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SU needle installation help


AlexS13

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I'm having some trouble setting the correct position of the needles. I have always used the methed of placing the needle on the piston and tightening the set screw just enough to cause some drag on the needle but allowing it to move freely, I then place the piston onto the carb and let the needle position itself. My problem now is that when I place the piston onto the carb I slowly lower it down and I can feel the needle bottom out, I then slowly pull the piston back out and the shoulder of the needle is sitting above the surface of the piston.

I have read another method is to use a straight edge along the bottom of the pistion and use that as reference for the needle height.

My question now is...do I install the needles with the shoulder sitting higher than the bottom of the piston, or do I manually set the needle height using the straight edge method?

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The factory manual says you should use the straight edge along the bottom face of the piston to locate the shoulder of the needle.

The shoulder should not be higher than the bottom of the piston... It should be perfectly even with it.

If you post what year are you working on, I can point you to the correct page in the manual.

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Thanks for the reply! They are '71 4 screw round tops. The only thing I can think of that may be happening is when I pull the piston back out the needle may be shifting at that point?

Foot in mouth, I just checked, and I don't have the 71 manual. Oldest I have is 72, but the needle setting procedure is the same. With that in mind, I've included page EF-28 (Engine Fuel) of the 72 manual below.

A little hard to tell from the blurry scanned pic, but you should use a straight edge to set the shoulder of the needle even with the bottom flat face of the needle. Don't set it to the depth of the narrow groove in the center of the piston face. Set it to the large bottom-most portion of the needle. In other words, If you pull the needle out of the piston completely, and then set the piston needle hole side down on a flat table... The surface that contacts the table is the surface you should use to locate the needle shoulder.

If done correctly, you should be able to see a small portion of the needle shoulder sticking out of the bottom of the piston when you're done.

needlesetting2.jpg

needlesetting1.jpg

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If by raising the piston you are extracting the needle some, it sounds like you are off center and the needle is binding on the oiifice of the nozzle. Long term this will wear the nozzle orifice egg shaped and wear down the needle.

Near term, I doubt setting the needle shoulder flush or fully buried in the piston will make enough difference to amount to anything. Other than keeping you busy, it matters not much.....

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Thanks for the reply! They are '71 4 screw round tops. The only thing I can think of that may be happening is when I pull the piston back out the needle may be shifting at that point?

Forgot about this part... I'm not really sure what problem you're identifying with the above, but I'll take a guess.

if you're trying to figure out why you're seeing too much shoulder when you use the "leave the lock screw a little loose and push the piston down onto the jet" to establish the needle position, it's because you've got the mixture screws turned down a few turns to where they should be to have the car idle properly.

Those two or three turns down on the mixture nuts will hold the nozzle jet down and that will add to the length of exposed shoulder when you use the "loose lock screw" method because you're using the nozzle to push the needle into the piston. The more turns on the mixture nuts, the farther down the jets, and the more shoulder you will end up having exposed.

Is that maybe what you're talking about?

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Ahhh ok never though of the mixture nuts. As Bruce mentioned I thought the needle may have been binding as I pulled the piston back out, I may have caused this by not pulling perfectly straight up on the piston causing the needle to bind. The mixture nuts are indeed a few turns out. I was simply just swapping out needles so I did not touch the mixture nuts. Either way I will use the straight edge method to set the height.

Thanks for the help guys!

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Make sure your needles are not bent by rolling them on a glass table. I always ran the needles flush with the bottom of the groove, but either way you can adjust out with mixture screws. Just set them flush and do the drop test to test for binding!

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