Topic: Is this a known problem?

[Dirty Steve]

Anyone ever come across the issue of the little tiny pilot jet outlet in the carb venturi clogging up, forcing the engine to draw more or less solid fuel from the main jet instead? I've got one carb that backfires at idle and the corresponding spark plug is sooty black, and fuel is pooling in the intake boot of that same carb.
Is that a known problem?
1975 CB750 K5 with stock carbs/airbox/exhaust.
Thanks...

[drumstyx]

Carbs get dirty in all sorts of ways. Pilot circuits especially. Pull it, clean it, install it, filter it (if daft punk were into bikes).

But seriously, did the bike sit a long while? If so, it's varnish. Otherwise it's debris. Either way, I'd recommend an in-line filter and an inspection of your tank.

[flybox1]

When gas isn't run through carbs on a regular basis, all bets are off.
Carbs need to come off for a thorough cleaning

[754]

I have a Planet blue 74 that got terrible mileage.. Around 20 ish..
Cleaned them a few times, ran wire thru the idles .. Still no luck..changed carbs it was fine..
 I suspect aftermarket brass.. Or float needles leaking..

[drumstyx]

Would you not have noticed it running incredibly rich?

[Dirty Steve]

I understand all these things and appreciate your replies, but I'd like to know specifically if when the pilot circuit is clogged the engine pulls solid fuel through the main jet/needle jet circuit.
Thanks.

[drumstyx]

I understand all these things and appreciate your replies, but I'd like to know specifically if when the pilot circuit is clogged the engine pulls solid fuel through the main jet/needle jet circuit.
Thanks.

In theory, all sorts of whacky things can happen from all sorts of whacky adjustments and states of cleanliness.

I would say though, that generally, no, that doesn't happen. With the throttle CLOSED, the vacuum is all on the engine side of the carb, and there's almost no vacuum at the main jet area.


Was thinking about CV carbs, my bad. It could happen in theory.

If you've got fuel pooling, I'd suspect the float valve too (and by extension the overflow tube, if it's not spilling over correctly...)

[Dirty Steve]

I'll set aside float valves for now-cleaned and replaced multiple times and replaced and adjusted floats (26mm)-and say that the overflow posts inside the bowls are clear, and the tubes as well, but I've never once seen fuel go out of them. I'm wondering if they even come into play in this situation, if the engine is indeed pulling fuel up through the main/needle circuit. There would still be a constant flow of fuel through the carb and no overflow happening.

[flybox1]

I understand all these things and appreciate your replies, but I'd like to know specifically if when the pilot circuit is clogged the engine pulls solid fuel through the main jet/needle jet circuit.
Thanks.

Depends on how bad the clog and if vacuum was equal across all carbs.
If only partially clogged, I would imagine that the fuel still pulled through he idle/pilot circuit would not be atomized as it was intended for a clean burn.
If fully clogged, I dont know that there would be enough vacuum to get gas up through the main/needle/emulsion tube.  If it was,  could only assume it would not be atomized.

Either way.  Carbs need to be detailed, followed by clear tube test to verify fuel height, and then a good vacuum sync (assuming all else is in order)

[mrbreeze]

Your last paragraph says it all Fly.

[754]

How long do plugs last I was fouling one every other day.. Thought I had a bad hole, it was the carbs.
 Pay attention to the floats, they can drag on the posts, and hamfisted techs can bend the tang at an angle causing the needle to move sideways.

[Dirty Steve]

Okay. Thanks. I didn't think about the possibility of solid or insufficiently-atomized fuel coming up through a partially-clogged pilot outlet.
Something I didn't mention before-I did a test to see if the float assemblies were sealing (hooked up a full tank to the carbs while they were still outside the bike and opened up the tap) and there was no leakage whatsoever. I even blew as hard as I could on the fuel line itself to see if I could make something come up into the venturies and got nothing.

[Dirty Steve]

How long do plugs last I was fouling one every other day.. Thought I had a bad hole, it was the carbs.

Not sure. The plug is sooty but I think it's still firing. I haven't tested it yet.

[flybox1]

I even blew as hard as I could on the fuel line itself to see if I could make something come up into the venturies and got nothing.

The carb atmospheric vents prevent fuel from going anywhere under these conditions.

[Dirty Steve]

I even blew as hard as I could on the fuel line itself to see if I could make something come up into the venturies and got nothing.

The carb atmospheric vents prevent fuel from going anywhere under these conditions.

Are you saying the fuel would go out the vents instead of through the jets into the venturi?

[flybox1]

I even blew as hard as I could on the fuel line itself to see if I could make something come up into the venturies and got nothing.

The carb atmospheric vents prevent fuel from going anywhere under these conditions.

Are you saying the fuel would go out the vents instead of through the jets into the venturi?

No, your breath goes out these vents, which prevents fuel levels from changing.

[Dirty Steve]

Okay, maybe this means something-there was fuel in the line as I was blowing on it, but I wasn't able to force anything through (fuel or air). It was completely pressurized.

[drumstyx]

Okay, maybe this means something-there was fuel in the line as I was blowing on it, but I wasn't able to force anything through (fuel or air). It was completely pressurized.

That definitely shouldn't be...the valves only gently seat, I'm pretty positive your breath pressure should be able to overcome them.

[flybox1]

Was there fuel in the bowls to begin with? 
If you answer yes, it only confirms your float valves are working (because the floats have closed them due to the fuel in the bowls)
You would have to pressurize with fuel enough to break this seal, and raise the bowl fuel level in the bowls >6mm with just the fuel in the fuel line.......but then there is the overflow tube it runs into first....so it would leak there right away
You would have to raise the fuel level in the bowls FASTER than it could evacuate through the overflows to get it to rise through the jets and into the venturis.

With only fuel in the fuel line, you'd certainly run out of fuel before the bowls got even close to the overflows.

[Dirty Steve]

Okay, maybe this means something-there was fuel in the line as I was blowing on it, but I wasn't able to force anything through (fuel or air). It was completely pressurized.

That definitely shouldn't be...the valves only gently seat, I'm pretty positive your breath pressure should be able to overcome them.

Okay, so your thinking is my breath should be able to overcome the float valves and push any fuel in the line through the bowls and out the overflow tubes and any air that comes after would pass through the vents?

[Dirty Steve]

I know this sounds pedantic, but it may help me get my head around this and understand the system better.

[flybox1]

^^...out the overflow tubes first....IF there was enough fuel in the fuel line to actually raise the bowl fuel level high enough.

so your pressurizing of the fuel in the fuel line into the carbs really tests nothing.
You didnt push enough to reach the overflows, and when you ran out of fuel, your breath just went out the atm vents....

[Dirty Steve]

Well, I meant that theoretically fuel would first pass through the overflows, then the air that followed would go out the vents (and/or the overflows themselves, I suppose), but I do appreciate your point. Getting back to my earlier point, though, I wasn't able to force anything through the fuel line at all. I didn't force any fuel past the float valves into the bowls and then blow air out the vents as you supposed. It was completely clogged.

[flybox1]

Doesnt prove it is clogged.
You are assuming you know what pressure it takes to force the float valves down.

Crack open a drain screw....let ALL the bowl fuel drain out.....then, turn on your tank fuel supply.
If more fuel flows out the drain screw, your fuel supply system works and is not clogged.
Close off the drain screw, and fuel should stop when the floats raise....and NOT flow out the overflows.

Either way.  Carbs need to be detailed, followed by clear tube test to verify fuel height, and then a good vacuum sync (assuming all else is in order)

[754]

Is it same cylinder fouling all the time.. Not sure but running vibration might let a float leak that checks out otherwise.. Pretty sure that is what mine was doing..