Topic: 350F Needle Jet Differences

[Dr. Frankenstein]

I was trying to clean up a 350F carb, but I don't know if it's going to be doable as far as the jets are concerned; the ones that were in the carb are pretty roached, and it's doubtful that I can use them again. That being said, when I acquired this carburetor, it came with a complete set of carb kits from the infamous people at 4-into-1...I wanted to use the original jets, but as I mentioned they are pretty much gone, and I think it would be expedient to use the 4-into-1 kits; the jets are marked #75 and #35, and everything is identical EXCEPT there is a difference with the needle jet - the OEM one that was in the carb has two little holes on one side, as compare to the 4-into-1 jet that only has one. In the pic, the OEM one is on the left, the 4into1 on the right. This pic was taken after I cleaned up the OEM one, it was pretty gross.

What difference will that make, if any? The 4-into-1 kits were marked for the CB350F (not that really it tells you anything), and they all appear to be the same, except the OEM has two holes on one side, whereas the 4into1 kit only has one, but is slightly larger - I 'm aware of 4into1's sketchy reputation, but has anybody used this kit in a 350 Four? I suspect there's a reason for the two holes in the OEM one, and I don't really want to mix-n-match parts - so for the "TLDR" version of this post, what's the difference between them?

[Bodi]

This is the emulsion tube, designed to mix air bubbles from those holes into the fuel going to the needle jet outlet at the "top", in the carb throat. The bubbly/foamy fuel evaporates better than without bubbles. Basically making the fuel droplets going into the cylinder smaller.
I expect KeiHin had a good reason to put the number of, and position of, the air holes where they did.

What is "pretty much gone"? These parts wear very slowly. The fuel flow lubricates the needle against the jet and worn out ones are pretty unusual - unless something was misassembled usually not before the bike is worn out, many many miles. I suppose some iron butt riders went coast to coast repeatedly but these small bikes generally didn't get to rolled over odometers.

I've had poor experience with aftermarket carb brass, along with many other owners. You can try them, but may have trouble with flat spots or over rich mixture. Only trying them will tell.

The OEM one, a kit with needle and orifice tube/jet - #1-6012-333-004 - is, of course, NLA and out of stock at the usual places.
Maybe you can find a good used set if yours are really due for retirement?

[HondaMan]

The one with 2 holes will aerate a little more than the 1-holer. This will cause slightly leaner-looking sparkplugs over time. The holes can be soldered up and matching ones drilled into the new emulsifier: I've done that a lot. The height (from the bottom end), size of hole, and number of holes controls how much aeration happens, and in what RPM range.

It works like this:
- when at idle the fuel is highest in the emusifier because almost no fuel is supplied form it: the idle jet is doing that job.
- By 1/8 throttle the emulsifier begins to supply the fuel as the idle jet is losing its ability to do so, due to laminar (still) airflow across the surface of the ID of the carb getting thicker as the air volume/velocity increases. The height of the top of the emulsifier inside the carb throat prevents its working until the laminar flow is as deep as that part of the emulsifier that is sticking up into the carb throat. At this point the fuel in the emulsifier is at the same depth as the float bowl.
- As fuel starts coming from this emulsifier, the fuel level in the emulsifier's tube starts to drop down, due to the restriction by the mainjet. The air bubbling into the emulsifier aerates the fuel as it rises from the bowl toward the throat.
- As engine speed passes 1/4 throttle the idle jet drops to doing nothing because the laminar air layer in the carb is too deep.
- As engine speed rises toward 50% of carb flow ability (this is an ideal number: actual is closer to 40% in the 350F/400F carbs) the fuel level inside the emulsifier drops to the middle of the tube's length. If the hole is in the middle of this range, it ceases bubbling the fuel and the air mixing must occur as the fuel leaves the top of the tube, in the carb's throat. Many emulsifiers have a number of holes along their length, often of different sizes, to increase or decrease the amount of air being added to the mix before reaching the carb's throat.
- As engine speed rises above 75% of the carb's airflow the fuel level in the emulsifier approaches the bottom 10% of the tube's length. Now all the emulsifier holes are increasing the turbulence inside the emulsifier to mix fuel with air because up at the carb's throat there is less turbulence and more free-flowing air. That wouldn't mix air & fuel so well, thus is falls to the emulsifier to make sure the mixing happened before it got up there.
- At full throttle the laminar flow across the emulsifier is so deep that only the holes in the emulsifier tube and the pressure of the air in the emulsifier can push fuel out of the top, into the airstream (in Keihin's carbs). So, mixing gets a little "clumpy" at full throttle in these tiny engines, and the last 1/8 throttle usually does not make much difference.

In the past Keihin made "power jets" for above-3/4 throttle control (CB/CL72 250cc bikes often had these, as did Honda's racers). In 2-stroke engines with Mikuni carbs there was a 'shroud' (a half-round, paper-thin wrap of brass on the downstream side of the top of the emulsifer's tube in the carb's throat) on the downstream side of the emulsifier to divert the air upward and over the top of this tube, crating a slight bit more lift at 7/8+ throttle openings. Racing carbs often had these, too.