Topic: Yet another carb question on PD41As.. or is it the engine?

[timmerk]

Forgive me if I sound like an idiot, but I'm new to the world of engines, carbs, etc. I just rebuilt my PD41A carbs (had to split them) from a CB750 K7, and am having one last problem I can't figure out. The floats are set correctly, and verified with the clear tube test, and the float needle and seats are 100% and not leaking, as tested with some very weak rubber bands holding the floats needles up.

Engine runs fine with the carbs, BUT gas flows out of carb #1's overflow tube at a rate of a few drops every 10 minutes. I attached clear rubber tubes to measure it, and there is a good 3-4 inches of gas leaking overnight. (By the way, carbs 2-4 don't leak even a drop.) When the engine is running, some "bubbling gas" comes out of the overflow tube and keeps bubbling. I might be explaining this poorly, but it's the best way I can describe it.

I double and triple checked the float needle and seat, and it's 100% sealed and not leaking. I double and triple checked the overflow tube for leaks or cracks... nothing. I even switched out the drain screw, o-ring and float bowl with another good one to be safe.. nothing.

I then saw a post where someone was having fuel leak between the carb and the pressed-in float needle seat. I added some fuel dye and used a black light - no leak there. Since I had the dye in the gas, I also filled up the bowl on the bench in a clear cup and monitored for leaks in case I missed a crack in it. Nothing.

This is where it gets strange: after adding the fuel dye and running the engine for a while, I expected to see the bubbling gas coming from the overflow tube turn neon yellow/orange. It didn't even after 20 minutes of running. Carbs 2-4 had the dyed fuel. Wondering what was going on, I pulled the carbs off, and noticed heavy staining on the intake manifolds (as expected), but only on intakes 2-4. Intake 1 was barely stained, suggesting gas was barely being drawn in. That might explain why the dyed fuel never came out - I suspect gas wasn't being consumed, so the dye-free gas was in there for a while.

To see why, I checked the spark plug in cylinder 1, and it looked fine. The gap was correct, there was strong spark, etc.

So, my question is: does this seem like a carb problem, or an engine problem which is causing the carb problem? Any pointers of what I should check next? Thanks!

[dusterdude]

Is the pipe on no. 1 getting hot when the bike is running?

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[TwoTired]

When was the last tune up check list completed?

Have you performed a vacuum sync of the carbs?

Have you performed a compression or leak down test?

Are the head pipes getting evenly hot?

When you rebuilt the carbs what parts did you replace and were they Keihin brand or aftermarket?

Have you carefully checked the stand pipes in each bowl for cracks?

When doing the mechanical float height check, do so with the carbs on their sides.  That way, the tiny spring pins on the float valves can't get compressed and alter measurements.

Cheers,

[timmerk]

Is the pipe on no. 1 getting hot when the bike is running?

No, I just checked and 2-4 got very hot quick, but I was able to touch #1 for a few minutes before it got too hot to touch.

[timmerk]

When was the last tune up check list completed?

Have you performed a vacuum sync of the carbs?

Have you performed a compression or leak down test?

Are the head pipes getting evenly hot?

When you rebuilt the carbs what parts did you replace and were they Keihin brand or aftermarket?

Have you carefully checked the stand pipes in each bowl for cracks?

When doing the mechanical float height check, do so with the carbs on their sides.  That way, the tiny spring pins on the float valves can't get compressed and alter measurements.

This is my first tune up, and haven't finished everything yet. I haven't done a vacuum sync or compression test yet, but I will. The pipes are not getting evenly hot - #1 is cool for a few minutes.

The carbs were rebuilt using only Honda and Keihin parts. The 77's specify a float height of 12.5mm, but I adjusted them to 14mm after the clear tube method was showing the levels at greater than the acceptable 3mm difference. The stand pipes have no cracks - I tested with fuel dye.

I will try to get ahold of a vacuum sync tool and report back. Thanks for the replies so far!

[dusterdude]

Is the pipe on no. 1 getting hot when the bike is running?

No, I just checked and 2-4 got very hot quick, but I was able to touch #1 for a few minutes before it got too hot to touch.

You're definetly not getting gas thru the number one carb or you have weak compression on that cylinder

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[HondaMan]

The leak sounds suspiciously like a cracked overflow pipe, or carb mount, in the #1 bowl.
Try this: get some of the clear fuel sealant in-a-tube from O'Reilly Auto Parts and dry out the #1 float bowl, then put some around the base of the little pipe, let it dry overnight. Then put it back on and watch to see if it is leaking.

The bowl drain screws on the PD41/42 carbs can also have a poor sealing face. Check yours for pitting (also the seat). I have a bunch of old K7 carbs around, maybe some still have a #1 bowl if you end up needing one.