Carby NT Carb Tuning Basics

DAMIEN1307

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What poor QC they must have in China
You are very correct...We the consumer have to do the QC on these when we build them and find their mistakes and then rectify them ourselves...lol...Thats what i do, and when its done, its damn near perfect...lol...I don't even get loose nuts or bolts...its scary its so reliable when im done...lol...DAMIEN
 

mcdanida

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I would, but the company is called Ankul. Not much info provided, probably they would rather not be contacted. It was thru Amazon message services. Amazon is the intermediary.
 

mcdanida

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I tried to save mine as a spare with "Seal-All" which is noted as being gas and oil resistant...after letting it cure for a full 48 hours, much more time than needed, i just let it sit in a sealed container with the same mix ratio of gas and oil i use in the bike...Sank like a rock...it doesn't work...just get the new carbie...less aggravation...The carbie that i recommended to you is a regular NT carbie that is universal for all of these China Girl engines, plus i trust Amazon over the other "off brand" vendors to supply this to me...its worth the 14.95 which might be a few little dollars more but its better to deal direct with Amazon if there is a problem than any of their vendor units...just my opinion...lol...DAMIEN
Haha, same here. The fix didn't work.
 

mcdanida

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Ahhh third party entity vendor...glad it worked for you, im always very wary of these third parties...lol...DAMIEN

This is a good beginning of the products from Ankul/Anbull, offered thru Amazon

Actually they went back on their word and said I need to buy one. Ummmm nope! Either send me a replacement or I will go through Amazon returns and rate them low.
 

DAMIEN1307

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Actually they went back on their word and said I need to buy one. Ummmm nope! Either send me a replacement or I will go through Amazon returns and rate them low.
You can also contact Amazon directly and tell them of the problem...888-280-4331

Ive done this before and have always gotten results when third party vendors don't honour their committments...Which is almost always...DAMIEN

Ps...This is part of what i meant about dealing with third party vendors...Never seems to fail.
 

mcdanida

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So when I threatened a poor rating and sought to return through Amazon, the vendor was hot on the trail to issue me a new carburetor. LOL. I have another on order, will keep the one I get later as a spare. Sounds like a good spare part, like the 415H master links I bought other day along with a lot of M6 fastening hardware. I am learning the ropes.
 

DAMIEN1307

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Sounds like a good spare part, like the 415H master links I bought other day along with a lot of M6 fastening hardware. I am learning the ropes.
Yes indeed you are learning quite well young padawan...lol...I also have spare carbs, masterlinks, kevlar tires and tubes, (M8 hardware since thats what most of mine is), the black and white heavy duty clutch pads and pressure plate bucking bar and bearing, spare cables for everything, and have 2 sets of spare disk brake pads on the way...Im stocking up on all these parts, partly due to a massive shortage of these things right now...On the disk brake pads, i was limited to one set only by Amazon due to short supplies in stock...found my way around it, but i encourage everyone here to order spares of what you dont have that you might need in future...DAMIEN

"He who fails to prepare, prepares to fail"

"Proper preparation prevents piss-poor performance"
 
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yuckfoo

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A few basics on the Happytime NT Carburetor:-

Some pics identifying the basic parts:-
View attachment 23593

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Float Bowl Gasket

View attachment 23596

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View attachment 23602

Also see srdavo's NT Carburetor Assembly Pics for a bit more info.


Brief Description Of Operation

The NT side-draught carburetor operates by drawing air into the air inlet past the slide needle jet. 'Venturi' action draws fuel from the float bowl upwards into the in-going airstream.

The slide regulates the flow of air into the engine. A slide needle extends below the slide into the slide needle jet to regulate fuel flow during low to about ¾ throttle openings. When the throttle is opened further, from ¾ to full throttle, the 'main jet', screwed into the bottom of the slide needle jet, regulates fuel flow. The slide needle is adjustable, up/down, to increase, (enrich), or reduce, (lean out), the amount of fuel in the mid-range mixture. To adjust ¾ to full throttle mixture, the size of the main jet must be changed, by replacement usually, although drilling or soldering and drilling is an alternative. Larger = richer and vice versa.

There is no idle circuit, pilot jet or idle mixture screw on these carburetors. The slide needle setting also controls the idle mixture.


Specifications

Specs vary with individual carbs and suppliers.

For a 66cc engine, I've heard of stock main jets sized at 0.6mm,(MBB Imports), 0.7mm,(Many US kits) and 0.79mm, (ZBox 66cc).

In the end, many seem to run well on about an 0.72 to 0.73mm main jet, with standard exhaust and porting.

Bore diameter: 14mm.
Float level: 21mm.


Tuning

The NT carb supplied with most typical HT engine kits is very simple and so should be easy to tune, but many have trouble getting it right, resulting in low power, low revs, smoky exhausts, fouled plugs and excessive buildup on heads/pistons if the mixture is over-rich or, in the other extreme, overheating, melted pistons, seized engines if the mixture is (way) too lean.

The most common problem is a lean mixture due to an air leak where the carburetor attaches to the inlet tube or where the inlet tube attaches to the cylinder.

The second most common problem is a rich mixture, (usually after sealing the inlet leaks), resulting in four-stroking and reduced power output, often in both the mid-throttle openings and at full throttle. See 'Four-Stroking', in 'Diagnostics' below.

N.B. Before attempting any carby adjustments, ensure that the carb to inlet joint is well sealed, the inlet gasket is in good condition, the carb is clean, the fuel filter and air filter aren't blocked and that the inlet tube nuts are tight.


Adjustments

There is only one direct method of mixture adjustment on these carburetors. This affects low to ¾ throttle openings. The slide needle has a series of notches near the top, (four on earlier carbs and five on the later ones), so that the attached 'C' clip can be removed and refitted to raise or lower the needle relative to the slide and slide needle valve.

When the needle is set lower, relative to the slide, less fuel is allowed into the mixture for a given amount of air, making the mix leaner and vice versa.

To adjust the mixture at ¾ to full throttle openings, the size of the main jet needs to be changed, smaller for a leaner mixture or larger to make it richer.


Diagnostics
One of the best ways to check the mixture is by doing a 'plug chop'. In effect, the engine is first warmed up to operating temperature, then a new plug fitted. Run the engine at the throttle opening for the area of concern for a bit.
ie. Slide needle setting - about ½ throttle.
Main jet - full throttle.

Now, without changing throttle opening, pull in the clutch, hit the kill switch at the same time and roll to a stop.

Check the plug color. If it's tan, the mixture in that range is OK. If black and/or wet, the mixture is too rich. If the plug is grey or whitish the mixture is too lean.
I had thought about including pics of plugs, but don't have any that aren't tan, right now.

One of the more common signs of a rich mixture is 'Four Stroking'. This occurs when the fuel/air mixture is so rich that the engine only gets enough air to fire every second revolution, reducing power and making the engine sound like it is only doing half as many RPM. An overly smoky exhaust is another indication that the mixture may be too rich.

An example of 'Four Stroking', (thanks Al.Fisherman, for pointing it out):-
Four Stroking (YouTube)

Any corrections or other suggestions?

(If this has been useful, please give it a rating.)

... Steve
Does anyone know the thread size of the idle screw? I want to replace it with one that has a knurled head so I can turn it by hand.
 
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