Odd Problem with a '81 CB750C

 Hello Folks,

I am very new to the newsgroup, but far from new to newsgroups and the
internet in general.

I purchased a 1981 Honda CB750C late in the fall last year. I mesed around
with getting it running a bit in the fall but had a problem I diagnosed as a
coil problem (bad diagnosis..)

First off here is the problem:

When you start the bike it idles fine - you give the throttle a quick snap
and it seamlessly revs through the RPM range - however this is when the nice
stuff stops.. after this there is little to no mid range. Cyls 3 & 4 flood
out and backfire.  The plugs are wet and like I say, it backfires from only
these 2 cylinders.

Having not really looked all that hard at the bike I assumed there was a
coil for each pair of cylinders - I was wrong. While there are 2 coils, coil
A is for cyl 1 & 4 while coil B is for cyl 2 & 3.

While all plugs do have spark I find that the spark isn't really anything to
get excited about. The spark is weak and jumps around - but none the less -
there is spark. I replaced the coils on my bike with a set from a bike that
had 10,000km's on it, so the coils should be good - but are giving me the
exact same problem as my old coils.

Anyone have any idea what would cause my cylinders to flood out at mid
range? I have had the carbs apart 7 (yes 7....) times checking for dirt etc
all with no luck.

Thanks in advance folks!

Andrew Barber,

Nova Scotia, Canada.

 Have you synced the carbs?
Are the rubber manifold boots old and hardened?
Have you looked at the valve adjustment?

 Does it have point ignition?

--
Paul Calman, Hathaway Pines, California

 I agree.
It's in the ignition system somewhere, not the carbs.
Check the points, also condensers, and on a scoot that
old, check for bad plug wires or caps.
Best of luck, shiney side up,
MadDog -" R75/5 forever!"

 
MadDog...@yahoo.com wrote:

I tend to blame the carbs...

I would believe that it was an ignition problem if #1 and #4 fouled, or
#2 and #3 fouled...

The problem might just be crud in the fuel lines above the fuel shut
off needle valves in #3 and #4 carbs. When the floats drop from a
little fuel being sucked out of the bowls, the needle valves might be
getting blocked by debris, and the fuel level in the bowls might be
rising too high...

And, how does the "choke" on a CB-750C work? Is it a starting
enrichenment valve, like Mikuni carbs have? The enrichenment valve has
a little rubber coated plunger that is pulled up unblocking an air
passage alongside the main bore of the carb. The EV sucks fuel directly
out of the float bowl through its own jet. Could it be that
two EV's are sticking open a little because they're out of adjustment?

The symptom that points toward the carbs instead of the ignition system
is that cylinders #3 and #4 foul out together. But #1 and #2 stay
clean...

On an engine with two 2-lead coils, one coil fires #1 and #4 at the
same time, with one set of points opening to cause the coils to fire.
One spark is on the compression stroke of the cylinder that needs to
fire, the other spark is wasted into the cylinder coming up on the
exhaust stroke

Then the other coil fires #2 and #3 cylinder simultaneously...