Author Topic: 1976 CB550F Leaking gas out of the carbs and into the airbox, why??? (Read 11752 times)

marty · « **on:** February 25, 2012, 11:14:10 AM »

I bought a CB550F super sport to restore, I've done everything I thought I should do but I can't get it to run right. I changed the oil and filter, set the valves, filed the points and adjusted the timing, cleaned and rebuilt the carbs, set the floats to 22mm. The carbs still leak gas into the airbox. It did this before I rebuilt the carbs and it still does it.

With the carbs on the bike I removed the air filter and looked inside the airbox, when I turn on the gas I gas see it start pouring out the back of the carbs and into the airbox. I've yet to isolate exactly which hole the gas is coming out of and on which carbs.

I pulled of all the float bowls with the carbs on the bike to see how much gas there was and all but one seemed right, carb #2 was a little lower than the other three, but it was definitely overflowing into the airbox. Not a drop of gas comes out of the overflow tubes which connect to the base of the carbs. What would cause this??? Is there some hole I neglected to clean properly? I left all 4 carbs connected to eachother when I cleaned them, do I need to separate them, and clean again? Or is there something I'm missing here?

Thanks in advance for any guidance.

marty · « **Reply #1 on:** February 25, 2012, 01:03:15 PM »

Update: I pulled off the rubber boots on the airbox side of the carbs. I turned on the gas and it started streaming out of the brass tube on the rear side of all 4 carburetors. So I sprayed some carb cleaner into one of them and it came out through the other 3 so I know they're all connected.

LesterPiglet · « **Reply #2 on:** February 25, 2012, 01:12:29 PM »

The overflow tubes? I've read on here they can develop barely visible hairline cracks. You may want to check that out.

marty · « **Reply #3 on:** February 25, 2012, 01:18:10 PM »

Cracks in all 4 overflow tubes? Would that cause gas to flow out of the brass opening at the rear of the 4 carbs?
Thanks for the suggestion, I will check them for cracks.

LesterPiglet · « **Reply #4 on:** February 25, 2012, 01:19:53 PM »

I thought you meant that was where the fuel was coming from.

marty · « **Reply #5 on:** February 25, 2012, 01:57:23 PM »

Sorry if my description was confusing: The gas is coming out of the brass tube at the rear of the carbs, not the overflow tubes at the base of the float bowl.
Here's a photo:

TwoTired · « **Reply #6 on:** February 25, 2012, 02:08:01 PM »

Fuel level is too high.

What float valve do you have in the bike? Stock? Aftermarket?

They should each have a spring pin to contact the float tab. The Float valve must be seated and the spring pin fully extended and just making contact when the float height measurement is made. The measurement is usually made with the carbs on theirs sides to relive float weight from the spring pin.

Did you change the float height from when the bike last ran?

marty · « **Reply #7 on:** February 25, 2012, 06:01:34 PM »

When I bought the bike it leaked gas out of the airbox and did not run well at all. I rebuilt the carbs with aftermarket rebuild kits (K&L) so the float needle and the seat are new. With all the float bowls off and the carbs on the bike I made sure each float needle was stopping the flow of gas at 22mm float height.

I set the floats heights with the carbs placed so the float just barely touched the float needle. The spring pin was fully extended.

My current thought is that the #2 carburetor's float looked a bit off center. I think the float may be getting hung op on the float bowl. However, if this is the case wouldn't the gas just leak out of the overflow tube at the base of the float bowl? All 4 overflow tubes are clear with no blockage.

TwoTired · « **Reply #8 on:** February 25, 2012, 10:59:56 PM »

The inlet air jet is at a higher level than the overflow tubes in the bowls.

If the carbs are level, the fuel must drain from the tubes before spilling out the air jet. Unless ... the carbs aren't level, rudimentary physics don't apply where you live, or the overflow tubing is blocked.

Can you make the liquids only occupy the right side in glass container?

I can't help when given false information.

Sorry,

marty · « **Reply #9 on:** February 27, 2012, 12:55:32 PM »

I'm not defying physics here nor am I providing false information. I realize the overflow drain tubes are lower than the air jet, however I am trying to clearly explain what's going on.

I pulled the carbs off the bike today, re-checked the float levels. Then held the carbs level and connected them to the petcock. Gave them a minute to fill with gas and there were no leaks. I waited another minute just to be sure.

I put the carbs back on the bike and low and behold they start leaking out the airbox again. Why on earth would they not leak when off the bike and leak when on the bike? I don't want to be a defeatist here, but I'm running low on patience with this bike.

The bike runs better with the airfilter removed and full choke than it does with no choke and the air filter on. But either way when I turn the petcock on it floods the engine and the bike dies.

bollingball · « **Reply #10 on:** February 27, 2012, 01:14:28 PM »

Why on earth would they not leak when off the bike and leak when on the bike?
My guess is the bike is not level. Longer fork tubes short rear shock? Park on a hill? Are the over flow tubes stock. Put a straight edge on the top of the bowl and measure how far above the top of the bowl they are.

Ken

TwoTired · « **Reply #11 on:** February 27, 2012, 01:50:22 PM »

I think you should review your fuel line connections.
The symptoms you describe are somewhat consistent with inserting fuel into the venting system.

Still, if fuel is coming out the air jet, it should also come out the over flow tubes. Can't deny physics.

When you connected them off the bike, were they on the same height with regards to tank fuel level as when installed on the bike?

Bike doesn't care about your patience. It needs what it needs and has no emotional investment. Deal with it on the bike's terms.
Humans are good at reasoning things out.

fastbroshi · « **Reply #12 on:** February 28, 2012, 12:49:54 AM »

I had the same problem after I cleaned my carbs and reinstalled on my bike. Took them off and rechecked my float levels and they were all wrong. Corrected that and no more leaky.

Since yours aren't overflowing from the overflow hoses that run from the bottom of the carbs, it can't be cracks in those. I've read others' opinions not to use the off brand carb rebuild hardware, namely the float valves, but you said it did that before you rebuilt them, so that possibly rules out faulty valves.

You never confirmed wether or not your suspension is stock. A raked out suspension could possibly cause your problem or exacerbate the floats being a bit off.

Could you verify with a picture how you measured your float height level?

lucky · « **Reply #13 on:** February 28, 2012, 07:23:55 AM »

Still working on this huh? 2nd post.

Why don't you show us some better photos.
And show or tell us how you checked the float level.

YOUR COMMENT
"I pulled of all the float bowls with the carbs on the bike to see how much gas there was and all but one seemed right, carb #2 was a little lower than the other three, but it was definitely overflowing into the airbox. Not a drop of gas comes out of the overflow tubes which connect to the base of the carbs. "

Very scary comment.

Is the fuel supply connected to the vent instead of fuel in to the float bowl???

You need to get an experienced MOTORCYCLE mechanic to show you HOW to check your float level.

Deltarider · « **Reply #14 on:** February 28, 2012, 08:28:22 AM »

Quote

Is the fuel supply connected to the vent instead of fuel in to the float bowl???

That's my guess.

marty · « **Reply #15 on:** February 28, 2012, 01:53:42 PM »

Thank you for all the responses, I am still working on this issue. My next plan was to change the float level heights from 22mm up to 24mm to see if that helps this issue. I haven't changed it yet because I was waiting to see what people would suggest. I am going to take another look at where the gas is entering the carbs but I can't see where else they could be connected. I thought there were just two points to connect the fuel line, not 4 options. Maybe I will take a photo of how they're connected.

I brought the carbs to a motorcycle shop that deals specifically with 70's honda's only and they thought one of my floats was getting hung up on the edge of the float bowl. Maybe I should have just left the carbs with them so they could sort them out, but I'm determined to resolve this myself.

Here's a photo of how I set the floats, I put the carbs in my lap and sit them at a 45 degree angle or so, so the float needle is just being touched but not depressed.

The bike's suspension looks stock. I haven't taken apart the front forks to confirm they're stock springs, but everything looks stock.

When I had the carbs off the bike and I connected them to the tank I held them at approximately the same height they would be if they were on the bike, maybe an inch or two lower.

fastbroshi · « **Reply #16 on:** February 28, 2012, 02:18:43 PM »

I wonder if it's just one carb or all that's the culprit. You said the "carbs" were leading into the airbox. I'm wondering if you mocked it up in place at any point and watched all four do this. If not, it's possible it's just one carb that's float is upside down or something silly like that.

TwoTired · « **Reply #17 on:** February 28, 2012, 04:10:31 PM »

I gotta ask...
The picture shows the measurement with carbs upside down. On each float valve is a spring pin and the weight of the float will depress it. The spring pin should not be depressed while the measurement is taken. I usually check float height with the carbs on their side.

The fuel should go into the carbs at the "T" fittings between 1&2 and 3&4 carbs. There are two much small nipples higher up on carbs 2&3 which are vent lines. These would bypass the float valve completely and certainly make the carbs overflow if connected to the gas supply.

Are the shiny new jets from Honda? As well as the float valves and seats?

marty · « **Reply #18 on:** March 01, 2012, 09:24:04 AM »

In the photo the carbs are in my lap. I tilt them until the floats are not touching the float needle at all. Then tilt them back until the tab on the float just touches the needle without depressing it. I then measure the float height and bend the tab to reach the desired float height. In this case, 22mm.

The new float valve and seat, needle jet, main jet, were all part of the 4 K&L rebuild kits, which I purchased at a motorcycle shop which works only on vintage hondas.

The fuel lines look to be connected correctly. They are connected to the T fittings between carb #1 and #2 and the other between #3 and #4.

harisuluv · « **Reply #19 on:** March 01, 2012, 09:53:22 PM »

Maybe we should start from the beginning. How thorough was the cleaning they got? Maybe you missed something--could be anything

marty · « **Reply #20 on:** March 01, 2012, 10:02:43 PM »

I cleaned them thoroughly, but you're right I could have missed something. I did not separate the carbs to clean them, I kept all 4 attached. I haven't fully dismantled all 4 carbs before because I've had good luck in the past just cleaning and doing partial rebuilds. This was my first full rebuild, I thought it would be simple. Just replace the stock jets and gaskets with the new parts and reset the float height. I set everything back to stock specs. Put the needle clip in the middle position, set the float to 22mm, brought the air mixture screw back 1 1/2 turns.

I used carb cleaner and several dentist style tools to clean out the carbs, they weren't too bad. I've seen much worse on a bike that ran pretty well!

I don't have an air compressor, maybe that's my problem. I blow carb cleaner through everything but maybe I need more pressure.

Duanob · « **Reply #21 on:** March 02, 2012, 08:32:22 AM »

Something still doesn't sound right but taking the carbs off and apart and going through them again is a good start. If you bought carb kits you should have all the o-rings for the fuel inlet T's. Get a K&L carb cleaning kit, it's a series of tiny wires for cleaning jets and passage ways. The pilot jet on these carbs uses the tiniest of them all and needs to be clean. I used canned air. You can squirt air into one passageway and figure out if it's clean if air comes out the other end. Make sure all your vent lines are clear too. Your floats should be plastic but make sure they're not 'gas-logged' and float the way they are supposed to. Just drop them in a container of gas and see how they float. If they submerge at all they need replacing. Also get a float tool to see where your floats are set with teh carbs on the bike. It's a clear tube that screws into the bowl drain. The gas line should be about 1 - 2 mm below the surface of the bowl/carb body. Other than that I can't think of anything else that could be the problem.

Bodi · « **Reply #22 on:** March 02, 2012, 02:10:00 PM »

Unless i miss my bet, the floats are in upside down. The flat should be up in your photo (down as installed in the bike). Still, the overflow tubes should dribble before the carb throat vents start gushing gas.

TwoTired · « **Reply #23 on:** March 02, 2012, 02:16:09 PM »

Quote from: Bodi on March 02, 2012, 02:10:00 PM

Unless i miss my bet, the floats are in upside down.

I suspected that as well. But, I checked one of my spare sets, and they were the same orientation.

Bodi · « **Reply #24 on:** March 02, 2012, 03:04:42 PM »

Maybe your spares are wrong? The 1976 550F fiche clearly shows the flats facing out, I haven't done a 550 for many years but the 400 flats face out. I remember I put them in reversed because it looks more natural, and setting the float heights is weird because there's no exact highest point - however the correct setting has them almost parallel to the metal anyway.

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Author Topic: 1976 CB550F Leaking gas out of the carbs and into the airbox, why??? (Read 11752 times)

bollingball