10-02-2012, 08:40 AM #1
- Join Date
- Mar 2012
'99 Kawa 1100 STX setting pop off
First time I will be setting pop off. I did rebuild the carbs months ago but I have a hard warm start issue. My research tells me that pop off could be the culprit. When I rebuilt the carbs I knew nothing of pop off, so the springs are gone and I put into the carbs what I put into the carbs.
my question what is the pop off pressure for a stock 1100 stx with Keihin Triples? I read one article here that said 17psi.
I am planning on using a hacked up bike pump with it's own prssure gauge. I will fill the carb with gas or wd-40. then I will cut or stretch the needle springs until the needle opens at 17 psi. each carb is to be tested individualy and since I cant dissconnect the lines I will hold down the other two carb's needls not being tested with my fingers.
does it sound like I'm on the right track?
If not, setting me staright will be appricated.
10-05-2012, 04:52 PM #2
from copy and paste, i did not write it, i think i ended up @ 28 psi.
Pop-off is the measurement of the point where atmosphereic pressure overcomes the fuel draw vaccum of the carb. The entire regulator chamber is nothing more than a big check valve. It is the step between the fuel pump, which is providing more pressure than the carb needs to deliver, and the jets, which meter a fixed amount, within a certain pressure range. The regulator chamber is in place to maintain that pressure range.
The whole regulator chamber gets filled with fuel. The fuel is coming from the pump, sits under the diaphram, and is flowing through the jets. If it was just an open flow, with no needle and seat, the jets would be supplied with too much pressure from the pump. As the fuel is sucked out the jets, the fuel supply in thereis drawn out, and the diaphram goes down with it. As it gets to it's low point, it contacts the lever arm, and lifts the needle from the seat. That allows more fuel into the chamber, pushing the diaphram back up, re-seating the needle and shutting off the flow from the pumps, so the jets aren't over-pressured. This happens many, many times every minute as fuel is consumed.
If the pop-off point is set too low, there isn't enough resistance on the needle to stop the fuel pump flow, and the jets are over-pressurized, giving you a rich condition you will never tune out. The same goes for leaking needles.
If the pop-off is too high, there isn't enough fuel pressure to overcome the vaccuum and spring pressure, and you will have hard starts, as the engine is not spinnging fast enough to produce enough pulse, to make the pump supply enough pressure to flow the fuel into the regulator chamber.
It is adjusted by changing the size of the N&S, and the spring tension. There are 4 general strengths of springs - the less the spring is rated the lower the pop-off pressure will need to be to unseat the needle. For example, a 65 gram spring may give you a pressure of 15 psi, where a 115 gram spring may give you 50 (not actual numbers).
You test the pressure with a pop-off pressure gauge. With the regulator chamber open, wet the N&S with WD-40 or gas. With the pump body removed, attatch the gauge to the fuel inlet fitting on the body. Pump the gauge until it 'pops' noting the pressure gauge. Repeat this 3 times to get a nominal reading. Installing a heavier spring will raise the pressure. To adjust in small increments lower, cut coils 1/2 coil maximum and re-test.
NOTE: You cannot accurately test with the pump body and fuel filter still attatched to the carb - you MUST disassemble it and test directly into the fuel inlet orifice, where the fuel filter sets. Otherwise you are testing through the pump checkvalves, throwing your readings off, too high. If you have an external fuel pump, this does not apply.
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