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Guys basic engine operation, if proper amount of fuel is introduced to the cylinder and even if that cylinder does not fire, that same amount of fuel will be compressed out of cylinder into exhaust.
If we have SO much fuel that is pouring out of intake boots than we have exorbitant amout of fuel which is filling the cylinder and time during exhaust valve is open at exhaust stoke is not enough to purge it thus fuel is filling the jug and got nowhere to go but out of intake boot.
My 2c
If we have SO much fuel that is pouring out of intake boots than we have exorbitant amout of fuel which is filling the cylinder and time during exhaust valve is open at exhaust stoke is not enough to purge it thus fuel is filling the jug and got nowhere to go but out of intake boot.
My 2c
Motogrady
Motogrady
It should.
I’ve had dirt bikes that had clogged tubes that go on the nipple of the float bowl get clogged. Fuel backed up into the motor.
But ya, if no vinyl tubes installed to get the overflow past the motor to drain on the ground, you should see fuel drip as long as the petcock is open.
But, as been mentioned, some petcocks of that era shut themselves off when engine shuts down and vacuum from the intake ceases.
Spawn of Satan petcocks some call them.
I think you're right about the fuel being expelled the exhaust, but the only way for fuel to exit the intake valves is if the piston is rising when the intake valves are open. And that doesn't happen.
Motogrady
Motogrady
Very tru guy.
Like I mentioned, I have one cylinder, I think it’s “3, that has a slow trickle thru the float valve.
Not enough to cause overflow, but enough to dump fuel into the bottom end overnight.
Been meaning to pull them and replace the float valve, really, but it’s too easy to just shut the petcock off……
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It would really be great if you would stop telling people to do this. Thank you.
True that doesn't happen but how we end up with fuel coming out of intake boots when piston on intake stroke creates suction!?
Chudman1
CB750 Enthusiast
Cylinders 1 and 3 spark plugs are black
2 and 4 are only slightly dirty
2 and 4 are only slightly dirty
My suggestion was the intake valves open to let fuel in but then close too soon and leaves fuel in the intake boot. It's just a thought.
Motogrady
Motogrady
Good question.
Maybe not strong enough to really vaporize the fuel like a running motor would?
I’ve never looked, one wonders what rpm a starter throws.
On the overflow thing.
If he’s got a vacuum shutoff petcock,
and it’s working, the carbs might not overflow without engine running.
He cranks it, vacuum is created, petcock opens, the out of adjustment or bad floats
flood the motor.
Just thinking out loud guys.
Chudman1
CB750 Enthusiast
Vacuum petcock is plugged
I don’t have a tank on atm so I’ve got a fuel syringe feeding straight into the carbs, nothing stopping it flowing into the carbs except the floats
Motogrady
Motogrady
I believe there are 2 ignition coils on that bike. Each coil supplies 2 cylinders.
I don’t know which ones.
Put the plugs in the wires.
Lay them on the motor so the sides touch metal to metal.
Turn the lights off ignition on and hit the switch.
All 4 should show blue spark.
They don’t move the plugs to different wires. If the weak or no spark stays with the same wires, swap coils around.
Motogrady
Motogrady
Home style fuel injection eh?
I love it.
You aren’t really squeezing that thing are ya?
Chudman1
CB750 Enthusiast
Alright, the issue is definitely too much fuel
Once the fuel ran out, the air rushed in and she went all the way up baby
See video.
Once the fuel ran out, the air rushed in and she went all the way up baby
See video.
Chudman1
CB750 Enthusiast
I cleaned the slides again, but it only cranked when the fuel ran out, could the floats be too high? I don’t think you can adjust these ones
Either that or it was less fuel
I’m an idiot and put bigger jets in but I’m gonna swap them back to the factory size
Either that or it was less fuel
I’m an idiot and put bigger jets in but I’m gonna swap them back to the factory size
Motogrady
Motogrady
Before you go any further set up a gas tank, let it feed by gravity.
I know one thing guys.
The 79/83 DOHC engines rev up and sound sweet.
If I’m lying I’m dying.
I know one thing guys.
The 79/83 DOHC engines rev up and sound sweet.
If I’m lying I’m dying.
Chudman1
CB750 Enthusiast
It’s being gravity fed
I’m just using the syringe as a vessel
Tank needs a good clean
I’m just using the syringe as a vessel
Tank needs a good clean
This thing you are calling a fuel syringe, is it applying pressure in the line? If so that will force the valves off the seats in the carbs and flood them. Go to a junk yard and get yourself a fuel tank off a lawn mower, and then hang it with a piece of wire from the handlebars and let gravity do it's thing. Also you mentioned earlier that you put larger jets in for some reason, so put the stock ones back in. This will eliminate some of the things that could be causing your problem. Get a fresh set of plugs also. You are probably going to have to change the oil at some point too because you will have fuel in the oil after all this flooding.
