Super Jet Pulse Line Material

[BrightE's]

Quick question over here - Is replacing the OEM pulse line with the much flimsier and thinner PVC (fuel lines) an adequate switch? The thinnner fuel 1/4" fuel line seems to fit the brass carb fittings just fine but I am concerned about it stretching (radially) and affecting my pop-off pressue.

I ask because my GF's 650 Superjet is all stock, so it has a pretty high stock pop-off pressure of 50psi.

Cheers.

 

[Matt_E]

Why are you replacing it at all? The OEM material (same for fuel lines and pulse lines) is better than any aftermarket material out there. Unless it's broken, I wouldn't replace it.

Also, the pulse lines have absolutely nothing to do with pop-off pressure. They only supply a vacuum pulse to the fuel pumps. Pop-off pressure is set via the needle and seat valve and spring on the other side of the carburetor.

 

[BrightE's]

There was some extreme water damage in the 650 when I bought it, and it caused a lot of the rubber to oxidize. I had to remove the pulse line to clean and rebuild the carb and when I put everything back together, the rubber line had become so hard it won't go over the brass fitting without cracking.

It was my understanding the the vacuum pulse from the crank case will vary depending on the resistance that the intake system creates (much like an aftermarket manifold and flame resistor would lessen that pulse, and require tuning of the pop-off pressure).

 

[BrightE's]

Maybe my understanding of the pulse line is backwards. I thought that pressure generated in the case opened the needle and seat and thus allowed more fuel in.

 

[Matt_E]

If you have to replace it, I'd recommend just getting black fuel-rated line from an automotive store. Should be 3/8 line if I am not mistaken.

And yes, you got it backwards. Pulse lines just supply a vacuum pulse to the carb mounted fuel pump(s) to draw fuel out of the tank.
Pop-happens via N&S and the spring. There is a hole on the diaphragm side of each carb. This provides atmospheric reference pressure. The other side of the rubber diaphragm is exposed to engine manifold pressure. As that goes down (engine revs up under load), the pressure differential will pull the center of the diaphragm towards the needle & seat arm, pivoting the needle open. Now fuel can flow from the pump side of the carb to the metering block side.
Only two things influence pop-off pressure: size of the needle & seat and the strength of the spring (well, arm angle also)

 

[BrightE's]

Awesome - thanks for the thorough explanation. Since we are on the topic. Where is the ideal reference point to start off with for setting up correct pop-off pressure. I don't like just going by the book (Repair Manual says set pop-off to 50 psi), but I like to know WHY??

I can understand why you might need to adjust it to compensate for differences in altitude, but what is the trigger that indicates what your pop-off pressure is supposed to be set at (other than taking the book's word for it).

 

[Matt_E]

It depends largely on carburetor size, whether there are one or two carburetors, flame arrestor design, and carburetor venturi design. The smaller the carburetor, the higher the pop-off you need (because the air will move faster through a smaller carb vs a larger one, and this decreases manifold pressure and increases the pressure differential).
More restrictive flame arrestors will require a higher pop-off than open style arrestors.
And so on.
Pop-off pressure can be adjusted in conjunction with the pilot jet - they overlap a bit in throttle range and may be used to compensate for each other.

Read the Mikuni owners manual here. It will answer lots of questions.

 

[Idok]

It depends largely on carburetor size, whether there are one or two carburetors, flame arrestor design, and carburetor venturi design. The smaller the carburetor, the higher the pop-off you need (because the air will move faster through a smaller carb vs a larger one, and this decreases manifold pressure and increases the pressure differential).
More restrictive flame arrestors will require a higher pop-off than open style arrestors.
And so on.
Pop-off pressure can be adjusted in conjunction with the pilot jet - they overlap a bit in throttle range and may be used to compensate for each other.

Read the Mikuni owners manual here. It will answer lots of questions.

and when youre done, read it again.....then one more time




and maybe once again, for good measure..unti you read it again

 

[norcal ex]

isnt it a 1/4 in line? 3/8 is the water lines.

 

[Matt_E]

Yah you're right.

 

[rwilliamtaylor]

Quick question over here - Is replacing the OEM pulse line with the much flimsier and thinner PVC (fuel lines) an adequate switch? The thinnner fuel 1/4" fuel line seems to fit the brass carb fittings just fine but I am concerned about it stretching (radially) and affecting my pop-off pressue.

I ask because my GF's 650 Superjet is all stock, so it has a pretty high stock pop-off pressure of 50psi.

Cheers.

Fuel line works great... I've had fuel line on all my skis with no issues.

 

[Matt_E]

For how long?
Most of that kind of line hardens over time, yellows, and gets brittle.

The Cold Fusion line I got last year seems to be holding up well, though.