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DannDZ

How Can I Set Jet Higher?

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I am in the process of tuning the round-top SUs on my 71. The first step is to set the idle screw and and jet level. The jet is supposed to be even with the "bridge," the flat part where the piston comes to rest when it is "at rest."

This is possible on one of the carbs, but I cannot raise the second jet high enough. It is about a 16th-of-an-inch below being the bridge, I cannot get it to a flush position. Is it possible to loosen the jet jet locking nut and then make a "gross" adjustment so that I will be able to set the jet to flush using the jet adjusting nut?

An other way of putting is, if I was rebuilding my carb how would I know where to place the jet before I held it in place with the Jet Locking Nut before I reassembled it and then made adjustments with the Jet Adjusting Nut?

In case it helps, here is an image from the web showing the jet in the bridge. Can't tell from this angle if it is flush or not, but this is what I am trying to adjust.

Image result for su carb jet position

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1 hour ago, siteunseen said:

Can you not bend the arm that connects to the jet or nozzle?  Maybe I'm not "on the same page"?

su arm.png

I don't think bending is the right action. I can check some diagrams, but I don't see how that will move the jet up. I need to be able to raise the jet with the Jet Adjusting Nut, but I have run out of turn to adjust it. So it seems to me that I need to move "the whole assembly" up a bit so that I can have some more room for adjustment.

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You might have a washer that is not the right size that may be impeding your ability to raise the nozzle so it lies flat to the bridge.  Check the lineup of nut, spring, washer, etc, on each side to make sure they are both the same.

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The jets should be 2 1/2 turns down, or so, at rest. I'd adjust the one that is all the way up down 2 1/2 turns and adjust the other one to the same depth which will probably be less than 2 1/2 turns down. They should run like that if the float levels are correct.

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16 hours ago, duffman said:

You might have a washer that is not the right size that may be impeding your ability to raise the nozzle so it lies flat to the bridge.

 

15 hours ago, Mark Maras said:

The jets should be 2 1/2 turns down, or so, at rest. I'd adjust the one that is all the way up down 2 1/2 turns and adjust the other one to the same depth which will probably be less than 2 1/2 turns down.

I will review both of these possible solutions. Thanks for your input.

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@duffman @Mark Maras @240260280

I followed your suggestions, busted out the trusty endoscope and took a look. There was nothing different about the parts in the two "stacks." Neither spring has any spring left in it. They both just stay stuck at the top. But I don't think that's the problem. What I think I need to do is move part (15) Jet Bearing up into the body further and then lock it in place using (17) Jet Locking nut. At least that's my theory.

When looking at the to (17) Jet Locking Nuts, it does appear that the one on the carb with the problem is not seated as firmly against the carb body as the other one. I'd like to turn it tighter, but am a little concerned about trying too hard. 

Any further suggestions? Does this diagram help?

Tuning SU Carbs p. 47.jpg

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I would not mess with (17) as the alignment of 15 will shift (which is no fun)

 

FYI: Having the jet go all the way up is just a nice way to set the needle relative to 0 turns of the jet.  The Jet will never be that high in operation so no worries if you follow these two steps:

 

1. Just use a depth gauge to set each top of jet 2.5mm down from the bridge and you will have a starting point.

2. The shoulder of the needle should be flush with the bottom of the piston, not the ceiling of the groove.

 

 

 

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48 minutes ago, 240260280 said:

I would not mess with (17) as the alignment of 15 will shift (which is no fun)

I hope your advice is not too late! I've given it a few partial turn. I'll get it "re-set" and hope for the best.

Thanks for the other tips. I'm getting a strong vibe that once you get it running, the rest is a fine art of balancing the variables. And that exactly what you need to do to get it in tune is different for each "instrument." (As an IT guy for years, I am really fascinated with the difference between "getting the bugs out" of an analog system, like the Z, and a digital system as found in any code. A great experience.)

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On 9/11/2019 at 2:38 PM, 240260280 said:

I would not mess with (17) as the alignment of 15 will shift (which is no fun)

And @madkaw, well I messed with 17! Not in defiance, but because I did it before I saw your reply. 

The jet has now fallen further in (15) Jet Bearing.(See picture) Am I beyond saving? Is there a way for me to raise it? I'm not asking for step-by-step instructions, just point me in the right direction. I have a copy of Tuning S.U. Carburetors, 2nd Edition in hand. Are there specific YouTube videos that might help me fix my problem?

Thanks.

Fallen Jet.jpg

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