Carb Tuning help - Technical Question

Mitch-90

Mitch-90

Hi All,

hoping someone who is pretty clued up can help me out here as I'm struggling to find anywhere to answer my question.

I'm wanting to raise my pop-off pressure, to do so I need to lower my needle size.


Has anyone ever gone so small that they actually starve the carby of fuel and high RPM and causes it to lean/cut out?

I ask, as I have a rich condition, and first thing I want to try is raising the pop off, however the needle size is already small compared to what a lot of people seem to run on big bore 2-stokes.

Lee
 
iQRTurbo

iQRTurbo

According to the Mikuni tuning manual a 1.5 seat is able to flow more fuel than the fuel pump can transfer, therefore the seat should not be a restriction.
 
Mitch-90

Mitch-90

CARBONX2 said:
It could
What is your reason for wanting to raise pop-off? What carbs? What engine?
Click to expand...

a few things.

so its got a 1000cc engine, with custom 48mm carbs. mikuni internals. its a surf ski.
mid to top is an absolute weapon!

when you want to do a setup wake on flat water, it takes too long to clear (4 strokes at low rpm) before you can use the power to kick, and you may aswell just not bother with it.

another scenario is if you ride around at about 1/4 throttle and rip it open, same thing, it 4 strokes and then clears.


I need to confirm (have only gone off what I'm told), this is the setup
Pilot 125
Main 130
Needle and Seat 2.3
Low 3/4
High 1/2

Going off other peoples setups for similar engine/carb size, I'm worried about going too small in the N&S, everyone runs 2.3-2.5 one for a reason (unsure what that is yet).
I do need to check the spring, but pretty sure its 95grams, so theres an option to raise pressure without reducing flow at higher rpm
 
Jr.

Jr.

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From what your showing here, try this

Same jetting your stating
2.0 n/s 95grm spring.
Being custom carbs, im betting restrictors are drilled out, which is why your feeling you need more pop
T both carbs together and run a 70 main jet as a restrictor. You will probbaly be very close.

To answer your origional question, i routinely run 1.8 n/s on 1200 cc motors without issue.
They flow plenty
 
I

Iconicsuperpoop

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How about checking actual settings and jetting, first. Give us real, confirmed info. Otherwise, just wasting time guessing and hoping.

Read the manual BK linked. Understand it. Come back with real info., and maybe get in touch with the original builder.

(insert sarcastic remark here)
 
Mitch-90

Mitch-90

@Jr.

Thanks heaps mate.
Thats the answer i was after. It confirma what i want to try.

Can i ask. Is the restricter youre referring to the one in the return line?
 
Jr.

Jr.

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Mitch-90 said:
@Jr.

Thanks heaps mate.
Thats the answer i was after. It confirma what i want to try.

Can i ask. Is the restricter youre referring to the one in the return line?
Click to expand...
Yes Restrictor in return line. Try to keep both return lines as even as possible in length, while being as close to the carbs as possible.
 
Mitch-90

Mitch-90

Jr. said:
Yes Restrictor in return line. Try to keep both return lines as even as possible in length, while being as close to the carbs as possible.
Click to expand...

I'm a little confused.

Hypothetically if the restrictor has been drilled out, wouldn't that cause the carb to have less fuel avail?

sorry mate just trying to educate myself as much as possible here
 
Jr.

Jr.

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Correct, Its reduces fuel pressure, thus less fuel avail for bottom end performance
When the engine needs it most. The correlation between pop-off , low speed jet and restrictor jet size effectively
Meter fuel delivery into the motor for off idle response. Fuel pressure is a very key item.
I prefer to run Full Spec 49’s on big motors. Its very common to have remote high volume fuel pumps run on those carbs. Im stating this because its an example of how key fuel pressure is for certain setups. Every setup is different.
You need to find what works for your carbs. If you want to go into more detail on them here, or privately?
Im happy to make suggestions to you.
 
Mitch-90

Mitch-90

@Jr. that would be awesome if you could please mate

do you have facebook. messenger is pretty easy to chat on.
Lee Mitchener is my name!
 
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