Super Jet 61x jetting question with single 46mm Blackjack

[Johnny340]

This engine hits hard from mid to top end but doesn't run clean on the bottom. The lack of low end power is really noticeable doing a tail stand for example.
Here is the setup:

Main: 160
Pilot: 125 (recently changed from 130 which helped a bit)
Low Speed: 2 T out
High Speed: 2 T out
Needle: 2.5
Pop Off: 16 PSI (black spring)

I was surprised to see both mixture screws so far out but that's where they ended up with tweaking on the water last time...

By the way it also has: FPP Limited B-Pipe, Lightened Flywheel, Group K Hammer kit with freestyle porting and intake mod, ADA head with 35mm domes, Advent ignition, hooker 10/16 impeller, etc.

 

[oxnard111]

I'd call the man hiimself and see what he has to say

 

[kevbo]

If it only does it in a tail stand then your popoff is too low for the tank being over the carb. 16psi is awfully low, make sure the tank is fully vented and not holding pressure.

 

[Johnny340]

If it only does it in a tail stand then your popoff is too low for the tank being over the carb. 16psi is awfully low, make sure the tank is fully vented and not holding pressure.

Funny you should mention tank pressure because it does hold positive pressure that I notice when I open the gas cap. The one way valve allows air in to offset the fuel burn so why would more air than that go in? Do you vent the tank both ways: in and out?
I agree about the pop off pressure being low. Rather than just change the spring, I'm also thinking that the 2.5 needle seems too big and by lowering the needle size, that would also raise the pop off pressure at the same time (just by putting in the smaller needle). Does that seem like a good idea?

 

[Matt_E]

Why not give Jetworks a call and ask for a jetting recommendation? The tank vent is one way, by the way.

 

[Johnny340]

Why not give Jetworks a call and ask for a jetting recommendation? The tank vent is one way, by the way.

Good idea. I have a message in with them now.
Any idea why the one-way valve would let too much air in and pressurize the tank?

 

[Johnny340]

Wow, is Art at Jet Works ever knowledgeable. He too said that the pop off pressure was low. He also mentioned the affects of the flow control valve setting (not opening too early) and adding a 4mm restrictor to the pisser off of the head to keep water pressure on the head pipe. Set the top pipe screw to 5/8 to 3/4 open with others closed. For my setup, I will get a 2mm spacer to move the impeller forward for low end. And finally, bore the nozzle to 84mm.

 

[Cannibal]

Art is the man, just make sure you have a half hour to kill when you call. I'd like to sit down and have a beer or six with that guy.