Help 1981 Honda 650 Custum Starting Problem

Thanks again everyone for you continued help. I will trying running
the Berryman's through.

To be more accurate, the big bang came out of the mufflers along with
some puffs of black smoke.

And the power loss is pretty severe- I can't get the bike over 30 mph.

 

That bike has a vacuum cutoff for the fuel as well as the manual petcock.
This vacuum widget cuts off fuel flow unless & until it gets engine vacuum.
(Yes, I know it makes no sense to have no fuel flow until the engine is
running, but I guess they figured you'd have enough fuel in the float bowls
to get it started. Me, I just removed the vacuum cutoff and run it the old
fashioned way with just the manual petcock -- instantaneous fuel flow when I
want it, and I don't need a gadget to take care of me forgetting to shut it
off.)
I'm mentioning all this just in case yours is preventing your carbs from
getting enough (or any) fuel to get started. Check that vacuum line for
leaks, and make sure the float chambers are full before you try to start it
up...

 

Something those bikes were (and still are) known for. Almost every CB650 you
find will have a dark area on the fins just under the head on one side. It's
a dead (from age) oil seal that looks bad but doesn't leak enough oil to be
worth the effort of a full engine rebuild to fix it... (Keep track of your
oil level, and if the leak becomes more significant, time for that rebuild.)

 

I'm less familiar with US models, agreed. In the UK, we got the more
basic earlier CB650 Custom, and we never got the upgraded model. So
apologies.

I owned a "straight" CB650 myself - bought one a few years ago. One of
the most under-rated bikes I've ever owned.

This one, though - if it's dipping on the left, it could be the gear
lever shaft seal.

 

You are so right. Virtually every SOHC Honda four I've owned has had a
dark area on one side of the head gasket or the other. My current 400
does, too. And as you say, it never looks bad enough to justify a
tear-down. Never has got that bad, either.

 

Just as an off-the-wall possibility, as Honda was evolving from using
ignition points to electronic ignition systems, their suppliers didn't
seem to know how to build an electronic ignition advance curve into
their ignition control units.

So your Honda has a *mechanical ignition advancer*.

Your advancer unit would be on the right hand side of the engine under
what Honda still called the "points cover".

It is just possible that your advancer is stuck in the full advanced
position. That would make starting very difficult, running down your
battery, and cause a loss of low RPM power.

A strategically applied drop of oil might fix a faulty advancer, or
maybe the return spring popped off.