Topic: CB550F Needle Jet/Emulsion Tube

[BRG-BIRD]

Hi all

1975 CB550F with 069A carbs, stock exhaust, stock airbox, properly set up carbs etc. I bought the bike this spring and it has run poorly and comsumes a lot of fuel ~26-27 mpg with a lot of surging from 1/4 throttle to 3/4 throttle and black sooty spark plugs. Full throttle the bike pulls hard and has no surging. The carbs originally came installed with 100 mains (from the PO) and have since put in the proper 98 mains. The carbs also have the stock needles with the Keihin part number 273004.

What I am highly suspicious of is the needle jet/emulsion tube, that it could be for a 550K 022A model. Two reasons: The 100 main jet that was in the carbs when I bought it, I think the PO bought the wrong kit and replaced the emulsion tube with the wrong one. I pulled a couple emulsion tubes out but there was no part number stamped on them. The other reason is I chased the same issue with another motorcycle when I blindly installed the Factory Pro needle with a stock emulsion tube and fuel economy was horrible afterward. Installed the factory Mikuni needle and it was corrected.

It appears there are two options. Buy an aftermarket set of emulsion tubes or used if they can be found or buy 022A needles. I’m not real jazzed about the second option.

I have checked the coils and they ohm out good, Dyna S Ignition System and the air screws need to be set at 2 1/2 turns out or it will not idle at all.

Looking for thoughts and opinions even if you feel I am not on the right track.

[Deltarider]

Maybe you are overthinking it. Not many will change the needle jets (aka emulsion tubes) and I don't see that the #100 main jets are an indication. #98 or #100, it doesn't make much difference. The number on the needles is correct for the #069 carbs, so why would one change the needle jets and not the needles? I think your problem originates elsewhere. What air filter element do you run and what's the condition? If you have the original paper filter element, know that the breather tube can transport condens to it, reducing its filtering performance. You can check this by examining the filter element. If it has this typical soft spot, often a bit brown, it may have happened and that would enrich the A/F. Also check the air tract for any obstruction like a forgotten cloth under the buddyseat. Since you worked on the carbs, are you sure the chokes open completely?

[Scott S]

 I have at least two sets of NOS 550F emulsion tubes/needles if you decide to go that route.

[BRG-BIRD]

Air filter looks new, white in color. I have been through everything regarding the carbs even setting the fuel level slightly lower than specs call for with little change. Seems to me the last thing to do/try.

[Scott S]

 Try a different air filter, too. I had one that looked NOS and wouldn't flow for crap.

[BRG-BIRD]

All the jets are new from Jetsrus, carburetors are clean. The feedback on the fuel level is appreciated and realize it is a form of bandaid similar to shimming the needles themselves. I could try running it for a short ride without the air filter to see if the surging goes away.

Replacing the 100 main to the 98 did make a slight uptick in MPG of ~1.5 mpg but that could be an anomaly.

[05c50]

I've been down that road before. I bought a 77 550f from a fella that "tried everything" to get it to run right. It acted exactly as you've described, ran rich, ate gas, fouled plugs. After trying all the normal things and some off the wall things, I started to realize that the guy that I got it from was pretty meticulous in his attempt to repair it. I eventually eliminated things one by one and stumbled on to the emulsion tubes. With a magnifying glass and a bright light, I could see that the inside surfaces of the emulsion tubes were uneven and appeared to be worn. A different set of tubes and the bike runs great- no fouled plugs, no fuel smell, decent gas mileage.

.....Paul

[Scott S]

 Same here. Same experience with a K model. Ran stinking rich no matter what. NOS tubes/needles cured it.

[BRG-BIRD]

Update. Based on the theory that the carbs had the wrong emulsion tubes I tried to find Keihin replacements and they are very difficult to find in a set of four. I ordered a set of reproduction emulsion tubes from 4into1 and they arrived today. They are different that what was in the carburetors. The emulsion tubes in the carbs have two extra “emulsion” holes per tube and the inside diameter of the emulsion tube was definitely larger.

After install and a test ride I am very happy to report that the running issues that have plagued this 550F seem to be corrected. More riding, new plugs and some final fine tuning should show if it is back to where it should be.

Thank you all for your thoughts and ideas!