I just looked as well, and either they updated it, or I saw it for a different motor. I know on the Mini Swift people would cut a notch it the top of the clutch cover without an issue.
Seems like this thread has been hijacked to a discussion about pumper carbs. Might be time to split it.
The last time I did a full clean and rebuild of the HL334AB carb, I spent a lot of time making sure every passage was clean, blown out and free of debris. While doing so, I started paying attention to it’s design and construction. The layers of the pumper body, the materials the diaphragms and gaskets were made of etc. Basically I was trying to work out if any gains could be made without changing the base components of the carb. A few things I noticed were that the pump chamber was relatively small compared to the metering chamber. Also the material the pump diaphragm was made of was stiff almost like a thin fiberglass sheet compared to the metering diaphragm which was soft and flexible appearing to be made of rubber or silicone.
So with the pump diaphragm being stiff and the small space it has to flex, it seems to me that it was made to handle short fast bursts compared to the metering diaphragm being soft and flexible was made to stretch gently within a larger space.
Now the pulse is sent from the block by the piston moving down with the reeds shut. It may have some negative pressure when reeds are open and piston moving upward, but minor compared to positive pressure sent when reeds are closed. Now at 1000 rpm, its pulsing 1000 times per minute. Hypothetically lets say you burn through about a gallon per hour. Most 2 strokes will likely average somewhere between 12,000 - 14,000 rpm (Min of 8,000, Max of 16,000), so lets say you average about 13,000 rpm over a single lap. Doing the math… if 1 Gallon = 128 oz, 128 oz / (13,000 pulses per minute x 60 minutes) = .00016 oz/pulse (.0044 mL). That does not seem that far off the mark using some rough figures. That being said, the pump diaphragm does not have to move much fuel per pulse, it just has to withstand between 48 - 256 pulses per second. Reed Valves do the same thing over a larger surface area with greater deflection and are made of thicker material. By using a thicker gasket on the pump diaphragm, you could basically increase the volume of fuel being pushed into the carb and change your flow rates. Conversely a thinner gasket would reduce the volume and reduce the flow rates.
The Metering Diaphragm on the other hand may feel the pulse of the Reeds opening and closing, but I think the overall pressure differentials are softened compared to the pulse sent directly to the pump, because of the sheer volume of air passing through the barrel tempers the pressure changes. I can only liken it to a fan blade. Ever put your face in front of a fan and feel the relatively steady stream of air blowing from it. Then you Talk. Suddenly you hear your voice broken in the choppy echos as the sound waves bounce off each blade as it spins. The reality is the air is not moving at a steady rate, but rather low amplitude waves as each blade pushes more air behind the last. The closer the gaps between blades, the the more steady the flow feels. I think a similar environment exists within the carb’s barrel. It creates a relatively steady pressure to hold the metering diaphragm open for a given throttle position. The pulse effect would be even lower at peak rpm just like the echo from a fan is more noticeable at low speed compared to high speed. I imagine this is why you have adjust low speed jetting more with pop-off changes as the metering diaphragm likely bounces more at lower rpm reducing flow rate to the low speed jet.
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Quote:KartingIsLife. " I’m curious how we can say the frequency of the carb diaphragm(s) match the frequency of the pulse"
Not sure what you are getting at here James.
The pulse acting on the pump diaphragm is surely the same pulse exiting the crankcase and reaching the diaphragm via the pumping orifice or pulse pipe (according to engine type) .?
The ‘frequency’ of the pulse is the same as the engine revs and using the example from Alvin earlier in the thread, at 14000 revs/min would be 233 pulses /sec.?
I’ll split this up shortly. What I’m saying is that (IMO) we can’t just assume there is a 1:1 relationship between pulse signal and a “flap” of the diaphragm. Example (admittedly with some flaws on relative mass\inertia). I can punch a boxing bag twice a second, but it’s not going to move/resonate at that same frequency.
Thanks James. Point taken and accepted.
Agreed. A punching bag is heavy and designed to absorb blows, not deflect/distort from them. lol.
I think of it more like the skin of a drum head stretched tight. When struck, it transforms the impact into a sound wave below. It doesn’t to deflect much to move the air below, it just have to make a change. The size of the head affects the volume of air moved. Different Drums can vibrate from 60 Hz up to 1 kHz (1000 Hz/oscillation per second ). Pump diaphragms are small and only need to move a tiny amount of fuel. They are made of a similarly stiff material. Why couldn’t they vibrate at those rates?
Even if I agree with everything you say, and I do, what does it have to do with fuel flowing from a fuel tank into a carburetor? It’s a steady pressure, about 15 psi, not a hit!
There’s no such thing as suck, everything gets pushed! Not that you said there was, just trying to point out a “basic fact”!
Sticking to the KA motor but changing the subject to the clutch. Maybe this is covered somewhere but what if any maintenance is required / suggested for the clutch assembly? What is a reasonable lifespan of the clutch friction material?
I don’t think I’ve ever touched my clutch on my KA. They’re pretty much bulletproof from what I’ve seen.
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Coming from a KT100, where the clutch is the Achilles heel, that is great news!
I’m with TJ on this one. From what I have seen around the track, IAME clutches are pretty well bulletproof. All I have ever done is inspected it every few hours for wear/cracks in arms and when I do a post-race clean up, spray some brake-clean to remove any oil/waxy residue from the chain lube. I am on year three of a second hand motor/clutch with no issues. The sprocket is the weak link. Like the chain, it will wear over time. If like the 09 Leopard/X-30 sprocket is separate from clutch bell. Even then, only replaced it once shortly after getting the kart. Worn chains tear them up faster.
More Carb questions.
Based on reading my understanding was you never want to move the needles very far on the KA to make adjustments, all they need is a little tweak here or there to get mixtures right for weather conditions.
Can someone quantify how much they’re moving the needles for a common adjustment? Since the manufacturer does it in “minutes” is 5 minutes a huge swing or fairly common? are your adjustments more like 2-3 minutes per change?
I’ve always been soo afraid of going lean on the motor and hurting it that I make adjustments that are small like maybe 2 minutes and it seems like I’m often times behind on the carb for the weather that day. For instance yesterday it was 94 degrees for our final and I leaned it out a little bit but EGT’s never broke 1000 the whole race.
I go by 1/8" increments.
I wouldn’t worry too much about going too lean. These things will take a beating and probably won’t run before they lean-seize.
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You’re close on the EJT, but could go a little leaner. We shot for 1050 and that was probably conservative.
At the end of each session, download the gauge and look at the EGT in the Measures Graph. What you’re looking for is the EJT not dropping before the end of the longest straight. If it does drop, you’re too lean and you’ve had pre-detonation.
Assuming that everyone has the same pipe, same header length, same compression ratio, same fuel, same octane, same air density and such, you want the EGT to read as high as it will go.
The EGT reads fairly fast, so “as high as it will go” applies to both coming off the corner and down the straight. It will read differently in both places. Same rule, as high as it will go. With practice and experience, you may learn to turn either needle less than 1/8.
Now that I am running with a EGT instead of CHT on the KA, I am trying to figure out what to do with these readings. Al, I believe you have said before that leaning the carb will actually lower the temp while riching the mixture will increase the temp. You need fuel to have fire and more fuel equals more fire. (correct me if I’m wrong) I would imagine this has a range then it will go in the other direction?
As for adjustments I find small increments do make a difference, but minutes may be a little too small of an adjustment.
What I actually said was; lean the mixture until you get the highest reading you can. If you lean it “too much” the temperature will drop. It will drop if you get too rich also.
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As TJ said, about 1/8 of a turn max at a time. Temps we reach are typically around 1100 degrees, but I also have the heavy duty EGT probe (I’ve heard they read about 100 degrees hotter, I have both sensors and going to do a test soon). If the motor is running good, but a little on the hot side, I richen the low needle a tad to help cool the motor. I don’t go too far because I want to make sure it still comes out of the corners.
If you go too lean on the high, it will sputter down the straight when it’s not getting enough fuel.
Here are two nice vids that describes what John is saying:
The air/pulse connection provides the “power” that charges the system with fuel. The flappers and pump diaphragm cycle at the same speed as the engine. It is just a very small positive displacement pump - a diaphragm pump.
The metering diaphragm does not work off the pulse. It works off of the vacuum created when air travels through the carb into the engine. When the carb “pulls” (not technically correct as Al noted, but I will describe it this way because it is more intuitive) on the fuel circuit, the metering diaphragm says “OK…I will open up the fuel valve to give you some fuel. But the amount I open the needle will be based on how hard you pull”. So the metering diaphragm is meant to be of a smart fuel valve. I say smart in that it opens a little more when the “pull” is high…ie high RPM. It opens a little less when the “pull” is low… ie low rpm. The metering diaphragm and spring work against each other. The more pull (vacuum) the more the diaphragm pushes on the lever. But the more you push on the lever, the more the spring pushes back. Spring force F = -kx, where k is the spring constant - an equilibrium displacement is attained.
If you make the spring stiffer (ie increase your pop off) what happens? The amount the needle opens changes a little bit. For a give vacuum level “pulling” on the fuel circuit, the needle valve will now open a bit less at a given vacuum level (RPM)
Now here is where things get COMPLEX. The losses across the metering valve seat ARE PART OF THE OVERALL FUEL CIRCUIT. Here is where the real tuning aspect of pop-off enters in. What are the friction or loss characteristics of a conical seat valve (that is what the metering needle is) as you vary the (axial) distance it lifts off the seat?
https://ars.els-cdn.com/content/image/1-s2.0-S0924424714004087-gr1.sml
This is obviously not a trivial question if PhDs are researching it still in 2014…
But suffice to say that as the axial position of the needle is changed, the friction across the seat is not going to be linear. What this means is that if you change your pop-off, you will have to adjust your HS needle a bit to compensate, but you will also alter the overall fuel curve as RPM changes. If you are richening up as RPM rises, and pop-off change in conjunction with a jet adjustment may give you a fuel curve vs RPM that is better (or worse) matched. This assumes the losses across the metering valve seat are significant relative to the whole fuel circuit losses. That is where pop off tuning comes into play.
How would you test this? OK…say you are running 9 psi pop off. Use a Lambda meter. Establish a base line “good tune”. Note what the Lambda is at 13000 RPM, and then at 15000 RPM. Then change the pop off to 12 psi. Now, ADJUST your HS needle till you have THE SAME Lambda (as when you were at 9 psi pop off using your base line tune) at 13000 RPM. Now look at what the Lambda is at 15000 RPM. It should be different (leaner) than when you were at 9 psi pop off. Now you will know how adjusting your pop off (with a compensating needle adjustment to return you to a base reference point) impacts the fuel curve vs RPM.
Note that on a 2 stroke, Lambda testing can be tricky. At lower RPMs, the pressure waves are coming back too soon (speed of sound does not change as the motor runs at different RPMs), resulting in hot burned gasses getting rammed into the cylinder. This messes up Lambda readings. So this will likely only be effective as a measurement tool in the higher RPM ranges on a two stroke. There is also the issue of fouling the probe due to oil in the fuel.
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Wondering what others are finding with changing gear ratios. Coming from running Yamaha’s, 1 tooth would make a notable change in max RPM (300 - 400 RPM), however with the KA, 1 tooth doesn’t seem to make much difference and really have to change 3 or 4 teeth to see that much change. Are others seeing this?