NT Carb vs Speed Carb

biknut

Well-Known Member
Sep 28, 2010
6,543
256
83
Dallas
I bought a Speed carb to replace my NT. I've got about a week under my belt using it so I thought you might be interested in my observations.

First off, a Speed carb looks almost exactly like a NT. About the only visible difference is, there's a built in petcock on the fuel inlet.

Closer inspection reveals the venturi's are different size in diameter.

NT carb........ = 13.74 mm

Speed carb... = 14.78 mm

Obviously the Speed carb is a little bigger.

So, to cut to the chase, does my bike go faster with the speed carb?

The answer is not really. It's seems to top out 38 mph with either carb. But there are other differences worth considering.

Such as, with all my 80 motors with a NT, they all seem to like to 4 stroke at partial throttle. The Speed runs much smoother at partial throttle. I'm running both carbs with the needles in the next to leanest setting. The NT was running a 60 jet, but the Speed likes a 64 best. The plug looks about the same now as either carb, which is a reddish brown.

It's hard to tell, but the throttle response might be slightly better with the NT at lower speeds. There's not much diff though. At higher speeds there's no difference that I can feel.

Starting is about the same either way. No choke, just work the throttle.

I think with a ported cylinder the Speed carb might offer a slight advantage powerwise, but with my stock cylinder I didn't gain any extra power.

So in conclusion, there's doesn't seem to be much difference in power, but the Speed carb cruises a lot smoother, with less 4 stroking. Just because of that I like it a little better than the beloved NT.
 

Al.Fisherman

New Member
Sep 9, 2009
1,966
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Calera, Alabama
Very nice review, ummmm so unless you have 4 stroking with the NT there should not be a reason to get the "Speed" carburetor. I've always been fond of (and had no problems with) the NT carburetor. Thank you for the review :>)
 

Drewd

New Member
Jul 25, 2008
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Colorado
Having a second fuel shutoff valve is a great idea as some stock tank valve dribble in the off position.

With several changes under my belt form stock NT to NT Speed carb, i echo OP's observation that transition and midrange are better with the NT carb.
 

Drewd

New Member
Jul 25, 2008
425
0
0
Colorado
Having a second fuel shutoff valve is a great idea as some stock tank valve dribble in the off position.

With several changes under my belt form stock NT to NT Speed carb, i echo OP's observation that transition and midrange are better with the NT carb.
I meant better with the NT Speed carb.
 

biknut

Well-Known Member
Sep 28, 2010
6,543
256
83
Dallas
The first time I tried a Speed carb was on a 80 motor. Now I'm going to try it on a 50.

All I did is remove the NT, and slap the Speed carb on. The 80 liked a 66 jet, so I'm leaving that in it.

First thing I noticed is my motor starts faster, and warms up quicker. It also feels like there's slightly less bottom end power, but a little more top end.

There's no trace of 4 stroking. I'd have to say the 50 runs a little faster now. Before it would get up to about 28 mph, and then struggle up to 30. A lot of the time there would be a little 4 stroking going on that would make it really hard to get 30. Now with no trace of 4 stroking it runs right up to 30 and beyond on a level grade. Seems like it's always good for 30 to 31 now without any struggling.

When you whack the throttle at low speed, this carb makes the acceleration feel smoother, just like on the 80 motor, but that could be because it makes a little less power at low speed.

Still, all things considered, I like the way the 50 runs better with this speed carb on it.
 

Alex Behan

New Member
Oct 21, 2021
24
10
3
17
New Bern, NC
The first time I tried a Speed carb was on a 80 motor. Now I'm going to try it on a 50.

All I did is remove the NT, and slap the Speed carb on. The 80 liked a 66 jet, so I'm leaving that in it.

First thing I noticed is my motor starts faster, and warms up quicker. It also feels like there's slightly less bottom end power, but a little more top end.

There's no trace of 4 stroking. I'd have to say the 50 runs a little faster now. Before it would get up to about 28 mph, and then struggle up to 30. A lot of the time there would be a little 4 stroking going on that would make it really hard to get 30. Now with no trace of 4 stroking it runs right up to 30 and beyond on a level grade. Seems like it's always good for 30 to 31 now without any struggling.

When you whack the throttle at low speed, this carb makes the acceleration feel smoother, just like on the 80 motor, but that could be because it makes a little less power at low speed.

Still, all things considered, I like the way the 50 runs better with this speed carb on it.
So would the speed carburetor work better on a 48cc?Also why does the engine warm up faster?