1979 CB750F Carbs

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1979 CB750F Carbs

ThomasF514
Today my 1979 CB750F started leaking gas from the float bowl overflow hose on 3 carbs all of a sudden. Really weird.
To date the carbs have never been worked on, wonder what would be the culprit, The float  pins not seating right anymore or the floats themselves.
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Re: 1979 CB750F Carbs

ThomasF514
I tried switching #1 float and valve pin that was leaking with #4 that wasn't to see it the leak moved to #4. I didn't, so, it appears something is strange with the carbs that are leaking from the overflow hose.
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Re: 1979 CB750F Carbs

gonebiking-2
These carbs can be tempremental at times. They can start overflowing for no apparent reason at all, however , from my experience, it only requires a tiny spec of dirt to get stuck in the float chamber to cause the carbs to overflow. I`ve had it happen to me on several occasions until I put an inline fuel filter just after the fuel  on/off tap. Problem solved. However, if you are not mechanically minded, dont even attempt to remove the carbs, as it a nightmare  of a job. Basically the fuel tank has to come off, then remove the saddle completely ,as it gets in the way.  Remove the airfilter and the housing , not forgetting to unscrew the clamps that hold the air filter inlet pipes to the carbs. Remove the battery and  now the fun begins. You will learn a whole new vocabulary in swearing that you never knew before, and apart from a few pieces of skin removed from the knuckles, eventually you will find the knack of getting the battery holder out and back in  - if you have to do it often enough. Remove both cables to the carbs, and do not alter the spring tension on the return side. Once everything has been removed, all four carbs will come off as one assembly ( make sure you have drained the fuel out of the float chambers before you pull eveything off ). place carbs on a clean surface upside down, and remove the screws that hold the four bowls in place. You might be surprised at the amount of dirt that is lying at the  bottom of the bowl. Clean everything thoroughly, and using a compressor blow throught the jets to dislodge any dirt. Dont remove the jets as they can be a right pain to get back in . Once everything is clean and you are satisified that its now OK to reassemble, the process is the reverse of dismantling. The tricky part is getting the carbs to go back into the same place that they came out of on the  inlet manifold side. Ther should be telltale signs of where they were seated. Connect up the the two throttle cables - make sure you mark which one is the  "open" and which one is the "return" cable. Can be a bit of a bugger to have to dismantle everything only to find you have connected the cables the wrong way round. Then have fun getting the battery holder back in place. After all that, everything goes back quite easily. Assuming you haven`t touched the mixture screws underneath, and you have not altered the butterfly openings, replace the tank, and if possible fill with fresh fuel. Give it a couple of minutes , and bingo she should start immediately.Otherwise if all else fails take it to a reputable bike mechanic who knows how to work on these four carbs ,and then get them syncronised, and the idle mixures set. She should be good to go after that, and 99.99% of your overflowing problems  will be a thing of the past.
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Re: 1979 CB750F Carbs

rickster
In reply to this post by ThomasF514
I have the same problem. Did you get yours fixed? What worked? Thanks!
Tom
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Re: 1979 CB750F Carbs

Tom
I ordered new float valve needles and cleaned the brass seats good inside where the valve seats when the float raises up. All better then, I also installed an inline fuel filter that I got from Auto zone for about 4 bucks. It's doing fine now.
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Re: 1979 CB750F Carbs

db45
In reply to this post by gonebiking-2
I'm not sure if the cb 750 k is that much different than the 750f but gonebiking-2 made the carb removal process sound way harder than it is. first of all, just speaking of my 79 750k you dont have to remove the airfilter, or airbox. you dont have to remove the battery tray or battery. a 10mm bolt on top of the airbox allows you to scoot the airbox back about two inches giving plenty of room for carb removal.
 As far as removing the jets, this goes for any carb cleanig job. ALWAYS remove the jets, blowing air through them will never get all the gunk of the back side of them. as far as being a pain to put them back in, if you can use a standard screwdriver, you can put them back in very easily.
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Re: 1979 CB750F Carbs

Steve
That's awesome, now that is the kind of admonition I was looking for to provide the necessary motivation to complete the job! I was thinking the same thing, why the hell wouldn't you take out the jets and give them a good soaking after going through all that trouble of the removal process? Then you know the job was done right! Thanks for the tip on just sliding the airbox back, that sounds better than taking it out!