Pop off question

[Kevin Pitta]

Anyone have a link to discussions on how and why you adjust pop off? I'm a newbie and trying to learn as much as I can so any info would be great. I have a parilla Leopard.

Thanks.

[al nunley]

You’re going to hear/read a lot about pop-off. My thinking is it makes so little difference you can ignore it for the most part. Like the weed eater, you need some, and as long as you have some, that’s enough. The weed eater needs some, because with most, the fuel tank is above the carb, and if there was no needle sealing the fuel source the engine would fill with fuel when not running. Some karts have the fuel level, when the tank is full, above the carb. No pop-off would mean a flooded engine.

Many disagree with me. I look at it like this; for pop-off to make any difference while the engine is running, the inlet needle would have to be opening and close on each cycle. I don’t believe this is possible. First off, if it did open and close on each cycle, the fuel supply would start and stop. How could the engine run with no, or an interrupted supply, of fuel? Second; I don’t think that needle could possible move that fast. Opening and closing at 7,000-18,000 time a minute?? I don’t think it’s possible!!

I believe you need some, (10 psi would be some) and more or less would do nothing.

As long as there is a vacuum in the engine crank case, the metering diaphragm is going to be pushed down by the ambient air pressure and this is going the hold the needle off the seat. If that is the case, and many will disagree with this, how could pop-off make any difference?

I’ve seen people set the pop-off to a very wide range. I’ve seen 3psi to more than my gauge could measure. Both ends of the spectrum were carbs built by highly respected National Champions.
_________________
If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting

[Kevin Pitta]

Thanks for the reply Al. The engine runs great so maybe next time I'm at the track I'll have the guys in the shop check it out for me. I have so much more to concentrate on right now like driving and general maintenance that I can worry about that later. Although I will probably get a chacker and learn how to use it eventually.

[Pete Muller]

Kevin,

Pop-off pressure changes how the carb works in transition (off/on the throttle) as well as a possible small effect on how the carb meters through the rpm range.

The reason is because spring pressure works "against" the diaphragm. Increase the spring pressure, and the diaphragm has to deform "more" until the inlet needle is lifted off its seat. Very generally speaking; the jets need to be open a touch more with higher popoff pressure to get to the same fuel flow.

Is higher or lower popoff better? Quite probably neither, though there are subtle differences when it's changed, and that's simply another tuning item (just like fulcrum arm height).

Pete

[Kevin Pitta]

So if the engine transitions fine and runs great is this something I don't need to be concerned about right now? The engine and carb are blueprinted, and I just got a Micron 4 2T so I can tune by egt, so I think I'm on the right track. Is this something that changes as the diaphram wears out? If so, how often do I need to check it and possibly replace parts? Thanks for the replies guys, I love learning about new stuff.

[al nunley]

One time I tried to touch the fulcrum arm and see if I could touch it light enough to not pop the needle. I couldn't. Ten pounds of pop-off pressure is easily over come by just a tiny amount of pressure on the fulcrum arm. In fact, as I see it, any pressure at all will pop the needle off the seat.

I use to bend the fulcrum arm tangs, one up and one down. Don't know if this did anything, but who knows. You know the fulcrum arm doesn't pull the needle off the set with a normal setting. The pressure of the fuel pump pushes the needle off the seat as soon as the fulcrum arm moves up even the slightest. Only when the needle gets stuck, from dried fuel and oil, does the fulcrum arm pull the needle off the seat. That's why it's captured. Chain saws and weed blowers sometimes sit around for weeks or months and the fuel dry’s out and the oil can stick the needle in the seat. The Mcs didn't have the forks and would get stuck all the time if you let them sit around for a long time.
_________________
If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting

[Pete Muller]

[Oscar Aguilera]

i agree with pete for the record he is the man. al might just be a little bit right?

What i see is that when a motor builder sets up a carb and tells you where he likes the needles set, it usually is the best way for that motor builders complete package.


i also found that different set ups made for different driving karts off the corner or down the straight.

my motor builder at the time ran higher pressure, a very open low speed needle and a very closed high speed needle. i did what he said, it worked for me.


what i would see when the pop off fell in pressure was the kart liked different settings. i would close the low speed and open the high speed needle more to compensate for the pump pressure.



so IMO just keep the pop off consistent and it will make tuning easier as you will get accustomed as to where your carb likes to be set.


IMO there is an art to set up on a carb, but it also goes with how the motor is built with port timing and that other motor builder stuff i really dont care to fully understand.

i always do what the motor builder says, i know the dyno will out perform me for testing every time. if i have alot of problems i know i need a new motor builder


i hope i added to what the other 2 guys said.

just my opinion

[al nunley]

[Steve O'Hara]

This thread needs some perspective. If you are just looking for mediocre performance that won't demand much attention go with Al's advice and pick a middle of the range setting and forget about it.
If you are a serious competitor and expect to spend any time at the sharp end of the grid in a national caliber event you will need to know how to adjust popoff and what effects changes make to your particular engine package and racing environment.
I have a trophy cabinet full of Duffy's and not a single one could have been earned with 10 pounds of popoff in my carb. In fact, just finishing the races would have been a challenge with that setting. The Walbro can be run with as little as 5 and as much as 20psi and still be competative in the hands of a tuner who knows what he is doing and has chosen his setting to compliment the other elements of his setup. However, either of those extremes can cause the novice tuner problems and a middle of the road setting is a better choice for ease of use and average performance.
Other carbs are much more sensative, for example the Mikuni we used on the TKM reed motors back in the 200cc Pro Kart era had to be run in a very narrow window of 5 to 7 psi. 10 psi in that carb and the motor simply would not start or run at low rpm. In the Mac engines, 10 psi would simply not provide enough control of the fuel to allow clean performance unless the duckbill circuit was eliminated and I used anywhere from 16psi to 28psi depending on the carb model and engine.
Saying popoff is "not important" when talking about a two cycle pump type carb is the equivelent to saying float level is not important in a Webber or Holley... it's just wrong.
Steve O'Hara

[JIM SILVERHEELS]

The article in the NKN issue of July 08 says that the pop off need only be enough to shut the fuel off as a float in a auto carb and enough to keep the metering cavity full of fuel. If the low press in the venturi regulates the flow through the needles and not injected by the carb pump, then why should it matter where the pop off is as long as the metering chamber is full of fuel? just going by what the article says
_________________
Don't get stuck in someone else's discarded chewing gum with your thought process.

[Steve O'Hara]

Jim,

The demand of racing engines are dynamic and change through the power band due to pipe configuration, port timing, ignition timing, etc. In addition, the output of the built in fuel pump varies over the rpm range so the conclusion drawn in the article you cited simply fails to recognise that the factors that determine the amount of fuel delivered change through the range of rpm the engines run.
Fuel moves from the wet side chamber into the throttle bore when there is a delta between the atmospheric pressure on the dry side of the diaphragm and the pressure in the throttle bore. That delta is not a constant for the duration of the compression stroke of the motor, rather if graphed it would apprear as a bell shaped curve for some motors and a much different shape for others. For example, a reed valve motor allows the intake tract to respond to a drop in case pressure limited only by the resistance of the reeds to open while a rotary valve motor or piston port design are mechanically controlled. The change in shape of the delta curve influences the length and intensity of the movement of the fuel through the jets and that would be the end of the story if we were dealing with a gravity fed system with an ample reserve of fuel available but we are not. The fuel pump adds another level of complexity to the system since it also is subject to variation in performance over a range of speeds and to make matters more interesting, the pump is out of phase with the movement of fuel from the reservior to the throttle bore. I'll explain... the fuel pump does not provide a constant pressure rather a series of pulses where the positive spikes occur when the piston moves down the bore and pressurizes the case. The movement of fuel from the reservior occurs when the piston moves up the bore and causes air to accelerate through the throttle bore thus you have a system where the reservior is drawn down and then replenished in alternating phases of the engine cycle. When the alternating events produce offsetting addition and subtraction of fuel from the reservior, the diaphragm and inlet needle will remain in a relative constant position, however, as the engine speed increases or decreases the strength of the signal at the jets and the output pressure of the pump change and the inlet needle position has to change to maintain the balance between the incoming and outgoing fuel.
The movement of the diaphragm and the inlet needle are restricted by the spring tension that controls the fulcrum arm. At one extreme, if that spring tension is too low, the pump can overpower the inlet needle and will add more to the reservior each cycle than the engine can use and the mixture will become overly rich unless the jets are screwed in. If the jets are screwed in to control the mixture at a speed where the pump is overpowering the needle and the engine speed changes to a range where the pump output is lower, the engine will go into a lean condition. The analysis is similar for a spring setting that is too high but instead of the pump overpowering the needle at peak output we see situations where the pump output falls too low to keep up unless we open the jets way up.
The old timers on this board will remember the days of running Macs on alky and having the jets fall out of the carbs while running on the track. That was simply a case of having to open them so far to compesate for a weak pump that they came loose and fell out. Those same folks will recall that we had to have a hole in the air filters to stick our fingers in the carb and shut off the main fuel discharge otherwise the engine would flood at low speeds. The Mac case is just an extreme example of the same conditions that occur in all of the modern carbs to a much lesser degree.
Most people on this board are likely to be interested in the Walbro as used on the KT100 and HPV motors. In my enduro racing days when slippy pipes were the norm my engine package performed best with the popoff set at 6psi. When I run my KT100 sprinter I find that I have best results with the popoff at 16 to 18 psi and I run my jets much different than most. My jets always end up with the HS needle at slightly more than 1/2 turn and the low speed will be in the range of 1.75 to 2 turns. Those jets settings will simply not work with popoff pressure of 10 psi. The reason is simple... opening the HS jet more creates a greater pressure delta on the diaphragm and that offsets the stronger spring tension.
Hope this helps you get a better idea of what's going on and why popoff does make a difference.
Regards,
Steve O'Hara

[JIM SILVERHEELS]

Thanks Steve,you have spent endless hrs as a guru researching how these carbs and motors work. Everytime I read your posts I realize how much I have to learn. thanks again for taking the time to respond.
_________________
Don't get stuck in someone else's discarded chewing gum with your thought process.

[Steve O'Hara]

Jim,

Thanks for the kind words. Frankly, I wish I were a more skilled technical writer. The subject of carbs is very tricky to express in writing and I never feel as though I have really covered it well. I do a lot better when I have one in my hands and can talk and point at the carb at the same time.
Regards,
Steve O'Hara

[Oscar Aguilera]

well im not even going to begin to act like i could answer like steve but....YEAH what HE said!

i know the motor stuff (timing, pipe, port heights) effect the carb set up for the motor. and i know that the motor builder had an idea of where to start. after that i did read my Digitron EGT and turn the needles as i raced. When my motor builder was there he usually set it up for me and i concentrated on driving. i seldomly had to mess with the needles.


it does make a difference from carb to carb and from motor to motor like steve said.


I am sorry i assumed the person asking he question was new since he admitted he needed to learn more.

you also said you drive a leopard. since it is a leopard the radically different motor set ups go away since it is a spec class. not as much variance from one motor to the next in way of building it.

the fulcrum arm set up will dictate the carb set up. if your kart is running good now then get a pop off reading and try to keep the pop off near where it is.


and back in the day al i didnt have a density guage nor did i need one. i could reach over and just lean it! if i was way off on the needles then the pop off had fallen or my chassis was bound up!

now adays i tune with adj alt and air density but with the shifter stuff.


like pete said, keep it clean and be anal about it so you dont get debris in the carb


I sitll like you al hope i didnt ruffle your feathers by not agreeing completely,

but its just my opinion

steve has done more clutch racing than me and at a higher level i presume. i ran rotarys, reeds, and kt100 in clutch and direct drive versions. i even ran the 3 needle ibea slide stuff. these carbs havent changed any over the years and EC Birt was an artist at what he did. my motor builder was no slouch either. i never got to participate at the highest end of clutch stuff other than the rotax and leopard of today.