1981 cb750 carburetor leaking gas

L

lifeon2

New Member
My bike wouldn't rev past 5,000 rpms, so I decided to do a cleaning on the carbs. I noticed a few things while putting them back together.

1981 CB750C Motorcycle
VB42A Carbs

1. While all of the jets are supposed to be 102, one is marked 68 (the hole is a different size too) and I have no idea why this is. Obviously someone worked on it in the past and put it in, but was it for a reason? Should I keep it in?

2. When I put everything back together with the original parts, the bike reved up to 7,000 rpms, which is better, however one of the tubes leaks gas profusely and I don't know if that is normal when starting the bike up for the first time after a cleaning. At first it started with the left two nipples, then stopped. Now the upper tube began to leak without stopping unless I turned off the gas switch. See video. Do I need to wait it out and eventually it will stop? What do I do in this situation?

https://www.youtube.com/watch?v=MBWi7gtyM6o

Thank you for your help!
 
1. No, get another 102, or, ideally a whole set of 102's as the actual diameter can vary between manufacturers. Better yet, do a full tune on the carbs and find out whether a 102 is even the right jet at all.

2. Probably crud in your float valve causing it to stick. Pull carbs and clean very, very thoroughly. Blow compressed air through all passageways and also check float height.

3. You didn't ask but you sound pretty new, get yourself a factory service manual, they are available in PDF format for free download on a number of sites (including this one I think). Go through the FULL tune up procedure. Check valves, ignition timing, points gap, carb synch, etc. etc. etc. This will give you a good basis of knowledge about how your bike works and should ensure that your machine runs well.
 
Read this:

http://www.dotheton.com/forum/index.php?topic=70017.msg821261#msg821261

Could be a few different thing's I've run into.
 
You really have to know your set of carbs. The 750 came with many. This will make your life easier..... http://www.hondachopper.com/garage/carb_specs/carb_specs.html

The pictures below the chart show you where to look for whichever set you have. Most likely the VB42A, but you'll never know if a Previous owner changed them out for another set that "should be the same" haha.

And a list of tuning procedures from the chopper site :) http://www.hondachopper.com/garage/carb_info/tuning101/carb_tips.html

What you didn't tell us, was if you set the float height....or the mixture screws and carb sync info. All 3 are critical to getting a nicely running machine. The float needles have springs on the "tail" do all of your float needle springs work? Any stuck in/out/varnished in place?

Get us some more info and we'll get you sorted out. Also, if you don't have a fuel filter on that fuel line, this can happen from little rust flakes or crap in the gas tank making it's way between the float needle and the seat.
 
MotorbikeBruno said:
You really have to know your set of carbs. The 750 came with many. This will make your life easier..... http://www.hondachopper.com/garage/carb_specs/carb_specs.html

And a list of tuning procedures from the chopper site :) http://www.hondachopper.com/garage/carb_info/tuning101/carb_tips.html

What you didn't tell us, was if you set the float height....or the mixture screws and carb sync info. All 3 are critical to getting a nicely running machine. The float needles have springs on the "tail" do all of your float needle springs work? Any stuck in/out/varnished in place?

Get us some more info and we'll get you sorted out. Also, if you don't have a fuel filter on that fuel line, this can happen from little rust flakes or crap in the gas tank making it's way between the float needle and the seat.
Click to expand...

I just updated the post. It is indeed a VB42A. What confuses me most is that coyote13 recommended that I change all of the jets to 102, but in the link you provided me http://www.hondachopper.com/garage/carb_specs/carb_specs.html, says "68/102" for the main jet. Could that one 68 jet actually be correct?

The float height is not alterable on these carbs. I did notice that while some of the floats "floated" well, some were sagging. I'm guessing I would need to get new float needles to fix this.

Mixture screws and carb sync... I still have to learn what these are...

There is a fuel filter on the main gas line.

Also, where might I find a manual for my specific carbs?
 
lifeon2 said:
I just updated the post. It is indeed a VB42A. What confuses me most is that coyote13 recommended that I change all of the jets to 102, but in the link you provided me http://www.hondachopper.com/garage/carb_specs/carb_specs.html, says "68/102" for the main jet. Could that one 68 jet actually be correct?

The float height is not alterable on these carbs. I did notice that while some of the floats "floated" well, some were sagging. I'm guessing I would need to get new float needles to fix this.

Mixture screws and carb sync... I still have to learn what these are...

There is a fuel filter on the main gas line.

Also, where might I find a manual for my specific carbs?
Click to expand...

Oh, you know what, I have one of those here at the shop. See attached pic. So you have a MAIN jet, and a secondary main jet. I just call the smaller jet the "mid" jet for easier clarification. The MID jet feeds into the pilot circuit. The Main jet (102) feeds directly into the float slide needle path.

Floats are designed to push on the spring loaded tang and hold the fuel from coming in. The spring part helps with bumps and vibration I'm sure. So make sure those are all "springy" haha. Generally they can be cleaned up and reused no problem. I don't like the aftermarket float needles much due to the specs being off. On adjustable floats you can fix that problem...on NON-Adjustable floats this can cause all sorts of havoc that you will chase down. So make sure those are free to spring nicely, clean out the float valves with brasso and a q-tip and you'll most likely be set.
 

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So it would be safe to say, since the floats are non-adjustable, the overflow situation would either be a faulty float or a dirty float needle, yes?
 
coyote13 said:
So it would be safe to say, since the floats are non-adjustable, the overflow situation would either be a faulty float or a dirty float needle, yes?
Click to expand...

Generally, it's crud that got inbetween the float needle and the seat. So usually dirty float needle/seat yes. It's a pointed part so if anything (even a small spec of sand) gets in there, it can hold it open and cause flooding out of the overflow tubes. I usually polish out the float needle seat every time I'm in a set of carbs. Not much, just enough to know there's nothing keeping the hole from being round and smooth.

Faulty float is possible, but not too likely in all of the carbs I've done with them, it's been the dirty seat/needle or the o-ring holding the seat in place (if it has one and isn't the pressed in version) But if it's cracked or doesn't float...that would definitely be an issue. I'm pretty certain you can still buy new floats for these...somewhere in the $20-30 each range yowsa! Or 4 into 1 saves the day!!
http://4into1.com/reproduction-float-pin-set-of-4-16013-413-851-honda-cb650-cb750-cb900/
 
MotorbikeBruno said:
Oh, you know what, I have one of those here at the shop. See attached pic. So you have a MAIN jet, and a secondary main jet. I just call the smaller jet the "mid" jet for easier clarification. The MID jet feeds into the pilot circuit. The Main jet (102) feeds directly into the float slide needle path.

Floats are designed to push on the spring loaded tang and hold the fuel from coming in. The spring part helps with bumps and vibration I'm sure. So make sure those are all "springy" haha. Generally they can be cleaned up and reused no problem. I don't like the aftermarket float needles much due to the specs being off. On adjustable floats you can fix that problem...on NON-Adjustable floats this can cause all sorts of havoc that you will chase down. So make sure those are free to spring nicely, clean out the float valves with brasso and a q-tip and you'll most likely be set.
Click to expand...

Thank you for the clarification. I found that someone mixed up the 68 and 102 jet when they last cleaned it up. So that's what threw me off.

Can someone tell me the name and function of the valve that is connected to a tube that leaks gas (in the video)? For some reason it keeps leaking regardless of me cleaning the float chambers and I'm starting to sense that the problem is coming from somewhere else...

https://www.youtube.com/watch?v=MBWi7gtyM6o
 
vandito.cb said:
spawn of satan, they call it. it's a vacuum assisted petcock, essentially.
Click to expand...

Yep that's it. Nobody likes them, yet we keep them. Well, typically I get rid of them on any bike I own, you just HAVE to remember to shut the fuel off (and most likely buy a manual petcock for your bike if it isn't already) Which I think yours is, so the automatic petcock isn't necessary really.
 
Williams said:
My 1977 CB750f is doing same thing. Fuel pouring out of satan
Click to expand...

Find God, get rid of Satan. God just happens to be beer in this situation :) Toss it in the bin, hook fuel directly to inlet on carbs from petcock, preferably with an in-line fuel filter installed in the correct orientation. I end up routing the line AROUND the throttle cables and then back towards the left side where the inlet is. Gives enough room for filter, and lifting the tank too!
 
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