There’s a big difference.
Hello guys,
In a study session about carb jetting I went to youtube and saw a video saying the following:
"changes in the needle angle of low rev influence both, low and high rev jetting, but the opposite is not true"
Anybody agree or disagree with this statement?
If you agree and have an explanation for that, could you please share it with us?
Thanks
“Of low rev influence”? What does that mean?
If we make changes to the low rev needle we are changing high rev jetting as well.
But if we make changes to the high rev needle, we are not making any change to the low rev jetting.
Is that correct?
Sorry for my English.
I think what @JR_Garcia is asking is why when you adjust the Low speed jet needle, it affects the High speed jetting as well, but not the other way around.
I agree and this is why. When you open or close the Low Speed Needle, it will increase or decrease the fuel supply into the engine with throttle open or throttle closed. When you change the High Speed Needle it only changes the fuel supply when the throttle is open. So changing Low Speed affects fuel flow overall and changing High Speed only affects open throttle (higher revs). If you look in the Venturi, you will notice a small orifice (nozzle) in front of the throttle plate and one behind. Low Speed is always closer to engine.
This is a simplified explanation, but the basic principle stands. I am sure there is much more to the “Black Art” of Carburetor Design and Manipulation than many realize. In auto racing, I have seen many try and only a very few succeed.
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I think it works this way;
the low-speed dumps were the throttle bore is straight, no venture effect. Essentially, it only responds to an I got and I will find that increase vacuum coming from the engine. The high-speed dump is right in the middle of a venture. This makes the high-speed more sensitive to increases in RPM i.e. airflow. In 2 cycle racing we have found, for instance, that the KT100 runs better with the high-speed almost closed and the low-speed open almost as wide as is effectively possible. At some point there’s no more needle in the hole. I have a story of a B-Bomb doing the same thing. I instructed the driver on my theory, he tried it, it worked, he went faster. He liked it! I learned most of this stuff on my dyno in the mid-80’s.
Here it is, almost 40 years later, and I’m still trying to convince people. I’m having fun though!
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This matches my experience. The low jet provides fuel throughout the range starting when the fuel signal (vacuum) is low and the high starts to provide fuel when the signal/demand is greater. The low continues to supply fuel at that time also.
You can almost see this in action on the stand if you take a video in slowmo and play it back.
Of course you have to consider your pop off setting too, especially for mid range/peak power.
I think running a KT with a warlbro on the low speed screw only (High speed screw closed) is pretty common if not ubiquitous. But that’s an outlier amongst two stroke kart carbs these days.
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I’ve seen that, high-speed needle closed, but only when the air density drops a lot. Where do you race, mostly?
I’ve seen it both sides of the Atlantic, colder days too 50’s (F) or so. This is with the warlbro carb only though. Mike Simpson in the UK had some cool tricks for that carb on JICA engines too involving piano wire for fine tuning the jet screws etc.
Thanks @GregF, @alvinnunley and @KartingIsLife
You´ve got exactly what I was targetting. Thanks for answering my question.
So, can I conclude we should always tune low speed jetting first? And high speed secondly?
I’m not sure exactly where you got that idea. Both are important.
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Agree with Al. Both require tuning, but it’s good to know what you’re actually changing with each needle. Like James mentioned, pretty common on the Yamaha to run with the high-speed closed or almost closed in some cases.
Depends on your engine, but for example on KA or X30 here, I very rarely touch the low-speed needle and almost only tune with the high.
TJ is spot on regarding the KA and X30. Getting the low speed needle right is the key to proper carburetion. I use an AFR sensor on my dyno, and it makes it easy to see the influence of the two jets. The low speed needle has a big influence across the entire AFR curve, and if it’s off, the effect on power is dramatic. It is clear why the engine is much more sensitive to changes in the low needle. I set the low speed needle on the dyno and never, ever touch it after that. Any adjustments at the track are on the high speed needle only.
My point is:
If I start jetting by high speed first and let the low speed needle to be adjusted secondly, I will mass the tuning I´ve just done.
But if I start tuning the low speed jetting first, when I go to the high speed needle I´ll not interfere to the adjustment I´ve just done to the low speed jetting.
Makes sense?
@JR_Garcia You might be over complicating it a little. Here’s how it’s generally done If your base settings are correct (ie pop off and screw settings)…
On the track start with adjusting the high jet on the track as this will have the largest gains. Then you may make a low speed adjustment if you need to. Then, depending on how much you had to adjust the low screw you might adjust the high speed very slightly.
Just a word to the wise; I would rethink that strategy. +
I presume you do this after getting the pre-track settings close? By running the kart on the stand, you can listen for a sharp change in the RPM from off throttle to full throttle. As I understand it, if the engine is sluggish coming up in RPM you may have to adjust the low speed slightly. Then once on track, you would do as prescribed above.
I’ll edit my post as I rarely touch the jets on the stand unless I’m absolutely backed into a corner and the engine is essentially not driveable. i.e. troubleshooting
Pre-track setting to me is your baseline settings for the carb, initially from the engine and carb manufacturers, then as you become familiar you’ll have your own personal variation… Or, ideally, settings that based on air density readings that you’ve documented.
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Did your findings with AFR line up with the power delivery curve too? My thought has been that AFR is a moving target because of the big swings in volumetric efficiency throughout the RPM range of a two stroke.
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The AFR curve doesn’t follow the power curve per se, but what it does makes sense and is consistent engine to engine. It’s lean at low rpms, and goes right through to slightly richer than stoichiometric at peak HP. It’s a huge time saver when doing dyno work. You can isolate carb or engine issues right away just by looking at the curve. Any two engines putting out their best power will have very similar AFR curves, so if the curve looks right, you can bet it’s not the carb.