787 Carb Pop Off

[Jnz]

Hoping to fix my fuel problems fixed. Got 4 new Mikuni kits to replace the aftermarkets and new needle and seats. My only questions are how to get the pop off adjusted right. Previously with the aftermarket kits I purchased black springs and had a pop off of 38. Now with after installing the new parts I'm getting 32 with the black springs. I want to be sure the arm is in the right spot and not bent to far? Where should the the arm rest. The manual says body, but there is a lower groove it could line up with also? If the pop is tested with the cover off and diaphragm out, is there a test with the cover on to be sure its working as it's supposed to?

Thanks for the help

 

[Jnz]

98 speedster is what this is for and the only part changed was the springs to black. Has 1.5 ns

 

[Coastiejoe]

MANY of the replacement springs simply are not accurate. I tell everyone to use your original springs unless they are defective for some reason.

 

[Jnz]

Boat came in pieces and carbs torn apart already. Used springs originally from aftermarket kit. Manual shows 38 +/- 12 so I went with the black springs.

 

[Jnz]

I'm just trying to figure out where the needle arm should be and is there a way to test and make sure everything's working once the diaphragm and cover are installed?

 

[Coastiejoe]

[MENTION=16022]Dr Honda[/MENTION] has good knowledge of these carbs. He can steer you straight. I will tell you this, if the carbs were rebuilt with an aftermarket kit you will be doing it again. They simply do not act the same and do not hold up.

 

[Jnz]

That's why I'm wear I'm at now I could never get anything to run right and been chasing problems which hopefully end here with Mikuni kits and new needle and seats since I'm at it. I just want to be sure its all good when I put it back together.

 

[Coastiejoe]

Did the right thing getting OEM kits....

 

[Jnz]

Yes, hoping this will take care of the issues.

 

[Jnz]

With new parts installed, 1.5 n+s, black springs I should be at 38, but 3 are close 30-32 and one is 28. In order for me to get these close I would have to bend the needle lever and right now they are all flush with the body. What should I do from here ?

 

[Dr Honda]

Pop-off isn't as critical as most people think. Why do you think it's a +-12 psi spec? a 24 psi spread is HUGE !!!!!!!

The reason is...

The pop-off test is bad. BECASUE... it's not how the carbs work. Not to mention... the number can change A LOT just depending on the oil, and how much you use. But unfortunately... it's the only way to check it on the bench.

When the carbs are mounted... the vacuum from the engine SUCKS the fuel from the diaphragm section... and in turn... that pulls the diaphragm down, and pushes the arm, and lifts the needle. (this is why things change a lot when you put on an aftermarket flame arrestor) In no way does fuel pressure play a role in this. (other than you need some to feed the carbs) That's why there is a bypass orifice in the carbs. This keeps the pumps from building excessive pressure in the carbs. So... the pop-off chart Mikuni made for us, is simply trials they did, and gave us a test.

When you use aftermarket kits... the springs are the wrong lengths, and the wrong tensions. Also... the aftermarket needle and seats are machined to the wrong tolerances. In turn... you may get them to "Pop" at the right pressure... but when in the carbs, you may or may not have enough Vac to make them open. OR... then may hang open with no pressure.

Because of that... the aftermarket companies would need to make up their own charts for the test.


With all that said... 28 psi, and 32 psi are both in spec. If you used the proper OEM parts... and you have OEM seadoo/rotax air boxes on the carbs... set the high and low needles to the book, and bolt them on. They will run just fine.


Oh... to the question on the needle arms... the book says to make them flush with the body. They will run perfectly right there. But I personally like them just a little higher than that. (maybe 1 mm) That helps with fuel at high RPM.

Hope that helps.

 

[Dr Honda]

Just a final FYI...

I've sent carbs out of my shop with a 3~4 psi spread with no ill effects. The big thing with the pop-off is if you see a big difference... then there is something wrong, and needs investigated. OR... if there is a leak... then you know it before trying to run them. (a leak is more of an issue that nailing down the pop-off at a very specific pressure)