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Pop off test
- Thread starter Eikari
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You are in the right forum.
Someone with specific knowledge will get to you shortly I would think.
Someone with specific knowledge will get to you shortly I would think.
I am no expert, but, the needle and seat must be wet to get the correct reading, some say WD40, some say no and to use the oil you use for the engine, I have tried both and they both seem to work, no wet, low pop off pressure. You did not specify how you are testing, that is important for the ones that know for sure what to do.
My 787 was at 28 psi if I remember correctly. If you can't build any pressure, your needle and seat may be leaking and need to be replaced. Also as mentioned earlier, spray a bit of WD40 into the needle and seat to moisten it then do the test.
kicker
Well-Known Member
As mentioned you need to use oil while doing the test. Where the carbs cleaned and rebuild kits installed? Are both carbs giving the same reading?
In the event that you did use oil, when it pops at 10 psi, does it give a pop or does it leak a bit of air first? To check for a leak around the N/S just look for bubbles coming up from the oil around the N/S .
What color is the spring, was there any corrosion? Did the lever get bent out shape?
I believe that you dont have an accelerator pump, so the factory spec would be the black 80 gram spring. It is possible that the spring is weak so you might have to replace it, or try the next size up, silver 95 gram spring. This is based on a 1.5 needle/seat which is stock for the 787.
If the above does not give a good pop between 25-42 psi then your needle and seat might be the problem.
Factory Specs for 787
MAIN jet-142.5
PILOT jet -70
N/S 1.5
Pop off 23 - 43
LOW speed - 1
HIGH speed - 0
In the event that you did use oil, when it pops at 10 psi, does it give a pop or does it leak a bit of air first? To check for a leak around the N/S just look for bubbles coming up from the oil around the N/S .
What color is the spring, was there any corrosion? Did the lever get bent out shape?
I believe that you dont have an accelerator pump, so the factory spec would be the black 80 gram spring. It is possible that the spring is weak so you might have to replace it, or try the next size up, silver 95 gram spring. This is based on a 1.5 needle/seat which is stock for the 787.
If the above does not give a good pop between 25-42 psi then your needle and seat might be the problem.
Factory Specs for 787
MAIN jet-142.5
PILOT jet -70
N/S 1.5
Pop off 23 - 43
LOW speed - 1
HIGH speed - 0
I'm not saying your doing your pop off test wrong, but you aren't listing much info on what you are doing. You said you can't seem to get anything above 10 psi. Are you sure you are connecting your air pump to the right nipple. There are two, one is fuel supply, the other is your return line. Make sure you are using the fuel inlet line (most carbs are marked with arrows to show their direction). Next, when you are doing the pop off test, the diaphragm and cover are removed so you can visually see the needle valve. It doesn't matter a lot of what type oil you use, as long as you are keeping the needle valve seat wet. I use the regular 2 stroke fuel, with oil in it (I get from my weed-eater). When you are ready, have your airline and gage attached, pout your fuel into the diaphragm/metering plate area, to fill the needle valve hole. Then, take the outer plate and just lay it across the carb so that when the needle pops, you don't get splashed with oil. Then, when you go to pressure it, do it fairly quick. Adding pressure slowly, will give you an inaccurate reading. Watch your gage and apply air fairly fast. Then, with a quiet shop, listen real well and you'll hear the "pop" of the needle valve. Note the pressure it popped. Then, do it a couple more times. You'll want the average.
Why do you think the pop off is wrong/bad? Though these motors have a fuel pump, they are not pumping fuel to the motor like a car engine. They are a pulse line pump that is only circulating fuel to make sure the engine has fuel available to "suck" it into the motor from the carb. When you turn the motor over, a vacuum is built in your intake, which in turn pulls on your diaphragm, which pushes down on your needle valve to open and allow fuel to come in. If you turn the motor over and over, with no start, then your pop off could be too high (usually no start, or it will start then die if you let it go back to idle). If you are turning the engine over and you flood it, or fuel dumps out the carb, then your pop off is set way to low. There is a range on the pop off for the 787 (and all others) for the pop off. I generally like to set mine around 24 psi, so I don't have to build much vacuum to get a start.
Why do you think the pop off is wrong/bad? Though these motors have a fuel pump, they are not pumping fuel to the motor like a car engine. They are a pulse line pump that is only circulating fuel to make sure the engine has fuel available to "suck" it into the motor from the carb. When you turn the motor over, a vacuum is built in your intake, which in turn pulls on your diaphragm, which pushes down on your needle valve to open and allow fuel to come in. If you turn the motor over and over, with no start, then your pop off could be too high (usually no start, or it will start then die if you let it go back to idle). If you are turning the engine over and you flood it, or fuel dumps out the carb, then your pop off is set way to low. There is a range on the pop off for the 787 (and all others) for the pop off. I generally like to set mine around 24 psi, so I don't have to build much vacuum to get a start.
Sportster-2001-951C-Stock
Well-Known Member
kicker's info is correct-o-mundo. Although 10psi of pop is adequate for many 2-stroke engines, in your specific case it's not to spec thus will likely make the low speed operation richer than it needs to be unless perhaps you're running an aftermarket flame arrestor and/or modified intake tract/carbs of some sort.
All else factory equal, factory spec is the target to shoot for.
Yours is in this list:
http://www.seadoosource.com/carbreference.html
Spring: 80gr black p/n Mikuni 730-03033, BRP 270500267
Needle/seat orifice: size 1.5 Mikuni 786-35015-1.5, BRP 270500241
Double check my work with your factory manual, to be sure.
All else factory equal, factory spec is the target to shoot for.
Yours is in this list:
http://www.seadoosource.com/carbreference.html
Spring: 80gr black p/n Mikuni 730-03033, BRP 270500267
Needle/seat orifice: size 1.5 Mikuni 786-35015-1.5, BRP 270500241
Double check my work with your factory manual, to be sure.
The idea wasn't to say "kickers" info was wrong. It was to double check that he is performing the test right. I've done this test a 1000 times, and with customers present, who inform me that they were trying to do it with the return line, not the inlet line.
Therefore, the idea that he still needs to check his procedure is valid.
Your best bet is to go to the Mikuni website and download the free Carb Shop manual. All the info is there for you to learn first hand, how to do this test correctly. The manual is free.
Sportster-2001-951C-Stock
Well-Known Member
Yeah, I know you were asking logical questions and I didn't see your response till I hit "send", I think you and I were answering at the same time and your answer was submitted first!
Good suggestions, as always.
Thanks for all the info. I didnt wet the needle / seat so i´ll try that. WD 40 should do it, or premix fuel.
I pumped through fuel inlet and blocked the return nipple. It is easier on PTO carb because it just has 2 nipples. The MAG carb has 3 nipples, there is fuel inlet close to idle speed screw and then 2 nipples on side of it. Where do i pump and block of these nipples?
The springs are silver, not sure about the weight of them.
Thanks in advance
I pumped through fuel inlet and blocked the return nipple. It is easier on PTO carb because it just has 2 nipples. The MAG carb has 3 nipples, there is fuel inlet close to idle speed screw and then 2 nipples on side of it. Where do i pump and block of these nipples?
The springs are silver, not sure about the weight of them.
Thanks in advance
Sportster-2001-951C-Stock
Well-Known Member
This selection chart is intended to help you select the components for your calibration:
http://www.seadooforum.com/attachment.php?attachmentid=23367&d=1389118366
This came from the Mikuni SBN manual.
http://www.mikuni.com/pdf/sbn_manual.pdf
BTW, if your spring(s) were silver, are they original? there are two silver spirngs of different rates, those may be the 65 gram spring with a 2.5 needle valve seat orifice(this combination makes 12psi pop-off, see chart). I believe your ski should have the 80 gram spring with the 1.5 orifice seat. In case of lighter springs and larger seats, this will tend to richen the calibration, especially larger seats flow more fuel in the top end to fatten the top as well, not just the bottom, but the larger seat doesn't give the repeatable results like a smaller seat does, so it's best to use the smallest possible seat for the particular application instead of large seat and heavy spring use small seat and lighter spring.
Yeah, use some type of lubricant and perform the test with the metering diaphragm removed. I think pre-mix is a good choice myself, if you can stand the odor, else a light machine oil or even WD-40 fish oil b/c it's a watercraft! Myth: WD-40 contains fish oil!
http://www.seadooforum.com/attachment.php?attachmentid=23367&d=1389118366
This came from the Mikuni SBN manual.
http://www.mikuni.com/pdf/sbn_manual.pdf
BTW, if your spring(s) were silver, are they original? there are two silver spirngs of different rates, those may be the 65 gram spring with a 2.5 needle valve seat orifice(this combination makes 12psi pop-off, see chart). I believe your ski should have the 80 gram spring with the 1.5 orifice seat. In case of lighter springs and larger seats, this will tend to richen the calibration, especially larger seats flow more fuel in the top end to fatten the top as well, not just the bottom, but the larger seat doesn't give the repeatable results like a smaller seat does, so it's best to use the smallest possible seat for the particular application instead of large seat and heavy spring use small seat and lighter spring.
Yeah, use some type of lubricant and perform the test with the metering diaphragm removed. I think pre-mix is a good choice myself, if you can stand the odor, else a light machine oil or even WD-40 fish oil b/c it's a watercraft! Myth: WD-40 contains fish oil!
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Sportster-2001-951C-Stock
Well-Known Member
27 is maybe a little low, perhaps that arm is slightly higher than the other(less spring tension)? That silver 65gr spring should get you very near a 32psi pop, 31psi I'd call close.
I think those carbs originally came with black 80gr springs (not really sure about that), but if no accelerator pump then the lower pop might be okay (to get fuel earlier).
I think those carbs originally came with black 80gr springs (not really sure about that), but if no accelerator pump then the lower pop might be okay (to get fuel earlier).
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