Topic: '81 CB650C, overflowing carbs, and vacuum tees

[adamb]

I have a 1981 Honda CB650C that I picked up for a song.  I have thoroughly cleaned the carburators, and they are extremely clean at this point.

However, since putting them back together, they overflow. Not out of the drain at the bottom of the carbs, out of the intake hole (the throat?).

There is one thing that might be changing things though - the vacuum "tee" hoses that connect carbs 1&2 and carbs 3&4 were excessively rotted, and so I have simply used a small rubber hose to connect the two carbs instead of teeing them to the single line that goes back to the airbox. 

Testing I've done:  The float valves work.  If I take the bowls off, send gasoline to the carburators, and lift the floats by hand, the gas shuts off to the floats I have lifted.  If I turn the carbs upside-down, and apply gas (from high above the carbs), no gas flows into where the bowls would be. If I lift the floats slightly, gas starts bubbling into the carburator.   It does not appear that the float valve seats are pitted or filled with junk.  The float valve dealies look just fine.  There is no way to adjust the float height - the floats are 100% plastic, and there's no way to try to adjust the float valve to adjust the float height.

I have first, this question:  The diaphragm above the carburators shuts off the flow of gasoline to the carburators unless the engine is turning over.  So, do the little vacuum tees somehow limit the flow of gasoline to the carburators when the engine is running? So, if there's a vacuum to the vacuum tube to the gas diaphragm thingy, maybe the vacuum from the airbox to those little tees on that connect to the carbs provides a way for the carbs to additionally shut off, or request less gasoline?

The only bit I'm missing is whether or not those vacuum "tee"s actually help hold back the flow of gas, or something.  Unless the floats are sticking once I put the carb bowls on, there's no other reason I can find that would cause all four carbs to overflow out their throats when gas is flowing to the carbs. 

My next test, when I can, is to take those joining tubes off of carbs 1 and 2, and see what happens when I let gas into the carbs. SEcond, is to connect one of the carbs with a tube, and apply a gentle vacuum when I deliver gas to the carbs... Unless you guys have a solution for me

[kslrr]

The tees have nothing to do with it.  Do the plastic levers that push against the valves have holes warn into them?  Maybe someone replaced the valves with the incorrect type which are too short.  Get yourself a manual (download from this site), a float level gauge and check the float height the right way.

[333]

I would also check a parts diagram to see if the floats haven't been put in upside down.

And while I'm not 100% on CV type carbs(I think that's what you have), I don't think those lines are vacuum.  Each bowl is vented.  They are vented so gas will flow into them.  The vents are tied together in this manner to minimize leakage if the bike is dropped.  A single line is then run parallel to the drain lines along side the drain/vent for the battery.

[Spanner 1]

Those lines with the tees are bowl vent lines and must have the tees installed, otherwise your just 'venting' one carb into the next ( closed circuit , no vent ) and the pressure in the bowls would hold the floats from rising all the way, i.e. overflow....... the vent hoses from the tees I think go up to / or over the airbox....maybe someone else can confirm that. Good luck.

[kslrr]

On the '81 and newer 650, the carb vents to the air box.  Part of the emissions control.

[Hush]

I had a similar problem on my 79 650 and it turned out to be the accelerator pump seal "o" ring, a small but vital piece of rubber that allowed gas to flow across the carbs and even back into the air filter box.
I think the newer 650 had that "0" ring built into the carb bowl gasket.
Not saying that is your problem but just another to look out for.
I would replace the tee joints before reassembling and refitting the carbs, it's hard enough taking these beetches off without having to remove them a second time just to fix something you could have done once.

[Tugboat]

Don't mean to hijack this thread..... I'm having a similar problem w/ carb #2 overflowing out of the overflow tube. No holes/cracks in the tube itself.. float bowl gaskets are new.. float level appears to be on par with other carbs' floats. Is there a similar o-ring on the accelerator pump seal for a '77 750F?

[Hush]

Sorry Tugboat, I'm not sure of the 750 set-up only the 650 but if there is an accelerator pump involved it could produce the same result.
It took me 12 times removing and rechecking my carb bowls before I tumbled to the fact that the overflow was actually coming from the accellerator pump and not the floats or the bowl seals themselves, the early model 650 has a seperate "O" ring for the pump shaft.
Since renewing that "O" ring I have not had the overflow problem......cheers...Hush.

[Spanner 1]

Tugboat, PM me or call me as your ' mailbox is full ' !?
Now back to our scheduled programme.........

[adamb]

So, addressing some various comments and so forth:

1. The hoses are for venting, but there will be a slight vacuum on them - they terminate at the airbox, which will cause a slight vacuum in the line.  The guy at the honda shop up here said they were for balancing the air flow of the carbs, but the connection from the tee goes right into the bowls.

2. No, the floats aren't upside-down. Trust me, I checked - but they will not install upside-down. The bowl ends up pressing on the float, and would never allow any gas into the bowls.

3. My accelerator pump rubber seems ok, or at least, it did when I removed it, cleaned it up, and then re-installed it.  I wondered if the accelerator pump had anything to do with my problem, but the accelerator pump is enabled when the choke is on, or the bike is close to 'floored' - not just at an idle, and so forth.  It will probably pay to replace my accelerator pump shaft/rubber thingy; it's 29 years old, and could probably use replacing.

Regardless, I still need to replace my carb Ts, and have done some looking around at hardware stores.  I've found a few possibilities, apart from paying for two kits simply for the rubber tees:
1. But a decent-sized rubber stopper, and cut it to fit inbetween the two carburetors. Drill it to be a "tee" connection. 
2. Find a 1/4 automotive rubber vacuum tee, and cut to size.  (they may not even make these in 1/4 size.)
3. Buy appropriate 1/4 hose, and then buy a plastic tee, and cut it to fit, and put the rubber tubing on it.


After I solve that problem, I'll worry about my accelerator pump.  bikebandit.com says the accelerator pump assembly is $73 (CRAP!)  , so we'll see about that one .  This $200 bike is getting expensive, that's for sure...

Anybody interested can take a look at pictures here:  (And yes, those carbs were BAD):  http://picasaweb.google.com/adambultman/1981CB650Cleanup#

[adamb]

So, I did the rubber stopper thing - $1.67 apiece for a rubber stopper, and about $2 apiece for a brass tee (I realized too late that I could just yank the metal one out of the old rubber tees, but those were also slightly smaller and less barbed)

It works... ok. It's probably about as secure and trustworthy as those silly rubber tees from Honda, that's for sure, and since I cut the rubber stoppers "too long", the carbs squish them together nicely. I don't know how it'll hold up with heat and vibration, but I'll be testing that as soon as I can.

Here are pictures of the 'final product', so to speak - they aren't good, but then, I wasn't about to take them back apart so I could get a better photo:

1. http://picasaweb.google.com/adambultman/1981CB650Cleanup#5423366867126556642
2. http://picasaweb.google.com/adambultman/1981CB650Cleanup#5423366875172753650
I'm basically finished with the carbs now; I bought some high-temp gas line and ran it on the carbs, there's a kink in the connection from the fuel valve to the main tube to the carb bodies, but I cant' find gas line stiff enough NOT to kink.

On saturday at the latest, I'll have the airbox reinstalled, and the carbs reinstalled, and I'll know if my handiwork is worth the money I spent.  

I also tested my compression - between 105 and 130. I was told "around 110" was just fine for a bike this age.  It runs decently (well, when there's not gas pouring out the carbs) but the real proof will be after breakup, when I can hit the road...

[Hush]

Hey if they work they work, you may have hit on a new cheap way to repair these T sections.

[MickeyX]

For your gas line..we got a unicoil. It works great on our 650. The kinking line is a thing of the past. It was worth the $$. We found them at Baxter Auto Parts near us.

[Buber]

Those lines with the tees are bowl vent lines and must have the tees installed, otherwise your just 'venting' one carb into the next ( closed circuit , no vent ) and the pressure in the bowls would hold the floats from rising all the way, i.e. overflow....... the vent hoses from the tees I think go up to / or over the airbox....maybe someone else can confirm that. Good luck.

And this is the real answer.
If you will leave those opening without those tees it would work too, only overflow would drip at the engine directly. Fastest trick is to connect to each carb a separate tube and put them somewhere "out of the way".

[adamb]

The line from the carb tees goes into the bottom of the airbox - which I suppose is a clever way to both let the carburators overflow, and take care of any emissions that might result from an overflowing carb.

I got my carbs back on the bike tonight, and it runs, and no overflowing!  I forgot to sync up the throttle plate on the #4 carb, so I have to operate it manually apart from the throttle - but I also don't have the airbox to carb rubber "connectors" on right, so it doesn't fit in there properly.  As a result of the throttle plate, the bike sounds like it has a cam in it or something.

So, once I fix those two things, and tweak my idle mixture screw, I should be back up and running properly - and I'll be able to move on to the next portion of this fixer-upper.

Thanks for all your help, but stay tuned, there'll be more!

[adamb]

Carbs back on, here's some video of it, and how it sounds:

Here's a second video, but this mostly me talking to myself about some other things I found:

On the plus side, when it was warmed up compression showed about 130 PSI per cylinder.

On the down side, I can't figure out how the heck to get the carbs on with the airbox.  I cleaned out the airbox, which included taking off the airbox to carb intake connectors, and I can't figure out how to get them on properly. When I try to install the carbs, it knocks those off the airbox, and when I do actually get them on, they aren't tweaked right, because they don't all match up very well.  I'll figure it out soon enough, I suppose, but if you have tips, I'd love to hear them.

[Hush]

Lots of smoke, you running 2 stroke in it?
I think you can probably glue the air box rubbers on so they don't move.
Starts up OK which is always a good pointer to tuning, my 650 custom was a #$%* to start.

[cb650]

Put  the airbox in first and strap it back as much as you can.  Then slide the carbs in.  You still might knock off a rubber but it will get easier. 

[Frankenkit]

Now, it was actually MickeyX who found this out, but at least on my bike (an 80) the airbox rubbers actually had a particular way they had to be turned in order to fit onto the carbs properly.  Try just rotating them about until you get a better fit.

[adamb]

Hush:  Might be a bit of smoke, but it's also because I had the garage door open to help vent the exhaust.  I had the garage heater at 60, and it was in the teens outside - so even my breath was showing up. 

Thanks for the tip about strapping back the airbox - I had thought of that, but ran out of time.  As for rotation - I was trying that yesterday, but without strapping it back, it was an exercise in futility

I'll let you know how I fare today

[adamb]

Got everything on.  Heated things up ahead of time with the 500 watt halogen work light, and that helped soften things up.  They aren't perfectly 'on there', but then, I don't think they even WOULD anymore. But, they're on, and they shouldn't come off.

I re-ran the hoses off my homemade 'tees' so that things would be cleaner and easier to manage; I ran them along the edge of the airbox inbetween the silver airbox covers and the plastic covers for the air filter/etc.

Also ran a sponge over it, and it looks purtier now:
http://picasaweb.google.com/adambultman/1981CB650Cleanup#5425318384886925010
It'll look better after I'm done with the toothbrush and the chrome polish.


Next purchases are a battery, the air filter cover and gasket, and I can't remember what else. I know I need a chain and some cables, but I have oh, 5 months before I can even think about getting on and riding, so I'll just hang out and wait for that.

[Hush]

5 months? should be a show stopper after that amount of love!

[Frankenkit]

hey, Hush.... my project took twoish years...

[adamb]

Dude, I live in Alaska - it's still too cold to ride in very late May. 

[D]

@ adamb
  When you cleaned your carbs did you dip them in any solution? If you did submerge them, did you separate them or did you leave all 4 carbs together. I want to dip mine and i am worried about the cleaner causing the carbs to leak where the fuel rail connects to each of them.