Topic: JET SIZING TIPS

[crazypj]

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

[lucky]

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???

[lucky]

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.

[crazypj]

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
 

[joerocket9992]

I have a 79 xs1100 with 4 into 1 exhaust and pod filters.  What should my main and pilot jet be changed to? 

[Variantbuilder]

ok guys, so i have a cb750 single pods on each carb and a 4 into 1 exhaust. HELP! haha

[Variantbuilder]

Stock says 105 and 40

[SKOL]

Stock says 105 and 40

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

[BobR]

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