Dripping Fuel: CB750 - Honda Motorcycles - FireBlades.org
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post #1 of 9 Old 06-13-2018, 12:09 PM Thread Starter
 
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Dripping Fuel: CB750

The Nighthawk doesn’t get ridden much, so I’ve decided that it needs a Battery Tender on it. It does have a little Stabil added to the tank because I know it sits. Before I got to the install, I got up the nerve to take my 5-yo for his first ride on the road. I had him get into his racing leathers while I pulled the bike out of the garage.

I turned on the choke and thumbed the start button and it turned over like a fresh battery should. But it wouldn’t start. Doh! Forgot to turn on the fuel selector, carbureted bike.

Did that and thumbed the starter again while I played with the choke setting to get it to catch. I noticed that it was dripping fuel from all 4 carbs like a sieve even while running. I turned off the fuel valve thinking maybe the floats were full from my failed start creating high vacuum. It continued to run. I let the bike warm in the OFF position and the dripping slowed. Turning the knob to RES also gave a fuel loss. But turning it off gave just a slow drip.

I decided (I hoped) we weren’t going to be the subject of a “never again” story with a bike catching fire as it seemed standing the bike vertical had a positive effect.

I turned on the fuel and away we went. I took a glance a couple times enroute and it seemed cured. About 20 minutes of running the bike through various speeds, gears, loads and RPM we returned with a lot of elation and gratitude from my son! He came over and hugged and thanked me for taking him riding.

Back home I noticed that parked, the engine will continue to run longer than I think reasonable with the selector in OFF.

1- Is this leaking something that I need to correct or a faculty of an incorrect starting procedure?
2- What’s the best way to clean the fuel residue from the bike?
3- Do I need to look at the selector valve?

-My 06 Fireblade Build Thread
-Her 02 CBR600F4i Custom Suspension
-My 07 CRF230F sold
-My 97 XR100
-07 CRF50F son's
-92 CB750 She lets me ride it, Jetted
-13 CRF110F, 01XR50 Daughter's

Because 12,000 RPM makes me feel better
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post #2 of 9 Old 06-13-2018, 12:58 PM
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Re: Dripping Fuel: CB750

1: Where was the leak coming from specifically? The float bowl drains? or the overflow tubes (higher up on the carbs).

2: Probably just have to scrub with soapy water - or spot clean with carb or brake cleaner or you could try this, which is a spot detailer, but I use it on everything and it works well.

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3: Probably. The bike will run (idle) longer than you would expect with the valve off. But there's a caveat to your fuel valve. The lever you turn could be bad (but it would likely leak out of it before it allowed fuel to flow unstopped into the carbs), OR the vacuum diaphragm could be old and hardened. It takes vacuum from the intake to allow flow to the valve. So in essence, you have 2 means of stopping fuel flow in one device. The diaphragm is replaceable, but a bit costly. Make sure it's bad by taking it apart and seeing if its "petrified".
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Last edited by IanDoohan; 06-13-2018 at 1:10 PM.
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post #3 of 9 Old 06-13-2018, 1:06 PM Thread Starter
 
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Re: Dripping Fuel: CB750

It looked like it was dripping from the bottom of the carb where a nipple should be located.

Did you intend to include a link to the spot cleaner?
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-My 06 Fireblade Build Thread
-Her 02 CBR600F4i Custom Suspension
-My 07 CRF230F sold
-My 97 XR100
-07 CRF50F son's
-92 CB750 She lets me ride it, Jetted
-13 CRF110F, 01XR50 Daughter's

Because 12,000 RPM makes me feel better
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post #4 of 9 Old 06-13-2018, 1:13 PM
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Re: Dripping Fuel: CB750

Quote:
Originally Posted by Jaybird180 View Post
It looked like it was dripping from the bottom of the carb where a nipple should be located.

Did you intend to include a link to the spot cleaner?
Yup! sure did forget that, but it's there now....just a photo, but I think Amazon sells it great product. I use it on my rims, fairings, everything.


The float bowls have a tapered screw to drain them. Sometimes they have o-rings on them too. Make sure they're not damaged, and make sure the tapered area isn't corroded.

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post #5 of 9 Old 06-13-2018, 1:47 PM Thread Starter
 
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Re: Dripping Fuel: CB750

I could see having one, maybe 2 simultaneous failures, but 4 all at once?!?? I will check with my wife but I think she would have said something if it leaked when she last rode it (scratches head thinking who rode it last).
IanDoohan likes this.

-My 06 Fireblade Build Thread
-Her 02 CBR600F4i Custom Suspension
-My 07 CRF230F sold
-My 97 XR100
-07 CRF50F son's
-92 CB750 She lets me ride it, Jetted
-13 CRF110F, 01XR50 Daughter's

Because 12,000 RPM makes me feel better
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post #6 of 9 Old 06-13-2018, 2:51 PM
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Re: Dripping Fuel: CB750

Quote:
Originally Posted by Jaybird180 View Post
I could see having one, maybe 2 simultaneous failures, but 4 all at once?!?? I will check with my wife but I think she would have said something if it leaked when she last rode it (scratches head thinking who rode it last).
Maybe they were left open after draining the carbs for the winter? I agree that all 4 leaking is strange, but it happens. Maybe the fuel is leaking from up top somewhere and just noticeable at the float bowl drains

I'm sure you'll get to the bottom of it

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post #7 of 9 Old 06-13-2018, 3:11 PM Thread Starter
 
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Re: Dripping Fuel: CB750

Are you saying they are supposed to be normally closed? We bought the bike this past Autumn. I haven’t turned a wrench or screw on it- not even washed it yet. Only put gas and Stabil in it this far.

After this mystery I plan to install the Battery Pigtail and put a cover on it. It’s probably due for an oil change just in the passage of time.
IanDoohan likes this.

-My 06 Fireblade Build Thread
-Her 02 CBR600F4i Custom Suspension
-My 07 CRF230F sold
-My 97 XR100
-07 CRF50F son's
-92 CB750 She lets me ride it, Jetted
-13 CRF110F, 01XR50 Daughter's

Because 12,000 RPM makes me feel better
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post #8 of 9 Old 06-13-2018, 4:58 PM
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Re: Dripping Fuel: CB750

Quote:
Originally Posted by Jaybird180 View Post
Are you saying they are supposed to be normally closed? We bought the bike this past Autumn. I haven’t turned a wrench or screw on it- not even washed it yet. Only put gas and Stabil in it this far.

After this mystery I plan to install the Battery Pigtail and put a cover on it. It’s probably due for an oil change just in the passage of time.
Yes, they should all be snugged up nice.

My 99 900RR Track Build Link:Track Build 900RRX

My Red Rocker 93 Build Link: Café 900RRP
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post #9 of 9 Old 06-13-2018, 5:41 PM Thread Starter
 
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Re: Dripping Fuel: CB750

Perhaps they vibrated loose. I’ll check them this evening. Thank you sir!
IanDoohan likes this.

-My 06 Fireblade Build Thread
-Her 02 CBR600F4i Custom Suspension
-My 07 CRF230F sold
-My 97 XR100
-07 CRF50F son's
-92 CB750 She lets me ride it, Jetted
-13 CRF110F, 01XR50 Daughter's

Because 12,000 RPM makes me feel better
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