Topic: Vapor Lock? - can't be.

[mycb750k6]

My 76 CB750 K has developed an annoying way of running out of gas. Suddenly there is no more fuel in the line which I have now run along side the tank to observe the fuel flow. When I pull the line from the tank, fuel flows readily from the tank but when I re-attach to the carbs - no flow. If I take an air compressor and blow in the line that goes to the carbs I hear and feel a small bop as if something is being blown back into place. From that point I can re-connect the fuel and all is well - until it happens again. ANYBOBY know whhat that pop could be and how I could service it? Float bowls? Gullys clogged?

BTW, fuel system is clean, carb bowls are clean, main jet cleaned.

It just behaves as a vopor lock that I can't get rid of until I blow through the line with compressed air and get that pop again. Oh yeah, I put a inline throw away filter in the line to prevent any outside influence.

What in the world is it?

[mick750F]

   Sounds like the vent in the gas cap might be clogged. I'd start with that.

Mike

[greenjeans]

   Sounds like the vent in the gas cap might be clogged. I'd start with that.

Mike

Agreed.    Mine was having trouble -  gas cap vent was clogged with gunk.  Give it a shot, easy and free.

[mystic_1]

Check the carb float bowl vents too.  These are the two smaller hoses coming off the carbs that vent to atmosphere down by the swingarm.  If these are plugged than there's no way for air to escape the float bowls, which would keep fuel from flowing in.

mystic_1

[mycb750k6]

Jeesh - thanks for these great ideas. I'll get on it as soon as the sun comes up.

[Steve F]

   Sounds like the vent in the gas cap might be clogged. I'd start with that.

Mike

Agreed.    Mine was having trouble -  gas cap vent was clogged with gunk.  Give it a shot, easy and free.

Not likely the problem, since he said that when he removes the fuel line from the tank, fuel flows freely.  Sounds like there may be a blockage in the tees between the carbs or something in the line, or like Mystic_1 said, check the bowl vents.

[goon 1492]

   Sounds like the vent in the gas cap might be clogged. I'd start with that.

Mike

Agreed.    Mine was having trouble -  gas cap vent was clogged with gunk.  Give it a shot, easy and free.

Not likely the problem, since he said that when he removes the fuel line from the tank, fuel flows freely.  Sounds like there may be a blockage in the tees between the carbs or something in the line, or like Mystic_1 said, check the bowl vents.

+1 on that he stated that they flow when unattached, it could be what Steve F said or even the float valve itself getting stuck closed. When mine would do that on my dirt bike it was like I was out of fuel, then I look at my 4 gallon desert tank and its full  .Thats when I had to go as far as having the carb off the bike and hooked up to the fuel line with the bowl off and opened and closed the float manually only ot find out that the valve itself had just enought oxidation buildup that it would hold it still and not let fuel in the bowl. A little 0000 fine steel wool and it cleaned up really well, I just had to make sure I got every last pc of steel wool off before installing it back into the carb.
Hope that got you some where.

[mycb750k6]

I can't service the tees between the carbs without removing the carbs can I?

If the float valve is getting stuck, would not that explain the 'pop' when I apply the compressed air?

If the float vale is getting stuck, what is the easiest way to service without removal?

It's light now but cold. I'll wait another hour to go out and play.

If I get it, do you all want to know the solution?

[Duke McDukiedook]

Sounds like you'll have to take your carbs off for these checks 
Sometimes you have to just hunker down and do it. Just make sure you label your carbs,
take some pics before disaasembly and get to it.
The hard part for me was getting those suckers back on.
Of course we want to know if it worked! 

[Soos]

If I get it, do you all want to know the solution?

YES 



l8r

[Deltarider]

I don't think you have to take your carbs apart.

1. Are your fuel lines in good order? No kinking, the right dimensions?
2. Any inline fuelfilters that can cause irregular fuel flow?
3. Sometimes a floatvalve can become a bit sticky. Since unleaded, gas seems less lubricous. A little seafoam in your fuel might help.

[mycb750k6]

OK I'm going out to tackle it.
Do you guys think the pop I hear when applying compressed air is the float valve being pushed back down so it will allow fuel to enter?

[Deltarider]

Could be. If you must use compressed air, make sure to open something like the drain plug so it can go somewhere. If not you might damage something in spite of the small vent tubes.

[mystic_1]

I would certainly look at the float valves at this point.

By testing with the hose disconnected, you've narrowed the problem to one of the following:

• Inlet Tees
• float valves
• floats
• float vents
• debris in the system (likely)

mystic_1

[goon 1492]

I can't service the tees between the carbs without removing the carbs can I?

If the float valve is getting stuck, would not that explain the 'pop' when I apply the compressed air?

If the float vale is getting stuck, what is the easiest way to service without removal?

It's light now but cold. I'll wait another hour to go out and play.

If I get it, do you all want to know the solution?

Listen to you with the "but cold" its a balmy 3 above zero here is KC
I forgot to mention that I have a 705k6 also and the popping you hear could be the float valve shooting open with the air pressure, careful that stuff isn't really made to take on pressures like that.
and + what they said

[tygrant]

I would bet money its stickey floats. my brothers bike was doing the same thing, its a v twin marauder but the front carb would run out of fuel just like that and he would have to hit it with the compressor.  when you take the carbs off i would only pop the bowls off and go at it from the bottom, very easy, less peices. less confusing. good luck and let us know.

[mycb750k6]

Well I've spent the morning out there. The weather's fine now ~ 78 degrees in San Juan Capistrano. But I digress.
mystic and goon, your're helping me a lot.
1. The floats are free moving.
2. The tees are clear.
3. The overflows are clear.
4. There may be dibris from before I added the inline filter but none is getting through now. I cleaned the jets and bowls last night.

That leaves the float valves I guess and likely not all of them I suppose. I don't know how they work or how to get them out. Can someone lend a hand here. I've got the shop manual open but still struggling.

So it I blow it out till it pops, it will run fine and fuel will flow but when I shut it down and turn off the gas and start over, the problem is there again. Can you point me in the right direction again? The float valves are staying open somehow?

[MCRider]

Well I've spent the morning out there. The weather's fine now ~ 78 degrees in San Juan Capistrano. But I digress.
mystic and goon, your're helping me a lot.
1. The floats are free moving.
2. The tees are clear.
3. The overflows are clear.
4. There may be dibris from before I added the inline filter but none is getting through now. I cleaned the jets and bowls last night.

That leaves the float valves I guess and likely not all of them I suppose. I don't know how they work or how to get them out. Can someone lend a hand here. I've got the shop manual open but still struggling.

So it I blow it out till it pops, it will run fine and fuel will flow but when I shut it down and turn off the gas and start over, the problem is there again. Can you point me in the right direction again? The float valves are staying open somehow?

No the valves are sticking closed, not open. Fuel levels in the bowl may be too high, forcing the valves closed hard and they get gummy and stick there.

Old style carbs had clips holding the float bowls on. Convenient. Yours are probably 4 screws. Put a phillips driver on a ratchet wrench to take off (without removing carbs, after draing bowls). Once off, you can push the floats up, watching the little valve plunger go up into its seat, then when you allow the float to fall that plunger should fall out of the seat as well, if not, it sticks. duh

So pull the float hinge pin and that little plunger will fall out on the floor. If lucky youcan just clean the point with lacquer thinner type product. If the tip is rubber someone may suggest how toi claean it. If the seat in the carb itslef is gummed up, it can be removed by unscrewing it, usually with a 10mm wrench. Its kinda like a small plumbing faucet seat. Then clean the gum out of it and screw it back in.

This is easier of the carbs are off, but its not necessary to remove them.

What we used to do was shut the gas off first before shutting off the engine. This would lower the level in the float bowl at shut down and the valves plunger would always be free of the seats and couldn't stick together. That wasn't the reason, but it was a result. The reason was to keep raw fuel from running into the engine after shut down. Maybe an unnecessary paranoia.

Are you parking on the sidestand? This often makes float valves stick, if gummy, but clean ones won't stick regardless. Try parking on centerstand.

[Deltarider]

BTW, parking on centerstand is always best. More oil stays 'upstairs' reducing wear at cold starts.

[tygrant]

Just to help in his description the "plunger" is the needle the seat is where it goes. this isnt identical to yours but similiar. when the pointed needle pushes against the seat it closes when its pulled away it opens. this is jsut to help visual understand whats going on in there.

[goon 1492]

That's it MCRider  I wasn't getting the explanation out right  , can't wait until I get a computer at home, working on one 8-10hrs a day and getting 15min. spurts of internet time three times a day is as much fun as a good sneeze in the middle of a good pi$$...

[Hush]

If you don't find the fault, and sometimes you just gotta chuck it all back together and ride til it screws up again.
Next time it runs outa fuel remove the carb bowl drain screws and see if you actually have gas in there!
A full set of carb float valves gost me $64 off the net and sometimes it pays to start with the basics, these little beggars only last so long.
Just remember that your bike has 4 of everything so one jamming float should make the bike run rough but not stop!
Some carb cleaner in the gas tank occasionally isn't a bad idea too.

[Spanner 1]

Jees!, these guys will have your bike in 1000 pieces in no time !
It runs fine until this event, no fuel in the line. right ?. Your carbs are fine,
I say its an air-lock in your tank....ride around with the filler cap popped open, see if that resolves it...back to your first reply. Please let us know the cure !!

[mycb750k6]

You guys are great and this is an awesome sight. The suggestions and pictures were top quality.
I wish I had something to report this morning but as of yesterday late, the problem has not returned but it's likely it will.
After sitting over night I will be at it again (once it warms up again  ) and now feel comfortable removing the floats and float valve with the carbs on the engine. I will finish this thread today if I have solved it or it's gone into hibernation.

[goon 1492]

Not sure why I didn't see this in the first post, must be because of the stomach bug I was getting over at the time.
Its your inline filter, espically if you have the screen on your petcock inside your tank still intact. It will trap air inbetween the inline filter and the petcock and when you give it enough gas the engine will drain all the fuel out of the carb bowls before the fuel line can trickle it down to replenish what was used, I know this because it happened to me. I can back this because when I had a clear inline fuel filter I could see the air pocket inside the filter and it was on the side between the tank and filter.

take your petcock apart and clean the screen good then put it back together without the inline filter and see how it flows , Where's TT at? He would have been on this one first thing.
Here's mine and I have a 750 k6 also.