I have a full mod zen 26 and 257carb.
What is the best pop of psi for my 257 carb ??

I kept hoping someone would reply with good info pertaining to the Walbro
257. Since nothing was posted, here is something which might help. This
was written by a member jmatak9 on JRCBD and it refers to the Walbro 813.
This should apply to many Walbro models.

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jmatak9 wrote:

"ok, for those that dont know, you can adjust the pop off pressure
of the metering valve, which dictates amount of fuel flow into your
carb at any point in the rpm range. by lessening the pop off pressure,
you allow for less fuel pressure to be needed to enter the carb, thus
makeing it easier for fuel to flow into, thus thru the carb, maximizing
fuel available for the motor thru the entire rpm range.

Here is how its done on a 813
Take the phillips screws out and you'll have the metering lever & spring
in hand. I took about 5 coils off each end. That was aprox half of the
total (close together) coils on each end. The 668 carb looks to have a
different spring than the 813 I've been messing with. My 813 spring had
more coils wound close together on both ends but was shorter in total
length so in theory would make it stiffer having more coils in a shorter
spring. Anyhow all springs will be different as will personal set-up's
so it's whatever works for your app and not a "coil count" as what to
take off. I found a utility knife works best to cut the coils off. Lay
the spring on something metal and put the blade in the spring where you
want to cut it and rock the blade back & fourth while pushing down. The
nail clipper way spreads the coils apart and bends them and was a PITA
IMO. Good luck !!

Stock spring popped at 30psi
Cut off 5 coils from each side brought it down to 26psi
Cut all close coils from each side= 20psi
Cut two more coils from each side and it's about 14psi

This can be a very important mod on a Reed case motor, but due to all carbs
being slightly different, it may or may not be an effect you have, or will
ever have. but its good information to know, if your maybe having some quirks
that you cant track down initiallly, and your swayed to think its fuel supply.
a good indication your pop off pressure is too high is if your primer bulb is
constantly being emptied out after a long run.

your primer bulb should basically be 1/2 full or more pretty much all the time.
if you run, and see your primer bulb is empty or near empty, you will likely
see great benefits to lowering your popoff pressure, thus increasing fuel flow,
thus fuel supply to the carb and motor. I have also done this mod to a 990 carb
that I had on a twin after realizing the fuel bulb was almost empty after some
long high speed runs. Lowering the Pop off pressure helped a great deal.

Here are some useful Walbro sites with good info
Walbro Carbs Revealed-
www.thegreenbug.com/bpracing/...revealed_1.htm

Walbro Carb Tune Up & Illustrated Guide-
tech.flygsw.org/walbro_tuneup.htm

Walbro Website-
wem.walbro.com/walbro/"

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Thanks and credit to jmatak9 for sharing this info. I hope it helps you as much
as it did me.

I'll second that ... 14 PSI

Works awsome on most Walbro 12 > 15mm carburators.

My 're-worked'/bat shafted' 1048's love that.