'84 CB700SC Nighthawk carb question
Norval Wallen
over three years. In bringing it out of retirement, I've tried to do
all of the "proper" things like all new fluid everywhere, new plugs,
etc. I put a can of carb cleaner through all of the jets, float bowls,
etc. The bike is 100% stock with about 17,000 miles. It ran good
before I parked it.
The shop manual calls for an initial pilot screw adjustment of two turns
out. Before cleaning the carbs, I counted 3.5 turns out as a result of
a carb overhaul done by a shop about 10 years ago. I decided to
compromise by starting with 3 turns. I don't know how accurate I am
because I turned them out one extra turn each with the carbs installed.
The bike fires up fine and sounds like it's running on all four jugs.
The shop manual also has you go through this routine about turning the
screw until it drops by 50 RPM, touch your toes, spin around, yada,
yada, etc. The problem is that (1) I don't have a tachometer that
accurate and (2) There's no way that I can adjust those pilot screws
with the carbs on and the engine running.
Here's the problem: The engine builds RPM rather slowly and is also slow
coming back down to idle. The carbs were synchronized before and I
didn't tear them down that far so I didn't think that would be a
problem. I've been running it so far with the intakes open because I
want to get everything dialed in before I button it all back up. Is this
a factor?
I guess my question is how far should I back out the pilot screws?
After the last overhaul, I noticed the bike would sometimes stumble
coming off idle on a hot day. I thought this might have been because
the idle/slow jets were the wrong size as I've heard that sometimes
bikes are jetted lean from the factory.
Would pilot screw adjustment affect this?
I live near St. Louis. Maybe 200 feet above sea level, 110 degree
summers, -12 degree winters. I probably only ride between 40 and 95
degrees.
Thanks for anyone's help
Norval Wallen
Chris BeHanna
> [...bike sitting three years...]
>
>The shop manual calls for an initial pilot screw adjustment of two turns
>out. Before cleaning the carbs, I counted 3.5 turns out as a result of
>a carb overhaul done by a shop about 10 years ago. I decided to
>compromise by starting with 3 turns. I don't know how accurate I am
>because I turned them out one extra turn each with the carbs installed.
>The bike fires up fine and sounds like it's running on all four jugs.
>
>
>Here's the problem: The engine builds RPM rather slowly and is also slow
>coming back down to idle. The carbs were synchronized before and I
>didn't tear them down that far so I didn't think that would be a
>problem. I've been running it so far with the intakes open because I
>want to get everything dialed in before I button it all back up. Is this
>a factor?
The lack of resynchronization could well be a factor, as could
overrichness, as could too high of an idle speed setting.
>I guess my question is how far should I back out the pilot screws?
2 turns, like the manual says. This is where you start.
>After the last overhaul, I noticed the bike would sometimes stumble
>coming off idle on a hot day. I thought this might have been because
>the idle/slow jets were the wrong size as I've heard that sometimes
>bikes are jetted lean from the factory.
If they're lean from the factory, then a hot day should cause
less problems than a cold day.
>Would pilot screw adjustment affect this?
Yes.
Here's what you do: start with the factory settings for the
mixture and idle speed. Synchronize the carbs. Now, turn the idle
speed down to 750-800 rpm. It will be more sensitive to mixture
changes that way. Now, adjust the carb mixtures one at a time, going
1/8th of a turn at a time. Each time the idle speed goes up, turn it
back down to where you started. When you reach a point where
continuing to turn the mixture screw causes the RPMs to decrease
rather than increase, then turn the screw back to its prior
(optimized) setting, and proceed to the next carb.
An offset screwdriver, or one of the really small common
screwdrivers, should work. If not, then you might have to get one
of those Lisle brand flexible jobs.
Set up a box fan in front of the bike and turn it on high so
that the bike doesn't overheat while you're doing this. The bike
should be warmed up enough to run well off choke before you synch
the carbs.
You should repeat this procedure after every valve clearance
check.
Regards,
***********************************************************************
Chris BeHanna "The fact that an opinion has been widely
Software Consultant held is no evidence whatever that it is
beh...@nospam.fast.net not utterly absurd." -- Bertrand Russell