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79 CB750 - Starting issues, carburettor?

Chudman1

CB750 Enthusiast
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East Coast NSW, Australia
Hi guys,

I bought a 79 cb750 (think it’s a k) about 5 years ago.
It was completely disassembled when I got it so it’s been a bit of a puzzle to put back together.
I re wired the bike with an M unit, I’ve checked all the sparks and everything seems to be working.
I’ve sprayed some starter into the airfilters and cranked it and it sputters and kicks so I believe the compression is okay.

Yesterday I took the bowls off the carbs and cleaned the jets, there were all clogged.
I put it back together and got some fuel back into the carbs. Tried starting again and nothing, even tried to give it some help with the starter spray but nothing more than a lite sputter.

Does the vacuum line need to be connected for the carbs to jet?


Thanks in advanced
 
There is a lot in that picture to discuss. Jets may be clean but what about the air passageways? You have pods... what are your jet numbers?
Compression is measured with the bike turning over and throttle wide open.
And we don't know if your m-unit was word correctly.
I don't want to be a downer, but we need to discuss each of these things.
 
There is a lot in that picture to discuss. Jets may be clean but what about the air passageways? You have pods... what are your jet numbers?
Compression is measured with the bike turning over and throttle wide open.
And we don't know if your m-unit was word correctly.
I don't want to be a downer, but we need to discuss each of these things.

Hi Brett
This is exactly the response I was looking for.
I have no idea if the carbs have been jetted to suit the pods, I’m going to assume not. Guy I got it off said it was stock and running before he disassembled it.
What would you recommend looking at first?
Is there a way of checking the air passageways without fully disassembling the carbs?
 
I cringed when I read you were using starting fluid. NEVER use starting fluid. You can break a ring land or a piston ring real easy. With any luck you haven't done that already. If the carbs were gummed up at all then they need to be fully disassembled and cleaned, and then cleaned some more. Then you move onto the next thing. Also don't use radiator hose clamps on the intakes. Use the proper clamps. Change them while you have the carbs apart. Also I see you don't have the choke hooked up, or the second throttle cable. You are going to need those.
 
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No on a 750 you'll have to remove the carbs to clean all the passageways. HOWEVER, if you know the pilots in all 4 are clear, then you need to know 2 things: the number of the pilot jets and the number of the mains. But you also need to know if those particular pods block the CV air circuit needed for the idle circuit. If those pods block the air circuit, the rest is immaterial.
 
I cringed when I read you were using starting fluid. NEVER use starting fluid. You can break a ring land or a piston ring real easy. With any luck you haven't done that already. If the carbs were gummed up at all then they need to be fully disassembled and cleaned, and then cleaned some more. Then you move onto the next thing. Also don't use radiator hose clamps on the intakes. Use the proper clamps. Change them while you have the carbs apart. Also I see you don't have the choke hooked up, or the second throttle cable. You are going to need those.
Is it detrimental not having the second throttle connected? The existing throttle tube was missing and the new one I got has a different sized fitting.
I had the choke connected previously

Noted on the clamps, I have removed the carbs again so I will get the correct ones.
 
No on a 750 you'll have to remove the carbs to clean all the passageways. HOWEVER, if you know the pilots in all 4 are clear, then you need to know 2 things: the number of the pilot jets and the number of the mains. But you also need to know if those particular pods block the CV air circuit needed for the idle circuit. If those pods block the air circuit, the rest is immaterial.
I’m confident the jets are clear now as I was able to spray the cleaner all the way through

How do you mean block the air circuit?
 
Is it detrimental not having the second throttle connected? The existing throttle tube was missing and the new one I got has a different sized fitting.
I had the choke connected previously

Noted on the clamps, I have removed the carbs again so I will get the correct ones.
I would strongly suggest the return throttle cable. And you will definitely need the choke cable hooked up. Good you have the carbs off so you need to completely disassemble the carbs and freshen up everything. If you do a complete job the first time then you won't need to revisit the carbs.
 
IMG_7262.jpeg
 
All the mating surfaces have to be clean. If you have a polish like Autosol, use that on the slide and the inside of the carb that it slides in. The needles have to be nice and shiny. you need to take all the primary and secondary and pilot jets out. The air cutoff valves need to be taken out. Go to Vintage CB750 website and get yourself everything to rebuild the carbs.
 
as mentioned, there is a lot in the photos worth discussing.

1: the second throttle cable is absolutely worth hooking up. the push/pull type throttle was used because of the fact that sometimes throttles can stick open and you need to be able manually close it so you don't lose control of the bike.

2: the vacuum line on the middle carburetor is used for the CB750K because of the vacuum operated petcock. its function held the petcock open under vacuum to keep fuel fed to the bowls. it looks like yours is just a piece of hose folded over. I would recommend using a vacuum cap, or at the very minimum use a longer piece of hose with a bolt stuffed in it to not prevent vacuum leaks.

3: all of the band clamps on the intake boots should really be changed out to the factory type. I'm assuming they were swapped out either due to damage or loss, but the wide band clamps could not be properly sealing the boots to their connection points on both the carbs or cylinders.

4. just because the jets are cleaned, does not mean the passages within the bodies of the carbs are cleaned. I assume this is where you problem lies. take the bodies of the carbs completely apart until there is nothing left except the throttle shaft/blade. I would recommend using a light scotchbrite to clean the vacuum slide surfaces to make sure the slides move smooth and free. make sure you also remove the emulsion tube under the jet that is not attached to the brass jet holder. just be careful because they can be locked in. it takes a flat-blade screw driver to remove it and it is very easy to strip if it doesn't want to move. I soaked my carbs in acetone and used a syringe made of 100% polypropylene and squirted acetone through each passage. one of my carbs had an idle circuit that built up pressure while doing this and when it finally cleared the clog, I could see a bunch of dirt/sand in the bottom of the container. doing this can also help you understand how the fuel moves through the carburetor.
 
Hey everyone, thanks for all your responses.

I ended up pulling off all the float bowls and jets out yesterday as I didn’t remove the jets the first time when I cleaned them, however I’m gonna go for round three to make sure everything is done properly.

I live in Aus so not much chance of finding the correct clamps for the boots locally so I’ll have to source those

Hoping I can give an everything a proper clean, then see if I can get it running while parts are on order.

I’ll update as I go
 
Update, got some autosol and gave the vacuum cylinders some love. Cleaned the jets again including the needle jets which I left previously as I was worried about stripping them.
Got all the vacuum cylinders springing nicely.
IMG_7267.jpeg
IMG_7260.jpeg
 
The choke spring assembly isn’t quite right, not sure if the spring is thrashed or something is missing so I need to have a look into that.

I got the carbs back on the bike and loaded them up with fuel.
Here’s where I’ve run into another problem, the starter was kicking and bike was sputtering and ready to crank over and either one of 2 things would happen.
Just before it cranked the starter clutch would slip, or the bike electrical system would short out.

I took some videos which I’ll have to compress and upload to a link if anyone wants to have a look.

Cheers
 
It sounds like your starter is getting fed up from continuous cranking. If the carbs are good and the choke is working it should start pretty quickly. Personally I think you should have completely stripped the carbs and rebuilt them. It is kind of like a cat chasing it's tail.
 
Sometimes the choke spring is falsely blamed for the carbs being misaligned and the shaft not operating smoothly. When this happens I loosen the screws that hold the carbs together and place them on my table saw (flat surface) then gradually tighten the screws, this aligns the carbs so the shafts work smoothly.
 
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