Topic: 1973 CB500 Four Running Rich

[Marv]

Evening all,

I am having some problems with 1973 CB500 Four which is running too rich.  It starts well but after about 5 minutes the plugs are black as black and no longer create a spark.  I have checked all the carbs and the current setting is....

Air / fuel screw:  2 and a half turns out
Pilot Jet #40
Main Jet #100
Needle 3 td Position Down
Exhaust: 4 into 1

The needle was on the 4th position down which I believe is standard but I tried the 3 position to make it a little leaner.

The 2 things which I am thinking:-

1.  The timing is out,
2.  The "O" rings on the main jet are worn and could be sucking through more fuel.

Can anyone point me to a timing for dummies page?  I have never tried to do it before so am a bit lost.

Am I heading in the right direction or has anybody got any other suggestions?

Thanks in advance.

[Desert Dan]

Do you have pods or a stock air box? What elevation are you at? And 2.5 turns out on the idle screws seems a bit excessive, try 1.25-1.5 turns out.

[Marv]

Hi Dan,

I'm in Gloucester (Uk) apparently 100 feet above sea level.

At the moment there is no air box fitted.

If I put the air screw at 1.5 turns out would that not make it richer?

Cheers.

[Desert Dan]

Hi Dan,

I'm in Gloucester (Uk) apparently 100 feet above sea level.

At the moment there is no air box fitted.

If I put the air screw at 1.5 turns out would that not make it richer?

Cheers.

The way I understand it is the mixture screws are fuel screws for the idle circuit, which lean the mixture when you turn them in (clockwise) and richen the mixture when you turn them out (counter clockwise), two strokes generally have air screws for their mixture screws, that work oppositely, in to richen out to lean. So going off that you turn it in to lean your mixture, but with a  4into1 and no airbox/No air filter? and at 100ft msl you should be running lean regardless of mixture screw settings. I concur with Cal in that you should check float level, or there may be other issues like oil leaking in to the cylinders fowling your plugs. If the bike smokes allot or smokes allot when it first starts you may be burning oil.

[Hersbird]

The float bowl overflows can be plugged in which case it will just overflow to the motor.
I need to do the clear tube method on mine as well clean the tank better as I seem to keep getting sticking open floats, then when reinstalling the bowls often somehow a float will get wedged shut instead. I had every one of them off at least 2 times this spring and now #2 is overflowing again.

[riverfever]

Hi Dan,

I'm in Gloucester (Uk) apparently 100 feet above sea level.

At the moment there is no air box fitted.

If I put the air screw at 1.5 turns out would that not make it richer?

Cheers.

Can someone confirm this? I thought with the screws being on the airbox side that backing them out would allow more air in and lean things out.

[Desert Dan]

Hi Dan,

I'm in Gloucester (Uk) apparently 100 feet above sea level.

At the moment there is no air box fitted.

If I put the air screw at 1.5 turns out would that not make it richer?

Cheers.

Can someone confirm this? I thought with the screws being on the airbox side that backing them out would allow more air in and lean things out.

I would also like to withdraw my statement and have some one more knowledgeable than I confirm this, the general rule of thumb being screws on the air box side are airscrews and ones on the manifold side are fuel screws. It's just a rule of thumb that most four strokes have fuel screws and that air screws are mostly on the air box side of carbs, but it's not written in stone. Ive succeeded in confusing myself and creating a Schrodinger's mixture screw quandary.

[riverfever]

Dan I only ask because this is the information I've been going off.

http://forums.sohc4.net/index.php/topic,5410.0.html

[Desert Dan]

Well that was very informative and led me to the download section where I discovered that these slide type carburetors are equipped with airscrews and not fuel screws. I apologize for spreading disinformation and assuming these used fuel screws, I based my assumption on my knowledge of of older car carburetors and a few 2 stroke bikes. Everything you could want to know about these carbs is here http://manuals.sohc4.net/general/ and this particular quandary can be resolved by reading the section titled "Idle and low speed system"
  With that resolved I still believe that Marv's richness issue is too sever to be resolved using the idle mixture screw and that it's either caused by incorrect float height, or some sort of oil contamination.

[thetar]

So where did we land with this topic?
Mixture screws clockwise makes it richer - by restricting air?

Having similar rich issue with my CB500 and not really sure what do do with the array of available screws to turn.
Thanks

[Deltarider]

Do the chokes close and - more important - re-open properly? Did the bike sit for a long period?
Then, in general, unless the cause is obvious, check the ignition first, all aspects. You'll find descriptions in every manual or here in this forum. If you really can't do it yourself, look for assistance. When the ignition has been checked (you have to do/learn it anyway), it's time for the carbs. Then all the above is worth checking.
If you happen to find the floatlevel not correct, look well what causes it (needle that sticks, small dirt particles?) and think twice before you start messing with the delicate floattangs. Very few POs ever tinkered with them.

The "O" rings on the main jet are worn and could be sucking through more fuel.

Could well be and it is often overlooked. I had this once. Some owners choose to drain the floatbowls every year before hibernation. Over time they can expect deteriorated O-rings. If you like to practice your surgical skills, here's a description of the maintenance you can do on these oldstyle carbs without removing them. http://forums.sohc4.net/index.php/topic,151734.msg1733813.html#msg1733813