Motorcycle Repair: Really boggy off idle, vacuum leaks, fuel accelerator

Question
QUESTION: So i have a 1979 honda cb650, it has the mechanical carbs, not the CV ones.  
I recently got it running well enough to take it out.  It runs well and fires on
all 4 cylinders during idle (the exhaust headers get hot) I have adjusted the
valves and the cam chain, cleaned the tank, gone through the carbs (multiple
times) changed the oil and spark plugs (twice), and it still seems to lag until
about 2.5 to 3k rpm.  If i keep the choke a little closed, i get much better
response and power off of idle.  Once the engine is warm and I push the
choke in it really has a hard time getting me rolling.  I have tried everything I
can think of, and I did check the charging system and the spark, and they
seemed fine.  It idles fine at about 1.5, but as soon as I put any load on the
engine, it really begins to stumble.  The only thing I haven't done yet is synch
the carbs since I don't have the right tool.  But I don't think that unsynched
carbs could cause such terrible response.  Another thing that happens is after
about 1/4 throttle the bike really picks up and runs fine.  I have tried
everything that I can think of so any ideas from an expert would be greatly
appreciated.  Once I did try putting tape over part of the air intake hole, and
it really helped my low end power, but sucked power from the top end, so I
think that it is running rich at idle, and I have looked for vacuum leaks and
haven't been able to find any.  The bike is all stock to, in case that helps.

Jonathan

ANSWER: Jon, the problem is still fuel delivery.  I believe on that year you had a fuel accelerator pump.  It could be poor adjustment on the injector pump.  It could be a broken diaphragm in the injection pump, or the inlet or outlet check valves may not be adjusted right.  You should get a small squirt of fuel at the instant the throttle is moved.  If you don't get the injection, the engine will bog.

Regards
Rich

---------- FOLLOW-UP ----------

QUESTION: I have checked the accelerator pump, and it seems to be working correctly.  
There is not a tear in the diaphragm.  Even under steady throttle conditions the
engine seems to bog at lower throttle openings, and adjusting the idle screws
doesn't seem to affect performance at all either.  Any other ideas?

Answer
Jonathan, the engine is running too lean meaning it isn't getting enough fuel as you tip in the throttle.  That year honda tried to intentionally lean out the carbs to meet emissions but they added the accelerator pump to address the problem you are experiencing.  There are four things that can cause this problem.  There could be an air leak down stream of the throttle slides, the float level can be too low, the low speed jets could be plugged or partially plugged, or the accelerator pump isn't working properly.  

To check the accelerator pump you have to pull the carbs and fill the float bowls with fuel.  Then open and close the throttle and watch when fuel starts to inject.  It has to inject fuel immediately when the throttle moves.  If the throttle moves any amount without fuel being injected, it will cause the problem you are experiencing.  


Regards
Rich