Si24 carb - float chamber main jet feed question
Post Reply    Forum -> Not-So-Modern
Author Message
Enthusiast
PX
Joined: 28 May 2020
Posts: 98
Location: Bognor Regis
Sun Apr 11, 2021 8:18 am quote
I'm trying to understand the logic behind the float chamber main jet bore hole on si24 p2 carb. From another post I made it was mentioned that the 1.5 mm bore hole is restrictive if putting in large main jet. I've just got my dr177 pretty much spot on with an ac140 be5 and 120 mj running on sip road 2.0 plug is chocolate brown on all running conditions, wot runs and pottering around town etc. I now want to move to a polini box I have. Jet suggested is three over sip road 2.0 so if I start about 125 and work down figured I would be safe. But will my carb be able to feed a 125 mj with that standard 1.5 mm feed? I don't want to drill it as it looks a right faff and don't have the tools. A 125 mj is 1,25 mm so is under 1.5mm right or does it not work like this? I have sip version 2 fast flow tap fitted so will this help? I'm not gonna do polinibox if I can't run with what I've got to hand so your help appreciated. Thanks
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2705
Location: London UK
Sun Apr 11, 2021 8:51 am quote
Yes, important. If you imagine putting a 150 jet in series with a 125 the resulting flow will be less than either alone. Like resistors in series but nowhere near as simple to work out.
Only need a 2mm drill bit. Not difficult to do.
Enthusiast
PX
Joined: 28 May 2020
Posts: 98
Location: Bognor Regis
Sun Apr 11, 2021 9:10 am quote
Thanks for reply. But pragmatically, will a 125 jet work? I've got .25mm free in theory if its 1.5 mm . I likely only need a 123 or 122 jet. My 120 jet is working fine so will a 125 really not work on a 1.5 mm bore? Its easy to do if you've got the equipment to do it and clean it out etc. Compressed air blower, drills etc. I don't want to buy stuff just to stick a polinibox on.
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5756
Location: So Cal
Sun Apr 11, 2021 9:22 am quote
Yes it will work.
Enthusiast
PX
Joined: 28 May 2020
Posts: 98
Location: Bognor Regis
Sun Apr 11, 2021 9:26 am quote
SoCalGuy wrote:
Yes it will work.
Thanks mate. I will work down from 125 . Much appreciated
Ossessionato
VSX, Stella 177, VNX1T, V9B1T
Joined: 20 Jul 2018
Posts: 2901
Location: Staten Island, NY
Enthusiast
PX
Joined: 28 May 2020
Posts: 98
Location: Bognor Regis
Sun Apr 11, 2021 9:56 am quote
Thanks Swiss. Thats great. I found this excellent video on how to do it https://www.youtube.com/watch?v=K7TJXHEAfaA
Ossessionato
79 P200E, 62 Allstate, 2008 Stella
Joined: 11 Jan 2006
Posts: 2072
Location: Florence, OR
Sun Apr 11, 2021 10:36 am quote
I put a 5/64" drill bit with a speed hex end in one of those screwdrivers that has four bits and did it by hand. Total control and nice and easy. You got this!
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2705
Location: London UK
Sun Apr 11, 2021 10:55 am quote
ferriswolf wrote:
Thanks for reply. But pragmatically, will a 125 jet work? I've got .25mm free in theory if its 1.5 mm . I likely only need a 123 or 122 jet. My 120 jet is working fine so will a 125 really not work on a 1.5 mm bore? Its easy to do if you've got the equipment to do it and clean it out etc. Compressed air blower, drills etc. I don't want to buy stuff just to stick a polinibox on.
As everyone has said, easy to do by hand and just blow it out. Is easier than it looks.

Putting in a bigger jet than needed will work. However, only at WOT. When at half throttle or less the bigger jet will have less flow and will work like a jet that is too big. Fluid dynamics, eh.
Member
1959 Allstate w P125x Engine
Joined: 29 Nov 2020
Posts: 49
Location: Los Angeles
Sun Apr 11, 2021 11:31 am quote
Jack221 wrote:
ferriswolf wrote:
Thanks for reply. But pragmatically, will a 125 jet work? I've got .25mm free in theory if its 1.5 mm . I likely only need a 123 or 122 jet. My 120 jet is working fine so will a 125 really not work on a 1.5 mm bore? Its easy to do if you've got the equipment to do it and clean it out etc. Compressed air blower, drills etc. I don't want to buy stuff just to stick a polinibox on.
As everyone has said, easy to do by hand and just blow it out. Is easier than it looks.

Putting in a bigger jet than needed will work. However, only at WOT. When at half throttle or less the bigger jet will have less flow and will work like a jet that is too big. Fluid dynamics, eh.
Is there any advantage to enlarging that 1.5mm hole with his 1.22mm mj?
Hooked
2003 PX200
Joined: 28 May 2010
Posts: 386
Location: Perth, Scotland
Mon Apr 12, 2021 11:34 am quote
ferriswolf wrote:
Thanks Swiss. Thats great. I found this excellent video on how to do it https://www.youtube.com/watch?v=K7TJXHEAfaA
Thatís the video I followed when I drilled mine.

Make sure you protect rim of float chamber with tape, as shown in video.

I also put a little bit of tape on drill bit to ensure you donít drill in too far. I used a needle to gauge the distance into jet hole and put tape on drill bit same distance.
The needle is also good to show you the angle the drill bit needs to be so you donít veer off.

After that it was actually surprisingly easy.
Enthusiast
PX
Joined: 28 May 2020
Posts: 98
Location: Bognor Regis
Mon Apr 12, 2021 11:45 am quote
cazshie wrote:
ferriswolf wrote:
Thanks Swiss. Thats great. I found this excellent video on how to do it https://www.youtube.com/watch?v=K7TJXHEAfaA
Thatís the video I followed when I drilled mine.

Make sure you protect rim of float chamber with tape, as shown in video.

I also put a little bit of tape on drill bit to ensure you donít drill in too far. I used a needle to gauge the distance into jet hole and put tape on drill bit same distance.
The needle is also good to show you the angle the drill bit needs to be so you donít veer off.

After that it was actually surprisingly easy.
You've got a kick ass 225, what main jet are you on? I'm using a rather tame dr177 so trying to establish the optimum main jet you can run on an undrilled Si24 as I'm likely not going to be higher than 124 with adding polinibox. Thx
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2705
Location: London UK
Mon Apr 12, 2021 2:30 pm quote
Ray8 wrote:
Jack221 wrote:
ferriswolf wrote:
Thanks for reply. But pragmatically, will a 125 jet work? I've got .25mm free in theory if its 1.5 mm . I likely only need a 123 or 122 jet. My 120 jet is working fine so will a 125 really not work on a 1.5 mm bore? Its easy to do if you've got the equipment to do it and clean it out etc. Compressed air blower, drills etc. I don't want to buy stuff just to stick a polinibox on.
As everyone has said, easy to do by hand and just blow it out. Is easier than it looks.

Putting in a bigger jet than needed will work. However, only at WOT. When at half throttle or less the bigger jet will have less flow and will work like a jet that is too big. Fluid dynamics, eh.
Is there any advantage to enlarging that 1.5mm hole with his 1.22mm mj?
By far the biggest advantage to enlarging the 1.5mm hole is that, the mpg gets more economical. Does exactly the opposite of what you might guess.
  DoubleGood Vespa T-Shirts  
Post Reply    Forum -> Not-So-Modern
[ Time: 0.1249s ][ Queries: 24 (0.0214s) ][ Debug on ]