Pop off pressure dual 38’s
- Thread starter TURBO JET
- Start date
- Location
- Riverside, CA
2 turns on the low (pilot) screw, and a low pop-off will kill top end.
Why do you Fricken think you need a different pop-off than the jet specs
for the pipe?? There is many things more important than pop-off. Pop-Off
isn't as important as some web sites claim.
Pop-off is inter-dependant on the low speed jet, pilot screw settings. Lowering
pop-off changes the jetting.
FPP makes a few mistakes in their instructions, and one common mistake
is on what springs are stock.
I see no reason why specs of 135m, 75low, 95gr, 1.5 seat, 3/4 pilot screw,
3/4 high speed screw, 43psi isn't a good starting point. Unless you are using an
impeller that has more pitch than recommended for your pipe.
If your engine starts, and transitions from the low speed to the high speed
circuits, the main requirements for pop-off have been met.
A carb with too much pop-off will not transition from the low to hi speed.
If your engine is hard starting, doesn't like to idle without eventually quitting,
(maybe it needs a fast idle) chances are you have a different problem.
Bill M.
IMO there is a lot of well meaning inexperienced advice in this thread.
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john zigler
Vendor Account
- Location
- wisconsin
Do NOT cut the springs...
Do NOT alter the needle arm too much. Doing so will affect the ratio at which the arm moves, and affect the tune. You want it level with the body of the carb.
75 low jet
140-145 main jet
1.5 needle and seat
95 gram spring.
John
Do NOT alter the needle arm too much. Doing so will affect the ratio at which the arm moves, and affect the tune. You want it level with the body of the carb.
75 low jet
140-145 main jet
1.5 needle and seat
95 gram spring.
John
- Location
- Riverside, CA
I really don't like making absolute statements. it is entirely possible that 2 mechanics
can have different skill sets and work experience that apply to the same subject. But this
nonsense about pop-off has been going on for more than 30 years.
First: Pop-off is a better tool for rebuilding carbs, than an individual circuit to be
modified.
1). Why, why do you think you need to reduce pop-off pressure more than once?
You lowered the pop-off pressure with the FPP pipe jetting(115gr to 95gr), and
then you want to reduce it again because it has aftermarket flame arrestors (FA)?
2). You are assuming that the flame arrestor is restrictive. But most Kawasaki,
SeaDoo, and Yamaha flame arrestors are not restrictive, and there is little too
no performance increase when you install an aftermarket FA.
3). The Mikuni Manual was written before the SBN was sold, and during the
70's, 80's, and very early 90's there were some restrictive oem FA's.
The graph in post #18 is another example of inaccurate documentation in
the Mikuni Manual. So is the idea that carbs dribble because the main jet
check valve is too thin.
4). Air is not sucked into the venturi, it is pushed in by atmospheric pressure.
5). I know what you meant to say, but: "because he has aftermarket flame
arrestors reducing the intake tract pressure" Reducing pressure implies
increasing negative pressure, hence increase ported vacuum (or air speed).
Both help the carb to increase the fuel to the venturi.
Another fun fact, fuel is pushed into the venturi.
Yamaha carbs were designed to run Hi pop-off pressures and lean low jets,
and they were equipped with a free flowing FA.
Yamaha carbs (compared to aftermarket sbn's) have a different throttle
butterfly and low speed by-pass configuration. They are not the same.
Stock FA are free flowing, it's loony tunes to change pop-off twice.
Which spec is that?
If I rejetted a super jet for a pipe, FA, head, porting and I kept the stock 115gr spring,
and the power delivery was "as expected," and you could hold the throttle wfo until
you were tired. Isn't that the goal??
It is possible because I treat the seat diameter and spring rate as team players, along
with the low speed jet and pilot screw.
Bill M.
Anyone can learn how to set-up their pop-off, low speed circuit, and pilot screw by
learning the Idle Drop Test. In addition the idle drop test will prevent the low speed
circuit from killing peak rpm, and make it easy to recognize when pop-off (and the low
speed circuit) is too rich.
can have different skill sets and work experience that apply to the same subject. But this
nonsense about pop-off has been going on for more than 30 years.
First: Pop-off is a better tool for rebuilding carbs, than an individual circuit to be
modified.
This is incorrect for many reasons.
1). Why, why do you think you need to reduce pop-off pressure more than once?
You lowered the pop-off pressure with the FPP pipe jetting(115gr to 95gr), and
then you want to reduce it again because it has aftermarket flame arrestors (FA)?
2). You are assuming that the flame arrestor is restrictive. But most Kawasaki,
SeaDoo, and Yamaha flame arrestors are not restrictive, and there is little too
no performance increase when you install an aftermarket FA.
3). The Mikuni Manual was written before the SBN was sold, and during the
70's, 80's, and very early 90's there were some restrictive oem FA's.
The graph in post #18 is another example of inaccurate documentation in
the Mikuni Manual. So is the idea that carbs dribble because the main jet
check valve is too thin.
4). Air is not sucked into the venturi, it is pushed in by atmospheric pressure.
5). I know what you meant to say, but: "because he has aftermarket flame
arrestors reducing the intake tract pressure" Reducing pressure implies
increasing negative pressure, hence increase ported vacuum (or air speed).
Both help the carb to increase the fuel to the venturi.
Another fun fact, fuel is pushed into the venturi.
Yamaha carbs were designed to run Hi pop-off pressures and lean low jets,
and they were equipped with a free flowing FA.
Yamaha carbs (compared to aftermarket sbn's) have a different throttle
butterfly and low speed by-pass configuration. They are not the same.
Stock FA are free flowing, it's loony tunes to change pop-off twice.
Just wanted to point that out not as a contradiction to what you are saying but just for clarity to others following along, pop-off
is just another component of tuning mainly to get your fuel delivery timing back on spec
Which spec is that?
If I rejetted a super jet for a pipe, FA, head, porting and I kept the stock 115gr spring,
and the power delivery was "as expected," and you could hold the throttle wfo until
you were tired. Isn't that the goal??
It is possible because I treat the seat diameter and spring rate as team players, along
with the low speed jet and pilot screw.
Bill M.
Anyone can learn how to set-up their pop-off, low speed circuit, and pilot screw by
learning the Idle Drop Test. In addition the idle drop test will prevent the low speed
circuit from killing peak rpm, and make it easy to recognize when pop-off (and the low
speed circuit) is too rich.
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- Location
- Riverside, CA
That is pretty close. The physics of a regular carb for a car and a float diaphragm
carb (FDC) are basically the same.
When a FDC carburetor has too much pop-off, it will start, but it will not transition
from low to high speed circuit.
Modern FDC carbs use a pilot circuit, not a low speed circuit that does both starting
and idling. So the screw that is commonly refered to as a low speed screw is actually
a pilot circuit screw for starting and idling.
Bill M.
@JKMike "Starting is controlled by the low speed adjuster and that is what you need to adjust to fix your hard starting"
On FDC carbs the most common problem is a vacuum leak. Vacuum leaks will barely
effect a car carburetor, but on a FDC the needle and seat will not open.
Make yourself a thread.
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- Location
- Annapolis, Maryland
So are you just saying the term "low speed circuit" is incorrect when referring to the OEM SBN Carbs and that its technically a "Pilot Circuit"?
- Location
- Riverside, CA
Yes, it is responsible for opening the needle and seat to start the
engine and control the idle mixture. It is not intended as a low speed
adjustment, but there is overlap.
A few older carbs had pilot circuits, but they were fuel only, and required
a different adjustment.
The newer pilot circuits on the SBN, SBN-i, and CDK2 carbs are fuel-air
circuits. These pilot circuits draw air from the low speed by-pass holes
and mix with the fuel at the pilot screw.
I am certain this was done for emissions testing, since all motorcycle and
watercraft were tested at an idle.
I am fairly certain the difference in the low speed circuits of Yamaha
carbs were done for emissions compliance too.
I have written a lot of info on the "Idle Drop Test" I used as a mechanic,
and the differences in tuning low speed starting & idling circuits, fuel
only pilot circuits, and fuel-air pilot circuits.
The idle drop test can also be used to configure the low speed jet, needle
& seat, spring rate, and pilot screw. You can also use it to determine changes
for weather and determine if a vacuum leak is present when screw settings
start requiring richer settings than normal.
Rich pilot screws kill peak rpm, which is very relevant in this thread.
Bill M.
The most common carb problem for all watercraft carbs is vacuum leaks.
FDC carbs can be affected by a vacuum leak on the main seal, besides all
of the obvious intake gaskets and pulse hose.
The problem with leaks on the vacuum side of the main seal is bound to
get worse in the future, as oem double spring seals are unavailable and
cheap Chinese main seals with a single spring and a dust lip become the
common replacement.
engine and control the idle mixture. It is not intended as a low speed
adjustment, but there is overlap.
A few older carbs had pilot circuits, but they were fuel only, and required
a different adjustment.
The newer pilot circuits on the SBN, SBN-i, and CDK2 carbs are fuel-air
circuits. These pilot circuits draw air from the low speed by-pass holes
and mix with the fuel at the pilot screw.
I am certain this was done for emissions testing, since all motorcycle and
watercraft were tested at an idle.
I am fairly certain the difference in the low speed circuits of Yamaha
carbs were done for emissions compliance too.
I have written a lot of info on the "Idle Drop Test" I used as a mechanic,
and the differences in tuning low speed starting & idling circuits, fuel
only pilot circuits, and fuel-air pilot circuits.
The idle drop test can also be used to configure the low speed jet, needle
& seat, spring rate, and pilot screw. You can also use it to determine changes
for weather and determine if a vacuum leak is present when screw settings
start requiring richer settings than normal.
Rich pilot screws kill peak rpm, which is very relevant in this thread.
Bill M.
The most common carb problem for all watercraft carbs is vacuum leaks.
FDC carbs can be affected by a vacuum leak on the main seal, besides all
of the obvious intake gaskets and pulse hose.
The problem with leaks on the vacuum side of the main seal is bound to
get worse in the future, as oem double spring seals are unavailable and
cheap Chinese main seals with a single spring and a dust lip become the
common replacement.
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The only problem I found with high popoff is on startup. Idle to 1/4 throttle is noticably (more) leaner until the pump side gets primed and up to pressure. The whole reason we measure the inlet side is because of the pump. Without that we would be measuring from the outlet and spring tension would be the only thing that matters.
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- Location
- Riverside, CA
I am not sure I understand what you are trying to say.
As I read it, you are inferring that fuel pump pressure has a significant role to overcome the pop-off,
similar too Klemm.
Bill M.
Have you ever deliberately repaired a vacuum leak as the sole problem on a Yamaha, Kawasaki,
or SeaDoo?
As I read it, you are inferring that fuel pump pressure has a significant role to overcome the pop-off,
similar too Klemm.
Bill M.
Have you ever deliberately repaired a vacuum leak as the sole problem on a Yamaha, Kawasaki,
or SeaDoo?
It is not very significant but definitely noticeable around 1/4 throttle. No big deal, after a couple of blips, 1/4 throttle mixture returns to normal. It just takes slightly longer with higher pop-off. At full throttle I have found that combustion temps do vary slightly with fluctuations of pump side pressure. One thing Ive noticed lately is some riders are installing check valves in their tank vents. The hot weather is pressurizing the tank and making the pilot circuit way too rich. The fix was either venting the tank or raising their (much too low) pop-off a little.
- Location
- Riverside, CA
Working on Kawasaki's during the 80's & 90's, I found that most problems
people associate with fuel pumps, are really caused by the needle and
seat not opening and the most common cause was a vacuum leak.
Sounds like you are talking about highly modified carbs having problems
with their pilot circuit.
Fuel tanks expanding is an old jet ski problem. Ski's would sit on the beach
and pressurize the fuel tank. Then the carbs would drain fuel into the engine.
SeaDoo solved the problem by using 2, one way valves. One to let air into the
tank, and a second 1 way valve rated at 2psi to allow pressure to bleed off.
Either way, a 1 way valve is a great way to prevent the fuel tank from venting
hydrocarbons into the engine compartment.
Bill M.
people associate with fuel pumps, are really caused by the needle and
seat not opening and the most common cause was a vacuum leak.
Sounds like you are talking about highly modified carbs having problems
with their pilot circuit.
Fuel tanks expanding is an old jet ski problem. Ski's would sit on the beach
and pressurize the fuel tank. Then the carbs would drain fuel into the engine.
SeaDoo solved the problem by using 2, one way valves. One to let air into the
tank, and a second 1 way valve rated at 2psi to allow pressure to bleed off.
Either way, a 1 way valve is a great way to prevent the fuel tank from venting
hydrocarbons into the engine compartment.
Bill M.
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I know a lot of guys using A/M tanks that vent inside the hull. My concern is always discounted. I run my vents outside with an OEM water catcher. I put an small asprin bottle over the vent hose with a tiny pin hole in it. This allows me to sub with very little, if any, water getting to the catcher. It took my X2 stranded inverted to get much water in the catcher.
I have always wondered if superjets need to have their tank pressurized to run correctly or the whole set up is to simply let air in to break the vacumn and let air in as fuel feeds the carb. If so, that SeaDoo set up really makes sense since you don’t end up with a “ballooned” tank on a hot day.
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- Location
- Riverside, CA
The simple answer is no. But just like the carburetors atmospheric pressure
pushes the fuel into the fuel pump. Atmospheric pressure pushes air into
the venturi, and atmospheric pressure pushes on the float diaphragm and as a result, it
(atmospheric pressure) opens the needle and seat, then pushes the fuel into the venturi.
The part I am leaving out is negative pressure, but with carburetion most everything
is pushed, not sucked.
Yup, lets air in (atmospheric pressure) to prevent a vacuum from forming inside
the tank.
During the summer, a small gasoline leak inside the engine compartment
will act like an exhaust leak.
Bill M.
"atmospheric pressure opens the needle and seat" This is why a vacuum
leak is so important. Atmospheric pressure doesn't suddenly increase to open
the needle and seat when there is a vacuum leak, so the needle and seat resists
opening, or while the engine is running (at a low rpm with a vacuum leak) the
needle and seat will close and the carb will just run out of gas (a common
problem).
Pop-off pressure is basically just a way to measure the spring preload with
air pressure. So if a vacuum leak is present, a lower spring preload might
allow the needle and seat to open despite the vacuum leak.
But Yamaha's came with a high pop-off stock, and without vacuum, leaks
stock Yamaha's run fine (as expected).
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- Location
- Riverside, CA
That is very similar to old 440/550's. The vent had a 1 way valve, and
ran up into the hand pole.
Bill M.