carb needles!

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J

Jonda500

Member
May 10, 2014
86
8
8
Canberra, Australia
I have been struggling to get one of my china girls to run nice. It has upgraded ignition from SBP's and a speed carb with a HD sports air filter (like all my bikes) - after trying 3 different jet sizes + all five needle clip positions for most of the 3 jets, today I left the #68 jet in but swapped the needle clipped #4 (from the top) for a standard NT carb needle clipped #3. This needle looks noticeably thinner than the speed carb needle but is exactly the same length.

Before the change she would 4stroke whenever I didn't have the throttle wide open and would only idle either very high or not at all.

She still has an unreliable idle but the mid throttle performance is suddenly superb!!!!!!!! thinner needle!! Who would have thought!

One of my other china girls was similar (kept 4 stroking) but without the idle issue - I had all but given up on it when I changed the exhaust for a longer poo poo exhaust (only because it wouldn't fit my other bikes!) and well blow me down - it suddenly had awesome mid throttle performance! What a nice surprise!!

John
 
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crassius

crassius

Well-Known Member
Sep 30, 2012
4,032
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USA
a lot of different needles out there - some shorter, some longer, some blunt ended, some well tapered - speed carbs often have a slightly smaller venturi than NT, NTTC, WH and the similar one with chinese chars on top

needle jets also vary in working length, 2 main types are the hex shaped outside and the round shaped outside

mixing parts among these is a pain

for last year or two, I've been getting the one with chinese chars and have not had to adjust needle or jet on a single one
 
J

Jonda500

Member
May 10, 2014
86
8
8
Canberra, Australia
the needle jet is the main jet? i assume these simple carbs don't have emulsion tubes(needle jets) like my old honda4's.
I have only three types of jets - and they are indeed quite different - the #66 hex type(speed carb), the #70 round(standard carb) and larger round SBP's in various sizes. My bikes like the #66 hex type better than the #66 round type from SBP's. I never thought about the difference in the jet profile affecting the outcome - assumed only the size of the hole mattered!- thanks crassius!
John
 
crassius

crassius

Well-Known Member
Sep 30, 2012
4,032
158
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USA
the main screws into bottom of the needle jet - in this tube is a shelf that varies flow as the tapered part of needle rises & falls (shelf is higher/lower on some)
 
J

Jonda500

Member
May 10, 2014
86
8
8
Canberra, Australia
so there are differences in the brass tubes(needle jets) that will affect how she runs too?!!
(I have been leaving the brass tube and main jet together as its easier to work with when the carbs not removed from the bike)
 
cannonball2

cannonball2

Well-Known Member
Oct 28, 2010
3,682
221
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Colonial Coast USA.
CG carbs are a crap shoot at best. In my experience you never know what may be used in them jet/needle wise. I to went through the whole 9 yards trying to make a CG run clean. Exhaust is fairly critical. I found increasing back pressure helped some. The best running stock type carb(on a 66cc) I had came from a 48cc with smaller jets and venturi. Problem is finding one that the seller is being truthful about being as advertised. I finally settled on a carb from a 50cc Yamahopper moped. This cost about $20 as I remember. It has a fully adjustable idle circuit. It also has a remote controlled enricher for starting which I like. no fumbling for the choke lever. Also it is a bolt on for the stock intake.

As I remember I dropped a jet size leaner and that was all I had to do. Runs nicer than any other carb I have tried and that was most common types.
 
Chaz

Chaz

Well-Known Member
Jun 3, 2012
1,004
72
48
Vancouver, British Columbia
Cannonball, I searched around the web and couldn't find the carb you mentioned. Could you post a link? People are always interested in carb options and it sounds like you found a winner.

thanks
 
cannonball2

cannonball2

Well-Known Member
Oct 28, 2010
3,682
221
63
Colonial Coast USA.
Heres a link to where I bought mine. The only mod I remember making to the carb was to plug the vacuum port that opens the fuel valve on the Yamahopper. I modded the stock airfilter to fit but it didn't fit all that great. The stock Yammy wont fit.

I tried to find a part of a thread where I installed this carb but haven't yet. Trying to remember if I used a sleeve from an SHA or if the one supplied worked.

http://www.ebay.com/itm/Carburetor-...ash=item3f5eceaf4b:g:zvoAAOxyBvZTUvaJ&vxp=mtr
 
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