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Carb Adjustment question

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Figures I can't find the bloody bookmark to the Autosport books page! :madder:


Nial, don't you have that? DKLawson does, fer sure.

JP, it's a matter of taper on the various needles. As it is lifted out of the jet, the mix is different at different heights (governed by RPM) due to differing tapers of the needles ("profiles").
 
DrEntropy said:
JP, it's a matter of taper on the various needles. As it is lifted out of the jet, the mix is different at different heights (governed by RPM) due to differing tapers of the needles ("profiles").
Click to expand...

That I get but unfortunately what I don't get is, so what?
 
JP

We need needle info bottom line. It can make you run too rich or lean. Its a two minute job to pull the needle out and post the number. Then we can compare to others with a 1500 cc engine and your carb setup.

Mark
 
OK all I could find on the needle was the number 5 does that make sense?

also, I presume the needle should be flat across the top and 90 deg from the collar sides. the top of mine looks a little worn, but, size stuff first
 
JP


So you have a 1500 cc with what SU carb?

Should be more then a 5 also any letters

Mark
 
HS4 carb didn't see anything but the 5 and I looked very close
 
The letters should be engraved on the needle. I am not sure of actual dimensions of the needles (except the jet "hole"....that's usually 0.090")

At some point (soon) I may have to get back into SU carbs. I've been away from them for quite a while.

My MGB has the crappy downdraft Weber. My 1500 Spridget racer has been running an S&S carb (copy of an American Zenith) with accell pump and adjustable main jet for about 6 years now. Real simple to adjust (but also simple to screw up).

From my reading of the HS4 carb, the AAA needle would be a good place to start.

The problem is that there isn't much known data on a single HS4 on the 1500 because it was never offered from the factory.

I will tell you that I just ran a common needle in mine ("AAA" needle) and it was about right for the street. Once I got to idle smooth by adjusting the throttle stop and jet nut, I didn't do much with it. I have no idea if it was running rich or lean at high speed. A year later, when I went racing with it, I switched to the S&S carb.

The good thing about SU carbs is that they are sort of self correcting, so you will usually have decent running if the needle is at least close.

Here is a needle selection program that may help

https://www.winsu.co.uk/getitnow.html

You might find it quicker to just call Joe Curto and see what he suggests (he sells SU needle). He would have a better idea than anyone I can think of.

https://www.joecurto.com/
 
JP

Yes call Joe. I was thinking it should be #ADT

Mark
 
abarth69 said:
JP

Yes call Joe. I was thinking it should be #ADT

Mark
Click to expand...

probably the best idea - let me get the mixture idle back to what it was so I can at least drive it.
 
we are very very close - pops a little on beginning but runs stronger I think than it did. want it to cool off now so I can check the plugs

In the meantime got a small drive in anyway lots of smiles!
 
Just ran through the 2 suggested sizes above on WIN SU - this is what I got - can someone hep me understand what I am looking at?
 
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