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After cleaning and re-build, the leak test has fuel coming out from the venturi/engine side of carb#2. All other carbs are sealing properly.

I just pulled the float bowls off #2 and 4 and both float heights are correct. All needle valves were replaced with OEM valves. All float bowl gaskets replaced. etc.

All the plenum orings were replace and in the correct location, per the Randakk notes. I did not do the main plenum seal since it wasn't flattened or hardened. I expect a failed seal would lead to fuel in the plenum, which there wasn't.

My assumption is the valve is not seating correctly in the seat. Is there any reliable way to check that? Any suggestions for other things to check on carb#2 that might be causing the leak?

how much pressure are you putting on it? I do my tests with a vacuum while I still have the bowls off. I lay the rack upside down, that way I can operate each float individually and see exactly when the valve opens.

Very similar to Whiskerfish' testing method, but I utilize a very low volume hand operated air pressure tester pump instead.
The witness of suspected piece of contamination between the float valve needle and seat <(its external oring, too) will be visually apparent with air pressure. A drop of liquid to show leaking air bubbles in that area will be required.
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Whiskerfish wrote: ↑Tue Sep 17, 2024 9:56 am how much pressure are you putting on it? I do my tests with a vacuum while I still have the bowls off. I lay the rack upside down, that way I can operate each float individually and see exactly when the valve opens.

I previously used an external tank with top of fluid height roughly 2' above the carb rack.

I've since swapped the valves between #2 and #4 and lowered the tank to 18" above the carbs. we'll see if it still leaks on this test. UPDATE - No joy. #2 still leaks

It has been a while but when I did the calculations I came up with 7 foot of head pressure to simulate the stock fuel pump.

toomanybikes wrote: ↑Tue Sep 17, 2024 7:25 am After cleaning and re-build, the leak test has fuel coming out from the venturi/engine side of carb#2. All other carbs are sealing properly.

I just pulled the float bowls off #2 and 4 and both float heights are correct. All needle valves were replaced with OEM valves. All float bowl gaskets replaced. etc.** the leaking carburetor Float valve seat filtering screen is in place, too? **

All the plenum orings were replace and in the correct location, per the Randakk notes. I did not do the main plenum seal since it wasn't flattened or hardened. I expect a failed seal would lead to fuel in the plenum, which there wasn't.

My assumption is the valve is not seating correctly in the seat. Is there any reliable way to check that? Any suggestions for other things to check on carb#2 that might be causing the leak?

toomanybikes wrote: ↑Tue Sep 17, 2024 5:59 pm

Whiskerfish wrote: ↑Tue Sep 17, 2024 9:56 am how much pressure are you putting on it? I do my tests with a vacuum while I still have the bowls off. I lay the rack upside down, that way I can operate each float individually and see exactly when the valve opens.

I previously used an external tank with top of fluid height roughly 2' above the carb rack.

I've since swapped the valves between #2 and #4 and lowered the tank to 18" above the carbs. we'll see if it still leaks on this test. UPDATE - No joy. #2 still leaks

" Is there any reliable way to check that?"
Yes. Flip the rack upside down, remove its fuel bowl and attach your filled and elevated test tank to the rack's inlet fitting .
You may discover on #2 carburetor, after all is said and done, its float valve set is not the source of leaking fuel,

gltriker wrote: ↑Tue Sep 17, 2024 7:42 pm
" Is there any reliable way to check that?"
Yes. Flip the rack upside down, remove its fuel bowl and attach your filled and elevated test tank to the rack's inlet fitting .
You may discover on #2 carburetor, after all is said and done, its float valve set is not the source of leaking fuel,

Good idea, I'll give that a shot.

Thanks

I haven't yet done the upside down leak test, but since the folks on this chat are offering very detailed carb advice, how do yo feel about using cotter pins to hold the various control arms in place? Any chance they will vibrate out?

What kind of rebuild kit did you use on these? A Randakk kit supplies new cotter pins.

I'm wondering too why you would not replace the plenum seal while you have the plenum split. Did you have the seal?

You do say you replaced the carb to plenum seals and say they are all in the correct place. Are you 100% certain? Since you swapped the float valve assemblies and still have a leak on the same carb that might be a good place to check.

robin1731 wrote: ↑Thu Sep 19, 2024 9:01 am What kind of rebuild kit did you use on these? A Randakk kit supplies new cotter pins.

I'm wondering too why you would not replace the plenum seal while you have the plenum split. Did you have the seal?

You do say you replaced the carb to plenum seals and say they are all in the correct place. Are you 100% certain? Since you swapped the float valve assemblies and still have a leak on the same carb that might be a good place to check.

I didn't use a pre-packaged rebuild kit. I have a stash of orings in multiple sizes that I supplement with bike specific parts, like float bowl gaskets.

I didn't replace that plenum gasket because when I pulled them apart it was still supple and not flattened down like the float bowl gaskets were.

I'm 99.9% certain the carb to plenum seals are correctly installed because I had the installation notes open as I rebuilt the carbs and followed those instructions. I even took pics and did use new urings there since the existing ones were brittle

It has been a day or two but I thought they were DRings not ORings for the fuel passage ????

Whiskerfish wrote: ↑Thu Sep 19, 2024 11:27 am It has been a day or two but I thought they were DRings not ORings for the fuel passage ????

I called them URings based on the Randakk's blog ... "Note: the side of these special “U” fuel seals with the radiused profile is installed facing the plenum. The flat side of the seals goes toward the carb bodies." ... but i can see where calling them DRings make sense as well.

Seals look right. Seal could have fallen out of position while putting bodies on plenum. You swapped valve assemblies and the leak stayed on the same carb. Float could be "water logged" (yes fuel, before someone "corrects" me) and sinking. Float could also be rubbing against the bowl gasket if it isn't in position.

Worse case, the carb body could be cracked in the valve assembly area but I have never seen that.

OK, after a few detours into cotter keys, rebuild kits, URings, etc. I believe the root cause has been discovered.

The valve seat screen on carb#2 was previously toast so I bought an aftermarket seat/valve/screen assembly and repurposed the aftermarket screen with the original OEM valve seat and newly purchased OEM valve. I'm all for OEM parts, but the Honda assembly was like $55 and all I needed was the screen.

Turns out the aftermarket screen was slightly longer than the OEM, probably less than 1mm, but long enough that it gets crunched a little when you install the seat. Most likely I felt the tension building and didn't want to over tight the brass valve seat.

After running the leak test for ~20 minutes, no leaks. Now to bench sync, assemble, and test start.

Thanks all