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79 CB750 - Starting issues, carburettor?

Drenched in fuel.
Won’t rev.

Next thing I’d do is pull a float bowl off one of the outside carbs.
It’s tight but you can do it with a real small or what some call a shorty Phillips screwdriver.

With the bike on the center stand put a rag under the carb that has the bowl off it.

Gently lift the float upwards with your index finger. It should move freely.

If it does hold the float up and turn the petcock on.
Float up no fuel should run thru the carb.

Lower the float and fuel should gush across your fingers. Lift it fuel flow should stop.

If it, fuel, doesnt stop make sure you have floats in right side up. Or there isn’t some thing from keeping the float shutoff valve from leaking.

A little tip on reinstalling carbs…….get a set of ratchet type tie downs from wal mart.
They are like15 bucks.
Get the carbs started in the boots as best you can. Maybe spray a bit of wd40 the necks.
Wrap a tie down around back of carbs and around the front of cylinders. Be careful to not place the strap where you’d put a lot of pressure on a single fin.

Gently ratchet the tie down in as you wiggle the rack of carbs back into the boots.
They will slide in very easily.

My ‘79 SS, it took me 4 times removing, cleaning, reinstalling the carbs to get it right. Now that bike runs and revs like a bat out of hell.

Hang in there guy. Yours will also.
You are getting close.
Hats off to you, you’ve come a long way, farther than most would.

You did pull the domes and slides out when you immersed the carbs in fluid I’d assume.
There are very delicate diaphragms up top
that lift the slides. Holes in those diaphragms or full of liquid would keep the slides from lifting.

Keep posting, I’m interested,

We will get this thing.
 
Was it the rear boots (airbox side) or the front boots (engine side) that were drenched in fuel?
 
I already full disassembled the carbs and ran through the ultrasonic, all new jets and seals.
Read the rest of the thread
Obviously you missed something or did something wrong! I rebuilt my carbs following these instructions once and my bike started right away with no issues and only needed minimal adjustment during sync procedure. Since you said you have carbs out I thought to share these instructions with you, you do with them what ever you want!
 
You can follow instructions with subpar parts and all end up with problems. In addition to my other question, was it all carbs that are having this problem?
True but that would still be on him and if that's the case he is paying for it now.
 
I wouldn't say it is all him. What I meant was if you follow the instructions to the letter, but use an aftermarket carb kit with crappy float needles, jets, etc. you will get less than desired results.
With that said, that's why I'm asking if he is seeing this on all 4 carbs; if he used a poor rebuild kit for all 4 carbs when they were out of the bike we might be able to pinpoint his issue faster.
 
Drenched in fuel.
Won’t rev.

Next thing I’d do is pull a float bowl off one of the outside carbs.
It’s tight but you can do it with a real small or what some call a shorty Phillips screwdriver.

With the bike on the center stand put a rag under the carb that has the bowl off it.

Gently lift the float upwards with your index finger. It should move freely.

If it does hold the float up and turn the petcock on.
Float up no fuel should run thru the carb.

Lower the float and fuel should gush across your fingers. Lift it fuel flow should stop.

If it, fuel, doesnt stop make sure you have floats in right side up. Or there isn’t some thing from keeping the float shutoff valve from leaking.

A little tip on reinstalling carbs…….get a set of ratchet type tie downs from wal mart.
They are like15 bucks.
Get the carbs started in the boots as best you can. Maybe spray a bit of wd40 the necks.
Wrap a tie down around back of carbs and around the front of cylinders. Be careful to not place the strap where you’d put a lot of pressure on a single fin.

Gently ratchet the tie down in as you wiggle the rack of carbs back into the boots.
They will slide in very easily.

My ‘79 SS, it took me 4 times removing, cleaning, reinstalling the carbs to get it right. Now that bike runs and revs like a bat out of hell.

Hang in there guy. Yours will also.
You are getting close.
Hats off to you, you’ve come a long way, farther than most would.

You did pull the domes and slides out when you immersed the carbs in fluid I’d assume.
There are very delicate diaphragms up top
that lift the slides. Holes in those diaphragms or full of liquid would keep the slides from lifting.

Keep posting, I’m interested,

We will get this thing.
The floats are definitely working and fuel is getting to the bowls.

I’m gonna hit the slides again and make sure they’re not getting caught on anything
 
Obviously you missed something or did something wrong! I rebuilt my carbs following these instructions once and my bike started right away with no issues and only needed minimal adjustment during sync procedure. Since you said you have carbs out I thought to share these instructions with you, you do with them what ever you want!
Didn’t mean any disrespect, I used the guide already, it was great
 
Brett if it isn’t firing, or is way out of time, one would think there would be a lot of gas at intake. Especially if he’s choking the heck out of it.

You’d think leaking float valves would show at overflow tube. But I have one that has a slow leak float. If I don’t shut off petcock, fuel will run down into a cylinder overnight.
Took forever to figure out why it knocked on start up then got quiet when that one cylinder cleared out.

I dunno.
At this point it’s turn the lights out, pull the plugs, get the shop dark and see if there is blue spark at each plug for me.
 
Yeah, I think you're right about the floats and the overflow tube but here we are.
So if the floats work fine, then I suspect valves but I would think the bike would run like shit, too.
Well, I reread the thread and it doesn't run well. I would give it a valve job. Perhaps the fuel is getting to get out of the carb just fine but the intake valve is too tight and closes too soon, leaving fuel in the boots?
 
Agreed.

This is where I take a walk on the wild side.

Shut off petcock.
Pull all 4 plugs.
If wet, crank with plugs out 4 seconds one time and leave open overnight.
Check for spark.
Check firing order, make sure the right wires go to the right plugs.
Next day reinstall plugs if good spark.
Take air filter out of air box.
Leave gas petcock off.
Spray 4 seconds starting fluid into air box.
Key on, bike in neutral.
Twist throttle open.

Hit button.

If bike starts, pretty sure it’s carbs/fuel.

If no start like that, time for compression check.
 
Last edited:
I have to disagree with the starting fluid. Should use carb cleaner instead. However,...
We know fuel is getting out of the carb, and we know it is getting into the combustion chamber because it fires. But because it doesn't run well and there is fuel in the intake manifold boots, I'm suspecting valves out of spec.
 
Good points guy.
Compression should find that.

Dry that thing out.
It might be half loaded up with fuel or flooding itself with bad float valves.
He’s gotta get those plugs clean or it will be hard to start.

A word of caution to all.
If the previous post brings the bike back to life, do not, even tho it’s tempting, keep shooting carb cleaner, starting fluid, hell, even gas out of a spray bottle into the air box to keep it running.
Let the bike die and find out what the heck is wrong with the fuel system.
 
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