tips for adjusting the popoff pressure

[segmentlime]

Hello,

i have a single mikuni sbn 44 carb, 61x case and 61x cylinders, i have a factory b-pipe. idk for sure but i think the head and domes are like stock. I guess also i should mention that i have a lowered hood and so I think maybe the ski still has the stock air intake filter and cover / flame arrestor? (is this right?) Please see pic

Is this chart ok to use for pop pressure targets?


If the measured pressure for a given configuration needs to go up or down but i am using the suggested spring and the measure pressure is fairly close, then is trimming the springs the way to adjust the pop-off pressure? Does shortening the spring by trimming some off a small piece of spring then reduce the pop off pressure?

Am i at least barking up the right tree?

Can the resultant pressure be changed just by stretching the spring (to get a higher pressure) or compressing it down firmly to make is shorter (for a lower pressure) if so?

Any tips for this process?

Again, as always, the expertise and knowledge so freely shared here is very much appreciated.

 

[john zigler]

Do NOT ever stretch a spring to increase pop off. It will not hold, and over time you will loose pressure.

You can cut coils to lessen pressure, and reduce pop off, but I will say we hardly ever have to do this.

May I ask why you want to reach a specific number? Usually on a set up such as yours, you will go with a specific needle and sea with spring combo, and will be good. Please know the pop off is just part of the jetting spec. Your numbers will work with your over all jetting. DO NOT get hung up on a pop off "number". How do you know your gauge reads the same as some one else on the internet that says you need to be at a certain number?

Please look for and read the Mikuni tuning manual. When you think you understand it, read it again. It explains all the circuits and how they are relevant in the SBN carbs.

We did a video on pop off......

 

[WFO Speedracer]

Defiantly not a stock head , stretching and cutting the springs is best avoided if at all possible, if you cut a spring it actually becomes stiffer overall , think cutting coil springs on a car , if you stretch a spring it always wants to return to it's original shape since metal has memory.

You are probably not going to hit those numbers in the chart exactly , but you should get fairly close , pure and simple if you need to increase or decrease the popoff you need to put in a different spring to accomplish that.

 

[waxhead]

Defiantly not a stock head , stretching and cutting the springs is best avoided if at all possible, if you cut a spring it actually becomes stiffer overall , think cutting coil springs on a car , if you stretch a spring it always wants to return to it's original shape since metal has memory.

You are probably not going to hit those numbers in the chart exactly , but you should get fairly close , pure and simple if you need to increase or decrease the popoff you need to put in a different spring to accomplish that.

I could not agree with this more. If some one ever tells you to cut springs, say thank you and back away

 

[waxhead]

Pop-off testing tip:
The pop-off should always be a clean pop, not a dribble. I see people testing them dry all the time, but this will almost always result in a dribble—even with brand new needle and seat assemblies.

Give the needle and seat a quick squirt of CRC or similar lubricant before testing. Then, pop the cover on (leave the diaphragm out), don’t bother with the screws—just hold the cover in place and go for it. The cover will protect your eyes and help you hear the pop clearly.

 

[kevbo]

With a single carb you can butcher the seat size and spring all you want. There is no second or third carb to match it up to. I have drilled seats and clipped/stretched springs back when I was exploring the limits of the “new” SBN carbs. The pop off effects mostly below half throttle. For instance, a 120 pilot jet will run as rich at lower pop off as a 122 would run with higher pop off. For single 44s I always use a 2.0 seat and start with a 95g spring. You can feel a big difference in part throttle jetting by changing the spring tension from that starting point. You WILL need to spend some time in order to find good response without being sloppy rich at part throttle.

 

[segmentlime]

With a single carb you can butcher the seat size and spring all you want. There is no second or third carb to match it up to. I have drilled seats and clipped/stretched springs back when I was exploring the limits of the “new” SBN carbs. The pop off effects mostly below half throttle. For instance, a 120 pilot jet will run as rich at lower pop off as a 122 would run with higher pop off. For single 44s I always use a 2.0 seat and start with a 95g spring. You can feel a big difference in part throttle jetting by changing the spring tension from that starting point. You WILL need to spend some time in order to find good response without being sloppy rich at part throttle.

Thank you for mentioning about the jets kevbo as the pop off charts just show needle and spring. i guess if dealing with issues during transition then distinguishing between too rich or too lean is key, and not just about where / when this is happening but how much fuel is there to come through?

 

[brapperdoodle]

Im assuming you are asking about pop-off pressure due to some kind of performance issue. I just want to mention, even though you are specifically asking about adjusting pop-off pressure it is important to make sure your fuel system is holding pressure and does not have any leaks. It’s also very important that your engine does not have any air leaks, both of those things can effect the way the ski performs and rideability. Also, your air box is facing the wrong direction. The long side should be facing the exhaust (turn it 90 degrees counter clockwise) Hope this helps!

 

[DylanS]

+2 on the clean pop for measuring pop off.

you can also make minute tweaks to pop off pressure by bending the arm but don’t go crazy.

 

[kevbo]

Practice on a twostroke leaf blower. They tune very similar to jetskis.

 

[segmentlime]

Also, your air box is facing the wrong direction. The long side should be facing the exhaust (turn it 90 degrees counter clockwise) Hope this helps!

With regard to the orientation of the air filter box, I have it turned as in the photo in order to get a little better access to the carb hi and low adjusting screws. Air comes in and goes down a round hole, do you think it makes any difference which way the long end of the rectangle is oriented on the hole?

 

[brapperdoodle]

With regard to the orientation of the air filter box, I have it turned as in the photo in order to get a little better access to the carb hi and low adjusting screws. Air comes in and goes down a round hole, do you think it makes any difference which way the long end of the rectangle is oriented on the hole?

No, but I do think that makes it easier for your engine to drink water. It's facing a different direction from the factory that's all I was pointing out. potato pot-ato

 

[WFO Speedracer]

Mine is turned this way on my 300SX because it's the only way it will fit, water intrusion is not a big concern on this one it usually stays dry as a bone inside , I don't even run a bilge pump , I did put in the oversized bilge system off of a TS sitdown in the ski when I built it.

That's not a ready to ride photo, it's sometime during the last build, notice no one way valve on the tank, no hose on the header pipe and no coupler cover, I am sure a few other things were changed and added since that was taken.