SB Needles to Resolve Lean WOT with SUs

[Duffman]

Have been thinking about doing that, Phil, any suggestions on what steps are best to make this happen?  What is best way to taper a needle?

[240260280]

http://www.terryhunt.co.uk/tech/picsb/pics.htm

[Duffman]

Thanks, Phil, good stuff!  I will rely on your station measurements above and give it a try.  I don't have a drill press, will have to figure out how to use my hand drill in some stationary format to get desired results.  Will get some 600 grit wet and dry sandpaper to taper the needle.  Might start with some old SM needles to practice on, move on to the RH.

[Captain Obvious]

8 hours ago, duffman said:

dropped the needle shoulder down so even with bottom of piston.

That is where the needle is supposed to be installed. If you had it pushed up flush with the groove in the piston, then you had it in too far.

You can do all the needle modifications by hand. You don't really need a drill at all. You're only looking to take off one thousandth or so.

[Duffman]

Hi, CO, I always slightly tighten the needle in the piston, raise the nozzle to top position (flat with base of carb), push piston down until needle shoulder is flat with the nozzle, then tighten the needle.  I believe this is the correct way to do it.  This position is slightly higher than the bottom of the piston, but lower than the trench in the piston.  So, I have actually lowered the needle from its standard position.

I guess I want to make sure I am uniform on all sides of the needle, am afraid with out some constant rotation may have flat spots, etc.

Edited March 21, 2019 by duffman

[Captain Obvious]

Yeah, the advantage to spinning the needle is a probability for increased uniformity. I do enough metalworking that I feel like I could just "wing it" if I didn't have access to machines to help.

Maybe lock your hand drill in a vice? Do you have a vice big enough?

[Duffman]

I wouldn’t trust me with winging it! Will have to get vise out and see what I can do. Just moved and most stuff in storage.


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Edited March 21, 2019 by duffman

[Captain Obvious]

Gotcha. If I were closer, I'd be happy to lend a hand. Problem is, this is going to be a "take a little off and try it... Take a little more off and try it again" kind of affair. Tough to do with two days shipping in each direction.

One thing you can to do to help yourself out is figure out where you are on the needle when at WOT. By that, I mean... When you are at full load and the suction pistons are pulled all the way up, there should still be some needle sticking down into the nozzle. You could push the piston all the way up by hand and reach into the carb throat with a marker and Sharpie mark on the needle where it enters the nozzle hole.

Then you would know where to concentrate your metal removal efforts. If you are running lean at WOT, you would want to concentrate your efforts where the WOT needle enters the nozzle and a little above and below that.

[Duffman]

Good point, CO, makes sense.  Will see how the tapering process goes.  Would be much easier to get one needle modified, versus getting two identical ones!  Will be an interesting test!  Will keep you posted.

[Captain Obvious]

Definitely keep us posted. It's an interesting project.

Do you have a micrometer? I've tried a couple different measurement devices on needles and have found a good ol' micrometer to be the best of the bunch. Most accurate and most repeatable.

[Duffman]

Yes, CO, have a micrometer, definitely makes it easier! Will keep you posted on results.


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[moritz55]

Just wanted to let you all know I have two(2) extra sets of "SB" and "SM" needles.   They were too rich for my L24 config so am selling at a good price for anyone who uses them.  

I am currently using "SR" Needles with very good results... will try the "SL" Needles which are the closest to N-27 stock needles at some point in the future.

Many thanks......

 

Edited July 9, 2019 by moritz55