Topic: Black Smoke cb550

[newcomtd]

I have a 1975 CB550 that I just took on a 160 mile trip.  It now blows out some black smoke when I rev up.  It has never done this before and I was wondering if something is wrong.  The bike has only 5,800 miles on it.

Thanks

[TwoTired]

1. Is the bike in stock configuration?

2. When was it's last complete tune-up?  List what was adressed.

3. How old is the air filter?

4. What color are the spark plug center insulator deposits?

5. Any carb work history?

6.  Has your gas mileage suddenly gotten worse?

Black smoke is usually unburned fuel, or soot from the same cause.  It can settle in the exhaust and shake loose under higher vibration pressures.

So, depending on your answers to the above, it may be something as simple as the muffler was completely dried out on your trip, and condensation that would normally hold the soot deposits inside are gone and the soot now blows out when you rev it.  Or, you may have a current running-too-rich condition.

These old bikes idle on the rich side.  Prolonged running at idle or very low throttle settings, tends to soot up the mufflers.

Cheers,

[csendker]

A crappy air filter will make it run rich.  My air filter looked pretty clean when I bought my bike (no clue how old it was).  However, it had been sittinng idle for over a year, soaking up ambient moisture and whatever else.  I swapped it out for a foam Uni filter and it seemed to breath much better.  I tested this by running for a short while with the filter remover (steer clear of dusty roads...).  5,800 miles on the bike & 2,500 on the filter - how old is it?

Does it blow smoke only at idle?  only at high rpm's? only while revving?  any or all?  Assuming soot = running rich, the carbs have idle jets, main jets, slide valves, etc.  You may be able to track down what's wrong by finding what is happening at different rpm's.

Pull all 4 plugs to see which cylinders are experiencing the problem.  You may have an issue with only one carb, but with a 4-1, you can't tell by the exhaust alone.  A plug chop may help --> http://www.sohc4.us/forums/index.php?topic=951.msg6073#msg6073

[TwoTired]

3.  Air filter has about 2,500 miles on it

A bit early for this to be a problem.  But, can't rule it out quite yet.  The paper filters work by trapping dirt, etc. and in the process become more restrictive.  This acts as partial choke making the mixture progressively richer as it traps dirt.  Usually the paper filter is back blown by compressed air at 3000 miles and replaced at 6000.  This schedule, of course, gets reduced if operating in dusty conditions and certainly if wet, humid and dusty as the dirt gets "glued" in and back blowing is ineffective.  You'll have to assess your current filter.  One test is to temporarily remove it to see how that effects your symptoms.

4.  Spark plug centers are just kind of sooty but not too bad at all

This is a pretty good indication of running too rich, particularly after a 160 mile trip at cruise speeds, it should be tan to light tan.
I forgot to ask what your plug numbers are.  If you've got D8EA in the bike, the heat range is probably too cold for the plugs to self clean properly.  If NGK Brand, they should be D7EA.

5.  Carbs gone through before I owned it about a year ago at 3,050 miles

Any idea if they were just cleaned or a "kit" of some sort used that replaced internal components besides the rubber bits?  Just wondering if throttle valve needles for a K bike were put in, or different mains, etc.  I'm assuming the bike in your avatar is the topic bike, an F model. Specific kits for these aren't the easiest to find.  Other possibilities include internal emulsion tubes not properly cleaned.

Do you happen to know what the Idle Air Bleed screws are set at?  Should be about 1 1/2 turns out from lightly seated.

Just to put your mind at ease, it's unlikely you have a serious problem with your bike.  Nonetheless, it can be made better and will probably make better power after correction.

Cheers,