Topic: IVE TORN OUT ALL OF MY HAIR....CB500 IDLE TROUBLES

[MADMAX]

finally got the charging system on my 1973 CB500 four sorted out.  now there is something else timing looks great with strobe, carbs cleaned, bench synced and floats set.  got new boots and K and N filter for stock airbox to replace my pods.  replaced plugs and it fired up and idled great at 1k. 

took it around the block and ran good until warm.  once hot, when i gave it throttle, rpms hung over 3k.  adjusted idle screw back a hair, rpms dropped and killed the bike.  unhooked cables and gave it throttle at the butterfly and it still hung up.  pull clutch and it sometimes climbs higher than 3k.    the idle comes down if i back out the idle screw but once it drops, i have to wind it back in to get the 1k idle again. 

plugs were black and sooty so i wound out air screws 1 and 1/8 turns and replaced plugs again.

soo i tore into the carbs again and replaced the aftermarket idle jets with clean stock ones.  checked float height and it looked spot on.  fired it up and immediately after giving it throttle (cold motor) there was that blasted hanging idle!

checked for airleaks with WD-40 today and noticed no change.  used clear tube method for checking float level as well.  all were sitting just below the top of the bowl. 

any help is greatly appreciated. i have plates for this thing and im ready to get out in this beautiful indiana spring!

[phil71]

Are you saying the linkage hangs or the rpm hangs independent of linkage?

[trueblue]

Check that your fuel hose isn't too close to the head, if it is this can cause vapour lock in the line and cause the idle to hang because it is leaning out

[MADMAX]

rpm hangs independent of the linkage.

fuel lines are routed correctly away from the head.

i should also mention that one header heats slower than the rest.

[trueblue]

The other thing I had with my 650 that caused a hanging idle was the cam timing out a tooth, once this was fixed it became a different bike.

[2manycbs]

     Sounds like an adjustment issue if you are not getting a vacuum leak where the manifolds connect to the head, or at the rubber boots.  When the rpm is hanging up the carbs have bypassed the idle circuit and are running in the power circuit main jet/throttle needle.  When you drop to the idle circuit it dies.  Usually this is caused by extremely lean idle circuit, as in vacuum laeks.   Again confirm no vacuum leaks with wd-40 at the head and boots.  Leave the air box off and check your throttle slides on all four to make sure one is not hanging up.  Check the choke plates off and on.  You should be able to do all this from looking at the back of the carbs.  Then set your mixture screws to two turns out.  Good luck, and remember it's just nuts and bolts.

[dave500]

youve only bench synced aswell,try to get hold of a vacuum set.

[MADMAX]

thanks for the responses.  i will try these things this afternoon. as far as the cam timing being off a tooth, how would one know?

[KB02]

     Sounds like an adjustment issue if you are not getting a vacuum leak where the manifolds connect to the head, or at the rubber boots.  When the rpm is hanging up the carbs have bypassed the idle circuit and are running in the power circuit main jet/throttle needle.  When you drop to the idle circuit it dies.  Usually this is caused by extremely lean idle circuit, as in vacuum laeks.   Again confirm no vacuum leaks with wd-40 at the head and boots.  Leave the air box off and check your throttle slides on all four to make sure one is not hanging up.  Check the choke plates off and on.  You should be able to do all this from looking at the back of the carbs.  Then set your mixture screws to two turns out.  Good luck, and remember it's just nuts and bolts.

+1,000

I had the same issue and after being told a million times that it was a vacuum leak, I finally found it to be a lean idle circuit.

[Lifeislarge]

My bike is doing exactly the same thing.  Power is great off idle, but finding an idle is impossible.  I'm waiting for the manifold adaptors to sync the carbs to come in the mail and will let you know if that makes any difference.  Should get them today.  I've done all the things you've done and had those damn carbs off 4 times now looking for things I may have missed.  I know now that I need to get a good sync on the carbs before I can fully dial in the idle mixture screws.  AARRGGHHHH!!!!!

[haill]

Float height seems to be one of the major problems when your getting soot on your plugs.
lest on my 71 cb500.
are you sure you've got them correct 22mm.
having them set higher 23-24mm might help your condition for they will shut off sooner allowing less gas in the float.
your obviously running rich. So if all you carb parts are stock and they're clean then float height is next.
your air filter is clean? and the airbox not restricted by anything? rag etc.

[trueblue]

thanks for the responses.  i will try these things this afternoon. as far as the cam timing being off a tooth, how would one know?

To check the cam timing on my 650 involved lifting off the rocker gear, but I would leave it till you are certain there are no carb or igniton issues.  If the cam timing is out you will also be down on power at the top end.

[ohiocaferacer]

Your mechanical advance behind the points plate is sticking.
Remove the points plate and spray some wd40 on it and make sure its moving freely.

[MADMAX]

they are all set at 22mm. 

air filter is brand new.

this sticking mechanical advance had not crossed my mind and makes perfect sense.

the beautiful indiana spring i was bragging about left me with a mean cold so i wasnt able to get to it today.  and so it goes. thanks for the help and ill let you know as soon as i get back to the garage.

[MADMAX]

advance was sticking . haha that wasn't so bad.  thanks ohio!

[Spanner 1]

With respect, MadMAX... an advance unit stuck = very neglected bike/maintenance . Time to do a 3, 000 mile tune-up... if you want your bike to be reliable... dont matter if you only ride within 'push home' distance 

[MADMAX]

duly noted!