Topic: CB350F is Rich

[steve]

I have a tuning problem that I could use some help with.  1st all the electronics/timing are correct - stock air cleaner and 4 into 1.
I have been through the carbs with standard jetting.  Bike starts and sounds great at 3K rpms or higher.  But it will not idle,  I check the plugs and they have a rich condition look.  If I crank the idle screw in to get the idle right it raises the slides too high.  This little 350 cannot be that complicated.  I have redone 15 CB550's and never seen this issue.
Hopefully someone else has experience the same thing.

[PeWe]

High fuel levels in bowls?

[newday777]

I have been through the carbs with standard jetting.  Bike starts and sounds great at 3K rpms or higher.  But it will not idle, 

 If I crank the idle screw in to get the idle right it raises the slides too high. 

"Been through the carbs", how? Process you went through them.

"Standard jetting", did you use aftermarket carb kit brass parts?

"If I crank the idle screw in to get the idle right it raises the slides too high" slides too high? explain what results

Have you checked for vacuum leaks at the rubber boots?
Have you replaced the orings that seal the manifold to the head?

Do you have aftermarket points and condensers in it?

[steve]

Honda Points and condensers.  New o rings and boots.  Standard jetting. rebuild kit from 4 into 1
The floats all measured correctly and exactly 50cc's of fuel in each one.  If there was too much fuel wouldn't they overflow?

[newday777]

Honda Points and condensers.  New o rings and boots.  Standard jetting. rebuild kit from 4 into 1
The floats all measured correctly and exactly 50cc's of fuel in each one.  If there was too much fuel wouldn't they overflow?

"Standard jetting " doesn't answer if you used the brass jets from the 4into1 kits or just cleaned and reused the original stock Keihin brass jets

[steve]

Brass jets from the kit

[HondaMan]

Brass jets from the kit

Put the Honda jets back in. None of the aftermarket jets are even close for these bikes, all of them. This had become a giant issue now.

[steve]

The carbs had after market in it already and the Honda jets are not available.  Options?
Also: I put in NGK 2923 plugs which are DR8ES-L. 

[willbird]

The carbs had after market in it already and the Honda jets are not available.  Options?
Also: I put in NGK 2923 plugs which are DR8ES-L.

David Silver Spares may have (10) original jets NOS ??

Bill

[steve]

Are the after market jets typically larger than the OEM even though they are marked the same size?

[willbird]

Are the after market jets typically larger than the OEM even though they are marked the same size?

Jets made properly should be flow tested. I'm sure they have a general idea what size precisely bored hole (bored not drilled) provides the flow for that size jet, but I would not trust the aftermarket people to do anything other than drill a hole, pack and ship.

My dad worked at Tillotson Carburetor in Toledo, OH when I was a kid. He always related a story where an Engineer flow tested a jet, then briefly rotated the point of a #2 pencil in the hole in the jet and it changed the flow of that jet.

Bill

[steve]

Should I buy #32 slow's and replace the 35's?  If it is running rich it has to be off a size.

[willbird]

Should I buy #32 slow's and replace the 35's?  If it is running rich it has to be off a size.

I would not bet $.02 about the flow characteristics of aftermarket jets. I did buy a set for my CB750K2 but that was before I knew how potentially bad they are. I have not needed to use them. The flow of an orifice is about more than the hole size, the entry and exit are equally important.

Bill

[HondaMan]

Are the after market jets typically larger than the OEM even though they are marked the same size?

My experience with the aftermarket jets made by Keyster is: they are 7% leaner than the equivalent Keihin number. For example, if the jet is labelled #100 Keyster it acts like a #97 Keihin IF USED WITH the Keihin needle. If the Keyster needle is used, it gets much worse: in the 750 roundtop carbs their needles make the entire mainjet range almost 10%-15% leaner at differing RPM, which is: at low speed (2500 RPM) where the needles start their work, the 750 runs 15% leaner than with the Keihin needles, getting slightly more rich as the throttle increases, but still too lean.

Here's the numbers as they have been worked out in the 750 roundtop carbs using Keyster jets and needles that would run up to about 95 MPH (they should be able to reach well over 100 MPH):
Keihin pilot jet: 40 / Keyster equvalent: 42
Keihin mainjet: 110 /  Keyster equivalent: 140-150
Kehin needle #27201 (750K1-K6) use middle notch for open exhausts or 4th notch for HM341 pipes
       / Keyster versions: there are 3 versions of thicker needles, all similar  - use bottom notch
NOTE: when using Keihin needles, use Keihin needle jets in carb body, and vice-versa. The Keyster needles can jam in the smaller holes of the Keihin needle jets, and the Keihin needles cannot close the flow for metering in the Keyster needle jets (their holes are too big).

Take note: in the CB350F the jetting (and air intake system) must be spot-on to work at all. The carbs are so small that they require perfect conditions, including the very specially-designed airbox system (all of it) to make the laminar flow mix fuel and air in those tiny venturis. In my experience, if any part of this system is disturbed it becomes a lawn-mower's carb in performance, barely giving any throttle control or response, and losing almost 50% of engine power overall. This is one big reason why the 350F appears in so many posts here in the SOHC4 forum with folks wrestling to get them right.

[Gibbon]

I'm in a similar situation to Steve.
 I've had trouble with poor high-speed running and fuel overflowing.

The previous owner of my 350F did a mild cafe racer conversion so I'm suspicious the jets have been played. While I'm in the process of de-decaffeinating it I want to refurbish the carburetors and re-fit the original airbox. Is there any way to determine if the jets and float valves are original?   

I can see a stylized K on the slow jet but cannot see similar markings on the other parts. The main jet is labelled 95 and there is 1.8 on the float valve.

YAMIYA claim to have genuine Honda float valves and jets. How reliable is YAMIYA? 

Thanks.

[Trevor from Warragul]

It should also be noted that the pilot jets are so small they will block at the slightest provocation! You can have them perfectly clean and clear, and a tiny bit of fine rust from the fuel tank will block them very quickly!

Also, a CB350F with clean and well adjusted carburetors will idle happily at 1050 RPM, and absolutely "sing" when revved out! It will also pick up cleanly throughout the entire rev range. They are quite sweet when tuned right...

[Floshenbarnical]

Brass jets from the kit

Put the Honda jets back in. None of the aftermarket jets are even close for these bikes, all of them. This had become a giant issue now.

I'm sure 4into1's jets are poor - what about the Keihin jets from JetsRUs? They don't have the logo stamp.

[HondaMan]

It should also be noted that the pilot jets are so small they will block at the slightest provocation! You can have them perfectly clean and clear, and a tiny bit of fine rust from the fuel tank will block them very quickly!

Also, a CB350F with clean and well adjusted carburetors will idle happily at 1050 RPM, and absolutely "sing" when revved out! It will also pick up cleanly throughout the entire rev range. They are quite sweet when tuned right...

Boy, have you ever got this right!
It's the only way I know to feel like I'm roadracing in city traffic.

[HondaMan]

I'm in a similar situation to Steve.
 I've had trouble with poor high-speed running and fuel overflowing.

The previous owner of my 350F did a mild cafe racer conversion so I'm suspicious the jets have been played. While I'm in the process of de-decaffeinating it I want to refurbish the carburetors and re-fit the original airbox. Is there any way to determine if the jets and float valves are original?   

I can see a stylized K on the slow jet but cannot see similar markings on the other parts. The main jet is labelled 95 and there is 1.8 on the float valve.

YAMIYA claim to have genuine Honda float valves and jets. How reliable is YAMIYA? 

Thanks.

The overflowing situation is usually due to tiny debris in the fuel tank that keeps getting into the little float valves, making them stick open.

While today's float valves for this carb set are a little more stiffly sprung than the OEM versions, I've found that setting the float level 1mm richer than OEM with those stiff ones works out well, so long as the fuel isn't gritty. Keep watch on the fuel line routing, make sure it doesn't go down and back up and then down again enroute to the carbs. Normally this isn't possible with this bike, unless someone has substituted a different gas tank ('cafe' bikes).

The OEM mainjet is 75. If yours has 95 then it is so overjetted that it is drowning. That will stop any form of revving above about 5000 RPM right quick. If it has that odd mainjet, check the needle jet (in the slide), too. They may be Keyster parts?


Yamiya's parts (that I have seen) have been spot-on for these bikes.

I should also mention: the O-ring on the mainjet must be a snug slip-fit into the carb's emulsifier hole, snug enough that plugging it in will make it stay while you shake the carb. If not, it is like so many of the Keyster parts, IT'S THE WRONG SIZE. Then it lets fuel leak past, making proper mixing impossible.

Most of the "cafe'd" CB350F bikes I have seen have failed. They might look like the owner wished, using non-OEM airbox equipment, but they don't run well. Or at all.