Correct Valve Needle for 1976' carbs

[bradenhall]

First post here, I recently purchased a 76 cb750K and i'm having issues getting the float valve needles to seat, which is resulting in overflow on one or more of the carbs. The valve needles are different than the stock ones in the manual, with a black rubber tip and a clips that hold them to the floats (just like these shown below)


The stock 76 valve needles are all metal, with no clips attached to the floats like the ones shown below.


I've adjusted the floats several times, but I keep getting the same problem. I'm wondering if these valve needles are the wrong ones for this year, and if i need to replace them, or if I just need to adjust to floats more.

Sorry if this was already posted in anothr thread, I wasn't able to find anything answering this question. Thanks

 

[madmtnmotors]

You have the wrong needles. The rubber tipped float needles were used in the 77/78 CB750 K and F models (some A models too I think).

 

[bradenhall]

Ok good to know, looks like i'll be ordering some new ones. I appreciate your help!

 

[madmtnmotors]

Actually, we should probably verify that you have the correct carbs for that model year first. A previous owner, hereafter referred to as "P.O." (in a contemptuous tone as well) might have swapped carbs for later model carbs. Or the whole top end might have come from a later model. Or the entire engine might have come from a later model. Can you post pictures of the carbs or at least read the model number off the side of one of the carbs? The serial number of the engine will be an indicator of the year the engine was made too.

 

[bradenhall]

The carburetor casting # is 657A, which seems to be for years 69-76. Found the engine serial number, which is 2060876; frame number is 2052995. I believe that would mean it's a 1972 frame and engine. This is strange though, because the title says 1976...hmm

 

[dirtdigger]

I use the later model needles in the older carbs as the rubber tipped ones generally seal better and longer then the metal ones. You do have to do a little bending on the tab of the float to get correct float height as the needles are different lengths. Did you use new seat also or just the needles?

As far as the engine and frame numbers is this a K or F model frame? If it is a F the frame number will be a cb750f-2052995 where if it is a K then it would just read cb750-2052995 If its a F model then it is a 76 but if it is a K then it is a 72, the DMV in some states has a very hard time keeping things straight when they dont know specifics about certain models. They only go by what there computer tells them. But by the numbers your engine is definitely a 72 K model.

 

[madmtnmotors]

Did you use new seat also or just the needles?

The way I read it, bradenhall merely tried to adjust the float level and discovered the rubber tipped float needles. The 657A would be correct for the earlier models (72-76). I would recommend new float needles and seats as a first step towards resolving overflowing carbs.

Do you have a rear drum or disk brake? How about some pictures of the bike too? We like pictures!

 

[Dave]

The solid needles are often replaced by the rubber tip needles in any float type carburetor, bike, auto, or farm tractor. The clip is added because the rubber can stick closed in some circumstances and the clip allows the float to "pull" the needles open. If they fit and work the type doesn't really matter.

You can easily test them by putting the seat in your lips and pressing the needle into it just enough to start compressing the pin/spring and then blowing.
(after washing them thoroughly in soap and water of course, gas tastes pretty nasty)

The real problem is quality which is sadly not what it used to be. Test the new ones you get before putting them in and save yourself a lot of time. Inspect them with a magnifying glass as well, look for machining errors. Last time I had this problem it turned out to be a tiny burr on one of the needles that would intermittently catch in the seat and prevent the needle from closing. It was easily filed off.

Also look for a ring impression on the rubber tip. The rubber tip will get this impression where it sits against the seat when closed. The combination of wear and age will eventually make it leak at that spot.

 

[amc49]

If they run ethanol in your fuel that can make that happen as well if bike sits for a bit before using it. Say couple of weeks. I personally don't care what the tip is made of, there are advantages to both types. The spring bail type can yank needle open if ethanol sticks needle to the seat. Rubber tip seals better but more susceptible to chemicals in fuel and often ethanol affects them. Chinese quality a problem with rubber tips too and bigtime. If the seat and needles are both new, if not matched to each other then they can still give fits, of course the guy who sells them won't say a word about that. The hole that needle drops in must be a certain size to let it move up and down easily with no hanging up but tight enough the needle cannot tip far sideways either, rubber bad in that case because a rubber tip will then seat angled sideways where a metal one will slide in a bit more to try to self-center and then needle closer to straight up and then seals better. A narrow angle contact point leaks less than a wide angle one. The hole that the needle actually shuts off matters too, that smaller hole deeper inside if too big then allows the needle to be overhwelmed by fuel weight easier and then erratic fuel levels again. Needle angle needs to be complimentary to the angle of any seat up in there. Sometimes conflicting angles cause the tip to stick in the hole. That's worse with rubber as the rubber 'sinks' into the other angle a bit. You can get issues like that with non-OEM parts.

You see there are all kinds of little things to mess up there.

I personally do NOT change those parts unless forced to, preferring to use them over and over again. I have to have solid proof they aren't working, I keep them matched to the individual seats and still use them over even if the tips show wear. If working fine it works fine after, don't go looking for trouble where none exists I say. No problems hardly at all in all these years. Ethanol is the biggest problem I've had with the parts.