You are here:

Motorcycle Repair/1971 Honda CB 500 will not idle


QUESTION: I have a 1971 Honda CB500 four cylinder motorcycle with 21,000 miles on it that I bought new. I obviously have not ridden it a lot, but it has always started on the first crank and idled quietly at 1000 rpm.  The only problem I was having with it was sometimes the carburetors would overflow gas when I shut it down.  So I took it to the only repair shop in Augusta, GA that works on these old Hondas.  They had it all last summer and it has not run right since.  Although he said he replaced the needle valves and seats, it still leaks gas and now if the idle falls below 2500 rpm it dies, and it does not want to start without ether.  I use to start it with the choke on and then in about 30 seconds open the coke fully.  If I left it closed the engine would chug or die.  Now, no mater what temperature it is, it idles much faster 5500 rpm with the choke closed.  This is what I have checked:

Points: OK

Timing: OK

Float level adjustments: they may not be perfect, but they are close.

Vacuum: (I have the Honda vacuum gauge set) The vacuum is suppose to be 20 inches Hg at 1000 rpm, but since I cannot get it to run at 1000 rpm I checked the vacuum at 3000 rpm.  They ranged from  4 to 5 inches Hg.  I can adjust any one carburetor down to 20 inches, but not more than one at a time without it dying.  I ended up setting them all at 7 inches Hg.

Idle Mixture: Screws were originally open 1.5 turns which is spec, but I was able to get the idle down from 3000 to 2500 rpm by adjusting them all to 1.75 turns more there way did not help.

There is no problem with the gas tank vent.

Choke position: I even took the inlet manifold off and verified that the choke plates were closed when the lever was in the closed position.

Does anyone know what is wrong with this bike?

ANSWER: Hi, I believe from the information you gave me that you still have carb ot intake leak problems

If you have a vacuum operated fuel petcock valve the diaphragm could be
allowing raw fuel into the engine.

The carbs should not leak fuel when properly rebuild. Sometimes aftermarket
float needles and seats can be fussy.
If you remove the float bowl and lift up on the float the fuel should stop
totally or you have a float needle/seat problem. Some float seats have
a small oring on the brass valve, if those leak the carb will flood fuel.

Another possibility is air leaks around the rubber carb mounts, if they leak
the idle will be lean and cause a high idle.
They can sometimes have cracks in the rubber as well.
Carb spray around the rubber mounts may indicate if they are leaking by a change in engine speed.

The slides inside the carbs should have the cutaway at the rear
and should all be sitting at similar heights when properly synced.
Try lowering the slides and then set them all so they are near bottomed
equally. Lower the slides and the idle screw and start tuning from there.
The vacuum will not read right with a fast idle.

Makes sure the choke is opening and closing well.
Also the throttle cable.

Check that the top caps on the carbs are not leaking vacuum,
they have a gasket and it must seal or you will get air leaks
and high idle from those.

On rare occasions the carbs air vent passages can create
fuel leaks, soaking the carbs in a good carb cleaner might help
those and using compressed air after.

If you know someone with an ultrasonic cleaner they can
be good at clearing small passageways in the carbs.

Just as a final note was the auto advance unit under the points
plate checked to be working and not seized in the full advance
position?  It should move outward slightly against the springs
and then retract when free.

Do the usual stuff like plugs, points, air filter.
Use a fuel filter on the fuel line always and new fuel.

Let me know if you find anything that looks off.

**Forgot to mention the valves should have clearance.
If they are too tight it will cause running problems as well.

Wayne S

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

QUESTION: I think I follow everything except the part about adjusting the slides.  I have a repair manual and an exploded view of the carburator, but not a clue how to adjust the slides.

Is the fact that all the carburators seem to be functioning the same any clue?  Or at least make some possiblities more likily?

Thank you very much.

Hi Craig,

When the bike is running normally the slides are the thing
that increases/decreases your engines speed.

If one is higher then the idle could be erratic.
Put your finger in the rear of the carb and
you can feel the slide lift when you twist the throttle.

The slides all have a cutaway at the rear bottom to allow
air into the engine. Also you can check if the slides
are sticking or if they lift and lower properly.

When the throttle is closed the slides should be near bottomed
out. The idle speed screw just lifts them enough to idle.

When you sync carbs what you are really doing
is making sure the slides are lifting at the same time
and closing in sync as well.

The slide is attached to your main needle that
goes through the main jet to control fuel flow.

If the slide is not down then the needle will allow fuel
to flow through the main jet. This is what happens at
higher speeds but at idle you want the slide and needle
near bottom.

Let me know if this helps.


Motorcycle Repair

All Answers

Answers by Expert:

Ask Experts


Wayne S.


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


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

Canadian Motorcycling magazine article

Licensed Motorcycle Mechanic

©2017 All rights reserved.