'91 CB750 carbs keep leaking

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1995 ST1100
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8866
The fuel is coming through the carb throats and is overflowing from the bowls. I've had the bank of 4 carbs off my brother-in-law's bike twice now. The most recent time we found we still had debris in the "fuel rail" portion of the fuel circuit. We have it clean, clean, clean now and it still leaks. Not all the time though. When off the bike they will hold over a 1' of vertical gas line just fine. Let it sit a day or two or mount it on the bike and you can see the gas going though the carbs.

The floats are non-adjustable and they float just fine. The Float pins are straight and not binding. We've done the q-tip with polish trick on the fuel valve seats again. Feeling desperate as the fuel valves LOOK good and the tips are still rubbery, so we've just ordered a new set of K&L fuel valves/needles in case that's what it is. Otherwise I'm stumped. Something we've missed?
 
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Its been many years since I had to address that issue, but I think the only two suspects are the needles and the floats (including the float pins). Either the needle isn't sealing, or the float isn't floating. It only takes a little spec of debris on the needle to leak, so I'm not sure that a visual inspection proves anything. Another option is to drain the carb bowls while on the bike, and hope that the fresh fuel flow clears out whatever is causing the leak.

Another question, have you verified that the inside of the tank is clean?
 
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slmjim

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I wouldn't trust the float needles to keep fuel from overflowing anyway.

The CB750 Nighthawk has a vacuum operated petcock. It has an OFF position. We bought a pair of his & hers '93 Nighthawks new. About five yrs. ago I had to rebuild the petcocks on both because they began leaking, failing to shut off even in the OFF position. They each failed about a year apart, at around 55,000 mi. on mine and about 52,000 mi. on hers. Symptoms were the same in both cases; overflow while sitting for a few days, petcocks OFF.
IMHO, relying on a vacuum petcock in the ON or RESERVE position to keep fuel from flooding the carbs is poor technique. It can lead to oil contamination, hydraulic lock or worse. Just my .02

I agree with dawlby too. Pull the drain plugs and let fuel flush the needle seats.


Good Ridin'
slmjim
 
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dkruitz
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We still have the tank off the bike and are filling the carbs with an auxiliary tank. This bike is a refurb project after sitting for 8 years, and a refurb of the petcock is on the list but so far it's out of the system and not a factor (yet.) We are working on cleaning the tank separately, and once we put it all back on, we will be adding an inline filter to go with the existing tank filter just-in-case. The float pins all seem to work when in place with no noticeable binding. Hoping the new needles/valves do the trick.
 
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Soak everything in some evaporust, then use an ultrasonic cleaner on small parts.
 
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Probably the trick is not to let it sit with the petcock on (or in your case, no petcock). Don't know that it's totally solvable as it seemed to be a feature back then. Yeah, I didn't like the vacuum petcock either.
 

slmjim

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We still have the tank off the bike and are filling the carbs with an auxiliary tank. This bike is a refurb project after sitting for 8 years, and a refurb of the petcock is on the list but so far it's out of the system and not a factor (yet.) We are working on cleaning the tank separately, and once we put it all back on, we will be adding an inline filter to go with the existing tank filter just-in-case. The float pins all seem to work when in place with no noticeable binding. Hoping the new needles/valves do the trick.
New information.

I've never the float bowls off of our Nighthawks, so I'm doing an educated guess here:
Do the float needles have a tiny spring-loaded pin at the end opposite the taper? That pin should be free to move in and out of the needle body. There is a spring inside the needle body that keeps the pin extended, but the pin should be free to move against the spring pressure. There's not much travel available to the pin, maybe 2mm., if that. If the pin is stuck in, I can see how flooding could occur. I've had success freeing needle pins by soaking them in some carb cleaner sprayed into a small vessel, such as a shot glass, then applying heat by setting the soaked needle on a hot light bulb. If the pin is free, work it in and out while immersed upright in carb cleaner to release the air trapped inside, then invert and tap the pin on a piece of white paper to expel the liquid. Repeat until no dark staining is visible on the paper, indicating a clean pin/spring chamber within the needle.

Good Ridin'
slmjim
 
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dkruitz
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New information.

I've never the float bowls off of our Nighthawks, so I'm doing an educated guess here:
Do the float needles have a tiny spring-loaded pin at the end opposite the taper? That pin should be free to move in and out of the needle body. There is a spring inside the needle body that keeps the pin extended, but the pin should be free to move against the spring pressure. There's not much travel available to the pin, maybe 2mm., if that. If the pin is stuck in, I can see how flooding could occur. I've had success freeing needle pins by soaking them in some carb cleaner sprayed into a small vessel, such as a shot glass, then applying heat by setting the soaked needle on a hot light bulb. If the pin is free, work it in and out while immersed upright in carb cleaner to release the air trapped inside, then invert and tap the pin on a piece of white paper to expel the liquid. Repeat until no dark staining is visible on the paper, indicating a clean pin/spring chamber within the needle.

Good Ridin'
slmjim
Thanks for the cleaning tips - they are all working with that little spring plunger. One was initially stuck when I tore into the carbs but they all move freely. Needles should be here tomorrow...
 
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From personal and shared 2nd hand experience, Let me start by saying... It seems like, with carbs, you always end up pulling them 3 times. Also, besides give them OCD level cleaning, needles and bowl gaskets are almost always gonna be needed on 'rebuild' level carb work. Also check the teeny tiny little o-ring on the bowl drain screw. Vacuum problems can sometimes cause these problems as well, but typically on the Nigthawks (every generation) fuel loss like you're seeing is almost always caused by leakage around the tip of the float needles or sometimes around the float bowl gasket. On the plus side, Nighthawks aren't prone to 'hydro-lock'. The worst thing with flooding on them is 'washing out' the cylinders and wearing out the rings.

P.S. I had major carb related headaches with my '83. Now that I've gotten them under control, I only use the valve on my petcock in the fall when I'm getting the bike ready for storage. It doesn't leak so much as a drop the entire season. YMMV, but it can be done.
 
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dkruitz
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Update: the new K&L fuel valves/float needles came in and we are now no longer leaking. Carb is still running funny, but from what I can tell it's the pilot circuit - it's only responsive on the throttle when the choke is on (ie it's running lean). Tank, lines and jets are clean - not sure about the pilot circuit. :(
 

ST1100Y

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Letting it sit for 8 years is a bit harsh... IMHO just about every rubber part could be cracking... incl the diaphragms...
(worked on a Nighthawk recently... among other issues was one diaphragm shot... rebuild kit avail so one doesn't need to buy the expensive slider/diaphragm combo...)
 
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We still have the tank off the bike and are filling the carbs with an auxiliary tank. This bike is a refurb project after sitting for 8 years, and a refurb of the petcock is on the list but so far it's out of the system and not a factor (yet.) We are working on cleaning the tank separately, and once we put it all back on, we will be adding an inline filter to go with the existing tank filter just-in-case. The float pins all seem to work when in place with no noticeable binding. Hoping the new needles/valves do the trick.
I subscribe to the Facebook forum 'Honda CB750 Four'. It's for fans of the single and dual cam CB750's. Lots of talent, contacts, part sources, etc. Subscribe and ask away. I am NOT doubting the assistance you are getting here, just a 'safety in numbers' suggestion.
 
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I was going to recommend checking out nighthawk-forums.com or ujmforum.com - both are Nighthawk oriented (more or less) and both have some great references and experience 'guru' types. I've also heard good things about the forum old bike is referring to... :)
 

ST13Fred

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Letting it sit for 8 years is a bit harsh... IMHO just about every rubber part could be cracking.
I had a 80 CB750F and 2 82 CB900Fs back in the day.

Mother Age is the big enemy of petroleum based products which are in practically everything out there. With age they harden and crack; not if, but when.
Carbs are particularly sensitive to this problem and foreign matter exacerbates it; one reason FI is more prevalent these days.

Glad you got it fixed......for now.

IMO, the black 82 CB900F with its 4 double walled (never discolored) header pipes flowing down in front of the black painted engine was the prettiest bike ever made. And it handled these WV roads with ease. :)
 
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The valve seats are a problem that pops up a lot and causes the same issue as you are seeing. Take a earbud soak it in carb cleaner put it in a drill motor and clean the seats. I don't trust the aftermarket float needles as the have given me nightmares over the years.i have been working on carbs on bikes for years and currently own a79 cbx been in to the carbs on that more times than I can count. Get the bypass circuit cleaned real good or its all a waste of time you can source the oem parts on line if they are kehine I think they may bee 018-880 about $ 12:00 each
 
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dkruitz
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Sorry for the delay - I forgot to follow up. The needles did the trick, until we noticed the o-rings on the fuel rails were now leaking. Got that fixed with a kit of Nitrile o-rings from Harbor freight, and then noticed seepage at the bowl gaskets. After 25 years, 8 years of sitting, and too much handling, we reached the failure point on the fuel rubber bits. New bowl gaskets are in and no more leaks and my brother in law has been driving himself to work now! Runs pretty well and we're happy campers!
 

slmjim

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Sorry for the delay - I forgot to follow up. The needles did the trick, until we noticed the o-rings on the fuel rails were now leaking. Got that fixed with a kit of Nitrile o-rings from Harbor freight, and then noticed seepage at the bowl gaskets. After 25 years, 8 years of sitting, and too much handling, we reached the failure point on the fuel rubber bits. New bowl gaskets are in and no more leaks and my brother in law has been driving himself to work now! Runs pretty well and we're happy campers!
Thanks for the followup. I've been curious.

Our two '93 Nighthawks have been pretty much trouble free for 65,000 mi+. Great do-it-all UJM's, reliable as anvils.


Good Ridin'
slmjim
 
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dkruitz
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Peoria, Illinois
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1995 ST1100
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8866
Thanks for the followup. I've been curious.

Our two '93 Nighthawks have been pretty much trouble free for 65,000 mi+. Great do-it-all UJM's, reliable as anvils.


Good Ridin'
slmjim
That's what I've heard.

Mosts recent problem - he had a front brake problem with the front binding after some use, and it turns out even after flushing the system, the master cylinder bore was filled with goo. (sitting for 8 years) I believe the relief hole was plugged, last night I opened the master cylinder up and cleaned it out with my ultrasonic cleaner. Upon reassembly, it was the easiest I've ever been able to bleed a master cylinder - less than 1 hour start to finish. We'll probably order a rebuild kit and refurb the entire brake system fairly soon.
 
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I truly love my '83 650 and the guys I know who have the 750s seem pretty happy. The Nighthawk was truly a great 'universal' bike. My biggest complaint about my '83 was the small tank/short riding range. 120 miles was the absolute max. It's really been fun reading your threads. Especially because your BIL had almost the exact same issues as he got his bike road-worthy as I had with mine. The great part about that is the familiarity he'll have with his bike afterwards. Hell, my neighbors think I'm some kinda 'guru' just because I've flushed and cleaned my own clutch and brake systems; pulled, cleaned, rebuilt, and tuned my own carbs; changed my own fork seals; etc, etc... It's nowhere near rocket science, but to the uninitiated, it my kinda seem that way. ;)
 
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dkruitz
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Peoria, Illinois
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1995 ST1100
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8866
We've got the forks set for a later on project, other than that it's been running well for him the past week and a half. So, fingers crossed! Cosmetics, are further down the line, as it could use some paint after suffering through some Texas UV damage.
 
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