Carb settings cb400f with 4-2 exhaust

[Erwin83]

Hi. Sorry if this is the gazillion-th topic on carb settings.

I'm currently overhauling my carbs, and they will go on my standard cb400f (normal air filter etc).
I've replaced my original 4-4 exhaust with a custom 4-2 which I built based on the headers of the 4-4, a custom Y-piece and 2 sporty but not completely open red-tip exhausts.
There are no balance pipes.

I'm wondering if I should deviate from the standard carb settings and jettings.
For instance a different clip height to have a slightly richer mid range?

I've destroyed a 350 engine already once because of too lean mixture. 'D like to avoid another of those 

Any advice?

Tnx

[Erwin83]

No-one with a savvy comment or link to information elsewhere on this forum?
 

[Laverda Dave]

I would go with normal settings for starters. Ride the bike to a dual carriageway, wait for traffic to pass, drop the bike into 2nd or 3rd gear and rev it to max revs for a 1/4 mile and then pull the clutch in and kill the engine at the same time, DONT close the throttle when doing this. Pull over and take the plugs out. If they are sooty the mixture is too rich, grey and it's too lean and you'll need to use a bigger main jet.
Its all trial and error so you'll need a pocketfull of main jets and plenty of time to keep doing plug chops. An alternative is to book a session on a dyno, it might be a quicker option. You are in unchartered territory here making your own pipe so there's no previous data to go on.
Good luck and let us know how it goes.

[Erwin83]

OK, got my Taiwanese carb rebuild kit (the one which people seem to be enthusiastic about). Hopefully I'll be able to get it fitted this weekend.
I'm going for the standard jets.

[Erwin83]

So, after installing the kit in the carbs, and placing the set back on the bike, the float overflow kept dripping. Quite annoying.
Engine runs immediately by the way with the base settings: standard jets, needle at 3rd position, 3/4 mixture out, synchronization eye-balled at peek-through, float heigth at 22mm (old setting was way-off by the way, at 26-28 mm...).

Took the carbs back off and checked if the float needles worked with compressed air. After setting the floats about 1 mm lower (23 mm), all seems well now.

Unfortunately the Taiwanese O-rings for the float cups seem to grow in size. I think I read something similar about the Keyster kits (brass parts OK, rubber parts NOK).

[Laverda Dave]

Float height should be 21mm. The Haynes manual is incorrect as I found out to my cost when the carbs overflowed! Thanks to the helpful people on the forum who know the correct measurement.
Sounds like it's going well though.

[Chris400F]

Unfortunately the Taiwanese O-rings for the float cups seem to grow in size. I think I read something similar about the Keyster kits (brass parts OK, rubber parts NOK).

I know various people have had a bit of fun fitting these O rings when using the Cruzinimage kit as they are not shaped.
A dab or two of sealant to hold various points in place seemed to help, and I seem to recall warming the rings to help soften them may also have been mentioned.

[Erwin83]

Float height should be 21mm. The Haynes manual is incorrect as I found out to my cost when the carbs overflowed! Thanks to the helpful people on the forum who know the correct measurement.
Sounds like it's going well though.

For some reason, I really needed to go to 23mm to get the float needles to work properly. 22 (or 21) didn't work well and made the carbs drip.
Just pressing the floats down (carbs inverted) I found that with 22 mm float height, the float hits the housing bottom before the needle fully closes the fuel flow (easily checked with compressed air). Of course that is with max compression of the little needle spring...

If it weren't for the huge amount of extra work, the best solution would be to do an actual check on the fuel level.

I guess (but I can't explain it) that the buoyancy of the floats may have reduced due to old age?
I tried 'em in the sink though, they all float the same way (height) in water... None of them seemed to be leaking.

Now, I hope my fuel mixture is not influenced to much by the possibly 1-2 mm lower fuel level.

[Erwin83]

Unfortunately the Taiwanese O-rings for the float cups seem to grow in size. I think I read something similar about the Keyster kits (brass parts OK, rubber parts NOK).

I know various people have had a bit of fun fitting these O rings when using the Cruzinimage kit as they are not shaped.
A dab or two of sealant to hold various points in place seemed to help, and I seem to recall warming the rings to help soften them may also have been mentioned.

Fresh out of the box, they fitted perfectly, even though they are unshaped.
However after 2 days fitted on the carb, they seem to get longer, maybe unsuitable material for use in fuel environment..?

I actually cut 2 of them and decreased the lenght by 3-4 mm to get them just right again.
I'm hoping this O-ring to U-ring conversion won't cause a leak, otherwise I'll have to get new seals.

[Tomb]

I usually treat the o-ring getting fuel on it making it swell as a sign that the fuel level is too high, As the fuel level should be lower than the bowl mating face, in an ideal world we shouldn't need an o-ring. For this reason I rarely replace the 0-ring.

[Erwin83]

I usually treat the o-ring getting fuel on it making it swell as a sign that the fuel level is too high, As the fuel level should be lower than the bowl mating face, in an ideal world we shouldn't need an o-ring. For this reason I rarely replace the 0-ring.

Yeah, the float bowl drain is clearly higher then the O-ring's position, so it's clear the fuel level was too high.
I hope this is solved by putting the float's at 23 mm now.

Let's see. I hope I don't get home finding my garage smelling like fuel later-on...

[Erwin83]

Continuing on this topic.

So, by now i've:
- set valve clearance
- changed points and set the point gap
- synchronised the carb
- tried several air mix screw settings from 1,5 - 2,5

Situation is:
Bike runs really well.
- good cold start
- good cold idle
- good through the revs

Once hot, everything is still Ok, apart from idle.

Plugs seem to read rich (carbon black, dry deposit)

I'm thinking float level, perhaps go from current 23 to 25 mm (lower the fuel level in the bowl)

Please comment and share wisdom 

[Brian G]

I do remember reading that float needle valves in some of the aftermarket carb kits have a slightly different overall height than the originals. That would certainly affect float level. For that reason, I used the original needle valves - which had next to no wear - with my new carb kits.

Regards,

[Erwin83]

I do remember reading that float needle valves in some of the aftermarket carb kits have a slightly different overall height than the originals. That would certainly affect float level. For that reason, I used the original needle valves - which had next to no wear - with my new carb kits.

Regards,

That is very useful information.

I tested something:
I closed the fuel tap and let the bike run (hot), hence slowly lowering the fuel level in the bowls.
It actually seemed to idle better after a short while.

So i think it's worthwile to try and set the carbs slightly different.
Let's see what happens.

[andut]

Hi Erwin,

Just seen this post and your resurrection of another, older post on the same topic.

I think your 'fuel off' test is probably an indication of a rich at idle condition and you may want to consider going down a size on idle jets (likely to give you a bigger tuning change than can be achieved by the idle air bleed circuit) if your float height adjustment does not provide a permanent fix.

I also had similar O ring growth with the Keyster kits and changed them for viton seals from ebay (seller gerben1956) that are much more durable.

Andy