Re: Advice on Honda MC problem?

On Mon, 4 Aug 2008 23:36:42 -0700, "Ted Mittelstaedt"
<tedm@xxxxxxxxxxxxxxxx> wrote:


"default" <default@xxxxxxxxxxxxx> wrote in message
news:5tgc941doba49ds5h7lf0odvo8t1eblrh8@xxxxxxxxxx
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.

I have a 1980 Honda CB750K. It goes fine 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.


I think your getting water mixed in with the fuel.

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.


You can get a 4 guage synchronizer for $45 that contains all of the
needed tubes and fittings to plug into the carbs, with guages that
are calibrated the same with each other, but each to his own.

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.


Vapor lock in that design is basically impossible. What vapor lock is, is
when the fuel in the fuel line changes to vapor due to heat, the resultant
vapor bubble rises to the highest point in the fuel line, which in a car is
the carbs, and you end up starving the carbs of fuel.

The vapor lock was manifesting itself in the paper fuel filter that
sat horizontally between the tank and vacuum fuel shutoff valve. I
could see it didn't have any gas in it. I got rid of the filter and
later took out the vacuum valve and plugged the vacuum hose nipple.

However in that design since the fuel tank is above the carbs, any vapor
that forms in the line simply rises into the fuel tank. Any vapor that
forms in
the carb bowls vents out the overflow tube and more fresh fuel simply comes
into the bowl to replace it.

What is more likely happening is simply that the engine is overheating.
That happens to mine when it's idling for a long time in traffic on a
very hot day. It's best to simply shut off the engine and wait in traffic
if you know your going to be idling for more than a minute in those
circumstances. You get the cylinders too hot and they expand to the
point they lose compression and your engine oil doesen't lube and cool
right, and the combination of increased friction and decreased power
at idle will stall it out.

First time it happened, I'd just covered about 50 miles at over 100 F
and another 50 before in the 90's. Slowing for a traffic light (first
in 50 miles) it died while still 150 feet from the light. I did the
usual WTF stuff and looked at the filter and it looked bone dry and I
knew there was still plenty of fuel in the tank.

Took the tank off and ripped out the filter and routed the line from
the tank directly to the carbs and went the rest of the way home.
Luckily, I had a couple of bottles of water along and a tool set.

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.)


And you know of course that doing this creates a rough surface
inside the jet, which your accounting for in your flow calcuations?
Liquids like flowing through tubes that have really smooth walls,
like the jets do when they roll off the assembly line. Liquids
don't like flowing through tubes with rough walls such as when
billy jim bob decides to save a few bucks by poking bigger holes
in his jets with his Black & Decker. You can end up with bigger
holes that flow less fluid, espically at higher flow rates at WOT.

But of course you know best with your 40 years of experience
so who am I to say anything?

It wasn't my intent to say that I'm master mechanic or walk on water,
but wanted to avoid suggestions like "did you try putting the key in
the ignition?"

You are saying a smooth round hole can't be drilled in brass? I
haven't done it, and from what I'm hearing there may be nothing to be
gained from it and I should be worrying about the idle air/gas not the
jets. Anyhow I do a fair amount of fabrication related to electronics
and have no problem drilling smooth round holes in material even with
hand drills. Brass can be a little tougher to drill than aluminum or
copper but come on this ain't rocket science.


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.


Mine has about 30K and I have a second in the garage I don't ride
much with about 70K on it.

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.


The muffler has nothing to do with it. Mine has holes in it, they all get
them. That design, the 4 into 4, was an untuned design that was for
one purpose, showing off a lot of chrome. They all rot out in the back
baffle areas and it just makes them louder.

The airbox has more to do with the tuning than the mufflers do.

Well that's why I'm asking for advice. What I read is that the jet
size should go up as either inlet or outlet restriction go down.
Further modified by don't sweat it because the jet ain't that
important unless you are racing the bike.

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

Unlikely.

Unlikely a guru or lean running?

The simplest explanation that fits all the facts is often the right
one. or "Plurality is never to be posited without necessity." William
of Ockham.

"Once we have eliminated the impossible, whatever remains, however
unlikely, must be the truth" Conan Doyle

The obvious thing is the high voltage but you replaced the coils, plug
wires and plugs and if it don't cut out with a spray bottle misting down
the high voltage parts, then it's not that.

It could be the ignitors, they are known failure points, but if they
are running fine when dry then there's likely nothing wrong with them.

I assume you have used a 'scope & water to make sure the ignition
pickup isn't shorting out.

I didn't put a scope on the pickups. The pickup coils are dry with no
sign of water getting in. I ran some silicone grease around the
gasket and put it back then taped the seams just in case.

So that leaves the fuel system. It sounds to me like you have a
water leak that's leaking water into the airbox somehow, then down
the carb throats. Or into an air bleed or some such on the carbs.
Or maybe even into the gas tank cap vent. Since water is heavier
than fuel, if you have your tank switched on to reserve, any water
that gets in the tank will go immediately into the carbs.

I think you need to expand your testing with a running bike and your
garden hose.

Ted

I've hit it with a garden hose and even dumped buckets of water on it
with it sitting in one place idling. This problem only occurs under
load with the bike moving - either loading or moving or both need to
be there along with the water.

Why I think it is a carburetion problem now. I never thought the low
voltage or high voltage circuits were at fault but replaced the coils
plugs wires & boots anyway. I would expect an electrical problem to
be there with the bike stationary unless it was something like water
being blown into contact with the HV wires. And the recovery time to
idle after dying is on the order of at most a minute - I'd expect an
electrical problem to be there until the water evaporates or boils
off.

I put some clear plastic lines on the float bowl drain valves and I
can see some droplets of water when the bike has been sitting parked
in the rain for a long time. Water collects in the bottom of the
bowls and gas is taken from the top - it would have to be a lot of
water to actually get ingested into the jets. In any event, I can and
have drained all four float bowls of gas to make sure there's no water
but if it is raining it will still die.
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