Stock NT carb (Modifications Guide)

curtisfox

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May 13, 2010
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618
This is KOOL! And maybe do the same for a 4 stroke. For what it is worth i used some base ball bat tube and made intake, ( witch is tapered ) with proper flanges to fit 4 stroke. Small 4 strokes 5/8 to 2 hp. All fun stuff.

Also seen it done with the larger part of the bat for velocity tube...............Curt
 


Steve Best

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Sep 22, 2012
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1,267
I didn't do much with the NT other than rejet it. The bore was the same size as the steel manifold and port so I didn't see a gain in enlarging it. What I did work on was intake tract tuning. This picture may look kinda crude, but so are many of my "proof of concept" experiments. It is a piece of copper pipe JB Welded to a cap over the carb inlet with just enough clearance to let the choke still work. Then I experimented with different sized plastic bottles to see their effect on power.

A long tapered bottle gave great low end power but killed top speed and speed on hills. For whatever reason this waterbottle gave the best top end power in the 7000-10,000 rpm range without hurting low speed power. It was better than no carb filter at all. The filter is simply open cell foam squeezed inside and didn't hurt performance either. Crude, not very pretty, but effective. KC must be cringing at my lack of aesthetic taste!

BlueOval, you are chasing the ideal. Modifying to what looks like a good idea. This is good, chasing a theory, but you have to test in between steps. So does it actually work better? or just look better to our mind's ideal? You have to test in an unbiased way to really know.
 

6blueoval9

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Joined
Aug 28, 2017
Messages
1,120
I didn't do much with the NT other than rejet it. The bore was the same size as the steel manifold and port so I didn't see a gain in enlarging it. What I did work on was intake tract tuning. This picture may look kinda crude, but so are many of my "proof of concept" experiments. It is a piece of copper pipe JB Welded to a cap over the carb inlet with just enough clearance to let the choke still work. Then I experimented with different sized plastic bottles to see their effect on power.

A long tapered bottle gave great low end power but killed top speed and speed on hills. For whatever reason this waterbottle gave the best top end power in the 7000-10,000 rpm range without hurting low speed power. It was better than no carb filter at all. The filter is simply open cell foam squeezed inside and didn't hurt performance either. Crude, not very pretty, but effective. KC must be cringing at my lack of aesthetic taste!

BlueOval, you are chasing the ideal. Modifying to what looks like a good idea. This is good, chasing a theory, but you have to test in between steps. So does it actually work better? or just look better to our mind's ideal? You have to test in an unbiased way to really know.
I'll have an untouched NT to compare the difference with...Nothing I've done so far has been tested NOT to work in different venues of use.

Removing abrupt turbulence is certainly an improvement with air flow in anything and a velocity stack is proven to help as it removes this turbulence...I'm highly expecting these mods to be an improvement without the need to test first but I will test the difference and report it.

That bottle mod you have I would consider (based on the taper) to have improved your performance based on the same principle a velocity stack would.

You saw in the beginning of the thread I wanted to get rid of barriers and edges inside the intake area of the carb to improved air flow...I'm sure the first two steps alone would have been an improvement but decided why not go all the way with it so, I completely eliminated all air flow turbulence by just creating a built in velocity stack..really it only amounts to two modifications when completed...removed the garbage center piece on ring and a built in velocity stack.

Cost me a whole $5.00 with tax from Walmart...I bought a package of the jb weld steel stick epoxy/ putty...gutted and scuffed up the area I used it on and rough molded it by hand then finished it with a sanding roll.
(Gutted)
IMG_20171018_222809.jpg



I'm actually considering some changes with the inside of the filter cover..I may make some changes to re route directly how the air is sent to the carburetor after being drawn in...The part I like best is it will all look like a stock carburetor installed which it would be, just an improved one.

As far as the bore, ultimately I'd like to be at 17mm once I take the build further so, most likely an NT speed carburetor with .5mm taken off the circumference should yield me 17mm and not effect idling or disturb the pressures to siphon fuel...or just a different carburetor all together but I will try the speed carb first.
 
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6blueoval9

Well-Known Member
Joined
Aug 28, 2017
Messages
1,120
This is KOOL! And maybe do the same for a 4 stroke. For what it is worth i used some base ball bat tube and made intake, ( witch is tapered ) with proper flanges to fit 4 stroke. Small 4 strokes 5/8 to 2 hp. All fun stuff.

Also seen it done with the larger part of the bat for velocity tube...............Curt
I've seen an aluminum bat exhaust on you tube.:cool:
 
K

Koz

Guest
Thank you 6blueoval9!
My bike wasn't getting past 26 mph unless I took off the filter. Then it would hit 32. I made the mods to the plastic cover, and now it breathes much better.
I was so frustrated, trying to change needle settings, etc.. It was that air flow turbulence issue!
 

Ratboi37582

New Member
Joined
Mar 9, 2020
Messages
3
For mod #2 I need to explain that this will eliminate your chokes function, possibly making it difficult to start your motor in colder weather...If you do not wish to eliminate your choke then do not perform this mod below.

My next obstruction I've found is the entry port of the carburetor.

Air is hitting the sides of the extended port entry, helping create turbulent air flow...Simply grind this extension down and radius the entry until you have a smooth transition for air to enter.

Now keep in mind that I'm not porting the entry way what so ever and the entry port still measures 14mm or close to that but will improve air flow quite a bit given its size limitations.

Here is the stock carburetors air intake side.... you can see the extended tube like extension of the bore ,this extension blocks air creating a turbulent flow into the bore...This extension is there so the choke flap has a surface to completely close off the bore and only breath through the tiny hole creating a very rich mixture that helps start your motor in cold weather.
View attachment 79468

This is what it should look like after grinding down the extension and giving the entry port a nice radius.
View attachment 79469
One more pic...notice how even the choke stop has been removed.
View attachment 79470

For stealth looks you can leave the choke assembled or simply remove the entire choke assembly and fill the lever hole with jb weld or epoxy so no garbage or water can find its way through.

Other than tuning I wouldn't consider anything more needed to get the most out of this carburetor.

If you want a larger carburetor I would suggest going with the 16mm "Speed" version of this NT carburetor and follow the same mods shown here if you wish.

Edit:

Upon some discussion below on the subject of boring the carburetor..I may try this to see how far it can be taken without idling issues or difficulties to tune...I'd like to keep this a simple but complete guide to gain a little more performance out of your stock 14mm NT carburetor.

About that high flow airfilter, in all honesty using an aftermarket air filter would help perform better as air isn't just drawn in from the bottom such as with the stock air filter cover but more so drawn in from 360 degrees providing a better direct flow of air in general to the carburetor and, will have better filtering capabilities.

Thanks for looking.

Edit: More to come.
Do you know the outside dimensions in mms of the intake housing on the carburetor? I am looking for a k&n conical air filter because the stock ones are crap from a filtration standpoint.
 

Steve Best

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Joined
Sep 22, 2012
Messages
1,267
Do you know the outside dimensions in mms of the intake housing on the carburetor? I am looking for a k&n conical air filter because the stock ones are crap from a filtration standpoint.
I experimented with all sorts of shaped intakes and used a piece of open cell foam for a filter. My best intake was a stubby water bottle:
93681


This was cobbled up in minutes and I tried dozens of different bottle styles and lengths, and included long hoses as well.

The length and diagonal of the cut was the result of trial and error as well. Find a good hill the bike will barely get up and start experimenting.
Bottles are cheap and easily formed with hot water. The foam is available everywhere. If you can breath through it, so can the engine. It will self oil with fuel puffing out of the carb. Shorter was better and the stock air box actually worked better than many of the bottles. The foam will filter better than the K&N.

The adapter is a piece of copper pipe and some epoxy putty grafted onto a stock aircleaner box lid.

93682


The copper pipe is a part of "Boost Bottle" testing (on a rainy day). Boost Bottles (or Helmholtz resonators) did have significant effects at specific rpm's but I could not tune it in to have a beneficial overall effect. It would boost power in a very narrow rpm band and kill it over a wide band.
 
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