Topic: what the eff is wrong with #2 carb suddenly???

[chippyfive50]

My modded 550 has run great all year, until season end here.  Alll of the sudden it is running very rich  on #2. If I tach it up it runs fine, but @ low/medium revs it is rough and sputtery. Ignition is strong, possibly a float issue??
I took them apart last night and saw no obvious problem. They were rebuilt last winter and are still visibly clean. I need to do some homework on these cb500 carbs anyway,  but hoping for some direction from the boundless knowledge out there.........thanks

[MetalHead550]

So its a 550 with 500 carbs?  Id say take a close look at your air jet.  Pretty small diameter and Id think that if it got clogged at all it would cause a rich condition at low throttle position.  This would make sense since it clears up at higher throttle and the fuel circuit changes to needle jet then main jet were it relies on the slide for air to create vacuum.  (Edited from earlier confused response)  

[chippyfive50]

does a clogged pilot jet cause the fuel to be drawn through the main jet at low rpm??  I am a carb rookie....      My setup is a 71 500 with a 550 motor 40's in the pilot and 115's in the mains, pods and open headers. It ran great all year until now......

[MetalHead550]

What year 550 carbs?

Check this out.  Basic intro to carb theory. 

http://hondaex.com/techtips/carb101.htm

Plus do a search on cleaning pilot jets, its been discussed a few times and I forget the details. 

[TwoTired]

All of the sudden it is running very rich  on #2.  possibly a float issue??
I took them apart last night and saw no obvious problem. They were rebuilt last winter and are still visibly clean.

How do you know #2 is running rich?  Sorry, but I have to question that assumption without reassurance, as it makes a big difference in causal factors.

Please consider thinking about carb throttle position rather than RPM when diagnosing carbs.

Problems at low throttle position/speed, are often low speed jet or related circuitry.   This is a separate fuel pathway than the main and needle fuel source.  But, the main and needle jet don't completely shut off at this throttle position and usually have low contribution.

A plugged slow jet would make low position lean.  Observed symptoms would be similar.
A clogged air jet, closed air bleed screw, or related passageways, would make it rich.
Stuck, blocked open float valve, or incorrect float height causing the bowl to overfill may cause over rich.
Perhaps, check the seal around the main jet of leakage.

 A bunch of other things could cause a lean condition. But, if it is rich there is no need to expound on those.

Cheers,

[Gordon]

does a clogged pilot jet cause the fuel to be drawn through the main jet at low rpm??

No, it doesn't.  A clogged pilot would cause that cylinder to run very lean at low rpm's, or not at all.  Hence the idling cold pipe syndrome when one of them is clogged up.  What leads you to believe that cylinder is running rich?

[MetalHead550]

A clogged air jet, closed air bleed screw, or related passageways, would make it rich.

Yes, thank you.  I was thinking about that..had my stuff confused.  Sorry for the misinformation.

My thinking was that for a carb to suddenly start running rich something would have to block air from the mixture at low speed.  air jet.

[MetalHead550]

Hey TwoTired, question for ya:  On older 550 carbs, say like the ones on my 75.  The mixture screw meters air.  On newer carbs like my 78 550 the mixture screw meters fuel.  Does this mean that when you adjust the screw on the 75 carbs you are allowing more or less air to travel through the AIR jet?  And on the 78 carbs does adjusting the screw govern how much fuel travels through the idle jet while the air traveling through the air jet remains constant?  Clear that up for me if you would.

[chippyfive50]

The #2 plug fouls(real sooty) after 10?  miles.  I did mess with the air screw, and got it to run OK(for a short while) after backing it way out. It runs like crap below say 3/8-1/2 throttle. I suspect it is the float/shutoff, since i neglected, and forgot to mention it began leaking some fuel, but only after I had it parked for about a month.  I suppose too  high of a fuel level will cause the richness at low revs and smooth out as the trhottle opens due to the increased vaccuum lowering the fuel level in the bowl to where it should be at that point??(bernoulli effect type thing??)
thanks for all the replys.......

[MetalHead550]

Well, rich for sure then.  If it was leaking then yeah thats a float/needle+seat situation.  Did you leave your petcock on while it sat for that month.  Good practice to shut it off every time with these old bikes and to drain the floats when they sit for a while.  And after about 3 turns or so out with the pilot screw its not doing any good and the prob lies elsewhere.  Yeah youre right about the fuel level=rich.

[TwoTired]

Hey TwoTired, question for ya:  On older 550 carbs, say like the ones on my 75.  The mixture screw meters air.  On newer carbs like my 78 550 the mixture screw meters fuel.  Does this mean that when you adjust the screw on the 75 carbs you are allowing more or less air to travel through the AIR jet?  And on the 78 carbs does adjusting the screw govern how much fuel travels through the idle jet while the air traveling through the air jet remains constant?  Clear that up for me if you would.

The principle of operation of the slow circuit remains the same for both carb types.  The adjustment screws have a different location in the circuit.
The newest carbs have a final adjustment screw for the idle mixture just prior to introduction into the carb bore.  It's a very small exit hole and the needle is very narrow leading to a very small, fine, and limited range of adjustment. This needle and orifice is fed by an air fuel mixture from the slow jet and its emulsion tube.  The emulsion tube is fed from its very own air jet near the mouth of the carb and, of course, its adjoined fuel orifice.

The earlier carbs have an air bleed screw that restricts the air from the air jet before it gets to the emulsion tube of the slow jet.  The K model 022A carbs have an air bleed screw with a hollow tip and cross drilled hole.  This limits the range of adjustment of the air bleed as the holes bypass the adjustment restriction.  The F or 069A carbs have a solid air bleed screw tip.  This can completly block the flow of air from the air jet to the slow jet emulsion tube.  The 022A carbs are less sensitive to bleed adjustment than the 069A carbs.
Remember the air jet is a restrictor in it own right on both carbs.  The earlier carbs can only diminish the flow from these.
On the other hand, the later carbs idle mixture screw restricts the flow from both the pilot fuel jet and the slow air jet simultanoeusly.

Does this clear things up?

Cheers,

[jdpas29]

The #2 plug fouls(real sooty) after 10?  miles.  I did mess with the air screw, and got it to run OK(for a short while) after backing it way out. It runs like crap below say 3/8-1/2 throttle. I suspect it is the float/shutoff, since i neglected, and forgot to mention it began leaking some fuel, but only after I had it parked for about a month.  I suppose too  high of a fuel level will cause the richness at low revs and smooth out as the trhottle opens due to the increased vaccuum lowering the fuel level in the bowl to where it should be at that point??(bernoulli effect type thing??)
thanks for all the replys.......

totally unrelated, but chippyfive50,  that is one of the best looking motorcycles i have EVER seen.  nice, man!   

[cbjunkie]

but wait! there's more...

http://www.sohc4.us/forums/index.php?topic=3317.0

[MetalHead550]

Does this clear things up?

A great deal.  I know of these orifices and tubes you speak of since Ive been through both types of carbs.  However I was ignorant of their function and simply shot carb cleaner through them, watched were it came out and wondered what the heck they were for.    Next time Im in a set Ill examine them more carefuly and keep your info in mind.  Thanks for taking the time!

Chippy always gets compliments on his bike and rightfully so.  For me its those white walls and that rockin' greeeen paint.  Gives me a hardon every time.

[egar]

I agree with Metalhead. Looking at Chippy's bike is like looking at porn.