Jetting Poll

rustycase said:

Biknut, my needle end up in the same position, also.
It occurs to me that perhaps for BEST tuning, maybe it is time to begin re-shaping the slide cutaway??

'They' do that, ya know....

I also heard that a small square may be cut on the downstream side of the slide to ease the transition from low to mid-range speed.
Never did it, myself, though.

and, Jeepers!

Just how quick can my Gutter-riding compadre travel? lol
It might be time to look into a smog pump from a sbc, belt driven off the crank, as a blower!

That'll kick 'er in the pants!
Youtube shows they do it for briggs...

Works well on aircraft for high altitude!

Best
rc



...We're hot-rodding some storebought 'Rocket Sauce' today...
Added some chiles de arbol enteros, habs, a little onion, carrot, and vinegar/salt.
Will see how it comes out next week...

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Thanks rusty, I appreciate your advise. Actually I'm pretty happy with the partial throttle already. Seems like I spend most of my time running WFO.

Whenever I cruise at partial throttle, so far I've been pretty satisified with the throttle response after just getting the needle set right.

machiasmort said:

I thought the needle controlled all gas comming into the block?

Click to expand...

I think not so much at full throttle. Here's a little basic information on tuning

The jet needle – Has the greatest effect between 1/4 and 3/4 throttle. It’s attached directly to the throttle valve. As the throttle is rolled open or closed the jet needle moves through the needle jet’s bore exposing different sections of the jet needle’s profile to the needle jet’s inner bore.

Six major elements determine the jet needle’s effect on fuel delivery - the diameter of the straight section, the length of the straight section, the jet needle’s taper, the clip position, the number of tapers and the length of each tapered section. The number of tapers is normally not changed from what was supplied from the factory.


The Main Jet- regulates the flow of gasoline from ¾ to Wide Open Throttle. Like the pilot jet the main jet has a precisely sized hole drilled through its center. Increasing the size of the main jet ( size of the hole ) richens the circuit by supplying more gasoline; i.e. removing a 152 main jet and installing a 155 richens the circuit

http://justkdx.dirtrider.net/printcarbtuning.html

machiasmort said:

Wow, thanks Biknut! Well written!

Click to expand...

Thank the WWW

I just slapped a Speed carb on for testing. A Speed carb is a little more than 1 mm bigger than a NT carb.

The NT was accurately jetted with a 60 jet. Since I have zero experience with a Speed carb, I bumped the jet up to a 64. Better too rich than too lean dontjewno.

Starting was about the same. After a 5 mile test ride, I just checked the plug.

I think we can say a bigger carb will run leaner. The reason I think this is because the bigger 64 jet was turning my plug black in the NT. It only got dark reddish after going down to a 60. Now the speed carb with a 64 jet, the plug looks almost exactly the same dark reddish color as with the NT.

Other observations,

The NT always liked to 4 stroke a lot till the motor got warmed up all the way. It was especially bad about it until I leaned it out to a 60 jet.

I don't think the Speed carb 4 stroked once, even cold.

Another difference could be felt at partial throttle. I set both carbs on the next to leanest needle position. The NT always feels like it 4 strokes a little at partial throttle, and is especially bad about it cold. The Speed carb feels much smoother at partial throttle.

Throttle response, which was always good, still feels good. My firse impression was that it was a little stronger.

My testing to follow.