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Carb needle questions [long]

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mrsprite

mrsprite

Jedi Warrior
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So.....just how long is an SU needle supposed to last? Is this something that should be replaced every few years or is it a 'set it and forget it' sort of thing?

Here's the scoop.....got refurbished carbs 2 years ago from a very reputable shop (these are the Datsun Hitachi SU's). Everything has been great until a few weeks ago. I had several small, unrelated problems pop up including a over rich condition on the back carb. The plugs were getting very sooty at a level that used to be very lean. I pulled the carbs apart and noticed a shiny spot at the base of the needle as if it were rubbing. What would cause this to happen all of the sudden?

I just got a Colortune and tried adjusting the carbs. The rear carb still runs slightly rich with the needle all the way up (fully lean....obviously related to the rubbing). Will this cause any problems until I replace it or should I just stop driving until I fix it?

Do I need to replace the needle or can I try recentering it? There was just the slightest bit of rubbing at the idle portion of the needle (station 1?), so maybe the needle is still saveable? Also, I did try to remove the needle, but it would not budge (yes....I removed the set screw). What's the secret to getting it out without destroying it?

I plan on calling ZTherapy tomorrow, but figured someone here could help me out. Sorrry this post was so long!
 
Do you have the spring-loaded needles or the fixed types?
The spring-loaded rub on the jet and wear, the fixed types (when they are set correctly) don't.
BillM
 
Sounds like you have a handle on it Mike. Rubbing needle probably wore the jet as well which would make a rich mixture at any opening. You might be able to compensate by raising the jet. ( I assume action similiar to SU's)
Kurt.
 
The needles are fixed (not spring loaded), so it seems something went awry for whatever reason and the needle started rubbing. Just my luck!

nomad.....the action is identical to real SU's. I've raised the jet all the way, but it still shows a slight rich mixture at idle with the Colortune. I'm still at a loss as to what happened all of the sudden to cause this.....I'll probably never figure that out.

BTW....do you still have any interest in my drivetrain? I am still seriously considering a modern twin cam swap (probably the 4AGE). Not sure if or when that would happen as I have no money to even think about a swap, but figured I would ask. I need to see if 'jayrz' ever finished his 4AGE Midget.

Anyway, I'll be giving ZTherapy (the carb guys who built them) a call in the morning and see what they can do.
 
I couldn't sleep, so I tore apart the offending carb. I couldn't tell if the needle was bent or just askew, but it was definitely dragging on the jet nozzle. I swapped out jets and it did the same thing, so I could rule them out (front carb worked fine with either jet). I played with the needle enough to where I think I have it square again (the piston now drops like it should....with a "clunk"). Will have to wait until morning to see how it runs.
 
'nother OCD problem solver!

A Quick-n-Dirty check to determine if a needle is bent: roll it around on a piece of glass tabletop or equivalent. If its bent it'll 'wobble' perceptibly.
 
The stock needle for the Midget with fixed needles should be a AN, with the jet stting down about .070" from the bridge measured with the end of dial caliper. If you're getting a rich setting with jet flush with bridge, thats not right for sure, you should be able with the dashpot pot, piston and dampner spring removed, turn the electric fuel pump on and see fuel just below the jet. It could be ignition timing problem as well.
 
It's hard for me to imagine how it could be rubbing without the piston sticking. Of course, if it's running rich, that might be part of the problem. Might be good to make sure there isn't something funny going on with the piston--too loose in the pot, maybe? A previous owner may have mixed parts from several carbs, and something like this conceivably could result, although I admit it seems improbable.

It's good to remember that the needle tapers are specified to 0.0001 inch, so it doesn't take much abrasion to change this significantly. Also, if the needle has been rubbing, the jet probably is also abraded.

If you are concerned, just get a new needle and jet. They're not expensive.
 
Yeah....I get a bit OCD every now and then. :smile: I finally got the needle out of the piston and rolled it around and I did not notice a wobble....doesn't mean it didn't, just that I couldn't see anything.

Hap.....I have a Datsun lump in my Bugeye that has twin Hitachi 38mm SU clones, so the needles are probably different than a stock Spridget. I don't have any calipers, so no measurements can be taken. I've checked and rechecked the timing, but it is good. Plus, only the back carb was running rich.....front one is fine, so if it were timing it would probably affect both equally.

The piston was sticking. When I lifted it up and dropped it it would get hung up before the bottom.....would never "clink" when it hit bottom. That's what I fiddled with late last night/early this morrning. I finally got it so the piston doesn't get hung up, but I still think it's rubbing (just not as bad as before).

I will be buying new needles and jets anyway, but now I think I can drive my Bugeye without fouling up the plugs. We'll see how it goes this weekend.

Thanks for everyone's help and insight.....I love tinkering with stuff, but admit that SU's are just one of those things that I know nothing about and quite honestly I was afraid of them. Well....afraid of screwing something up anyway. :smile: I think I have them figured out now.....no more mystery there for me.
 
Hey Mike, I'd still be interested in the drivetrain. As to the carbs it sounds as if the jets were'nt centered with the needles. Looks like you got it figured out by this time anyway. Might cost you a new jet and needle though.
I'm curious as to what happened to jayrz's conversion as well.


Kurt.
 
Hey Kurt.....I think I have it straightened out. What a PITA! I removed the jet and the needle and fiddled with them for a few hours early this morning then again this afternoon. After driving around and checking things out it would appear as though everything is back to normal. I'm still going to buy some new needles and jets, but I don't think it's as pressing to swap them out as it seemed yesterday.

I heard from Jay a couple of months ago when I was still living in Wyoming, but haven't heard cfrom him since I moved back to Colorado. I PM'd him, but he may or may not see it anytime soon. I do know where he lives though, so I may stop by someday soon to see what's going on.

I'll let you know if/when I ever do the engine swap. I think I was a little frustrated when I asked if you wanted it......whenever the engine acts up I always threaten to rip it out and start over. :smile:
 
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