Topic: Perplexed! please provide input: photos attached

[drdreas]

Hi,

My bike is a CB750 K7, purchased it last spring and rode it about 2 thousand miles with NO issues. Engine runs and starts smooth as butter. Went to ride it for the first time all winter a couple weeks ago, and though it no longer leaked fuel, one cylinder wasn't firing, and after debugging found that the carb on that side of the bike no longer provides any fuel.


So after posting on this forum about how to take off the carb bowl and check the float, I did so today, and I am totally stumped at what I found!!

Here is a picture of the problamatic carb and its float.


It seems as though the float is sitting too low to ever allow the bowl to fill up. I manually allowed the bowl to fill with fuel (float valve works fine), re-attached the bowl, started the bike up, and it ran perfectly for about 2 minutes, until the fuel in that bowl was used up and that cylinder stopped firing.

I couldn't figure out how I could adjust the bowl to be high enough to allow the bowl to fill, so I decided to check the carb on the opposite side of the bike, to see what it looks like, and this is what I found.....



What the heck is going on!! I have NO experience with carbs, but this can't be right! It looks like its the exact same bowl, same float, same everything, except the Float is upside down compared to the problematic carb's float. This one seems to be oriented correctly, as it is high enough that by the time the fuel level makes it level, the bowl is full.


What is going on? I though maybe the carb model was different, and indeed the markings on it are a bit different, but the inside components and bowls look identical. How is it possible that for 6 months i rode the bike and it ran perfectly with no issues if the bowl was upside down?

One thing I noticed was that the problematic carb had three holes open to air. It doesnt look like metal was broken, but on the other side of the bike these aren't open. Here is the problematic carb:


Does that look normal?

Here is the carb on the other side of the bike:



As you can see, it only has two of the three "holes" that the other carb has, but they are closed, not open to air.


Please help me figure this out, I have NO idea what is going on here! I was going to jut flip the float on the problem carb, but I've got to know why its upside down and how was it working perfectly for 6 months!? I have not touched the carbs since I got the bike.

Thanks

[KingCustomCycles.com]

Float is upside down on one carb.  That little tab in the picture is designed to keep the float from dropping too far open.  The film on everything could use a good cleaning with Simple Green.  I have no idea how the bike ran well like that.  You may be surprised at the performance with all four properly running.  Maybe the float needle valve leaked enough to keep some fuel in the bowl?  Your pictures are awesome by the way.

[drdreas]

thanks for confirming that its upside down. I guess its possible it leaked, but now it won't leak at all, when the bowl is empty that cylinder just shuts off. I guess the PO installed it upside down, I'm just baffled at how the bike ran for so long.... idles smoothly at 1100, pulled strong... hows it possible?

So to flip the bowl back upright do I push the pin out? Is there some special attachment to the valve? Will I be able to flip it while the carb is still on the bike? I'd love to just leave it on there and do the flip.

thanks

[drdreas]

Another thought... from the scum line on the float you can see that it was installed in the correct orientation for a considerable period of time, letting it sit in the fuel and build up the brown color. I've had it for 6 months, if it were inverted that whole time would the lower portion be brown already like the correctly oriented float?

[Hush]

Yes just push the small pin out with something smaller in diametre, the float and float valve will probabaly fall out into your hand so be ready to catch it.
The top picture has the float in upside down by the way.
Just reassemble by pushing the float valve back up into the hole it fell out of (sometimes they attach to the float with a small spring like clip) and holding the float up replace the pivot pin.
I put all 4 of my floats in upside down, my bike didn't run at all ha ha ha........

Those other holes you are worried about are production blanks and the ends of the choke butterfly axles I think so just leave them.

I'm really suprised the bike ran so well, maybe as King C said the float valve leaked enough to fill the bowl.
If you have only owned the bike for 6 months I'd say the prvious owner may have replaced the carb float valves and got confused as to how the floats go in, that "scum line" is a few good years of being in the "right" position.
4 cylinders gotta be better than three and a half anyway ha ha.........Hush.

[Gordon]

4 cylinders gotta be better than three and a half anyway ha ha.........Hush.

I'm always surprised at how well the 750's will run on only three cylinders.  So well that someone who's not familiar with these bikes won't even know that there's anything wrong, but you'll definitely notice the difference once you get all four running!

[drdreas]

Thanks for your help. I just flipped the float, reinserted the valve and pushed the float pin back. I checked the the valve operated, and the float level seems to be the same as the functioning carb on the other side.

I reattached the bowl and turned on the petcock and noticed that fuel immediately started pouring out of the bowl. Initially I couldn't tell where from but it seems it's coming from the back near the valve. What's wierd is that it leaks even when the bowl is empty. It's like with the valve open now it just leaks straight out. I checked the gasket and it looks fine. No cracks in the
bowl or carb I can see. Any ideas? Is it possible I reinserted the valve incorrectly? Even if that is the case how would that cause fuel to bypass the gasket and leak?

[Markcb750]

sounds like the tab on the float that actuates the shut off is way out of position due to the improper installation.

you need to set the point at which the needle contacts the seat by re-bending the tab on the float.  Generally the tab is located between the two floats.

[Toxic]

+1
I bet that is your problem.

Your bowl is over filling.

[drdreas]

I took the air filter boot so I could get a better look at the carb and where fuel was leaking.

I verified that the leaking is not due to the bowl overfilling. Even when there is no fuel in the bowl if I reattach the bowl and turn on the fuel petcock, fuel leaks. After removing the air filter boot, I found that fuel is actually leaking above the float valve, and looks to be exiting from a fitting in the intake. Here is a picture:


Fuel seems to be leaking out of the brass fittings pointing horizontally to the back of the bike. Again, this occurs even when the bowl is empty, and I double checked to see that the o-ring is in good condition. Any ideas?

Here is another picture of the intake, and below that, a picture of the float now that Ive turned it upside down.







Thanks

[TwoTired]

Those brass tubes are air bleed inlets for fuel mixture purposes.

Have you checked that the float valves are operating.  As the float rises, it should close a valve to keep the bowl from over filling.  The float height spec is where that point is adjusted.  If someone adjusted it fror an upside down float, it is almost certainly wrong for when it is installed correctly.

Cheers,

[Bodi]

Reversing the float wouldn't change this odd behaviour: if fuel leaks from the carb throat with an empty bowl, the float valve isn't in the picture. If it didn't leak before you reversed the float, Occam's Razor suggests that something you did has caused the problem.
Those brass tubes supply air to the emulsion tubes around the main and pilot jets. If the carb gets seriously overfilled, the gas will run back out through them, as well as through the needle jet and slow orifice.
There is an overflow drain in the bowl, the brass tube sticking up from the bottom into the bowl. This does get plugged with crud sometimes, but gas should dribble out there before it gets to the pouring-out-the-throat stage.
Is the float valve needle actually in there? it's held in by the float tab alone and may have just dropped out when you pulled the float off. Without the needle, flow through the valve is pretty impressive and could fill the bowl quite rapidly.

[drdreas]

Bodi, those were my thoughts as well. I don't think the float adjustment is the problem, because it leaks even with an empty bowl. I will check the overflow tube and see if it is plugged. I don't remember there being a separate float valve needle. There was a piece of metal, about 1/8" in diameter with a sharpened point that I thought was the float valve needle, and it is clipped to the float, which in turn pushes it up and down in a vertical cylinder, and this regulates the fuel flow. It is in there and functions correctly; when the float is lowered fuel pours down around the valve, and when the float is raised, the flow of fuel stops. Its just that when I put the bowl on, fuel leaks out of those brass tubes in the picture.



Thanks

[MRieck]

Occam's Razor

Is that from "Galaxy Quest"

[jsaab2748]

There was a piece of metal, about 1/8" in diameter with a sharpened point that I thought was the float valve needle, and it is clipped to the float, which in turn pushes it up and down in a vertical cylinder, and this regulates the fuel flow. It is in there and functions correctly; when the float is lowered fuel pours down around the valve, and when the float is raised, the flow of fuel stops. Its just that when I put the bowl on, fuel leaks out of those brass tubes in the picture.



Thanks

You may want to re-read Two Tired's most recent post  It may make more sense now.

[Hush]

drdreas, you just very accurately described the "float valve" the wee squarish thing with the pointy end.
If that pointy end does not seal into the brass hole it enters then gas will flow.
You could check this by removing the float valve from the float and just hold the valve up into the brass hole then turn on the gas, it should seal, if not I'd replace the valve.
I got a full set of 4 for $46 kiwi last year so in USA they should be cheaper.

[KingCustomCycles.com]

Remember, there are two carbs potentially feeding that vent tube.  Maybe the other carb bowl is flooding with a plugged drain.  Another thought is the fuel inlet line fitting is cracked and fuel is flowing out and around the carbs. or the fuel line just isn't sealing.  Keep at it, the knowledge you are gaining will always be with you. 

[Spanner 1]

Agree with TT, bowl can not over-fill if the float valve is closing...also fuel should flow from overflow tube before it appears at the carb. throat, is it clear? A blocked overflow could cause a pressure situation in the bowl which could hold the valve open ??, as it also vents the bowl to ambient air.
You really should check float height...with the valve lightly closed on your bike ( K7  with 41a carbs.)  measure should be 12.5 mm...top surface of float to bowl flange surface ( where bowl fits to when installed ). Let us know if any result !

[754]

Any grit or crud on the needle seat or needle tip could be holding the needle open & cause the leak..

[drdreas]

Another update: I decided to check the vent tube in the bowl. I tried blowing in both sides and it didn't seem to be clear. Also I checked the vent hole at the very bottom of the bowl... There as what looked like a little ball of paper stuffed down there blocking it. Weird.

So I took of the bowl from the other side of the bike to compare: blew into it and air passed through easily both ways. I put this good bowl onto the problematic carb and voilla! It filled up fine with no leaking.

So now I know the problem is the bowl. Back to it: I filled the bad bowl up with water, then loosened the drain screw. Nothing. So it seems as if the fitting at the bottom of the bowl is clogged, doesn't allow the drain pipe to drain, or the little drain hole in bottom of the bowl to drain. Does this sound like it the problem?

Any ideas how to unclog it? I tried pushing some wire in there but it feels like it hits metal. Tried blowing compressed air. Nada. Any ideas? Thanks all for your help!

[KingCustomCycles.com]

Does this help?

[drdreas]

In the drawing, the overflow tube is blocked. There is a fitting at the bottom of the bowl that is the exit for both the overflow tube and the drain at the bottom of the bowl. This fitting is blocked. I can't figure out a way to get it unbocked, and it doenst look like I can replace it.

[KingCustomCycles.com]

overflow tube should be clear, use a wire to unjam it.  Bowl drain screw should flow also when unscrewed.  I am starting to lose clarity on what all of this has to do with your starting and running? 

[drdreas]

Conclusion!

I checked the overflow tube, and indeed something was jammed in there from the bottom. This is what I pried out from the drain bolt hole after 30 minutes:



Put the bowl back on the bike and found that it was dripping fuel when the bowl was filled. Put some water in it, blew into the top of the overflow tube, and found that the tube was cracked and thus leaking. Soldered it and everything works now!

So the picture comes together like this.... I think the PO saw that the bowl was leaking, and rather than fixing the tube, decided to plug the drain hole completely. I don't know if he purposely flipped the float or not, but in any case, it works now. The big question is have I been riding it all this time on three cylinders? It definitely sounds odd when I pull the plug on that cylinder, so I don't think Ive been riding on just three.

However, when I re-installed everything with the float right side up, no leaks, and started the bike up, it sounds COMPLETELY different. It is much more high pitched, snarly, and sounds more like a current inline 4 motorcycle than it ever did before. I took it for a spin, and noticed it pulled like crazy, way more than it ever has before, however, it seems like low end torque is down... It was bogging a bit when I let the clutch out and I almost stalled a couple times... once I got going above 3k though it pulled like it never has.

Is it possible with the float upside down that cylinder was only partially running, and the PO tuned the bike for this condition? Its still a mystery to me, I'm just so happy I've got it all fixed now.

[Gordon]

However, when I re-installed everything with the float right side up, no leaks, and started the bike up, it sounds COMPLETELY different. It is much more high pitched, snarly, and sounds more like a current inline 4 motorcycle than it ever did before. I took it for a spin, and noticed it pulled like crazy, way more than it ever has before

See what I mean?

And yes, it's possible that cylinder was supplying partial power as long as it was still getting a small amount of gas, but I don't know how likely it is the P.O. did any tuning to compensate for this.  He probably just turned up the idle stop screw.  Check your plugs for carbon/gas fouling, and set the idle mixture screws.  Are you absolutely sure the float level is set correctly on the carb that had the inverted float?

[KingCustomCycles.com]

No comments on my drawing that looks like a 3 year old did it?  I am losing faith in the abrasive nature of this forum...

[Gordon]

No comments on my drawing that looks like a 3 year old did it?  I am losing faith in the abrasive nature of this forum...

We generally try not to poke fun at something that another member obviously spent hours and hours of hard work on and pored his soul into.  You could really hurt someone's feelings doing that!

[Spanner 1]

O.M.G....I believe I solved it!!...blocked vent-tube...the give-away was fuel appearing in the carb. throat, that should not happen 'cos the top of the vent-tube is below  this and would drain any excess fuel.!

[TwoTired]

O.M.G....I believe I solved it!!...blocked vent-tube...the give-away was fuel appearing in the carb. throat, that should not happen 'cos the top of the vent-tube is below  this and would drain any excess fuel.!

The stand pipe behaves like a vent until there is something in it, like fuel, etc.  All the bowls have a separate vent (sometimes ganged with others) at the top of the float chamber.

Cheers,

[KB02]

Occam's Razor

Is that from "Galaxy Quest"

I think it's "Igthar's Hammer." 

Never give up! never Surrender! - Hey, that's a good slogan for our bikes as well. 


My one thought as to why the bike was running well(ish) with the upside down float is that the PO somehow got it locked open so there was always fuel in there.

Sounds to me like all you need now is a good carb sync and tune up and ride the heck out of it. 

[Spanner 1]

Surely, TT, the 'stand pipe' is also designed to drain excess fuel in the bowl to the ground via its drain tube, before the fuel level rises to the carb, throat and spills on yer motor.
Otherwise why would it be there if there is a " separate vent in the top of the float
 chamber "?

[TwoTired]

Surely, TT, the 'stand pipe' is also designed to drain excess fuel in the bowl to the ground via its drain tube, before the fuel level rises to the carb, throat and spills on yer motor.
Otherwise why would it be there if there is a " separate vent in the top of the float
 chamber "?

In normal operation there are two vents to outer atmosphere for each carb bowl.  The stand pipe behaves as one until it contains "something" that prevents pressure equalization.

The vent at the top of bowl chamber connects to the adjacent carb via a tube or "log" type manifold that interconnects with all four carb bowls.  Other Keihin designs used on the SOHC4 also provide a separate tube as an atmospheric vent.  While the  PD41/2 series does not appear to have the tube to atmosphere directly (it is awkward to examine these closely in my garage at this time), this interlink manifold ensures that all four stand pipes must be plugged before pressure equalization can cease.  I suppose it is arguable whether this specific carb design assigns primary value to the standpipe as either bowl vent or overflow safety function.  However, Keihin's other carb designs on the 500/550, 350/400, etc. since 1971 had separate tubes for bowl vent function, making the primary function of the standpipe on all these other designs, overflow safety.  I've not examined Cb750 carbs prior to 77 for the bowl vent implementation.  So, I don't know for certain if they had a separate bowl vent tube to atmosphere.

Cheers,

[Don R]

Congrats on a job well done by all involved.  I would be very surprised if the float and it's metal tab had not been bent in some manner to make it work upside down or just bent by the act of bolting the bowl on with the float like that. If it's working OK you must have gotten it though!