Topic: CB750 smoke

[xcvz]

Hey guys, CB750 here. After winter I adjusted the valves and cleaned the carbs. It started nicely, but the sound was a bit rough (didnt notice it at first after winter and all)  and after a minute or so harsh gas smelling smoke started to show graduately (white color maybe a little to the gray side), was not smoking at all for the first minute, choke off. I thought it was the carbs so i overhaulled them once more, polished the float needle wells, changed the needles, checked the float heights like 5 times,everything is checked thoroughly, the mixture screw is set to default turns (even 1/4 to the lean side as i though it was to rich) the carbs are not leaking with fuel connected, everything is fine. Put everything back and got no change, same smoke after one minute of warming up time.

Help me real quick guys, maybe i set the valves wrong, can it cause this condition?

P.S. filters, spark plugs are brand new.

[BenelliSEI]

Has winter condensation filled your tank with water in the gas? Try dumping it or running off a remote tank with fresh gas........?

[HondaMan]

changed the needles, checked the float heights like 5 times,everything is checked thoroughly, the mixture screw is set to default turns (even 1/4 to the lean side as i though it was to rich) 

First Q: Which carbs are these? (PD or round-tops)?
Second Q: what needles are you using?
Third Q: what is the setting on the air screws?

[cjespersen]

Did you check the plugs? Do the plugs look fouled and wet do they smell like gas? Did you use aftermarket parts for your carb rebuild?

Look through this thread.

http://forums.sohc4.net/index.php?topic=105310.0

[xcvz]

Has winter condensation filled your tank with water in the gas? Try dumping it or running off a remote tank with fresh gas........?

The fuel is changed but i use an aux tank so it doesnt matter.

changed the needles, checked the float heights like 5 times,everything is checked thoroughly, the mixture screw is set to default turns (even 1/4 to the lean side as i though it was to rich) 

First Q: Which carbs are these? (PD or round-tops)?
Second Q: what needles are you using?
Third Q: what is the setting on the air screws?

1.Round top

2. The needles are aftermarket ones.

3. 1turn

Did you check the plugs? Do the plugs look fouled and wet do they smell like gas? Did you use aftermarket parts for your carb rebuild?

Look through this thread.

http://forums.sohc4.net/index.php?topic=105310.0

I cant get the engine to show something on the plugs because of the fu@#ing smoke screen covering all my yard, I have to shut it off early.

I used aftermarket float needles.

So you guys think that non oem needles can be causing this? Like it starts working fine when cold and then little by little starting to flood one or two cylinders?

[cjespersen]

These aftermarket float valves have a longer spring/plunger so it can throw off all the OEM measurements. You may need to tweak the float to get it to compensate for the longer float valves. I had the same problem. I put in aftermarket parts adjusted my floats to the OEM measurements and I had gas coming out of the overflow. I had white smoke and fouled wet plugs. After messing with carbs for days, I finally put the OEM parts back in using the OEM specs and it ran great. You can also make or buy a Honda Carburetor Float Fuel Level Gauge Tool off ebay that will screw into your carb bowl drain bolt and you can see the gas levels in the clear tubing.

[ekpent]

  How long did you run it after starting up ? Did you drive it around for awhile ? Hopefully long enough to burn out whatever condensation,possible fuel leaking in from a faulty petcock over winter, etc. that may have collected in the pipe.  One minute or so would be just getting up to temp to start cleaning stuff out.

[PeWe]

Floats might be wrong or stucked so fuel fload. The description fits.

 Measure heights does not ensure correct function even if it should work.
Only way to verify correct float height is to see it by the clear tube procedure.

My last CB750 K2 build were carbs floats carefully measured on the bench, it smoked a lot when started.

 Adjusted float tangs by feel and smoke issue fixed. I have checked the heights today and adjusted a little higher after I have seen the level by clear tube.

[xcvz]

I noticed that these aftermarket needles are sticking a bit in the seats. I polished the seats thinking it would help a little, and yes little less smokey now, but the bike still acts funny, when I try to rev up,  it stumbles and wants to die, the revs are picking up realy slow, when turning on the choke it doesnt make a much diference ( it should rev up instantly but it actualy stays at same rpm then shoots up then goes back to idle again).

That are the symptoms of very rich mix condition right?

I am guessing that those needles are #$%*. Ordered new genuine ones, I will install them on friday, readjust the floats and let you guys know how it went.

Appreciated for the help!

[PeWe]

I bought cheap carb restore kits from cruzinimage some years ago

I did not trust the new needles, my K6 stock carbs had done around 90.000km and ran OK last time so I used all old jets after cleaning.  Needles looked OK and the new needles taper was different. I used the old float valves, pilot jets and air screws too.

Only thing I used from the kits were the rubber gaskets for top and bowls. Top gaskets are too wide making  pain to screw on the carb tops

Ordered Keihin main jets 110.115,120,125 to have some to test for stock and future needs

[Tracksnblades1]

Have you checked your oil tank for gasoline dilution...?

[Tracksnblades1]

Xcvs,

We are all talking about the needles like the one in the attachment...?

Or are we talking about aftermarket float needle and seats....?

It sounds as your talking float needle, correct...?

[HondaMan]

The aftermarket needles are mostly made by Keyster. They run more than 10% lean until 7/8 throttle, so this causes a large 'flat spot' to occur at about 1/8 throttle or so, until more than 2/3 throttle is reached. If you have the OEM needles, use those, for sure.

If the float level is too deep (for various reasons, one being the bike is on the sidestand and not the centerstand), it will make the mixture rich until the semi-stuck float needles dry the bowls out too much, at which point it will die. The only SOHC4 that will idle on the sidestand for very long is the CB350F and some of the early CB400F bikes. These carbs actually rely on some roughness of the pavement and more-or-less vertical orientation of the bike (or equivalent G-force during turns) to feed the fuel to the carbs.

Don't overlook the venting to your fuel supply. If the tank (or bottle) is sealed it will start out rich and dry out soon afterward, going lean. These tanks have vented gas caps to let the fuel drain to the carbs, but the vents can corrode shut in a single winter, especially if gas with ethanol is in the tank (it just happened to mine again this winter).