Carbs from hell!

Status
Not open for further replies.
H

Hoiyay

New Member
I can't get these carbs to pop off right at all.

1996 seadoo GTI.

Problem is, the needle and seat won't seal up right. I rebuilt them once before with SBT kits. When setting the pop off pressure, it always leaked just a little before the pop off pressure would hit. I didn't think it was a big deal because it was just a little and only a few psi before target pop off. Threw them back on the ski and its got a no start now...I'm assuming the engine is flooding because the needle/seats are leaking.

Came here and did more research and bought genuine mikuni kits. They do NOT come with the needle/seat like some people say. So I bought the needle and seats and the springs from SBT.

I bought 1.2, 1.5, 2.0 needle seats as well as all 4 types of springs. Tired of waiting on shipping.

I have literally installed every single needle/seat into the carb with at least 95-115g springs...and they STILL leak right out of the box. My lever arm is even with the chamber. I fill the seat with WD40 before I even put the needle in. I then fill the ENTIRE chamber with wd-40. And it STILL leaks with like 5psi. I have to remove the entire needle (removing the lever too) and literally just try reinstalling it about 4-5 times each before it won't leak below 10psi. Yet, it always leaks a bit before target pop off.

WTF. Are the SBT needle/seats really just that terrible garbage? Am I doing something wrong? Where can I get better quality needle/seats?
 
OEM Mikuni carb parts are necessary. I'm not sure why you ordered 3 different types of needle seats either, you should use the one that is made for your particular model.
 
I ordered the one that was for my ski. And orders others to just make sure I have whatever I needed. When shipping is taking 3-4 weeks...then its worth it. I was reading you can mix and match seats/springs if necessary.

So where do you buy mikuni seats from?
 
Pwnage1337 said:
Osd Marine has them and has awesome service
Click to expand...
Cool.

I hate websites that don't have a phone number. Normally not an issue, but I need to know if they are shipping within 2 days or 3 weeks. And no way to contact them.

If I order through a dealership...will it be aftermarket or mikuniMl?
 
Yes, the SBT carb parts are junk. That link Miki sent you is an OEM part, so you can go that route if you want to have Amazon customer service. Honestly though, OSD is super responsive, and I get parts from him way faster than Amazon here lately...
 
Double check with OSD, the last time I spoke with him he was out of needles and seats and there is a shortage for some reason....
 
Hoiyay said:
I ordered the one that was for my ski. And orders others to just make sure I have whatever I needed. When shipping is taking 3-4 weeks...then its worth it. I was reading you can mix and match seats/springs if necessary.

So where do you buy mikuni seats from?
Click to expand...
That is a very interesting statement...." mix and match seats/springs".....I have NEVER READ that and I don't believe you can/should mix & match......EACH carb has a specific Needle & Seat size and a Spring strength.

A '96 GTI twin carb probably used the 80 gram springs (this info comes from a very reliable source).
 
Correct, don't mix and match springs to get pop-off, just use the springs and seat size that your ski calls for in the parts diagram.
 
mikidymac said:
Correct, don't mix and match springs to get pop-off, just use the springs and seat size that your ski calls for in the parts diagram.
Click to expand...


Originally, I did do that (with aftermarket). And my pop off was pretty damn high...like 80psi. That was the few times I could actually get the needle to properly seal/seat. Tried lower springs and they were really low, like 10psi lower than spec. Was going to try changing from a 1.2 to a 1.5 with original spring to see if that would help. Seems like it did, but like I said, the needle/seats were leaking most of the time.

I may not understand enough about these pop off carbs, but I don't see why it would make that much of a difference. From what I understand its just a way to allow fuel to flow into the metering chamber. The pop off pressure is what allows fuel into the chamber, a larger needle would just allow it to so faster.

I'm not arguing, just saying how I think it works. Maybe somebody smarter can chime in explaining how it works better.
 
I did call OSD and they are out of a lot of mikuni stuff. I did get some needles from amazon..hopefully here tomorrow.
 
Seat and spring size make a big difference and makes the entire carb settings richer or leaner. Think of it like a master valve supplying different fuel amounts to the other jets and screws.

Pop Off Pressure - The pop off pressure is not an adjustment that exists on the carb, but rather a specification that is a combined function of the needle valve size, and the spring rate of the float arm spring. This specification has a wholesale effect on the fuel metering in the 0 - 40% fuel range. Because the pop off pressure has this far reaching range that overlaps with several other metering ranges, we consider it a fundamental starting point.

The term "pop off pressure" 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 the Mikuni and Kiehin carbs (unless otherwise specified) should be no less than 25 psi and no greater than 35 psi.

Needle Valve and Seat - The popular size Mikuni needle valves are 1.5, 2.0, 2.3, and 2.5 (these numbers designate the orifice size in mm. Some stock boats come with other sizes as well: 1.2 in the GP 1200, 1.8 in the 950 Sea Doo. The larger seat diameters will yield lower pop-off pressures. As a rule of thumb, we recommend to use the smallest diameter needle valve/seat that allows the pop-off pressure you are seeking. We recommend this because the smaller diameter seats are less susceptible to vibration leakage/damage, and therefore far less likely to turn into "dribblers". Always use the same size seats in multi-carb arrangements.
 
mikidymac said:
Seat and spring size make a big difference and makes the entire carb settings richer or leaner. Think of it like a master valve supplying different fuel amounts to the other jets and screws.

Pop Off Pressure - The pop off pressure is not an adjustment that exists on the carb, but rather a specification that is a combined function of the needle valve size, and the spring rate of the float arm spring. This specification has a wholesale effect on the fuel metering in the 0 - 40% fuel range. Because the pop off pressure has this far reaching range that overlaps with several other metering ranges, we consider it a fundamental starting point.

The term "pop off pressure" 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 the Mikuni and Kiehin carbs (unless otherwise specified) should be no less than 25 psi and no greater than 35 psi.

Needle Valve and Seat - The popular size Mikuni needle valves are 1.5, 2.0, 2.3, and 2.5 (these numbers designate the orifice size in mm. Some stock boats come with other sizes as well: 1.2 in the GP 1200, 1.8 in the 950 Sea Doo. The larger seat diameters will yield lower pop-off pressures. As a rule of thumb, we recommend to use the smallest diameter needle valve/seat that allows the pop-off pressure you are seeking. We recommend this because the smaller diameter seats are less susceptible to vibration leakage/damage, and therefore far less likely to turn into "dribblers". Always use the same size seats in multi-carb arrangements.
Click to expand...



Think I read that directly in one of the mikuni brochures as well.

I don't understand how it makes it richer/leaner though. Even in the brochure all it says the needle/seat does is simply act as a gate that allows gas into the metering chamber. It doesn't actually meter (or allow in) more fuel to the intake/engine.


The needle/seat allows gas into the chamber, but isn't it still up the actual metering jets to meter it correctly into the engine?

Theoretically if the pressure needs to be at 40psi, how does it matter if I obtain 40psi via a 1.2 or a 1.5 as long as I have 40psi at pop off? I understand a 1.5 would allow fuel in quicker, but it would also just seat again quicker.

Like I said...im not arguing...just trying to understand. Thanks for the post.
 
Remember, these are floatless pressure operated carbs... Everything is operated by either pressure or vacuum rather than being gravity fed into a bowl. Mixing and matching needles and springs to get the same pop off with two different needle sizes will make them open at the same time, but you’ll have a higher flow rate in the one with the larger orifice. With the way these operate, the needle and seat have a much more significant effect on metering than a needle valve in a float and bowl design...
 
JeremyD615 said:
Remember, these are floatless pressure operated carbs... Everything is operated by either pressure or vacuum rather than being gravity fed into a bowl. Mixing and matching needles and springs to get the same pop off with two different needle sizes will make them open at the same time, but you’ll have a higher flow rate in the one with the larger orifice. With the way these operate, the needle and seat have a much more significant effect on metering than a needle valve in a float and bowl design...
Click to expand...


Right. But just like a bowl/float carb...the needle/seat doesn't actually meter any fuel. It simply allows there to be fuel at the ready to be metered.

In these floatless/pop off carbs...there is the meter chamber with jets in it that do the metering. That orifice size doesn't change. The only way it would meter more or less is if the pressure varied. So if the pressure from the needle/seat stays the same regardless of seat size, then what is causing it to meter differently?
 
This might help.

POP-OFF PRESSURE AND LOW SPEED JET
How do pop-off pressure and the low speed jet work together?
These two circuits overlap, although the low speed jet continues past 1/4 throttle where pop-off pressure has little to no effect. In general, if your pop-off is slightly too high, you can compensate by increasing the size of the low speed jet. The opposite is also true; if the low speed jet is slightly too small, you can compensate with less pop-off pressure. Once you get to the point where you think each is adjusted correctly, it's best to try varying the two to make certain you have the best combination. For example: If you have pop-off pressure of 30 psi and a 67.5 low speed jet, you should also try a pop-off of say 35 psi and a 70 low speed jet. To verify that you have the correct combination there are two things to test:

When personal watercraft come from the factory they have fairly high pop-off due to the fact that they also have somewhat restrictive air intake systems that cause the engine to generate very high manifold pressures; the higher the manifold pressures, the higher the pop-off pressure required to properly regulate the fuel delivery to the engine. As you modify or change your watercraft's flame arrestor to a less restrictive type you will most likely start to experience a lean hesitation caused by a decrease in manifold pressure. This change will require an adjustment in pop-off pressure to regain crisp throttle response. Because most aftermarket flame arrestors are less restrictive than stock, you will need to decrease pop-off to compensate.
 
mikidymac said:
This might help.

POP-OFF PRESSURE AND LOW SPEED JET
How do pop-off pressure and the low speed jet work together?
These two circuits overlap, although the low speed jet continues past 1/4 throttle where pop-off pressure has little to no effect. In general, if your pop-off is slightly too high, you can compensate by increasing the size of the low speed jet. The opposite is also true; if the low speed jet is slightly too small, you can compensate with less pop-off pressure. Once you get to the point where you think each is adjusted correctly, it's best to try varying the two to make certain you have the best combination. For example: If you have pop-off pressure of 30 psi and a 67.5 low speed jet, you should also try a pop-off of say 35 psi and a 70 low speed jet. To verify that you have the correct combination there are two things to test:

When personal watercraft come from the factory they have fairly high pop-off due to the fact that they also have somewhat restrictive air intake systems that cause the engine to generate very high manifold pressures; the higher the manifold pressures, the higher the pop-off pressure required to properly regulate the fuel delivery to the engine. As you modify or change your watercraft's flame arrestor to a less restrictive type you will most likely start to experience a lean hesitation caused by a decrease in manifold pressure. This change will require an adjustment in pop-off pressure to regain crisp throttle response. Because most aftermarket flame arrestors are less restrictive than stock, you will need to decrease pop-off to compensate.
Click to expand...


Gotcha. Ok thanks....good info
 
And... the needles and seats do meter fuel under 1/4 throttle since you don’t have a needle on a slide like a motorcycle carb so they are metering the fuel along with the low speed jet and low speed screw. On these carbs the fueling circuits are cumulative, at wide open throttle the needles are fully open, the LS and HS screws are flowing and the low and main jets are flowing.

Also the pop off pressure is just a test and not how the carbs actually work when the engine is running. You have the fuel pressure trying to open the needles like the pop off test but the big thing is the engine vacuum pulling the diaphragm against the springs to open the needles.

So a small engine with a restrictive air cleaner is going to need a smaller seat because it doesn’t need much fuel at low rpm but needs a higher spring to resist opening too much with the high vacuum.
A big ported 951 would starve for fuel with the little seats from a 580 and not be able to open the needles with the strong springs because the air cleaners and porting cause low vacuum at the low throttle settings.

The other problem with big seats and light springs is that they are known to dribble fuel from engine vibrations and flood an engine at idle.

So even though some combinations of seats and springs overlap in pop off pressure it’s really not the same in how they actually work when on a running engine.
 
Like Miki was explaining, not all of the fuel being delivered is flowing through the main and pilot jets. Those jets would provide the same amount of fuel independent of seat size, but the fuel delivered by the the other circuits would not.
 
Not to hijack, but I think this is within the scope of this discussion. If you test your pop-off and it is barely within the specification, should you try to adjust it to be higher in the range? In mikidymac's carb rebuild tutorial (thank you for that btw), it is suggested that it might be better to be on the upper end of the range; however, Seadoosource.com states:

Pop Off Pressure Test
When rebuilding carbs, people constantly want to mess with popoff when in most cases it isn't necessary. I think they listen to too much bad advice telling them it is a step that has to be done. If your engine is stock and you're rebuilding your carbs, you should be reusing your oem springs and if installing a new needle, using the same size as oem. If you do that and adjust the arm height properly, you should never have to mess with popoff. Spring size and needle size pretty much determine popoff so why mess with that? Put it back together the way it was and run. If you rebuild the carbs and it doesn't run right with the same needle/spring combo you had when new, you've done something wrong in your rebuild or there's another problem in the system causing your issues.
Click to expand...

For example, my 95GTX has a specification of 19-35 psi pop-off with a 1.5 needle, and is currently popping off at 18-20psi (elevation = 519'). Assuming everything is stock, would it be better to change springs and increase the pop-off to be in the middle to upper part of the range, or leave everything as it is?
 
You’re better off leaving it alone as long as it’s within the specified range. Pop off testing is just a verification. The spring tension is what really matters while the carb is operating, and you’re only checking pop off to confirm that the tension is within range and noting is sticking or binding.
 
Status
Not open for further replies.

Similar threads

Landen Koch
Seadoo 951 pop off questions
2
Replies
29
Views
1,006
Landen Koch
Landen Koch
Dreed3345
92 580 won’t start
Replies
2
Views
140
mikidymac
mikidymac
Zig
Leaking carburetor after rebuild
Replies
2
Views
74
Zig
Zig
C
1997 GTX wont go past 1/3 throttle
Replies
1
Views
75
mbellomy
mbellomy
G
  • Locked
Carb Fails Pressure And Pop Off Test After Full Rebuild
2
Replies
28
Views
2,419
POPPS
P
Back
Top