Topic: CB350F help

[itsahonda4]

Hey all,
I have a 72 CB350 Four, exceptional as-found condition.
Rebored to first oversize after a short history of smoking after an ill advised hone with new pistons and rings.
Reassembled with 2 thou bore to piston clearance and it started and ran perfectly except it is running ultra rich…and I mean rich!!!!!

Choke arm is set with lots of friction so that it can’t possibly slowly ease itself on, so that’s not the problem. Carbs have been dismantled three times and all passages cleaned thoroughly . My son and I own CBX’s so we understand just how clean carbs must be so I’m satisfied we have the carbs spotless. Float height is 24mm I believe (stock height), no change there.
I fitted an electronic ignition system this morning and it ran even better than with the points but is still running extremely rich and I’m unable to get to the bottom of it.
Filter is a new factory Honda paper type; spark plugs are new D8R; coils are Dyna with new wires and resistor caps ; needle clip is set one higher than stock ( leaner) and all jets are stock.
Yesterday I went for its first running-in 80 km run and it used almost a full tank of fuel while not exceeding 4000 rpm.
Do anyone have any meaningful commentary to help me did what must be something so obvious I’m failing to see it ?
Al

[scottly]

 
http://forums.sohc4.net/index.php/topic,205195.0.html

[HondaMan]

Hey all,
I have a 72 CB350 Four, exceptional as-found condition.
Rebored to first oversize after a short history of smoking after an ill advised hone with new pistons and rings.
Reassembled with 2 thou bore to piston clearance and it started and ran perfectly except it is running ultra rich…and I mean rich!!!!!

Umm...2 thou piston clearance? They are supposed to be 25% of that much, around 0.0004"-0.0006" (4 ten thousandths to 6 tens thousandths inch). I have rebuilt one 350F with 0.0002" clearance, and it ran very well, too.
Large amounts of bore clearance in these tiny engines make them unable to seal the bores very well, making them use lots of fuel. The fuel also tends to end up in the oil after a while, giving the oil a distinctly "gassy" smell (and I discovered once that it will even burn if a lit cigarette is put to some of it when spilled on the ground! I don't recommend that test, though...).

[scottly]

Hey all,
I have a 72 CB350 Four, exceptional as-found condition.
Rebored to first oversize after a short history of smoking after an ill advised hone with new pistons and rings.
Reassembled with 2 thou bore to piston clearance and it started and ran perfectly except it is running ultra rich…and I mean rich!!!!!

Umm...2 thou piston clearance? They are supposed to be 25% of that much, around 0.0004"-0.0006" (4 ten thousandths to 6 tens thousandths inch). I have rebuilt one 350F with 0.0002" clearance, and it ran very well, too.
Large amounts of bore clearance in these tiny engines make them unable to seal the bores very well, making them use lots of fuel. The fuel also tends to end up in the oil after a while, giving the oil a distinctly "gassy" smell (and I discovered once that it will even burn if a lit cigarette is put to some of it when spilled on the ground! I don't recommend that test, though...).

Mark, even with the largest piston and the smallest bore available from the factory, the clearance would be .0004", and the largest bore with the smallest piston would be .0016". These are the tolerances deemed acceptable by Mr Honda when the bikes left the factory, and the service limits have much greater clearances. Also, just like the OP's other thread, he has a carburetor problem, NOT a problem with ring seal. He's already had the engine re-bored needlessly on account of bad advice, and has the same problem he started with.

[itsahonda4]

I might be wrong on the piston clearance - my machinist does lots of bikes ….
Ends of pipes are sooty not oily and plugs are covered in dry soot so I’m presuming my cylinders are good - yes I think
I have a carb problem but I’m lost as to what the problem actually is ……
This has been a thoroughly frustrating bike to work on!
Al

[scottly]

Al, do the main jets have the proper o-rings in good condition? If they leak, gas can by-pass the jet. Do you have the leaf springs that hold the jets in place? Is the air filter new? Have you tried running the bike without the filter?

[scottly]

Here are the specs for your carbs:

[Deltarider]

Let's start with the simplest. Are you 100% sure air intake is free and unobstructed. I once made a worried CB350F owner happy by simply opening the seat and removing a cloth she had put there.

[itsahonda4]

New Honda paper filter , lid is on the airbox, intake tube is clear with no obstructions, O rings are new, spring clip is in place and all seems perfect but now it’s stumbling at 3-3500 rpm which which would suggest the transition from pilots to main is compromised . I havnt taken it over 5000 rpm so have no idea what it does at those revs. At 4500 it was pulling very strongly ……
I’m pulling the carbs off again today to look for any obvious issues but before I do that I’ll run it with the seat up and air filter out to see what happens ……
Al

[itsahonda4]

Ok got my machinist on the phone and he confirmed the clearance was .00015 thou. I confirmed that with a photo taken of all pistons showing his clearance written on the top of each piston.
I’ve ordered a complete set of new brass, floats and gaskets for the carbs from Japan .
Stay tuned ….
Al