Topic: which air mixture screws are better?

[boost190]

So quick question guys. I have a 1978 cb750 with stock carbs, I have an extra set of carbs for a 76 750 that came with my other bike. My question is which air mixture screw do you guys feel is better the air mixture screw with holes in it at the bottom or the screw with out the holes. The reason I ask is I was cleaning the 76 carbs and pulled the screws out and noticed they had holes.And I remember cleaning the carbs that are on my 76 750 right now which are the exact same carbs but they have no holes in the air mixture screws. So again my question is which would be the better air mix screws?

[enwri]

Pull one from the 78, should answer your question. should be different enough to not work.

If you're just swapping between the 76's, I don't know which are better.

Now I think about it, I can't help, probably shouldn't have bothered replying at all. .

At least you got a bump.

[boost190]

Haa haa its fine . Yeah im gonna run the 76 carbs instead of the stock 78 carbs . I have a 76 750 as well and on those carbs the screws are solid and in still spare bank of stock 76 carbs I got with my bike those screws  have holes in the bottoms. Just wondering if one was better than the other.

[evanphi]

Will putting the air screw with the holes in them change tuning? The ones I just took out were solid, and the replacements have holes.

If yours were solid, and you think they are original that way, then we're into the "weeds" for verification. The little brass hole in the bell mouth of the carb is smaller if the idle air screws have a hole in their tip: it is bigger if Keihin installed solid-tipped air screws. The difference is .006" hole diameter. I don't know if I have a set of 7A carbs around, I'll take a look to give you a "good number" to check.

Most of these carbs have the solid-tip air screws. All the "new factory" carbs came that way, while the "old factory" bikes were the source of the ones with holes in them. I've found (by accident first, experimentation later) that the ones with holes in their tips will adjust from 1/2 turn out to a little over 1-1/2 turns out, while the solid ones only adjust between 3/4 and 1-1/4 turn. Beyond those limits, there is not further "authority" and the mixture does not change further. If you go more closed, it just stops the fuel altogether: if you go more open, it can't lift any more fuel than it does at the max rating, so it doesn't get richer (it does, however, get irregular and "clumpy" when open too far, making black sparkplugs quick!).

All this is a busy way to say, "If you change from solid to hollow tips: it won't tune the same". You'll find it has to be tinkered before it will settle in. I've usually found the Keyster hollow-tipped air screws to be very rich, making the carb idle settings run nearly 3/4 turn out (instead of nearer 1 turn) in order to keep from wetting the plugs too much at speeds below 3000 RPM.

The Keyster needles for the slides are also much leaner than the Keihin needles (number #27201 should be the correct ones in yours), and are sometimes also longer. This tends to make for flat performance in the 2500-5000 RPM range, which is right where I usually ride.

There are also 3 distinctly different needles in the Keyster kits. I have samples of all 3 around here somewhere. That's just bizarre!

[enwri]

Will putting the air screw with the holes in them change tuning? The ones I just took out were solid, and the replacements have holes.

If yours were solid, and you think they are original that way, then we're into the "weeds" for verification. The little brass hole in the bell mouth of the carb is smaller if the idle air screws have a hole in their tip: it is bigger if Keihin installed solid-tipped air screws. The difference is .006" hole diameter. I don't know if I have a set of 7A carbs around, I'll take a look to give you a "good number" to check.

Most of these carbs have the solid-tip air screws. All the "new factory" carbs came that way, while the "old factory" bikes were the source of the ones with holes in them. I've found (by accident first, experimentation later) that the ones with holes in their tips will adjust from 1/2 turn out to a little over 1-1/2 turns out, while the solid ones only adjust between 3/4 and 1-1/4 turn. Beyond those limits, there is not further "authority" and the mixture does not change further. If you go more closed, it just stops the fuel altogether: if you go more open, it can't lift any more fuel than it does at the max rating, so it doesn't get richer (it does, however, get irregular and "clumpy" when open too far, making black sparkplugs quick!).

All this is a busy way to say, "If you change from solid to hollow tips: it won't tune the same". You'll find it has to be tinkered before it will settle in. I've usually found the Keyster hollow-tipped air screws to be very rich, making the carb idle settings run nearly 3/4 turn out (instead of nearer 1 turn) in order to keep from wetting the plugs too much at speeds below 3000 RPM.

The Keyster needles for the slides are also much leaner than the Keihin needles (number #27201 should be the correct ones in yours), and are sometimes also longer. This tends to make for flat performance in the 2500-5000 RPM range, which is right where I usually ride.

There are also 3 distinctly different needles in the Keyster kits. I have samples of all 3 around here somewhere. That's just bizarre!

It's detail like that you could go a lifetime and never notice. Thanks Hondaman, thanks Evanphi.

[evanphi]

That's why he's called HondaMan!!! Get his book!