Thread: Pop-off pressures
02-25-2010, 10:17 PM #1
This is a subject that newbies probably have no clue about. Usually one would buy n fly around the lake, and never give it a thought. what is it? where is it? How much is enough? Does jetting play a role in it? what else does? Whats the best tool? and how do you actually do this procedure? Does it take long? How many time a year should you check it? once/? 30 times? how often? Where do you hook up the tool to check? are there high ,low tolerences or does it have to be bullseye? how do you figure out how much is enough for a given ski? Well thats a start! lol
Can anyone describe this in human terms? So I can do it! lol A pic of the tool and or adapter would be nice too. I thinks it's a mity vac correct? Thats about all I know. I know each ski has its own pop-off pressure.
What happens if its too low or too high?
02-25-2010, 10:39 PM #2
- Join Date
- Jul 2007
- near Toronto, Canada
Have you looked in the Tech section?
02-25-2010, 10:40 PM #3
I'll take a look and see If I can make heads or tails of it.
02-25-2010, 10:56 PM #4
what is it?
It is the point where the diaphragm in the carb depresses the needle arm, causing the needle to release from the seat and allow fuel to flow into the carb.
where is it?
Fuel side of the carb
How much is enough?
What ever the exact tolerance is for the ski you have. This can vary if you have a heavily modified ski.
Does jetting play a role in it?
what else does?
Needle, seat, and pop-off spring. These are usually matched like a 2.0 needle and seat with a 95 gram (gold) spring. That's the stock combo for my ski that gives right about 23PSI pop-off pressure.
Whats the best tool? I thinks it's a mity vac correct?
A pop-off pump is the best tool. Randy has the pump you need right here:
How many time a year should you check it? how often?
IT's kinda like RonCo "set it and forget it". Unless your ski is a mod monster, once you have the carbs set you can pretty much set it once and be done. Now, if the ski takes a ton of tuning, you may need to play around with needles and spring combos until you find the sweet spot.
are there high ,low tolerences or does it have to be bullseye? how do you figure out how much is enough for a given ski?
The manual for your ski will tell you the values for your engine and carb combo. For example, my '96 SLT780 is suppose to be between 22 - 24PSI. If the pressure is too high the needle will open too late thus starving the engine for fuel upon acceleration. If it's too low, the fuel comes in too soon causing a rich condition.
and how do you actually do this procedure? Does it take long? Where do you hook up the tool to check? Well thats a start! lol
It doesn't take long at all. Remove the diaphragm from the carb, cap off the return fuel hose, spray some WD40 into the fuel inlet on the carb, then attach the pump. Pump the hand lever watching the gauge the entire time. Pressure will continually build until you hear a spurt sound and the needle on the gauge drops. The max reading you see on the gauge is your pop-off pressure. Perform this in triplicate to ensure you get consistent readings. Make sure the needle arm is set correctly (level with carb body) and reinstall diaphragm. Test pop-off again to make sure diaphragm isn't pushing on the arm causing pop-off to be lowered.
I know my explanation here is a bit simplistic, but it will give you some basic info to chew on. The link below is to a PDF file that is the official Mikuni SBN manual. I found this really helpful as I was trying to learn and grasp all the info on these carbs. I think your ski has Mikuni's.
Also, here is some info from Group K on carb tuning.
02-25-2010, 11:01 PM #5
In the tech section, theres only 2 threads on how to build a tester// but I see John has got the info !! Thank you!
Nice info. I actually understand now how it works/ thanks. it's kinda a off beat subject.. But I see it's very important..
02-26-2010, 12:08 AM #6
Here's what group K says. Whats your opinion on this procedure? You said you had 23psi pop-off, they say at least 25 no more than 35. Why is there a 10 psi difference to play with? Is it the different springs/ why 1 over the other? I mean why would you want a little higher pop-off pressure? or lower for that matter but still within the 10 psi range? I thought a spec for a certain ski was....25 or 30 or..whatever it is?
Pop off pressure - This term refers to the amount of fuel pressure needed to push the float needle valve away from it's sealing seat. Pop off pressure is checked with a hand pump that is fitted with an in line gage. The pump is connected to the fuel input fitting of the carb. The return line fitting is then sealed off with one finger while the pump pressurizes the float chamber. The pressure reached on the gauge when the needle gives way is called the pop off pressure. "Adjusting" the pop off pressure is discussed below. If you don't have a pop off pressure gauge, you should get one that has a gauge and pump capable of 30 psi. (Most Mikuni distributors carry them) At the beginning, it's only important to check that the needle holds the pressure back with no leaking up to the point where it pops cleanly away from the seat. Perform the pop off test several times to confirm the actual pop off pressure. Initial pop off pressures on round pump carbs should be between 8 - 12 psi. Initial pop off pressures on the square pump Mikuni and Kiehin carbs (unless otherwise specified) should be no less than 25 psi and no greater than 35 psi.
How do you adjust it? springs?
02-26-2010, 12:29 AM #7
The transition range of the new generation carbs is adjusted by changing the pop off pressure. These pop off pressure adjustments are made by way of various combinations of needle/seat sizes and float arm spring tensions.
In short terms, the transition circuit is richened by reducing pop off pressure with larger needle/seat sizes, as well as shorter or weaker tension float arm springs.
So these adjustments in pop-off pressure dictate how the mid-range power band of the ski acts.(richer or learner by changing springs and seats). And retesting pop-off again till it's spot on.
Back to Group K.....
The various combinations of larger needle valves and weaker springs result in a wide range of lower pop off pressures. Measuring the pop off pressure is how you determine whether you have made the transition circuit richer or leaner (higher pressures are leaner, lower pressures are richer) Most racing engines prefer very low pop off pressures because they demand so much fuel in the mid range. Unfortunately these low pop off pressures can often create an undesired (and nonadjustable) rich condition in the 0 - 30% range. This nonadjustable rich condition takes place at about 9 psi on the Mikuni square pump carb and about 13 psi on the Keihin. Running pop of pressures this low is not recommended. Ideally, you want to run the highest possible pop off pressure that permits "hesitation free" mid range throttle response.
So..... On your average Pro785, what would be the average? Guess it depends on everything.
Could you all name your pop-off pressures and and combos in your seat and needle configs just for comparison?
So many of these Pros are Modded. So everyones will be different.
Ok take a poll. who really has done thier transition range needle and seat checks? Who just rides and has never checked? lol
Honestly myself.. if the ski ran, I wouldn't of checked it. ( I bet after I bought another motor I would)!.lol
02-26-2010, 12:42 AM #8
- Join Date
- Jul 2007
- near Toronto, Canada
Here is something to ponder;
Atmospheric pressure is less than 15PSI (often stated as 14.7PSI at sea level). That means that if there is a total vacuum on one side (zero PSI), the outside air presses against it with about 14.7 pounds of force per square inch.
We know the Mikuni pulse fuel pumps put out about 6PSI at high engine output RPM, and often closer to 4 PSI (that is, 4-6PSI above the 14.7 PSI atmospheric pressure).
When you have a carb with a pop-off pressure of 25PSI, where on a running engine is the pressure/suction coming from to create a 25 PSI differential?
02-26-2010, 01:27 AM #9
the restricter in the return line or in the carb..... creates back pressure for the carb so it doesn't flood out. when it hits 25 it lets go.(the pop-off). and lets fuel in. Heck I don't know! Just a guess.. well then again too, the pulse (the vacumn from the crankcase )being turned over creates negative and positve pressures to suck fuel in the cylinders and creates a pressure pulse to the diaphram to the carbs. Did I get any of it right?/lol
02-26-2010, 08:53 AM #10
What I have been struggling with is: How do you test pop off pressure on each individual carb when they are linked together with a common fuel rail? In my manual it says when you are checking pop off pressure you block the fuel return line for each carb and check them individually, if they are checked without blocking the fuel return lines between the carbs, the results will only show the lowest pop off of all three carbs, in other words there is no way to tell if all three carbs pop off. I'm sure someone has run into this before.
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