Topic: Cb550 Carb problems

[Stretch.jbm]

I've searched these threads high and low for this issue, couldn't find any answers.
I'm working on a 1974 Honda CB550f, full carb rebuild. They were incredibly dirty so so I put them in the ultrasonic cleaner to get them cleaned up, they now look brand new with new jets, float needles and seats. After putting them together I benched tested them and there were steady drips coming out of the overflow tube. So I took them apart again and double checked float height and made sure there were no cracks in the floats, still didn't fix the issue. So I ordered another set of needle and seats and installed them, pressure tested them with the mighty vac, they hold 7psi of pressure, float heights are at spec, put them back together and gas pours out of the two left side carbs, out the overflow and through the throat of the carbs, I've swapped floats, 3 sets of needles and seats, but I cannot get the carbs to stop leaking.. Ive been working on bikes for many years and have rebuilt 100's of carbs and have never ran in to this issue. Anybody have some suggestions of what might be going on?

[DaveBarbier]

Try swapping around the float bowls. It's possible that the overflow tubes have cracks. The problem will follow the bowl in that case.

I've heard of the rubber gasket moving and sticking inside the bowl and not allowing the float to move freely. Another thing to check.

Also, sometimes the float tang has a dimple on it. If the floats are empty, sometimes the dimple can keep the float from rising. As it's filling with fuel, give a tappy tap tap on the side of the bowl with a screw driver or something to help free them.

Lastly, original Keihin jets are better than new aftermarket. Nothing to do with your issue, but it may give you problems once you get this bike running. Most here advise cleaning the original brass as nothing usually damages them. They just need a good cleaning and will outlast you.


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[TwoTired]

No such thing as a 1974 CB550F  Only K models available in 1974 for the USA.  First year for the F model was 1975.  The Capital F on "Four" does NOT make it and F model.

Compare the spring pin strength of the aftermarket float needles with your originals.  Strong springs make factory float height specification useless.  And then only a clear tube up the side and connected to the drain will reveal true fuel level in the bowls.

Unless damaged, you are better of with the original Keihin brass parts.  They were proven to be of the correct functioning specs and dimensions.  Aftermarketeers take liberties with dimensions and don't check vendors supplied dimension to save on QA expense and rejection costs.

Do ensure the bowl gasket stays in its rail.  I use Gasgacinch for this purpose.

Cracked stand pipes must be repaired, or they drip.

Watch for even the most minor of divots around the bowl pivots to interfere with free bowl swing movement.

Cheers,

[evopgt]

I had this problem on my 78 cb550.  I simply tightened the heck out of the carb bowl drain screw and it stopped. The one drain screw must not have been seating correctly until I tightened the heck out of it. 

[DaveBarbier]

Tightening the heck out of two soft metals probably isn't the best idea.

Besides, I believe the earlier carbs don't drain from the overflow tubes when you crack the drain screw, so in his case, that wouldn't do anything.


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[northern]

Have you used clear tubing in the drain plug holes to confirm your float height? A high float will cause a drip out of the overflow tube.

Place a section of clear tube into the drain hole, then hold the other end higher on the carb. Open the pet cock and see how high the fuel goes up into the hose. I believe it should be 2-3mm below the top of the float bowl.

The other issue could be cracked overflow pipes inside the float bowls.

[Bankerdanny]

I had this problem on my 78 cb550.  I simply tightened the heck out of the carb bowl drain screw and it stopped. The one drain screw must not have been seating correctly until I tightened the heck out of it. 

That is the sort of thinking that gets you DPO (damn prior owner) status when you sell and the new owner strips out the head of the screw because you "tightened the heck out of it" instead of properly cleaning the orifice so it doesn't have anything keeping the screw from sealing properly then replacing the o-ring. There is no pressure in the bowl. There is absolutely no reason to crank down on that screw.