Topic: CB500 runs lean

[vincentvega]

Okay so at my wits end here i hope someone can help me out. First the measurements of the old girl 1972 CB500, 105 main jets, 40 slow jet, 22 mm float height stock pipes and air box.

The bike was serviced by the dealer when the last owner (friend of mine) first got it. Inside of the carb looks perfect, i didnt touch the needles when I took it apart or crack the idle screws as I dont have an easy way to sync the carbs after since I live in an apartment, if i need to do anything with that I will have to rip them off again and figure out how to bench sync.

If I start the bike and leave it on less than a quarter choke it runs fine but as soon as i take it off choke fully i loose all power. I cant see any leaks in the system but the air filter is old, I am thinking of replacing it. Also one of the exhaust pipes has the rear plate that holds the baffle in rotted out but I dont see any other holes in the pipes. I was having one cylinder take a bit to fire when it was colder out but that was likely due to a leak in the system i have since repaired (my own fault, I left a clamp on one of the rubber adapters loose last year when I had the carbs off again).

Any help is much appreciated.

[Duanob]

Sounds like the carbs should come off the bike for a good thorough cleaning. Not sure where you are (USA or Europe) but the main jets are larger than stock (for stock pipes and airbox). The idle/air screws won't do anything to the sync just make sure they are set all the same ~ 1 1/2 turns out. It sounds like you have a vacuum leak somewhere you should be able to eliminate it once you pull teh carbs off. There are o-rings between the intakes and the head that get dry and brittle and crack. its a pretty common place for leaks. Check the intake boots as well for cracks or rot. A clogged air filter will make it run more on the rich side, have you checked your spark plugs? Whitish, black?

[vincentvega]

Carbs came off the bike and I did a full inspection, not a spot of varnish as they were cleaned by the dealer two years ago. I was able to blow air through the passages for the slow jet and I had the main jet and emulsion tube out and they were all clear. The slow jet was also clear.

I am in Canada so I am not sure if the bikes up here used bigger jetting or not due to colder conditions or something but this is what the dealer put in when they rebuilt the carbs. I wouldn't think the bigger jet would cause a run lean issue would it? Plugs were replaced last year by me and I had been riding it with the choke cracked on slightly maybe 18/th or 1/4 on which would richen it i would imagine causing black plugs but I will pull the plugs and see what I have as well.

I will try the propane torch trick on the sections where the O-rings are and see what that does or squirt some WD40 to see if i can get a temporary seal or just pull the carbs off again (sob). I inspected the boots on the engine side and on the airbox side for cracks and didn't see anything but I did find that the engine side boots were very rigid rubber when removed, no visible cracks or anything though.

I am wondering if I should see what the needle position is, maybe the dealer didn't set it to the right default clip position or if the jet is too big as you state maybe the default position isnt correct for this jet size. I didn't want to risk having to resync these things again so I was afraid to muck about with this.

Also the Engine is stamped CB500E I dont know if this is important or not.

[Scott S]

 Are you SURE you are operating the choke properly? It can be confusing on the 500. If the lever is horizontal, the choke is ON. When the lever is down, the choke is OFF.

[CB500_k2]

I ran into this problem when I was getting my CB500 back to like.  I found that the float bowl level was too low.  With proper adjustment, 2 mm below the bowl O ring all was made ok.  I used a plastic tube sight glass my motorcycle mechanic brother gave me to observe the actual level in the bowl.  It consisted of a drilled drain plug with a hose attached which I could connect to each carb and actually measure the liquid level.  With much trial and error I was able to set each carb to a proper level.  Since then she has behaved perfectly.

[Duanob]

Propane Torch trick??? Around gasoline filled carbs? Good luck with that. its easy enough to unbolt the intakes and pull them baqck enough replace the orings.

[iron_worker]

Propane Torch trick??? Around gasoline filled carbs? Good luck with that. its easy enough to unbolt the intakes and pull them baqck enough replace the orings.

The propane torch is NOT LIT. You allow the gas to flow around the outside of the intake tracts. If there is a leak the propane will be drawn in which should cause the RPMs to change alerting you that you have found the leak spot.

 

IW

[vincentvega]

Propane Torch trick??? Around gasoline filled carbs? Good luck with that. its easy enough to unbolt the intakes and pull them baqck enough replace the orings.

The propane torch is NOT LIT. You allow the gas to flow around the outside of the intake tracts. If there is a leak the propane will be drawn in which should cause the RPMs to change alerting you that you have found the leak spot.

 

IW

Yes exactly unlit

I think I may be an idiot and using the choke wrong as previously stated. Ah well at least I know what my carbs are jetted for and my bike should work perfectly now that I know how to use it *facepalm*.

Just need to fix a charging issue caused when the headlight is used now (diodes i think).

Thank you all for the advice, safe travels and happy riding!