Topic: cb500 running rich at idle.

[paul_D]

I am still working on my bike, but I have been extremely busy since summer with my engineering degree (side note: 3rd year going onto a coop with SKF engineering team for a year)

Anyways, my cb500 turns nice shiny spark plugs black in a matter of minutes at idle.
The carbs are 649A with 100/40 jets.
The filter is uni foam.
The rubber boots and runners are all soft and no holes with proper clamps.
I am using a filter cover/tray with the little metal screen.

I have tried air filter on/off, no effect.
I have tried air screw from 0 to 3 turns out with no effect.
I have changed the main jet from 125 (idk why so big) to 100 with no effect.
I have recleaned and checked every part and passage in the carbs 3 times now.

I have never changed the float needle or the seat.

I plan on ordering new float needle and seat set from the Honda dealer, and installing and checking with a clear tube method.  I believe my problem is with the fuel height even though I've checked and rechecked the float heights several times. Installing new spark plugs again.

My questions are, what else should I be checking (I will try and let you know if I've already tried it, Its been awhile and might need my memory refreshed on what else I've tried).
2ND question is can the carbon build up inside the engine from the 3+ previous test cause any problems ? (id rather not open it up again)

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[dave500]

is the filter over oiled?try a 38 slow jet.

[Deltarider]

The little O-rings around the main jets, do they seal well? Any cracks in the towers?
BTW, no 500 or 550 needs a main jet over 100.

[paul_D]

is the filter over oiled?try a 38 slow jet.

The air filter has the lightest coating of oil i could do using K&N red aerosol air filter oil. I did run it without the air filter to see if it would lean it out, it did not.

The next step will be 38's after I try my new needle valve and seats from honda.

The little O-rings around the main jets, do they seal well? Any cracks in the towers?
BTW, no 500 or 550 needs a main jet over 100.

No cracks around the towers, some pitting. I think the o-rings seal well none the less. But im having rich problems at idle so the main jet should have minimal effect right ?

And I know the jets were way too big at 125, swapped in new 100's.


I thought I found some rust in the carbs even though my tank leaks very clean, so I did a 24hr metal rescue soak to be sure.

Finally I think these needles in the attached picture will show part of my problem, also I'm pretty sure they got mixed up when I originally stripped the carbs so that the worn seats did not match the worn needles causing it to overfill even with the floats at 22mm measured from the float just touching the needle without depressing the spring. (hence new ones being ordered)

Furthermore I plan on checking the new valve and seats with the clear tube method.

[paul_D]

By the way, is there any way of telling if the floats are missmatched ? Considering I know the previous owner had been messing with the carbs. They all look identical.

[dave500]

mixing the floats wont hurt,mixing the float needles shouldnt either if they arent worn,make sure you blow compressed air through every hole you can find in the carbs every which way.

[haill]

make sure your float height is spot on make yourself a gauge and with the float arm just resting on the float valve check the the floats are equal to themselves not twisted and at the correct height 22mm or 0.89in from the carb body for too much fuel in the bowls will make your bike run rich at idle.

[Deltarider]

No cracks around the towers, some pitting. I think the o-rings seal well none the less. But im having rich problems at idle so the main jet should have minimal effect right ?

It's not about the main jets themselves. If they don't seal well or if there's some other leak (crack maybe) the high vacuum at idle will make the engine suck very hard whatever fuel it can get through any little hole.

[paul_D]

make sure your float height is spot on make yourself a gauge and with the float arm just resting on the float valve check the the floats are equal to themselves not twisted and at the correct height 22mm or 0.89in from the carb body for too much fuel in the bowls will make your bike run rich at idle.

made a gauge out of cardboard and measured each carb float height.. all spot on 22mm.
Checked each carb with a clear tube and 6mm carb sync adapters that were wrong pitch but worked and all the bubles were just below the top of the bowl.

No cracks around the towers, some pitting. I think the o-rings seal well none the less. But im having rich problems at idle so the main jet should have minimal effect right ?

It's not about the main jets themselves. If they don't seal well or if there's some other leak (crack maybe) the high vacuum at idle will make the engine suck very hard whatever fuel it can get through any little hole.

Got it. checked all the posts and 2 of 4 show corrosion but only on the outside not in the seat for the jet. So they all sealed well.




Now that the battery is fully charged and gas tank recleaned. New float valves installed. Float height hopefully finally set perfectly. I am going to go to my friends place where my bike is and reinstall everything and try starting it. Its a little cold here though, -2 celsius.  Need to warm it up to drain the oil for other problems (see oil pump seal and heavy as hell clutch).

[paul_D]

bike fired right up no problem.

looking for new clutch cable and maybe a lifter.  Clutch lifter rod was 10 1/16". so 1/16 short.