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Setting Needle

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MathewM

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I have taking the big leap an took my HS4 carb off to rebuild. I watched the video "Just SU" and felt confident that I could handle this repair. The only thing that is driving me nuts is how to set the needle in my pistion. Now the version of HS4 that I have has a spring loaded needle. This is where I need some direction. I have been trying to guess the height of where the needle should be located and once the dome is back on and move the piston up and down the piston gets stuck. I have been bending the linkage for the jet nozzle to help it line up with the needle but with it being spring loaded it moves with the adjustments. The only thing I didn't do was a jet alignment barrel. This is the part the the adjustment spring and screw get threaded into. The reason I did not do this was the piston movement before the rebuild was ok and the video said that if you don't have to take it apart don't. Any ideas would be great. I have been dealing with a idle and backfire problem since Oct 06 when I got the car. Timeing, Points, Vacumme Lines, and Clean Gas all good. So the only thing left was the carbs. After doing this repair I did find out that the stock valve and seats were bad and replaced with Grose sets. Also the linkage that is connected to the float jet was screwed to the wrong side(see Pict) Well got to go. Thanks /bcforum/images/%%GRAEMLIN_URL%%/grin.gif

Sorry I can't figure out how to load the pic. Yeah I am a Newbie
 
Centering the jet so that the needle moves smoothly up and down without binding is one of the trickier procedures, and should not, in my experience, require any bending of linkage parts.
Slacken the locknut on the jet adjustment allowing the jet to move. You can then center it by eye by wiggling the jet back and forth. Then gently drop the needle into the jet. My experience has been that slightly wiggling the jet back & forth will quickly allow the needle to drop all the way down in the jet. This must be done with care, as you do not want to score nor bend the needle.
As you have probably discovered, tighting the jet locknut invariably causes the needle to bind again, but you have to keep monkeying with it until you reach a point where the needle will drop all the way down, and the locknut is tight.
Moss has a jet centering tool, but I have never used it, so I don't know how well it works.
Perhaps others have a better way of centering the jet, but this is what I have done.
With respect to the idle/backfire problems, are you sure the throttle plates close fully? If they are partially open it will be impossible to set the idle properly, and can cause the backfire problem.
 
Centering the jet with a spring loaded needle isn't as critical as it is with a fixed needle. Are you sure you asssembled the dashpot and piston exactly the way they came off? Try rotating it 180* on the carb body and see if things improve. Or, remove the needle, and make sure there is no binding of the piston in the dashpot.
All bending things will do is get you a bunch of bent up and misaligned parts.
Jeff
 
Sorry if my long-winded discourse on jet-centering was inapplicable. I've not had any experience with spring loaded needles, indeed I didn't know there was such a thing.
 
I haven't worked on the carbs with spring loaded needles either but I think you'd still need to have it close to centered correctly. I have seen the jet centering "plugs" listed on other recent threads on this board but I am not aware of how to use them on carbs with spring loaded needles. Perhaps they would be used the same as they are with the fixed needles?

Jeff's suggestion about rotating the position of the vacuum chamber should help if you left the jet tube in place. If you've moved the jet tube at all then you probably need to recenter it even with the spring loaded needle.
 
Doug and Ron. On my carbs that run spring needles, I generally center the jet using a bushing and fixed needle. With the spring needles, there is always contact between the jet and the needle, usually resulting in a groove wearing into the jet tube over time. One supposed advantage to the spring needles was that it mostly eliminated the problem of centering the jet. However, now the jet becomes a wear item. Not a valid trade off in my estimation.
I've never seen a spring needle bind due to a jet centering problem. It's usually either dirt in the suction chamber / piston interface, or the dashpots have been incorrectly assembled to the carb body.
Hard to diagnose this one on line, but I'm sure it's a simple fix, provided things aren't totally distorted by bending parts.
Jeff
 
Adjusting the jet tube will be somthing I will try tonight. The reason to bending the linkage was that it was bent before and screwed to the wrong side of the jet. It is possable to replace a spring loaded needle with a fixed needle? The bushing that the set screw locks into is hallow expect. So there I would need to replace that part aswell. That bushing is also something that I am not sure if it is to be flush or resessed. Flush allows the needle to get stuck more because of the steps in the needle. Once I find out how to load a picture I will show you all the unique parts that I have.
 
It's possible to convert spring needle carbs to use a fixed needle. Joe Curto sells the bushings needed.

https://www.joecurto.com

The needle should be set flush with the bottom of the carb piston.
There should be no "steps" in the needle. It should be a smooth, tapered profile. It's possible that misalignment has worn the needle to the point where it's useless.
Jeff
 
It sounds like the taper on the needle might be worn into steps from being misalighned and tightened...or are the "steps" just marks or ringed stains from assembly and disassembly? Either clean it up or replace it, but when you do, straighten out the bends you have made, install the springed needle and reassmeble, making sure it stays clean while you set it. Tighten about snug, then back off about half way and then snug it all up. Put it all back together carefully, snugging and backing off slightly to make sure the needle and spring do not hang up on anything in the tube. Patience and a light touch are necessary to get it together with the needle flush with the piston ...and a generous use of carb cleaner will make it easier to see and feel the fit. Then let it dry completely (go have a cold drink) and check for snug again (not too tight now) reassemble the rest and crank her up. I have always experienced a little rough running for a few minutes, until everthing seems to fall into place. Then make the other adjustments and take her for a drive.

Good luck!
 
It was suggested dashpot orientation could be wrong which is a issue on a HS2s (Midget carbs) as it has two dashpot to carb body attachment points and can be put on either way, and if put on wrong can cause a bind, but with the HS4 (MGB carbs) you have three attachment points on the dashpot and it can only go on one way. Check all the movement points of the piston/carb body, and make sure they are burr free, I use stotchbrite to clean the inside of the dashpot, and the outer contact surfaces of the piston, also make sure you don't have any "hang ups" in the damping rod and the piston damping tube, I often hone these areas with small flex hones just to make sure I have nice, fluid motion. Generally if you didn't disturb the jet bearing, the needle alignment is not changed. I have use marking fluid before to find out where things are rubbing though, every once in awhile you run into a carb that just wants to be a "bugger".

The best advice I can give you is to be patient, play close attention to the carb and you will fiquire out where your bind is. I once read in one of these MG forums I go to that a SU carb is a very precision instrument, I almost spit coffee on my keyboard from laughing, these folks used to make meat grinders /bcforum/images/%%GRAEMLIN_URL%%/smile.gif and a SU carb is far from being precision, and about the simplest carb on the face of the planet, just study the thing and fiquire what makes it ticks, then it will all make sense to you and you'll have a better general overall understanding of what make these very simple carbs work. Hope this helps
 
OOPS!! My bad, Hap. I guess I've been messing with HS2's too long. I should have grabbed an HS4 before I shot my mouth off. /bcforum/images/%%GRAEMLIN_URL%%/blush.gif
Jeff
 
Well I took apart the jet tube and relized the the needle was not centered. So I took the piston out and removed the needle and want seemed to be the problem was the small sping on the needle was on half way. So once on the piston it stuck into the jet off center. I got everything bolted back on and then I noticed that the spring that are inbetween the piston and the dashpot were differnet lengths and not the same number of coils. I had been keeping this part separate and never notice until now. Should I try to buy new ones to be on the safe side? There isn't any paint left on them to know if they are correct. Even with this problem I hook all the fuel lines up and she started. I know what I did helped the flow of fuel better with rebuilding but idling is rough and I can't seem to tune them to work together. I think I will take a break from this rebuild and come back another day with a clearer mind. Thanks everyone for your advice!
 
If both pistons return down at the aproximate same rate then don't worry about the the piston return springs, use what you have, if they look like the drop rates are quite diffrent from one carb to the next, then order a new set of springs from Joe Curto.

https://www.joecurto.com
 
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