Topic: float needle / seat

[ahofam123]

Hello all,
I have a '78 cb550 that has been giving me carb problems. I have already rebuilt the carbs and have adjusted the float height numerous times, but after a couple of minutes running, fuel starts to seep out from the overflow tubes and the idle ports.

Is the problem likely with the float needles?
Also, it looks like the needle seats are pressed in. Is there any way that I can check to see if the problem is with the seats and not the needle?

Thanks,
Tom

[oldbiker]

according to my parts book the seat is pushed in with an o-ring to seal and hold it. I don't know about availability but the needles can certainly be obtained from a guy in New Zealand who advertises fairly regularly on e-bay.

[MetalHead550]

I just went through a set of carbs on from a 78550.  These are different than other carbs Ive worked on in that the needle has a spring loaded "nipple" that contacts the float tab, like the carb on a car, whereas other carbs have a needle that sits on a spring in the seat.  Make sure those little nipples move freely and have enough "spring."  I had a couple that were stuck in the uncompressed position and was able to break them loose with needle nose plyers.  Perhaps a couple or all of yours are stuck in the compressed position and therefor allowing way to much gas in regardless of float hight.  As long as the needles move freely in the seats then the seats should be fine.  As for the seats, I think theyre pressed in.  I tried to pop a couple out and they didnt seem like they wanted to budge so I let em be.  Hope I helped some!   

[MetalHead550]

Wait you said rebuilt..not cleaned..so those needles should be fine.  Did you put the little wires that connect the needles to the float tabs back on?  Beyond that, I've got nothin.

[ahofam123]

Yes, I reattached the little wires connecting the float tab to the needles. Also, I'm 99% sure that the needle's internal springs are able to move freely, but I'll verify this later on today.

How common is it for the needles to just stop sealing correctly? I've looked at the tips of my needles and couldn't find any imperfections. But then again, it doesn't take much to mess up a seal.

Thanks,

Tom

[MetalHead550]

Hell the needles in my carbs are the originals, I just cleaned em up and they work great so Id say not that common.  If your sure your floats are adjusted properly, then Im stumped man.  They should be level with the body of the carb where the float bowls attach when just touching the needles.  Mabey one of the older pros will jump in and help.

[Bob Wessner]

I went back and reread the original post. You say thay 'all' begin leaking after a time? If this is the case, it sure sounds like some sort of global issue. I find it hard to believe all carbs would have the exact same defect such as a stuck needle spring, etc. You say you checked the float height more than once, boy don't I know that feeling. I've been in and out of mine so much.. well, that's another story. I spent a good deal of time last year cleaning, rebuildind and measuring the float heights. Thought I finally had them squared away, just in time for winter storage.

This spring I went out and one of them was intermittantly dribbling fuel from the overflow. Talk about being p_ssed-off. So, off they came again. The offending float was dead-on the required 26mm as were the others. You can't argue with the facts, it still leaked some, so I just tweeked the damn thing a hair and it solved the problem. Like other things involved with these old 4's sometimes the recommened spec is a starting point I guess.

[phylo101]

Tom - STOP! - hold everything......

This is one of the problems that was created when this new version of the site was created, the OLD threads werent archived and copied across.

THIS is the answer. The problem is actually VERY simple, and eventually occurs on a LOT of 550s.

Its NOT the needle seat or needle, ok? And the dead giveaway is that its only from ONE carb, they would all have worn roughly at the same rate if at all, and they dont wear that much.....

The answer is actually ABOVE the needle seat.Remove the float in that carb by pushing the little pin it floats on out of the two alloy columns with a matchstick or something soft. Inspect it carefully......

And what you'll see is a number of rings on the little pin. What has happened is that the float has worn the centre section of the pin, ok? THEN as time passes the ENDS of the pin fret veeeeeeeery slowly in the two alloy columns, and the ends wear, and float back and forth a little...which means that as it moves thru to one side or the other (it cant go all the way out dont worry) the portion of worn pin from INSIDE one of the mounting posts peeks out into the centre between the two posts....and although worn it has worn at a different rate to the centre part where the float rubs....and so theres a "step" or "shoulder" for one end of the float pivot section to hang up on! If you could get it under a VERY powerful microscope the pin would now be shaped faintly like a dumbell.

Ok so far? Now, you can get new pins...but you have to buy as part of a float kit, VERY expensive but you only need ONE. And you get a new float at the same time lol. Now, the only issue is that as the fretting in the mounting posts occurs, the ALLOY wears as well as the steel pin, so its still free to move...see the problem now? itll still move a round a little. So positively locate it with a tiny bit of Loctite - I mean the cyanolatic glue not the studlock lol JUST a little, so that the pin doesnt move again. Believe me, theres NO other reason you should ever have to move the pin again if you fit the new float at the same time.....

Re-fit carbs, switch on, ride and enjoy.

Happens on a LOT of high-mileage 550s, same as the breaking countershaft sliding third gear. Oh,has noone mentioned THAT yet?

Phylo