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Honda CB750 - running very rich and thirsty - any ideas please?
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Ok I think I might be getting closer with my 1976 CB750F1. It's all
MOT'd 'n' taxed 'n' stuff now and I've actually been out on it a few
times (like Popham the other weekend).
On anything but small throttle openings it actually runs pretty well,
always on four cylinders now, which is a bonus. Tickover is hit or
miss, sometimes it will behave, but others it is revving at about 2.5K
and takes ages to slow down. It pulls well on WFO upto as fast as I
want to ride it.
I have: Set valve clearances. Checked timing (with a strobe + dwell
meter). Dressed points. Replaced: plugs, air filter, main jets, idle
jets. Measured float heights (by where the tang touches the float
valve). Cleaned carbs in an ultrasonic cleaner, several times, for
several hours. Made sure that all the carb drillings are clear by
spraying carb cleaner through them and checking it exits where it
should.
Now to where I need help. It is running very rich. Black smoke on
tickover. Black exhaust. (No smoke on/after a run) It has that rich
smell of unburnt fuel. It is only doing 23mpg, I think it should do
over 40mpg? (Can anyone confirm that please?). But the occasional over
revving on tickover makes me think it's a low float height (eg, if I
turn the fuel off, the rev's slowly rise), so that's got me stumped too.
What else could be wrong? I am wondering if it could be the float
height - could that have such a dramatic effect on mixture/fuel
consumption? I have not replaced the float valves - perhaps that should
be my next step? I have set the height statically, I have been trying
to get some tube to make a float height gauge so I can see the real
level but so far I've not been able to find a way to do that - anyone
else solve that? I'm reluctant to replace the entire float valves cos
they're quite expensive (12 quid + VAT EACH at Dave Silver's)
There's a guy advertising in this month's Classic Bike Guide - Paul -
"Berkshire Bike Repairs", says he specialises in 60's-70's Jap bikes -
has anyone used him/know if he is any good?
TIA, angle grinder at the ready...
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you do have the right sized carbs, jets etc for your bike don't you ?
take the airfilter out to see if it leans it up a bit. Its not clogged
up is it ?
needle settings correct ?
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It's all definitely standard, it's even still got a stock exhaust on
it. (I hate to think what that might cost to replace). Air filter is
brand new so shouldn't be the problem. I will check the needles again
but I'm fairly sure they are in the right place.
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Bad form etc. I checked the plugs and no surprise they are all black.
searched the web for a lot of posts (many of them by TOG) and am still convinced
it's something worn/clogged up in the carbs. I refuse to be beaten,
there MUST be a way to get previously gummed carbs to work. I have
three sets of carbs in total for this machine but unfortunately none of
them have come off a recently running machine so they are all as
good/bad as each other.
I have taken the plunge and ordered some float valves. These guys
Silver.
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New float valves and seats didn't cure it. I took one of the other sets
of carbs I had apart, boiled them up in the ultrasonic cleaner, this
time firstly with some washing powder, then a dishwasher tablet. In
addition I blasted them with loads of carb cleaner and compressed air
through all of the little passageways. Changed all the bits over from
the other set of carbs. Set float heights to 26mm from the lip of the
carb.
Sync'ed the carbs on the bench by checking the pistons all closed at
exactly the same time. Set the idle mixture screws to their mid-setting.
Put them all back on, the bike seems to run -much- sweeter now without
that petrol smell/black smoke. I haven't checked the mpg yet.
As I changed several things at the same time it's hard to be
conclusive, but it does seem like maybe slightly restricted internal
passageways in the carb body was causing the rich running.
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To be continued.
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