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79 CB750 - Starting issues, carburettor?

Good thinking, I’ll check

That being said the ones that were on the 750 carbs had turned to mush, so I’m not sure they’ll be much use.

Anyhoo, here’s the carbs after the clean, I popped the bowls on to set the gasket in.
Didn’t have time to assemble the bank today but we’re getting there.
Come a long way since the beginning of this thread

Please ignore the mess that is my workbench 🤣

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They look great! Do you know if the float height is set correctly? Would be good to know that before connecting them to each other.
 
Ultrasonic seemed to have done a good job getting some of the varnish off
Blasted all the air circuits with cleaner and polished the float seats

Didn’t have time to piece everything back together today, but it’s getting close now
While you are there HAND polish vacuum piston sides, bore and cap (inside). This will really improve your throttle response.
For your missing rubber plugs go to Amazon like Brett said, they have variety pack of silicon plugs, I would test their resistance to the gas first before putting them in!!!
 
While you are there HAND polish vacuum piston sides, bore and cap (inside). This will really improve your throttle response.
For your missing rubber plugs go to Amazon like Brett said, they have variety pack of silicon plugs, I would test their resistance to the gas first before putting them in!!!
I gave them a light hit with some scotch bright to make sure they were moving freely
I’ll probably double back on them again though
 
Alright I’m back!

Just so happened the 900s carbs had usable slow speed jet plugs, so that’s a bonus.

I got the 750s carbs fully assembled today, they’re looking damn good too if you ask me.
I stole the rail off the 900 set too and cleaned it up.

Now, I know everyone is going to have the shits with me for this next part…

Rather than use the airbox, I got some bigger jets when I did the rebuild.
I swapped the slow speed jet for a 45.
The main for a 115 and the secondary main for a 75.
Now with all this in mind, see the video.

 

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If it runs with that set up, that's great! Pods have a particular issue with these carbs, and that is blocking the air circuit at the face of the carb body. Make sure that is open. Once you get the bike running, start checking the plugs for a lean condition so you don't melt valves, etc.
When your carbs' circuits are unblocked, and you find the right jets, refer to the plugs for confirmation it has the correct fuel/air mix and you're set to ride!
 
Hey Brett, I did check and they don’t block the air circuit
However I got the bike running a few times but even with the throttle fully open and the choke closed it was completely bogged?
All the pipes were hot which is a good sign

Also there’s a decently loud knocking
Not sure if you can hear it in the video
 
Didn't hear the knocking. Great news on news on the carb circuit.
You still may not have the right jets. Check the spark plugs to see what the engine is telling you.
 
Those pleats in that style of pods causes turbulence in the air entering the carbs. Changing jets WILL NOT solve that problem. Your slides will not work smoothly with that air turbulence. Long FOAM filters will not have that turbulence.
 
Great point. I think that's right. UNI filters seem to be a popular option. But so do DNA even though they have pleats.
 
Alright I’m back!

Just so happened the 900s carbs had usable slow speed jet plugs, so that’s a bonus.

I got the 750s carbs fully assembled today, they’re looking damn good too if you ask me.
I stole the rail off the 900 set too and cleaned it up.

Now, I know everyone is going to have the shits with me for this next part…

Rather than use the airbox, I got some bigger jets when I did the rebuild.
I swapped the slow speed jet for a 45.
The main for a 115 and the secondary main for a 75.
Now with all this in mind, see the video.

Did you put new throttle cables and a choke cable on it or are you still trying to take shortcuts? Put the airbox on. Get the hose clamps off it and put the proper clamps on. Why do I get the feeling we are all wasting our time with you?
 
If I was still taking shortcuts would I have spent 20hrs rebuilding the carbs?
The goal was to see if the bike would start, which it did.
The clamps and new throttle cables etc are on the list of things to get
 
You very likely have vacuum leaks because of those clamps, but we already told you that. Put a choke cable on it, put the airbox on it and get it to run. Running for a few seconds isn't a victory.
 
Pods.
I’ve seen guys tape off 2/3 of the pods filter area to get them to work.

Clean the jets till yer blue in the face.

Unless you get the gunk/varnish/crap out of the very small passages that from the idle screw you are wasting your time.
A guitar string to act as a snake, acetone squirted thru a small tube from a squirt bottle, along with 100 psi, not 40, not 50, to
blast that crap out is recommended.

If the fuel does not squirt into the throttle body from the idle aircrew hole you do not have it clean.

The dohc bike loves a well charged, fresh battery.

If it doesn’t start within 4 seconds, after 3 or 4 times, there is a reason.
No need to fry the starter.

Put the air box on.
Leave the filter out.
Spray starting fluid in the box for about 4 seconds.
If it lights up, it ain’t the carbs.
 
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