Topic: CB750 F2 Carb Clarification and aftermarket breadbox

[Taelonorigin]

Hi guys, I'm finishing up some work on my 1978 CB750 F2, and I had a few questions about the carbs that I was hoping someone here could answer.

Second, as far as I understand it:
-The pilot/slow/idle jet is up to 25% throttle (oddly, this one seems a bit rich, and it's the only part of the powerband with any throttle)
-The needle jet and the jet needle both do fuel from 15-80%
-The main jet kicks in from 60-100%

So, if I'm low in mid-RPM, then I should up the main jet size, AND add a few washers to the needles?
Also, these are round slide carbs, does that mean they're not CV carbs?


Second dilemma: I fully rebuilt the carbs using some aftermarket kits on vintagecb750, and dropped in 115 mains instead of the stock 105. The PO had ditched the airbox and put on those cheap metallic (EMGO probably) pod filters. The bike runs but seems incredibly mediocre in its performance (then again, this is my first motorcycle so I have no idea what to expect).

I have an old airbox i picked up at a junkyard, but the rubber seal between the two halves is shot and I'm not sure if the old box is for a K or an F, so I was looking at alternative options, right now I'm thinking the velocity stacks from steel dragon with some kind of sock filter, or the aftermarket breadbox from steel dragon. Does anyone have experience with these? Specifically, does the breadbox have similar airflow to the stock airbox or will it require major rejetting? It seems like the easier, lower maintenance option but I'd like some firsthand experience before i drop $150.

Thanks!

[HondaMan]

Wow, so many things here... 

First: you're close on the proportions of 'which jet does what, when', except the mainjet is in play from 1/8 throttle on up. This meters the fuel to the needle jet system, and the slide cutaway controls how much of that system is in play.

Next comment: these are very small-bore engines. Throwing a 10% fuel increase into them will drown them so badly they cannot light the fuel, and quickly. If the bike feels "mediocre", then something else is probably wrong: a good-running F2 will smoke the rear tire and lift the front wheel, stock. If it doesn't, then something is amiss?

Start with this: set the mainjet to #105, the needles where they belong (if yours lacks clips, just remove any shims you installed). Set the float height to 14.5mm or 14mm. Make sure the tiny O-rings and washers inside the air screw ports in the bottom of the carbs is in good shape: the O-rings is on top, with the washer next, the screw compresses the washer to seal the O-ring. This has major throttle effect until 4000 RPM in that carb arrangement.

Make sure the spark timing is correct, and advancing as it should: the full advance should be reached at 2500 RPM on the F2 engine, and not before that: later is better, up to 3500 RPM. It must return to the "F" and "LF" marks at idle speed, about 900 RPM.

The air screws should be set according to the model of carb: there are 2 different ones for the F2 carbs, depending on which carb they are. Look for a number like "PD41", PD42", PD42A" or PD42a" on the carb bodies, to determine what setting your air screw needs. You can get the chart with these settings in our "FAQ" section.

Don't come at these small-bore engines like they are a Ford or Chevy big-block with 4"+ pistons: they are barely 2" bores, like a lawn mower! They start producing their power above 4000 RPM, and make a bunch of it at 7000 RPM.

[Taelonorigin]

Thanks Hondaman! Yeah sorry it was a bit of a text wall

I'll give this a go today. You think I should go back to the 105's even with pods installed? The bike has an electronic ignition and I haven't messed with the timing at all, but I'll pick up a timing light and check it out! Mine are PD42s, so I'll check the FAQ.

[Taelonorigin]

Well, found one problem! The main jet had come unscrewed and fallen in the bowl in the #1 carb. Are there o rings around the main jets?

[harisuluv]

Well, found one problem! The main jet had come unscrewed and fallen in the bowl in the #1 carb. Are there o rings around the main jets?

no

[Taelonorigin]

Damn, I was hoping that collar around them was for an o-ring. I'll just have to crank them in tighter.

[flybox1]

There shouldnt be a collar around them, and dont crank anything anywhere on these carbs....please!
Main jet screws into a brass emulsion tube, which screws into the carb main jet post.
pic from another thread

[Taelonorigin]

By collar I meant that non threaded area around the main jet head, and don't worry, only a light cranking! I just don't want the main jet falling out again

[flybox1]

ok,  yeah.   no o-ring needed.  The press-in pilot jets are more apt to fall out with improper/frequent installation.
Just hand tight, with a screwdriver and a (8mm,IIRC) spanner.

[harisuluv]

no cranking, no light cranking.  no kind of cranking. 

hand snug.

[Taelonorigin]

ok,  yeah.   no o-ring needed.  The press-in pilot jets are more apt to fall out with improper/frequent installation.
Just hand tight, with a screwdriver and a (8mm,IIRC) spanner.

Thanks. Now with setting the float height, i've read all the articles and I'm still not getting it. I want to put the carbs sideways (like shown) then move the float until the tank on the float is just touching the spring loaded part on the back of the valve (#1), and I want that to be happening at 14mm in the area shown?

[HondaMan]

Yep, that's how the floats are set! You're on the right track, there.
Did you notice: on the needles from your carb kit, do they have a number like "D37" or "D17", or do they have a much longer number, with a tiny "K" in the collection (usually at the beginning)? If you have the former, they are Keyster needles, which will cause the bike to fall on its face so badly at midrange that you may fall over the handlebars when pulling open the throttle...