CB750C carbs

Friend got it cheap after it sat for five years, 31,000 kilometres.
Local technical school dis-assembled and cleaned carbs, swapped forks
from parts bike, changed oil & filter, adjusted valves, put in new
battery, front pads, new plugs and possibly wires (can't be sure - they
look new). not sure about the coils, swapped chain, carbs synch'ed.

The day after he drove it home it wouldn't start so I tried to check
charging and discovered that rotor was shorted (great pointer from Mike
Nixon at www.motorcycleproject.com). Swapped rotor and stator, now
charges well enough, I think. However it still took up to twenty long
bursts to get it started (just as before). Choke (old-fashioned air
choke) seems to close nearly all the way, plugs after about a 50 km ride
to his place were fairly clean on the electrode but black on the nose.

Found manual on internet and noticed it says to start the fuel pilot
screws from 1 1/2 to 1 3/4 turns out, then continue to turn out.
Noticed that screws were set at about 2 1/4 turns. Smelled a little
gassy on start attempts but for no good reason other than I have
sometimes had success by leaning the pilots, I turned them in to 1 3/4
and it seemed to start quicker after waiting for a couple of hours.
(The thing with this bike is that once you get it started, even if you
run it for only five minutes and then wait three hours, it will start
right away without the choke, so I the next morning I went over and it
started after about six tries.) Then I turned the pilots down to 1 1/2
turns and 'phoned him the next morning. He had already been trying to
start it for some minutes, but had forgotten to turn the petcock on.
Once he did it started on the second attempt. I told him if all is good
it should start after several seconds or less on the first attempt, so
he asked me to turn the pilots down to 1 1/4 turns. Tomorrow morning
we'll know if that made any difference.

I'm always afraid to lean out air-cooled bikes too much. We plan to
find an oil cooler but the expected rider for the next couple of months
is a retired guy who misses his Goldwing that he got rid of some years
ago. I guess to be more careful about it, I really should measure the
float height even though these floats are the non-adjustable kind as
well as double-check the float valve seats. The older gent is quite
anxious to take it home and get on it and get back into the riding habit.

I'm wondering should I worry that I may have leaned the pilots too much?
After I set them to 1 1/2 friend took around neighbourhood (I wouldn't
do it because it is un-licensed at the moment) and told me it pulled
well in all gears, even in fifth going up hill at city speeds). I don't
mind taking an hour or two to pull the carbs and disassemble the
bottoms. Thought of putting a can of Motul fuel system cleaner and
seeing what happens after a tank or two but from what I'm seeing there
could just as easily be a slow-speed air blockage.

(Oh, also, at 1 1/2 turns, bike idles around 2,000 with choke on, you
can take the choke off after less than a minute and it settles down to
about 1,200 rpm. Whereas when pilots were at 2 1/4 turns, after you
took the choke off and gave it throttle to 3,000, most of the time, it
would not return to idle, which is another reason why I had the feeling
it was too rich, ie., float too high in one or more of the jets.)

Thanks for any comments,
p

 

Oh, a couple of other things I forgot to mention - the school put in a
new in-line paper fuel filter and I have no idea without dis-assembling
whether the carb has stock jet sizes.

p

 

Oh, sorry for all the posts, another thing I was wondering besides did I
make the idle too lean, is whether I should pull the tank and try to
spray WD40 on the rubber manifolds?

p

 

You want to start at the point the manual says.....You should also clean
the carbs.. give them a complete cleaning.... all the circuits.. even if
they "look" good to you....those passages are very tiny and can clog up
extremely easily....it doesn't take much...its also pretty easy to
do......you will find that the bike will start up and run the way its
supposed to when done properly....you donh't say what year the bike is, but
am assuming it is of the vbintage years....one other thing I would do is
replace the fuel rail o-rings as they will eventually go on you....
Each rebuild kit is about 30-35 bucks...worth every penny of it too if
you ask me.....If your intake manifold gaskets are cracked, replace
them....same with the air boots.... this will only save you time later...
make sure you have no air or gas intake leak....change the air filter, and
plugs too.
Once you have cleaned the carbs, have them back on the bike, new plugs
etc....give it a try starting... then take one of the plugs out and "read"
the plug......too white??? turn out the idle screws 1/4 turn at a
time...too black?? turn idle screws in 1/4 turn at a time... nice an tan??
Leave them alone.....

Hope this helps...

Fwed

 

I would say that at 2 1/4 turns on the pilot screw, it "hangs up" but
does not "surge". It is interesting to me that manual advises to turn
out (ie., if I understand the manual correctly) when adjusting, whereas
what you are saying seems to gibe with what I am guessing).

thanks,
p

 

Thanks, I haven't been able to find out whether the rubber/o-rings was
replaced (even though I was a student at the same school - three times
as old as some of the others, just took it because it was always
someting I wanted to do and there's no way anybody would hire me for a
normal job, I wanted to work on this bike but another student who didn't
have many "customers" got it whereas I had my own scooters to pass the
tests on! On the other hand, I didn't care about passing the course,
rather I wanted to understand the "physics" of bikes.).

I am going to take the carbs off, look at them with my own (poor) eyes,
and apply some OMC fuel treatment (which is what we generally used at
school) to clean them (although I've read that ordinary vinegar or
Pinesol has some effect after a few days!).

I am still pondering Albrecht's advice as I think I like these bikes and
might try to find one that has a moderate fairing for myself, just for
comfortable highway riding, although I occasionally succumb to ads for
old Goldwings which might be more comfortable to see friends who are at
the end of the highway, in pretty much any weather except for snow and
ice, than my Burgman 400!,

p

 

Sorry, the bike is an 1982 CB750C (aka "Custom"). Very little rust, all
surface I'd say, pipes and mufflers look solid. Nice long wheelbase for
"us" old guys, good sprockets and chain, quick brakes, extremely stable
at speed, seems to have lots of torque (as I found out when I released
the clutch while standing beside not knowing that one of the neighbours
had put it in gear) and horsepower, since friend rode it from school to
home at pretty much 160 kph all the way (and he's older than I am and
seems to be thinking that he likes it better than his two-year-old BMW
650!).

p

 

I've got an old 79 750 L.... I am in the process of rebuilding the
carbs.... only cuz I did it before and never replaced the o-rings.... well
guess what? I had a leak after the rebuild... at first i thought it was the
float bowl gaskets...come to find out it turned out to be the fuel rail
o-rings...so off came the carbs again...anyway...
Be sure to check the spring loaded air butterflies when you pull them
apart... make sure the spring doesn't unload like mine did.... after I put
them back together, only two of the air butterflies worked and it took me a
half hour to figure out the spring needed to be recoiled one turn... works
perfectly now...
So...make sure alwso that the carbs are synched properly... this may be
a problem area too....keep in mind that it isn't a big carb, and if it isn't
running properly, you have to clean out the carb better....it doesn't take
much for the fuel passages to clog up...also make sure your fuel filter
doesn't have any particles trapped. If it does you probably have little
tiny rust particles going thru the fuel tank to the carbs.... those little
rust particles will also clog up your carbs and you will be back at square
one....I had that prob as well, so I kremed the tank and guess what?? all
gone....

Hope this helps..

Fwed

 

Thanks, Albrecht. Following your lead below, today I took the tank off
and pulled the carbs up high enough to see the butterflies and you were
right, on each carb, the leading transition hole was showing. So turned
the idle knob out (and stupidly, forgot to count the number of turns)
until the butterflies were well past hiding those ports.

Started at 1 3/4 turns on the pilot screws. Got a bit of stutter when I
blipped the throttle at 2 turns and 2 1/4 turns, couldn't notice any
change at 1 1/2 and 1 1/4, still nervous about going leaner.

Bike definitely idles rougher than before and I'm pretty sure there is
less exhaust pressure from cylinders 1 and 2 (I simply put my hand over
the ends of the pipes, just a seat-of-the-pants impression). Didn't
ride it, still no licence plate/insurance for it.

What puzzles me is how does one synchronize these carbs for idle
balance? As far as I can see (and manual is no help, just says it is
sync'ed at the factory), it has no vacuum outlets to attach sync tubes
to. I wonder if it can be done at the bench with feeler gauges?

(By the way, the manual's procedure seems to imply, although it doesn't
say so directly, that it is quite possible for the final adjustment to
end up with each of the four pilot screws turned out a different amount,
which is a possibility I hadn't thought of before.)

p

ps: again, it is a 1982 CB750 Custom.

 

Oops, thanks, didn't think to look on the head.

p