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Motorcycle Repair/1981 honda cb750 custom

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QUESTION: Having a hesitation problem at'6000 rpm.tank vent open,removed automatic fuel valve,cleaned fuel filter,replaced wires caps and plugs,cleaned carbs thoroughly,checked ignition timing and pulses gap,spark advancer is moving freely,I battery at 14 volts at 2000 rpm and up.if I slowly open up throttle it will rev up to 8000rpm smoothly.if I go wide open from 4500 rpm and up it bogs and slowly works it's way up to 7000rpm.cleaned out fuel peacock.i am lost for the next step.

ANSWER: Hi Ken,

I would suggest doing a spark plug color reading with clean plugs
and after a quick run at the problem rpm.

Shut the engine  off quickly and read the plug colours.
See if it shows signs of being too lean or too rich in fuel/air mixture.

High speed throttle openings are most affected by main jet size.
Also by the jet emulsion tube openings above the main jets.

Total air flow is also important at high speeds. This would be affected
By the air filter and exhaust system

Holes in a carb diaphragm can also be an issue.

Make sure the fuel has no moisture in it.

A plug reading may direct you to the problem
area. Sometimes a minor change in jettimg may help.
See if any plug is a different color than the rest.you may have one bad plug or coil.
Make sure compression readings are similar also.

Plug chart here:  
http://www.verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm

WS
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---------- FOLLOW-UP ----------

QUESTION: Blew out emulsion tubes today,oddly,running with choke on does not change problem nor does adding washers under the large fuel needles(just runs a little less responsive)I am baffled why it will accelerate under mini al throttle but bogs out at  quick open throttle? I disconnected the accelerator pump to see if  that was the problem but did not improve the situation

ANSWER: Hi Ken,

Whenever you quickly crack a throttle open you are making more
air available to the carbs and engine. It automatically
leans out the fuel mixture until the fuel/air mixture equalizes.
If the hesitation is severe then you have a situation where
there is either too much air flow or not enough fuel available.
The engine rpm is also a factor.

The solution may be to provide a more free flowing air
supply to the carbs by opening up the air box or air filter.
The other solution depending on plug colors is to enlarge
the main jets. If it needs more fuel this will help.
If the plugs are running dark or black colored then the air flow
increase may help rather than main jets.

Another possible problem is leaking intake rubber between carbs and head
which reduces the engines total vacuum which also affects carb operation.

The carbs also have a cutaway air valve on the side under
the three screw plate. This has a rubber diaphragm under it
that could crack up. It is basically an antibackfire valve
but it can affect the fuel mixture if faulty.

Always use plug colors as a guide to what is happening in the combustion chambers.
They indicate the true conditions inside the engine and may hold clues that will help
pinpoint a problem. Occasionally a rich or black plug can also indicate a weak ignition coil.
If carb and air flow changes don't help then spark may be the next area to investigate.
All the above is assuming an engine in fair mechanical shape and valve clearance
and ignition timing set correctly.

WS
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---------- FOLLOW-UP ----------

QUESTION: I sprayed wd 40 at the carb boots but nothing changed. The main jet is a 68 and I think the secondary jet is 112. The bike has a kn air filter in factory air box and a 4-2  factory exhaust system.i have changed the side diaphragm valve on carb 1.

Answer

Hi Ken,

have you tried the bike without the air filter in or with the air box lid blocked open?
It almost sounds like you are not getting enough air into the engine at high speeds.
If the plugs are black or dark colored this might be true.
If the plugs are running light colored then you may have a fuel flow
restriction or need larger main jets.

If altering the air flow doesn't change anything then I would look
to the ignition coils. These bikes often need a strong battery to
get full ignition power.


WS
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Wayne S.

Expertise

Licensed Motorcycle Mechanic, Knowledge of motorcycles from 1960 up, Japanese, British and most other brand motorcycle repairs.

Experience

Worked for Yamaha, Honda, Suzuki, Kawasaki British: Triumph, Norton, BSA Other: Most Scooters and Mo-Peds

Publications
Canadian Motorcycling magazine article

Education/Credentials
Licensed Motorcycle Mechanic

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