Carb Problem Honda CB650sc Nighthawk

Recently bought a 1983 Honda Nighthawk 650 which had been sitting
without use for over a year. it has 26,000 miles.

I tore down the carbs and I carefully cleaned them, jets, pilot screw
the whole shebang. Replaced the intake manifold o-rings, drained the old
gas cleaned the petcock.

It started up fine with the starter valve(choke) engaged. Idle smooth
also at around 2-2500 rpm. When I backed the choke off it would idle but
a bit rough. When I attempted to rev up the engine with the choke off
the engine would die. With the choke on it would rev up.

When I looked inside the carb as I am attempting to rev it up it seems
like the carb piston/needle is trying to raise just before it dies.
I checked the diaphragms and none are torn or leaking. They are properly
seated. I have been futzing with these #%@^# CV carbs for several days
and the best I can determine is when the throttle butterfly opens the
carbs go lean as the pressure drops and there is insufficient fuel to
transition to the main jet. But I can't figure out whats causing this
condition. I have adjusted the pilot (idle screws) but this has no effect.

If any of you carb guru's can have mercy on my soul and solve this
riddle I would appreciate it.

The carbs are Keihin with rubber diaphram pistons.

thanks

 

Well... lets see... I guess we need to know how long you are letting it
warm up, but I have to assume you are letting it warm up for at least 10
minutes....next, are the carbs synched? and what do the plugs look like?
Have you put new ones in? It sounds like you did a complete job on the carb,
but no mention of replacing the plugs....how are they burning? are they all
a brownish color? Are the pilot screws all turned out the same amount of
turns? What abut the float levels?? Have you checked them? do the carbs
give a sort of popping sound?
My guess is you need new plugs (as part of your problem) if you haven't
already put a set in....or let us know a little more about the plugs and how
they are burning etc. etc..

Hope this helps....

Fwed

 

Thanks Fwed,

I didn't replace the plugs. The plugs look like they are running lean ie
no carbon, metallic appearance and not the brown color thats expected.
No doubt new plugs could help.

I did test each spark plug and did get an arc, though not very intense.
There is no popping noise from a lean burn though. The carbs are not
sync'd. The pilot screws are turned out about 3 turns manual calls out
for 1 1/4 turns to start.

These carbs have the plastic floats which have fixed tabs and as far as
I know they are not adjustable.

I did let it warm up for over ten minutes and what's wierd if I really
gun it with the choke on it will respond with the choke off sometimes.
At one point I was able to ride it though it was a little hinky.

It has a linear response with the choke ie more choke rpms go up (to 3K)
less choke they go down. This type of carb uses a start valve which I
think is a separate carburetor circuit. Once I cut the choke completely
out it will idle (1000 rpm) but once I start twisting the throttle even
slowly it dies. It will start back up and do the same.

billybob

 

This is the most common problem with carbureted motorcycles, other than
dead batteries. The gasoline evaporates out of the float bowls and gums
up all the tiny jets and passages.

The idle fuel/air mixture passages are all plugged up. Buy a can of
Berryman's B-12 Chemtool Choke and Carburetor cleaner (the aerosol
type) and spritz through the idle mixture aduster screw holes, with the
carbs off the engine.

The carb cleaner will come out through the idle mixture port, the three
transition ports, the idle jet and the pilot air jet. It has to squirt
freely out of all those holes. Put your fingers over two of the holes
while you're spritzing the passages in order to get the carb cleaner to
squirt through the passages with the most pressure.

Alternatively, you can drain the carburetors by opening the float bowl
drain screw and refill them with a 50/50 mixture of B-12 and gasoline
and let the mixture sit in the float bowls for half and hour. Then
start the engine and let the engine burn the B-12 for fuel as it cleans
the idle passages.

 

Actually I did all that you suggested. I am aware of the little holes in
the venturi that are tied to the pilot jet and pilot needle screw. I
vigrously flushed these passages with B-12 followed by a compressed air
chaser. I took out the pilot (idle jet) and soaked it for a 1/2 hour in
B-12 then blew air through it. I actually saw the B-12 come out of the 4
holes in the venturi.

My only other thoughts is that it might be a leak in the diaphragm for
the piston between the lip of the bladder to the carb body. It seemed
like it seated well tho.

Thanks for the suggestions though, good advice nonetheless.

What I'm wondering if when I crack open the throttle and it hesitates
and dies, is going lean and dead or going too rich and quenching the
spark plug?

billybob

 

Well, a set of spark plugs might be in order. When the supposedly high
voltage at the spark plugs is too low, down around 10KV to 12KV, the
engine will operate as though the mixture is LEAN, because the spark
kernel doesn't find much burnable mixture in the narrow gap. The spark
plugs might be clean looking, but they would SMELL like unburned
gasoline because the engine can't light off the lean mixture.

I would think you should get a bright blue spark that's at least 1/4 of
an inch long, but if the ignition system is supposed to be a higher
voltage system, you risk puncturing the insulation inside the
electronic parts.

If your motorbike has one of Honda's CDI systems with a low voltage
source coil in the alternator stator, you might want to find out what
the resistance of that coil should be and check it out. Maybe it's
partially shorted?

OTOH, If you have the idle mixture screws open 3 turns instead of the
1-1/4 turns recommended, you have to compensate for an excessively RICH
idle mixture by opening the throttle butterflies too far. That uncovers
the transition ports too soon and the engine gets too much fuel at low
RPM.

But, in that case, I would expect the sound of the exhaust to be
"toot-toot, toot-toot, toot-toot" until the engine RPM builds up enough
to blow the excess fuel out. The spark plugs would be sooty black.

There is the possibility that your fuel tank evaporative control system
has a leak in the canister purge hose that goes to the carburetor, if
you have a hose going to the carb.

You could try disconnecting that hose and plugging the fitting on the
carb if you have such a hose. The other evaporative control system
fittings collect fumes from the float bowls. That part of the system
does have to vent to atmospheric pressure somewhere so the float bowls
can fill up by gravity and the carbs can suck fuel out of the bowls
without fighting "vacuum".

 

If the plugs look like they are burning lean, then the plugs would be
white ish in color.... a brownish color plug is what you are trying to
achieve here... that means you have the proper adjustment. If the plugs are
anything but brownish, then your mixture isn't adjusted properly.... a dark
or blackish color plug means its too rich..... if you need to turn out the
pilot jets any more than 3 turns to get rid of the lean or whitish looking
plugs, then I would go up one size on the jet. After a certain amount of
turns out, the adjustment does no good and a larger size jet is needed. One
other thing you didn't mention is your exhaust system. Is it stock or do
you running a different type? This will also make a difference.... For your
info.. I have a 79 750 which I also did the carbs on and was having a slight
problem with a flat spot at around 6- 7000 rpms. It turned out to be a lean
mixture, and I still have a slight flat spot, but not nearly as bad. I have
the stock jets in there and the screws are turned out about 2 1/2
turns....so its borderline on the jet size. I also have a similar problem
with the starting.. i need to keep the choke out all the way until it warms
up a bit, then I can push the choke back in...although it doesn't stall when
I give it gas. Anyway a set of new plugs will help... then just
monitor them to see how they are burning... if they are black, turn the
screws in a little at a time and run the bike for a while (maybe 50 or so
miles) then check them again and make the proper adjustment...I would highly
recommend synching the carbs though...as for the floats.... if you can check
them to make sure they float (have no cracks so gas can get inside of them)
that would give you rule anything like that out....other than that, how does
the bike perform while you are riding it? does it skip or sound like it
wants to go faster, but can't? Another thing might be to check
compression.... make sure all cylinders are within 10 ft lbs of each
other... if not, then that is a problem area....way back when, I had a
Kawasaki kz400.... which kept blowing the head gasket and kept giving me
trouble until I told myself not to beat the shit out of it all the time...
lol...I ended up trading it in for a different bike....
Another thing you might want to check is to make sure you don't have an
exhaust leak at the manifold.... this might also cause something like you
describe....

Let us know..

Fwed

 

Thanks fweddy and Pale fire.

I change the plugs and took it out for an extended ride. The plugs
helped it idle a little more stable. Ran it around and it pulls great
between 2-4000 rpm's. Fast reponse.

Whats wierd is if I rev it with the choke on it doesn't die though it
idles high, but if I rev it a few times in this way I can take the choke
off and it respond normally.

After the vigorous ride guess what it now seems to run ok. It possible
that it may have still been clogged because it is now a different
animal. This bike may be very cold blooded and maybe needed to get a
little hotter than I was running it, but it seems fine now.

Thanks again for the great info

 

No problem... just keep an eye on the plugs and check them once in a
while... make sure they aren't running too rich....or lean..

Fwed