Topic: Stumped by clear tube test [SOLVED]

[bertoxxulous]

Hi,

I've been struggling with my CB750 F2 (PD 41B carbs) where cylinder 4 did not fire at idle (but would power up when giving it some throttle for a few seconds). I checked the plug, and lo and behold, it's completely black. I pulled the slow jet, and it looks perfectly clear. I then proceeded to try and check the float level with a clear tube (which I've done several times before with success), but this time, when checking cylinder 4, the level in the clear tube went up all the way to the fuel petcock level. What's even weirder, is that no fuel was flowing outside of the carb. To my understanding of basic physics, the level in the tube should reflect the level in the carb, but clearly this is not the case, as I would expect fuel to be flowing out of the air inlet. I pulled the carbs, and made sure the vent is clear by blowing compressed air through it, which also worked fine. I can also easily blow air through the overflow tube.

I'm now completely stumped, and really do not understand what's going on here. What would make the fuel level in the clear tube rise that high, when the fuel inside the bowl is clearly not that high? I have the carbs off of the bike, and can take pictures of whatever if needed.

[willbird]

Is the overflow tube in the float bowl plugged up ? ??

[scottly]

The PD carbs have a teeny tiny vent hole on the side of the carb body. Make sure they are open.

[Medyo Bastos]

The PD carbs have a teeny tiny vent hole on the side of the carb body. Make sure they are open.

+1


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[bertoxxulous]

Is the overflow tube in the float bowl plugged up ? ??

Overflow tube is clear, and I can easily blow air through it.

The PD carbs have a teeny tiny vent hole on the side of the carb body. Make sure they are open.

Vent was also my first thought, and it took me quite a while to even find it. Mine does not look like the one you're showing.

I've attached photos of the offending carb, with arrows pointing to what I surmise must be the vent (which is actually a tiny brass tube). Don't mind the missing tube connecting the accelerator squirter channel - I messed that up as I was removing the carbs.

[scottly]

The inside of the carb looks the same as my K7 carb, but like you say, yours has a tube out the side where mine just has a hole. How big is the hole through the tube? The hole in my carb is only .014" or so, and I had a hard time finding it, too.

[bertoxxulous]

The inside of the carb looks the same as my K7 carb, but like you say, yours has a tube out the side where mine just has a hole. How big is the hole through the tube? The hole in my carb is only .014" or so, and I had a hard time finding it, too.

Yeah, 14 mil sounds about right. I attached a tube to it, and confirmed that it's clear using compressed air.

[willbird]

The only carbs I have worked with are round tops, but the bowl vent and the overflow are two different things. But I really ponder sometimes about some of the angst about the bowl vents when the overflows also provide some bowl venting IMHO.

Linked picture just for discussion

https://i.ebayimg.com/images/g/KO8AAOSwaoZl1ZUe/s-l1600.webp

[jonda500]

must have moved the clear tube (up) after letting fuel into it?
John

[bertoxxulous]

must have moved the clear tube (up) after letting fuel into it?
John

Yeah, I'd have thought so too, but I've repeated the exercise multiple times with both carbs 1 and 4 and it's only 4 that's given me this issue and it's the same result every time.

[rotortiller]

Did you physically move the float to shutoff position and see the fuel stop flowing past the needle valve? If that seems normal I would next check the buoyancy by swapping floats from one carb to another to cover the bases. If it all checks out and the float chamber is vented to the atmosphere you should be good for the correct fuel level in question.

[jgger]

^^^^This is what I was thinking too. Also if that float is saturated you may want to smell the oil for a hint of gas.

[bertoxxulous]

So after spending a few more hours in the garage, I eventually discovered what was up. I was completely out of options after swapping almost everything possible from carb 3 to carb 4, with the issue still persisting, so I ended up removing carb 4 from the rack for further inspection, and going back to square one.

Well, turns out that while the vent was not completely clogged, and it did allow some air to pass through it, it was gunked up pretty bad, and the effective diameter was probably one third of carb 3's. After finding a suitable piece of wire and clearing the hole as good as possible, I reassembled the rack, and lo and behold, no more gasoline flooding on my table!

Thanks for all the support.

Edit: Also, I will definitely be replacing the oil regardless. I know that the airbox was flooded at least a couple of times, so it's not impossible the cylinders have been, too.

[scottly]

I would drill those holes out to 1/16" or so.
BTW, some folks have reported problems with the PD carbs when riding in the rain, with the drain/overflow hoses routed in front of the rear tire. Move the hoses away from the tire so water flinging off the tire doesn't spray on the hose ends. Also, in the event of a flooding carb, you don't want gas dripping in front of the tire.

[HondaMan]

Edit: Also, I will definitely be replacing the oil regardless. I know that the airbox was flooded at least a couple of times, so it's not impossible the cylinders have been, too.

Smart move!
Those little bowl vent that have a spigot on them should always be connected to a hose (or the adjacent carb) that goes over and behind the airbox for quiet air, too. I often find these missing on the K7/8 carbs, and it causes variable float bowl levels when riding interstates in crosswinds. This feels like a real slow "surge" of power (or loss thereof).