Author Topic: JET SIZING TIPS  (Read 24269 times)

0 Members and 1 Guest are viewing this topic.

Offline crazypj

  • I'm brill, me
  • Old Timer
  • ******
  • Posts: 4,464
  • first 100,000 miles. 1977 CB550F
Re: JET SIZING TIPS
« Reply #25 on: May 05, 2012, 06:50:01 am »
Yes, as a starting point go rich and work down.
Short open pipe really messes with carburation.
Aftermarket open headers are even worse.
You can get a pretty major increase in power but, only at or after redline (plus, it really makes your ears hurt)
For drag racing it works on the street its a dumb idea
I fake being smart pretty good
'you can take my word for it or argue until you find out I'm right'

Offline lucky

  • Really Old Timer ...
  • *******
  • Posts: 6,721
Re: JET SIZING TIPS
« Reply #26 on: May 05, 2012, 10:37:22 am »

Righto - I am scouring the web looking for jet size advice.

I have a 1977 CB750F2 wit ha stock engine, running a 4 into 2 into 1 set of pipes finished off with a Lossa Engineering muffler.

I am also running one of Tintop's antipod filters.

The carbs have 115 mains in - but after today's ride and some sound advice I reckon I need to upsize the jets.

Based on the theory posted here - if the stock jets were 105:
 - the 4 into 2 into 1 pipes add 2 jet sizes
 - the antipod probably counts as individual pod filters and add 2 jet sizes
 - and then I subtract one jet size

I should go up three sizes from the 105's which I think makes it 112 (105, 108, 110, 112, 115, 118, 120)

- but the 115's aren't doing the job.

If you use a #42 idle jet,  a #120 main jet, needle in the middle position,
Mixture screw out 3/4 turn ,you will be very close.

Possible to  go down to a #40 idle jet since you have adjustable needles.
The use 1 to1-1/4 open on the mixture screws.

The Antipod is the wild card.

Important question. How much total square inches
of intake area is the Anti pod??????

Offline lucky

  • Really Old Timer ...
  • *******
  • Posts: 6,721
Re: JET SIZING TIPS
« Reply #27 on: May 13, 2012, 05:14:58 pm »
13:1 for max power, 14.7 is stochiometric ratio if it was possible to get 100% efficiency at all rpm/throttle openings
Engine will often be running at around 16~17:1 at cruising high gear, medium rpm small throttle opening
Idle/pilot jet rarely needs much if any increase as the air filters are not restrictive at low rpm

I think what is  happening is that the larger idle jet is helping by bleeding over into that first part of the 1/4 throttle position. that is why it works.

I suppose lifting the needle one notch would do that also.
As soon as i get some 1977 needles i will try it.

Offline crazypj

  • I'm brill, me
  • Old Timer
  • ******
  • Posts: 4,464
  • first 100,000 miles. 1977 CB550F
Re: JET SIZING TIPS
« Reply #28 on: May 13, 2012, 06:33:50 pm »
The idle/slow/pilot jet delivers fuel even at max rpm on most carbs (except Kei-Hin 3 jet - most stock Honda's, all with CV carbs) , but, even though it delivers fuel, total flow of pilot jet is limited by size of primary main jet.
The transitions through different area's really changes fuel delivery at different air flow and throttle positions
It's the reason Kei-Hin CV are so difficult to tune for pods and why the +1, +2, -1 up-jetting doesn't apply
 Mikuni , AMAL, and AMAL style carbs (CB350f 400f, 500f, 550f, 750) have pilot system and main system so total fuel is much easier to control
 
I fake being smart pretty good
'you can take my word for it or argue until you find out I'm right'

Offline joerocket9992

  • New Member
  • Posts: 1
Re: JET SIZING TIPS
« Reply #29 on: September 27, 2012, 07:31:45 am »
I have a 79 xs1100 with 4 into 1 exhaust and pod filters.  What should my main and pilot jet be changed to? 

Offline Variantbuilder

  • Full Member
  • *
  • Posts: 13
Re: JET SIZING TIPS
« Reply #30 on: March 05, 2017, 05:26:09 pm »
ok guys, so i have a cb750 single pods on each carb and a 4 into 1 exhaust. HELP! haha

Offline Variantbuilder

  • Full Member
  • *
  • Posts: 13
Re: JET SIZING TIPS
« Reply #31 on: March 06, 2017, 12:39:29 pm »
Stock says 105 and 40

Offline SKOL

  • Hot Shot
  • ***
  • Posts: 271
Re: JET SIZING TIPS
« Reply #32 on: October 18, 2017, 11:43:11 am »
Stock says 105 and 40

Given this and your original sizes... it would be 112.5 Main and 42.5 Slow

Offline BobR

  • Hot Shot
  • ***
  • Posts: 356
Re: JET SIZING TIPS
« Reply #33 on: November 10, 2017, 07:48:50 pm »
Stock says 105 and 40

Given this and your original sizes... it would be 112.5 Main and 42.5 Slow

 Soundsclose . I'm running a 4/1 with minimal baffling and a drilled airbox with 115's on my K4 that comes stock with 105's. Bob
Reality is merely an illusion, albeit a very persistent one. Albert Einstein
 CB750K4(F2 engine)

 

;
Honda