Honda MC question

Hi there,

I know this isn't the motorcycle group, but I am looking for some general
info on how to tune the carbs on my 79 Honda CM400 Twin. 395CC engine, 5 spd
trannie.

I have searched the net up and down to no avail. I think my only option will
be to break down and try and find a manual at a decent price. So far no luck
on EBay.... There is a Clymer one, but I wouuld prefer an actual Honda
manual.

So what I am looking for is

A) Advice on how to tune up the carbs on this bike OR

B) Link to a website that shows diagrams and a step by step procedure on how
to do this.

The current symptoms include:

1. Brown smoke coming from the exhaust, indicating that the bike is
overfuelling and running very rich.
2. Intermittently runs on one cylinder, indicating overfueling and fouling
of one cylinder.
3. Gas occasional pours out of the overflow of both carbs, indicating a
problem with the floats or the fuel shut off valve.

Thanks for your input.

t

 

news://rec.motorcycles.tech

 

Don't know if you have actually visited news://rec.motorcycles.tech, but it
SUCKS.

A helpful link, PLEASE.

t

 

Yeah, I read it often. There was a discussion just last week regarding
tuning carbs.

Don't like my link, don't follow it, but the data you seek is there.

 

well, your symptoms certainly /do/ point towards float problems and/or
needle valve leakage. you need to strip and clean to see what's up.
needle valve failure/leakage is very common on older high mileage carbs.

regarding subsequent adjustment, i can't say because i've not stripped
those particular carbs, however you can balance between chokes/idle
circuits with an old pen and a glass bead. pens usually have a slightly
conical center [aids mold removal during manufacture] and therefore a
glass bead trapped therein will rise according to airflow from bottom to
top, if you see what i mean. that and a rubber tube and a pin [to
prevent accidental bead ingestion] and you can now balance relative
airflows between carbs. it doesn't give you a setting for what the
actual flow rate /should/ be, but it allows you to balance them
perfectly and costs almost nothing to make. or you can buy the tool
that does the same job!

 

It seems unlikely that both float valves would be hosed, but maybe some
dirt found its way from the tank and screwed them up.

Pull the carbs and remove the float reservoir bottom end caps.
Dependiong on the design, you should have instant access to the float
and valve(s). There is probably a simple pin to pull/slide that will
release the floats. Pull them, watching for jumping valve components as
you do so. Check the valve seats and the needle valve for wear, dirt,
corrosion, etc.

-Greg

 

I guess I am reserved to buying the book, every resource I have looked at so
far requires that you measure the baseline before disassembly. IE Float tab
angle, air screw and fuel screw settings. I am pretty sure that they are all
wrong, probably been taken apart already and messed with.

Without a vacuum gauge, I guess syncing these carbs will be difficult.

I have pulled the jets and all removeable gummed up parts and let them soak
in carb cleaner for a couple hours. That includes the float needle, which
seems to be working much better now that its clean. However, I hope I didn't
wreck it by letting it soak in solvent, there are probably rubber seals
inside it. I gave it a shot of some high quality assembly prelube to help
lubricate any gaskets inside.

The carb has 2 jets, #76 and #112. Which one would be have a bigger jet?
Also, can I assume that the one with the bigger hole is the main jet?

Thanks for the response!
t

 

Also, with regards to the pen tool, good idea! I will try to make one.
perhaps more questions to come!

PS Do the jets get screwed down all the way? or is there another setting
there to help atomization of fuel? Or does that occur only at the needle?

t

 

depends on the jet. if it's simply a fixed orifice, yes. if it's a
needle jet, with a fine thread on the adjuster screw, no. usually, they
get screwed in GENTLY to the stop, then backed out a number of turns.
but what that correct turn number is is anyone's guess. before
disassembly, see how many turns it takes to the stop for each jet. if
they're both roughly the same, that's probably what you want to use
again [and balance from there]. if they're way different, you're back
to guess work.

jets and floats are the big things. some floats puncture and stop
floating or their float level has been "helped" into the wrong
position.

one trick for cleaning jets is to use copper wire to clean them out.
copper is softer than the brass of the jet, so won't abrade and enlarge
the hole, but is sufficiently strong to bust out most grime that may
have accumulated. if you have a magnifier, check the jets for signs of
previous damage. if these carbs have been stripped and "helped" before,
there's a danger someone's damaged them.

sorry, can't help on the settings - no idea for this machine. the book
is the way to go. it may be expensive, but it sure is cheaper than
paying for a shop to do this stuff. and it has resale value.