Topic: CB350F carburettor adjustment questions.

[Gibbon]

Hello.

In the process of de-decaffeinating my CB350F I have refurbished the carbs, replaced the jets with correctly sized genuine parts, new o rings etc. and set the fuel level using a transparent tube to about 3mm below the bowl joint. The needles are set to the mid position and the pilot screws were set to 7/8 turn out. Original air-box and air filter has been re-fitted.

The bike starts easily and idles smoothly at about 1000 - 1200 RPM but at partial throttle it seems to hesitate or stumble.

The spark plugs are darker than I would expect and I it smells like it's running rich.

Would lowering the fuel level make the mixture leaner ? and if so how much should i lower it?
Would adjusting the pilot screws help?

Thanks.
 Gibbon.

 

[jonda500]

Why not try turning out the pilot screws more? I don't think the mixture really changes much with the fuel level at anywhere between 0 and 4mm below the bowl top?
John

[HondaMan]

The mainjets should be #75 and pilots #35 - are they? I have yet to see a correct "rebuild kit" for these carbs, from any vendor but Honda.
The air screw is correct at 7/8 turn, but: did you make sure the aeration holes from the bell area of the carbs into the mainjet's emulsifier holes are clear? I have seen them blocked (because they are so small) with aluminum casting dust: this can happen if the bike was run during the era when we had MTBE in the gasolines, which etched the surface of the insides of these carbs. It sometimes takes some extra care to clear those small passages fully, and if you add a bit of oil to the gasoline once in a while it will prevent this from re-occurring.

Also: if someone [before you] has over-tightened the air screws before, this widened the seat for the screw in the carb's body (this happens often with the CB750K0-K6 carbs). This then means the 'correct' setting for the air screw will be closer to 3/4 turn than 7/8 turn.

Don't fall into the internet trap of "open the screws more for leaner mix", as this is exactly backward. These idle-mix screws are adjusting the amount of fuel-air mixture from the pilot jet and its emulsifer for each intake stroke, not the air-fuel mix. The pilot jet's size, plus the tiny holes in its emulsifier tip, are what set the air-fuel ratio - the screw only controls how soon it starts feeding into the intake stream on each intake stroke. The 350F (and its little brother the 250F in Japan) are VERY sensitive to small adjustments here.

[Gibbon]

The mainjets should be #75 and pilots #35 - are they? I have yet to see a correct "rebuild kit" for these carbs, from any vendor but Honda.
The air screw is correct at 7/8 turn, but: did you make sure the aeration holes from the bell area of the carbs into the mainjet's emulsifier holes are clear? I have seen them blocked (because they are so small) with aluminum casting dust: this can happen if the bike was run during the era when we had MTBE in the gasolines, which etched the surface of the insides of these carbs. It sometimes takes some extra care to clear those small passages fully, and if you add a bit of oil to the gasoline once in a while it will prevent this from re-occurring.

Yes mains are #75 genuine Honda from David Silver and pilots are original fit #35.
I did not check the aeration holes. Hopefully they will be OK.
I don't think it would have ever been exposed to MTBE (had to look this up) as it was off the road from 1993 when un-leaded fuel was not a thing here in New Zealand, until 2009. MTBE was banned here in 2003.

Also: if someone [before you] has over-tightened the air screws before, this widened the seat for the screw in the carb's body (this happens often with the CB750K0-K6 carbs). This then means the 'correct' setting for the air screw will be closer to 3/4 turn than 7/8 turn.

Don't fall into the internet trap of "open the screws more for leaner mix", as this is exactly backward. These idle-mix screws are adjusting the amount of fuel-air mixture from the pilot jet and its emulsifer for each intake stroke, not the air-fuel mix. The pilot jet's size, plus the tiny holes in its emulsifier tip, are what set the air-fuel ratio - the screw only controls how soon it starts feeding into the intake stream on each intake stroke. The 350F (and its little brother the 250F in Japan) are VERY sensitive to small adjustments here.

This is quite possible, the carbs have been worked over by previous owners. I was not sure which way to adjust the pilot jet.  I'll try turning the pilot screw in to 3/4 turn.

Thanks.
 

[Trevor from Warragul]

On "modern" fuel I found my 350F ran perpetually rich (I've had it long enough to know how well it ran on leaded petrol). I went one size down on the main jet and that cured it.

[HondaMan]

On "modern" fuel I found my 350F ran perpetually rich (I've had it long enough to know how well it ran on leaded petrol). I went one size down on the main jet and that cured it.

This is an issue with all of these SOHC4 engines. I ran my 750 on only premium until the advent of these 'blended for low emissions' fuels, which all burn much slower than the 1970s versions so they will light off catalytic convertors faster in vehicles. That's why you'll often see me suggesting that someone with fouled sparkplugs switch to using Regular instead of Premium fuel, and the like. In my 750, which has slightly raised compression (about 9.2:1) from engine mods I have done over the years, I never run Premium unless I am out on the interstates (speed limit 80+ MPH around here) and carrying heavy loads like 2-up or camping gear (I don't go camping much anymore, though!). It just fouls plugs quickly because the chambers get wetted with unburned fuel from it burning too slow.

[Gibbon]

Also: if someone [before you] has over-tightened the air screws before, this widened the seat for the screw in the carb's body (this happens often with the CB750K0-K6 carbs). This then means the 'correct' setting for the air screw will be closer to 3/4 turn than 7/8 turn.

Don't fall into the internet trap of "open the screws more for leaner mix", as this is exactly backward.

Thanks HondaMan. Turning in the pilot jets improved the hesitation considerably.   

On "modern" fuel I found my 350F ran perpetually rich (I've had it long enough to know how well it ran on leaded petrol). I went one size down on the main jet and that cured it.

What size did you go to Trevor ? And where did you get the jets ?

[HondaMan]

On "modern" fuel I found my 350F ran perpetually rich (I've had it long enough to know how well it ran on leaded petrol). I went one size down on the main jet and that cured it.

What size did you go to Trevor ? And where did you get the jets ?

I've done that with the larger bikes, too. It's easier, as it takes a #5 step size. The little 350F might need half that?