Topic: Please don't 'throttle ' me

[nixon]

Hello
I've read as much as I can on this,  and had carbs off several times,  and happy to keep removing and adjusting till its running good.

It starts ok,  not even needing much or any choke.
It will idle ok, but will die if give gas, until warms up
Once it warms up,  it has noticable stumble from 1/4 to 1/2 throttle.

3/4 and wot it pulls really week quite  clean

Also,  the air/fuel screws on airbox side,  it will ONLY run with them pretty much closed.

I have not properly synched carbs. Only bench synched.
It's a cb750a, with round top carbs. Im running the top half of a factory air box, with a k&n air filter secured on the bottom,  with an alum sheet blocking off the bottom. The bottom half I couldn't make fit the auto

Please help obi wan and company.

I want to either raise , or lower the needle clip, I can't recall where its  at,  and or adjust slow jet.

But could it be balancing? Should I synch while running and only then make adjustments?

Also noticed these severely shrunken tubes,  middle one is fine? Are these must replace?

[BenelliSEI]

Those “shrunken tubes”connect the outer carbs to the inners. They are for venting the float bowls to the atmosphere. In your top photo, you have a hose looping the two inner carb vents shut. If you are trying to run that way, start by putting two hoses in there, venting up and back towards your seat. If they are not open ended, gas cannot flow into the float bowls at the rate it should. Also, are the shrunken tubes sealing up, or are those cross connections open?

Ensure the bowls are properly vented, or any tuning efforts are a waste of time........

[ekpent]

 Good eye on those vent tubes 

[nixon]

Wow ok!  That was how I got them years ago and wrongly assumed " that's how it's meant to be" do they normally vent  into the air box or something? Really appreciate your response!!!!! I'm very excited now . Can you replace the shrunken tubes without pulling the carbs off the rack I wonder? I found I can adjust the needles by loosening the carbs off the plate they attach to,  gives enough room.  So hopefully I can? I can't really tell if they sealed shut those shrunken tubes so better to replace.  Feeling stupid but happy

[WhyNot2]

Those shrunken tubes should be fine as long as they are allowing air to flow thru them freely.

That looped tube should be 2 tubes just free flowing to the atmosphere, but can and should be run to an area that can secure them from getting caught in any moving part.

Mine are long and loop up over the air cleaner down the back of it.

[ekpent]

 Just an E-bay pic but it shows some stock vent tubes. As for the vent tubes going between the carbs maybe possible but I say very difficult without splitting the rack.  Stock ones are black so yours were changed at some time.

[nixon]

Just managed to replace inner ones.  Put the small pieces in  boiling water,  silicone on metal and seems ok

[nixon]

So back together and much much better when warmed up.  The major stumble 1/4 to 1/2 Its pretty much gone.

But the air screws... why is it only really running ok when they are pretty much closed or  1/4?

I'm about too go on a long ride and will try aiding adjusting ..could i be doing damage to motor running like this? Apart from  more open mufflers that's only mod, dyna

[goodtryer]

Sounds to me like they’re getting air through some other path. Try the trick with spraying starter fluid around the carb rubbers to test for air leaks.

[nixon]

Well after 300km ride it's very strange.

- if adjust air fuel screws 1/4 or half turn out..it runs progressively worse. From 1/8 to 1/2 thottle.

If closed,  it still is noticeable heasition from 1/4 to 1/2.. but not nearly so bad.

Also - with zero throttle.. none..it will really cruise along now,  especially down a hill.  Like cruise control,  but you will slow on a flat.. but it really doesn't drop off.

If no carb leaks are present, Should I be checking,  adjust slow jet, needle? Which way?

Falling in love again with this bike but it's not doing it justice at moment

[nixon]

I changed the slow jet from 40 to 42. That allowed a 1/4 turn out of the air screws,  and starts easy.  Idles well. Still not right 1/4 to 1/2 then will pull strongly and cleanly . So I understand the needle clip Should be adjusted .  Does it need more fuel so drop clip 1? Or lean it out by raising? Or is it just trial error best bet?