Topic: a carb question

[nunnik6]

Just got my '70 750 out today, and I fired it up. After letting it warm up for awhile, I revved it up a few times and it sounded great. However, when I throttle back too fast, the engine dies and I quickly have to throttle back to get it to fire up again. Any suggestions? I know its probably an easy fix, but just to clarify...

Also, does anyone know when they started/stopped making the "tin" floats in the 750's? My '70 750 has tin (or metal) floats, while my '74 has something different. Just curious...

[smrshl]

I had the exact same problem with by CB750A.  Took it to the repair shop and had the carbs completely rebuilt ($500).  Tried it after they rebuilt the carbs, it still did the same thing.  They finally figured out that the points where dry and the dwell was off.  Once they freed the points up a little bit and adjusted the dwell, it runs like a sewing machine.

[eldar]

Do a full tune up first before jumping into the carbs.

Adjust your cam chain and set your valve lash.

Check your points and replace if needed and set timing and dwell and point gap. Check plus and replace if somewhat old.

Change oil if you have not done so yet.

See how it runs at this point. your problem could be gone now.

If you still have it, then pull the carbs, You probably do not need to rebuild them and a shop will not be able to do a much better job than you could yourself.

Clean your main and idle jets. Actually pull them out and get the crud out of the little holes.

Do a bench sync then reinstall the bank.

Now set the idle mix to factory settings and your bike should start. Warm it up and hook a set of gauges to it and do a sync and st your low speed according to a manual.

This will probably take care of your issue.

[HondaMan]

The brass floats disappeared during 1971 in the CB750, somewhere after the later K1 and the first K2 models. I had a K1 (late 1970 purchase) with brass floats myself, then when I bought my K2 it had the plastic ones. The plastic ones let the level "float" a little higher, I think, because a common malady of the plastic-floated carbs is a bit of gas dribble from the overflow tubes when on an extreme leanover sidestand situation. We always told the owners to turn off the gas when parking on the sidestand to prevent the puddles. My K1 never did this, but the K2-K4 were famous for it.

 I saw a K4 with brass floats one time, but the owner did lots of work himself, so I suspect they were a transplant, or maybe it had been modified with K0/K1 carbs. Startled me enough to write it in my notebook though! I didn't jot down the carb numbers, or that would have told the whole story.

[jaknight]

Hi Guys, ....... Hey HondaMan,

     Just a bit of trivia info for you........ I have a '74 CB750 K4.........
     I bought it with 16,000 original miles.  To the best of my knowledge it has
     not had any modifications........ all stock.  But, it does not have plastic floats in
     the carbs.  Go figure............

     Perhaps tomorrow during daylight I can get specific info off the carbs and add
     that info to this posting.........

     ~ ~ ~ jaknight ~ ~ ~

[HondaMan]

Jaknight:

See if you can get the carb numbers of off your carbs. You have one like the odd one I spotted that day. I wonder if Honda ran out of plastic floats for a while, or resorted back to metal ones, for the leakage reasons, for a while.