Topic: I know, I know... another carb problem

[nitro7476]

I need help!  I had my '73 CB750 running enough to take me for a ride at about 40 mph, but it wouldn't go faster.  Big dead spot above about 1/4 throttle. So I figured the carbs weren't as clean as I thought.  Took 'em off and tore 'em apart.  Sure enough, little pieces of gunk in the jets.  Cleaned everything up with carb cleaner, a strand of copper wire and a toothbrush.  Put it all back together and tried to start it.  Got it to start and run finally at full choke and half throttle!  won't idle,  won't stay running without the choke.  Adjusted air screws from 1/2 turn to 2 1/2 turns, it worked best at 2 turns.  Still not good.  I don't know what to adjust next.  Please help!

Thanks,

nitro7476

[TwoTired]

A clean carburetor wil return to the dirty state if fed dirty fuel.

Repeated cleanings of carburetors is a very slow way of flushing bits of crud out of the fuel tank.

[eldar]

Install a filter to prevent contaminated fuel from entering the carbs.
When monkeying around with carbs. It is best to cover a few basics right away.

1: set you float height and make sure you float valves are good and the seats are clean.

2: I suppose this should have been #1 but hey, you write when you think of it! Make sure EVERYTHING is clean. Some people dip their carbs, use the yamaha cleaner for this as supposedly it does not eat the rubber seals. Get a jet cleaning tool and VERY CAREFULLY clean the jets and all other little passageway you can. Make sure the needles are not badly scored , same with the slides and that the jets or not worn.


3: Start with your needles set to the same clip position and do a rough "bench" sync with a smooth, round piece of metal and set the slides to the same height. Turn you idle screws about 1.5 - 2 turns out but set them all about the same.

4: now reattach carbs and hook up your cables carefully to prevent binding or any pull as you turn the handlebars. Make sure choke is at full open when you connect and tighten down the cable.

Following these steps should at least get you started and will make it easier to track down problems.

Use clean plugs and maybe new plugs is these are old. Check your points for pitting.

[nitro7476]

OK, I got past my reluctance to rip the carbs off AGAIN, and tore 'em down to the last screw.  I wound up building 1 set out of 2.  The tops of my slides were so bad that the screws wouldn't come out.  So I robbed the slides out of another set.  I took everything apart and let it soak in carb cleaner overnite.  Put everything back together this morning, and fired it up.  It runs good when sitting still; quick throttle response, revs up good, idles well.  Kick it into gear, and it goes, with enthusiasm!  UNTIL, I get out on the road and try to keep it tached up to go up through the gears.  It dies out at about 1/2 throttle in second, third, etc.
This time I think I know what to do.  I need to reset the jet needles to run a little leaner.  If I am wrong about this, please let me know before I tear the carbs down again.  My kids forgot it was Father's Day because they haven't seen me since Thursday night. (I spent most of the weekend in the garage ).  So it may be a couple of days before I tear into it again. 

Thanks for your help,

nitro7476

[nitro7476]

GRRR!  I couldn't wait, so I reset the jet needles after work today.  Now it runs good at idle up to 1/2 throttle.  when it starts getting rough, it gets better if I choke it, runs up another 1/8 to 1/4 throttle, then I have to back off the choke a little and it takes off.  If I leave it choked and slow down, it dies.  If I slow down and choke down at the same time, its fine.  When I was getting this one going, I took 4 individual air cleaners off another bike and put them on, would it help if I put the original air cleaner box back on?  I don't want to have to tear the carbs down again to reset something if I don't have to, although I am getting ALOT faster at it.

Thanks,

nitro7476

[eldar]

Well now, what are the rpms at half throttle when it starts to die? If you need choke, at that speed, it gets difficult. You would need to do a plug chop to sse this. Runn as fast as the bike can then kill the engine BEFORE it revs down. Stop and pull the plugs and look at them. IF they are white or very light colored you are running lean, then you may want to raise your main needles by lowering your clips to richen the mains.
If there is alot of carbon, then lower the needles to lean the mix. I suppose this could be done with carbs on the bike but it would probably take longer than just removing them.

[nitro7476]

I put the original (I think) air box back on, and the only rough running is at idle 'cause I have it set for the pods.  I will fix that tomorrow.  Thanks for all your help, eitherin response to my questions, or in others' posts.  I have 4 more of these bikes so I'm sure I will have more questions.  At present I am going to enjoy riding this one.

Thanks again,

nitro7476