car engines, with a thermostatically controlled water
jacket on the inlet manifold. If this ad stopped being heated, it'd
explain the problem.
Some aero engines, accept the slight loss of efficiency involved, and pass
the inlet in through the warm oil in the sump (Lycoming units), making
them _much_ less likely to have problems. Compared to a Continental engine
without this, carb ice is perhaps 50* less common...
The choke, shouldn't help, but if there was physical ice in the venturi,
movement here, could simply displace the ice. However I'd be much more
suspicious of the actual fuel jet being where the problem lay. Perhaps
icing here, which leans the mixture off even further, letting the choke
help.
Historically, this was more of a problem in some countries, and in the
past there was a different carb/inlet setup, supplied in some countries to
help with this. You might want to look and see if there is such an
alternative installation on your machine.

Best Wishes

 

[GPG]

The reason that 'carb ice', is rare in cars and bikes, compared to in
planes, is twofold. The first is that it is normal in these vehicles to
heat the carburator to prevent it (this has the 'downside' of slightly
reducing engine efficiency), and the second is that it is rare to run for
prolonged periods at steady throttle settings (and particularly at low
throttle settings).
Fuels have changed, modern stuff, often contains ethanol, which can make
this much worse. However your problem sounds 'extreme'.
Has the fuel system been fully drained (if the fuel was left standing for
a while, it can pick up moisture).
Does the bike have any system to heat the carburettor, or the incoming
air?. Is this working?. If the bike is water cooled, then it may be like
the common system in car engines, with a thermostatically controlled water
jacket on the inlet manifold. If this ad stopped being heated, it'd
explain the problem.
Some aero engines, accept the slight loss of efficiency involved, and pass
the inlet in through the warm oil in the sump (Lycoming units), making
them _much_ less likely to have problems. Compared to a Continental engine
without this, carb ice is perhaps 50* less common...
The choke, shouldn't help, but if there was physical ice in the venturi,
movement here, could simply displace the ice. However I'd be much more
suspicious of the actual fuel jet being where the problem lay. Perhaps
icing here, which leans the mixture off even further, letting the choke
help.
Historically, this was more of a problem in some countries, and in the
past there was a different carb/inlet setup, supplied in some countries to
help with this. You might want to look and see if there is such an
alternative installation on your machine.

Best Wishes- Hide quoted text -

- Show quoted text -

I have worked in a honda shop as a mechanic, as I said above the most
likely problem is the charging coil.(Or its connections) Spraying with
water while sitting in the driveway under no load will not show the
problem. When on the road much of the water comes from underneath. The
information you have supplied definitely makes it an ignition problem,
you do not say what model.

 

[default]

Check/replace the CDI charging coil.

What are you calling a CDI charging coil? The Clymer's manual calls
it a CDI system, but it isn't. There is no source of HV or separate
capacitor charging coil. The alternator is an excited field type with
electromagnet and brushes - three phase output (and I even saturated
that with water although it will start and run with no alternator at
all).

There are two pickup coils that turn on the transistor switches. The
coils are mounted to what appears to be one end of the crank or
camshaft and are 180 degrees apart. The switches themselves are small
(less than a cubic inch) aluminum modules with wires. There is a
battery connection to both modules, the kill switch interrupts the
power from the battery to shut the ignition off.

 

[default]

I know you said you drenched the engine while running and it would not
stall.
I'm curious if you ever tried reving the engine and then aspirating it
with saturated (wet) air?
Then let off the throttle.

If that proves useless, then perhaps it is a combination of things.
Moist air, engine load, and physical weight on the bike.
Also, what altitude are you at? Could that be a variable here?

Now that I'm thinking carburetor ice, I'll have to try inducing it
artificially (misting the air intake while revving and closing the
throttle) I did mist the air intake while idling - but probably just
got some water on the air filter.

I tried water where ever I thought it didn't belong like the
connections between carbs and engine.

What gets me is that the carbs and ignition are more or less
independent systems the only things they share are common gas supply
and the same +12V to the transistor switches. What ever is stopping
the engine is managing to stop redundant systems (four carbs and two
ignitions).

 

[default]

Fuel filter?

Well it does have a fuel filter I added. I was having a problem with
rust from the tank keeping the float valves from sealing properly.

It is between the tank shutoff valve and engine shutoff valve and
horizontal with inlet and outlet on the same plane. When the bike
sits for an extended period I can see the fuel level in the valve and
when cranking it fills with fuel.

What hypothesis suggests it could be clogging when it rains outside?

When I run out of gas (normal - no fuel gauge) the bike will drop
power as one carb runs out then slow more as a second one drops out.
Most of the time that takes 1/4 to a 1/2 mile while I fumble with the
reserve valve and is likely to happen while going uphill.

This failure is much quicker than normal running out of gas and it
recovers faster - just switching to reserve will often take another
1/4 to 1/2 mile before it smoothes out again.

There may be a coincidence with going uphill and encountering the
problem - but there are so few hills here I can't really say.

 

[default]

Also, what altitude are you at? Could that be a variable here?

Altitude is between 34 feet above sea level and at most 71 feet when I
visit the wife. The land is flat for the most part, under water when
the earth was formed, sand peat and no rocks to speak of.

Carbs are "constant velocity" types. Twist grip opens a butterfly
valve but the slide doesn't go up until the vacuum builds enough to
raise it, opening the air path, and withdrawing a needle from the main
jet allowing more gas into the airstream.

It has been my experience that this type of carb can work 0-11,000
feet with no adjustments and little loss of power. (unlike the old
'72 Triumph)

 

[default]

The reason that 'carb ice', is rare in cars and bikes, compared to in
planes, is twofold. The first is that it is normal in these vehicles to
heat the carburator to prevent it (this has the 'downside' of slightly
reducing engine efficiency), and the second is that it is rare to run for
prolonged periods at steady throttle settings (and particularly at low
throttle settings).
Fuels have changed, modern stuff, often contains ethanol, which can make
this much worse. However your problem sounds 'extreme'.

That it is

Has the fuel system been fully drained (if the fuel was left standing for
a while, it can pick up moisture).

Well it is normally very humid here. I just filled the tank two days
earlier from nearly empty before trying it yesterday in the rain. The
fuel level was only inches from the cap. Normally the water in the
fuel, if any, will be at the bottom of the float bowls and each bowl
has a drain screw and I can see any water globs in the transparent
tubes when I open it. - but if they are adding alcohol to the fuel it
may mix with the alcohol and not be so easy to spot?

No that doesn't sound likely. I remember reading that the way to tell
if a fuel has alcohol is to put it in a graduated cylinder and add a
known quantity of water to it. When the water mixes with the alcohol
it drops out of solution with the gas and the oil / water separation
level is higher than the specific quantity of water would justify -
but that may just apply to a lot of water . . .

I don't use a lot of gas, but take a 250 mile trip once a month and
use the bike for grocery shopping (12+ miles) at least once a week,
about 15 gallons a month go through it.

Does the bike have any system to heat the carburettor, or the incoming
air?. Is this working?. If the bike is water cooled, then it may be like
the common system in car engines, with a thermostatically controlled water
jacket on the inlet manifold. If this ad stopped being heated, it'd
explain the problem.

No heat at all for the carburetors. When the engine is hot, cooling
air from the fins can flow past the carbs or rise from the hot
crankcase while standing still. Coupling between the carbs and engine
ports is via a piece of reinforced rubber hose about two inches long
with hose clamps at the carb and port. (and I tried spraying water
around the area of the connections while troubleshooting it
stationary)

Some aero engines, accept the slight loss of efficiency involved, and pass
the inlet in through the warm oil in the sump (Lycoming units), making
them _much_ less likely to have problems. Compared to a Continental engine
without this, carb ice is perhaps 50* less common...
The choke, shouldn't help, but if there was physical ice in the venturi,
movement here, could simply displace the ice. However I'd be much more
suspicious of the actual fuel jet being where the problem lay. Perhaps
icing here, which leans the mixture off even further, letting the choke
help.
Historically, this was more of a problem in some countries, and in the
past there was a different carb/inlet setup, supplied in some countries to
help with this. You might want to look and see if there is such an
alternative installation on your machine.

Best Wishes

What I need is a good way to rig a water mist for the intake while
riding the bike. Some kind of nebulizer or atomizer rigged right to
the air box opening. Try hard enough and I'll induce a problem even
if it isn't the one I'm troubleshooting.

 

[default]

I have worked in a honda shop as a mechanic, as I said above the most
likely problem is the charging coil.(Or its connections) Spraying with
water while sitting in the driveway under no load will not show the
problem. When on the road much of the water comes from underneath. The
information you have supplied definitely makes it an ignition problem,
you do not say what model.

1981 Honda Custom 750 DOHC - two pick up coils for each ignition
system - 12V via the ignition switch, and kill switch is applied to a
pair of transistor modules that feed the coils.

No separate HV capacitor charging circuit or energy transfer ignition.

I've sprayed water around the pick up coil housing - I've a frozen
screw holding the cover on and no way to get it off short of cutting
the head of the screw off or drilling it out. It has three counter
sunk oval head Philips screws holding it on. Two of them I could
loosen with my impact driver - the third still has the tip of the
impact driver bit imbedded in it - broke off before the screw turned.

I even moved the little rubber cable gasket away and misted directly
into it. The outside of the housing mates closely with the engine
case and to keep water out I added 360 degrees of aluminum tape around
the seam. (after waiting weeks for any moisture I induced to dry out)
This tape is real aluminum metal and has a very aggressive adhesive
and soft enough that it will smooth over contours for a good seal.

So, while I can't get to the pickup coils, I doubt they are getting
any water in them.

The manual's schematic shows a connector somewhere between the pick up
coils and connector at the modules. It would seem likely it is in the
starter motor housing since the PU coil wires are routed through there
and the schematic shows it picking up another single wire for the oil
warning light.

Any idea where that connector is on that model? I'd like to inspect
it. I had the starter motor cover off - the gaskets are in good shape
and it appear clean and dry in there - but I'd still like to find that
connector and look at it.

 

[Greg Neill]

1981 Honda Custom 750 DOHC - two pick up coils for each ignition
system - 12V via the ignition switch, and kill switch is applied to a
pair of transistor modules that feed the coils.

Have you checked that the kill switch is not intermittent
or shorting when wet?

 

[Jim Thompson]

No vacuum to distributor, no distributor. Two independent ignition
systems firing four cylinders - wasted spark on one cylinder when the
other is firing.

Only one external vacuum line and it is used to operate a fuel shut
off valve so that gas can't flow down (gravity) unless the engine is
turning over or running (vacuum turns on the valve).

Sounds like you found the problem ;-)

Certainly fits your starved fuel symptoms.

...Jim Thompson