Topic: 1972 CB350F problem

[itsahonda4]

Heya all,
Been a while since I’ve had a problem that has stumped me and my engineer son but mows the time to fess up …
I found a very nice barn find 350-4 with faded paint and stock perfect exhausts BUt it was pudding a small amount of smoke out of the cam box vent hose.
I checked compression which was low and a couple of the valves were leaking.
I had my local bike rebore experts lightly hone the cylinders; I fitted a couple of new valves, decoked the head, installed new viton stem seals, polished the valve heads and lapped the valves.
Also added new Dyna coils and genuine points and condensers.
Ultra sonically cleaned the carbs then made sure all passageways and orifices were clear; installed new genuine jets and set the fuel level at 21 mm, which proved to be too high and so, based on Hondamans commentary we set the foot height at 21mm

After the new pistons and rings were fitted and with the bike cool not cold compression is130 on every cylinder and 138 to 140 when hot.
Needles were initially set with the clip on the centre as per standard specs.
Now the fun starts…..
Bike starts readily on the choke but slowly stumbles and dies while spewing black smoke in copious amounts. Starts again easily and after revving it to clear it a little same thing happens - slowly stumbles and dies while spewing.
I lowered the needles one notch, made sure the drain tubes in the carbs were clear and weren’t split and the hoses were free and clear, rechecked the valve clearances, checked the cam timing, ignition timing, fitted new plug wires and fitted new resistor caps and resistor plugs just to be certain I hadn’t missed an obvious problem. Oh and checked the choke operation to ensure the choke wasn’t the problem.
So back to the startup - I removed the plenum so I could confirm the choke wasn’t slowly operating  and once again the bike starts easily on the choke but as it warms up it starts to blow smoke and slowly stumbles and dies while spewing smoke.
So after all the work and rechecking, the operation is identical and the problem is identical…..
What are we missing? It’s gotta be something obvious and simple but I can’t see it for the black smoke …..

[itsahonda4]

Photo attached
Errors - ‘ Initial float height was 24mm, then set to 21 mm’

[scottly]

 set the fuel level at 21 mm, which proved to be too high and so, based on Hondamans commentary we set the foot height at 21mm

The black smoke comes from too much fuel, so it sounds like your float setting is wrong. You can check the actual fuel level in the float bowls using the "clear tube" method.

[HondaMan]

Wow, that looks like Sochiro's own 350F! He liked that green color (Bacchus Green, I think it's called?).

Could one (or more?) carbs' float valve(s) be sticking and making the bowls too deep? Like, are just 1 or 2 of the cylinders blowing black?

By chance: did you measure the piston-to-bore clearance after [re]honing the bores? It has to be incredibly tight in this engine: if it is more than 0.0008" (less than 1 thousandth inch) then this engine will smoke big-time. I set them at 0.0004" when I rebuild them (and the CB400F).

[itsahonda4]

Yeah I remembered what you had told me about the bore clearances and we didn’t measure them so all along I’ve suspected that’s the issue.
I think given everything else we have done I’ll just pull the head over the weekend and check it and my uneducated guess is that’s the problem.
Yep it’s a beautiful little bike and the paint has faded so you can see the blue undertones in the paint and it’s just magnificent
Thanks again
Al

[scottly]

You have a carburetor problem!!!! Do not tear the engine apart!!! Your compression is fine!!! Black smoke comes from too much fuel or too little air. Oil smoke is more blue or white. Pull your spark plugs out: I'll bet you a cyber beer that they are either black and sooty or black and wet.

[M 750K6]

I went back to the original brass on my 750's carbs, as the replacements were too rich. Starting too easy could suggest it's too rich on the idle circuit. Put your old pilot jets back in to check.

Are you taking the choke off as soon as it starts? Again, a 750, I start it on choke, straight down to 2/3 choke for a couple of blips of the throttle, then choke off, hold the revs on the throttle and take off.

[Mark1976]

You have a carburetor problem!!!! Do not tear the engine apart!!! Your compression is fine!!! Black smoke comes from too much fuel or too little air. Oil smoke is more blue or white. Pull your spark plugs out: I'll bet you a cyber beer that they are either black and sooty or black and wet.

Thank you,
It's not rocket science. Go back to the basics.... starts way to easy (rich), gets worse as it warms up (rich), pours out 'copious' amts of black smoke (rich). Tearing down the topend is going to solve anything but your curiosity....

[itsahonda4]

Thanks everyone for the input. On reviewing the start up it’s characterized by blue smoke for a short period and then heavy black smoke .
It only takes me an hour to pull the cylinder block so off it comes this’d weekend to check the piston to barrel clearance. This engine had done 14,000 miles so I think it’s obvious it needs a rebore and a hone was never gonna do the job. I shoulda known better……
Al

[scottly]

The BLACK SMOKE is caused by too much fuel!! Fix the f#%king carburetor problem!!! 14,000 miles isn't that much, and all that fuel washing the oil off the pistons and rings isn't helping things.

[M 750K6]

My 750 had been run on the choke by my bikes PO, because it always stalled when he took the choke off. He only managed a few short rides. The bores were like mirrors.

But 14,000 miles is nicely run-in. Mine was 41,000 miles and I just gave it rings and a hone. Good compression, no oil use and runs well since then (2 years and 4,000 miles later).

[scottly]

Any updates?

[scottly]

 set the fuel level at 21 mm, which proved to be too high and so, based on Hondamans commentary we set the foot height at 21mm

The black smoke comes from too much fuel, so it sounds like your float setting is wrong. You can check the actual fuel level in the float bowls using the "clear tube" method.